Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/27/20

Area Forecast Discussion

National Weather Service La Crosse WI

 1000 PM CST Sat Dec 26 2020

.SHORT TERM...(Tonight and Sunday)
Issued at 200 PM CST Sat Dec 26 2020

Light snow moves through Sunday causing some slippery travel for
some. Forecast hasn`t changed much in the last 12 hours.

GOES water vapor imagery at 1830Z showing good cyclonic shortwave
energy in the intermountain region north of the polar jet racing
into CA/NV. This energy develops low pressure in the High Plains and
tracks it across northern IL Sunday. Deep weak-moderate QG forcing
accompanies the shortwave trough, and extends northwest into a
northern stream shortwave dropping south from Canada. It appears two
main frontogenetic areas will form - to the northwest from southern
MN into northcentral WI, and east-central into southern WI. The
southern area appears stronger and more prolonged - and should
promote more precipitation in the deformation area of the low. This
is the most consistent signal for this system. However the signals
among various deterministic models develop various transient bands
and locations. The latest 26.15Z RAP has intensified the northern
band and the 26.12Z NAM is much stronger with the southern
deformation band ~0.25" QPF - very similar to the 26.12Z ECMWF.  The
26.12Z SPC HREF snowfall paints a mean snow band amount of 2-4" just
south of the WI Dells area running from NE>SW, but the max in the
ensemble is 4-6" showing the upward potential of the southern band.
Thus, locally the average on the northern side paints about 1-2" in
central/swrn WI. WPC snow probabilities only yield about a 50%
chance of over an 1" of snow in central WI from the 60 member super-
ensemble. Thus, amounts near 1 inch seem reasonable, but the
frontogenetic band will need to be monitored for position and
intensity - as obviously amounts could be higher.

So, the bottom line is it looks like decent forcing, fast moving
system, with the net snowfall more in WI as the system develops and
strengthens. Areas of northeast Iowa will probably see only very
light amounts of snow if any as the system struggles with
saturation. Much of the area is in a 0.5-1.2" range.

One final note is that the light winds, inverted profile in the low-
levels and 5-8F dewpoint depressions this afternoon could lead to
low clouds and fog forming this evening should higher levels clouds
remain thin. Worst case scenario could be low visibilities in fog
forming in the 7-10 pm time frame.

.LONG TERM...(Sunday Night through Saturday)
Issued at 200 PM CST Sat Dec 26 2020

Light snow mentioned in Short Term section should end Sunday
evening, exiting west to east. Good subsidence noted behind it with
some cold air advection.

Besides a quiet but colder Monday, focus is on the system midweek.
No surprise - 2020 isn`t going to leave quietly.

Still a bit early /Days 3-4/ but this has potential to cause
significant travel impacts Tuesday night into Wednesday. Ensemble
model members continue to focus on our area and vast majority
suggest warning criteria snow is certainly possible. If trends
continue, by this time tomorrow, we could be switching from possible
to probable to likely.

Similarities in medium range guidance include large amplification in
upper flow across the CONUS Tuesday and significant moisture
advection northward ahead of deepening short wave trough. Model
differences include how wave is handled /open wave versus closed/
and related speed or precipitation onset. Of course speed could also
impact total accumulations. In addition, track of cyclone could have
some precipitation type issues in southern and eastern parts of
forecast area with hints of warm layer advecting north in such
strong flow. A lot will depend on storm track which has not come
into agreement quite yet.

Agree with winter desk at WPC that latest GFS runs may be too open
and fast with this system. Ensemble approach suggest it might be
better to use blend of ECMWF/NAM at this point, although storm just
starting to enter NAM realm. Noticed 26.12z ECMWF was a little
faster, more in line with NAM. In broad terms, range of 4 to 8
inches across parts of the area certainly possible but details will
be worked out next day or two. Messaging will continue to be broadly
focused on expected impacts midweek.

Differences in speed of storm will certainly impact how quickly
things calm back down to what looks like a quiet end to the upcoming
week. At some point forecast temperatures may start getting cut or
lowered too with possible snowpack in place by then. Started this
process, especially late week with high moving in and lighter winds.

Hints at another short wave dropping through in neutral to northwest
flow next weekend that could be another light snow producer. Will
watch in time.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 1000 PM CST Sat Dec 26 2020

Cigs: mvfr/ifr cigs will move in Sun afternoon, holding through the
evening with some improvement possible overnight.

WX/vsby: Areas of light snow set to move in Sunday as a couple
shortwave troughs swing through. Meso models favor more impacts from
snow at KLSE (stronger shortwave), but amounts look to be an inch or
less either way. Expect vsby impacts with 1-2SM in any moderate snow
band but will hold with MVFR for the moment.

Winds: light this evening, picking up a bit from the northeast
toward 12z Sun. Winds continue to uptick some in the afternoon while
becoming more north/northwest.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...Baumgardt
LONG TERM...Shea
AVIATION...Rieck



Area Forecast Discussion

National Weather Service Binghamton NY

 940 PM EST Sat Dec 26 2020

.SYNOPSIS...
Minor to moderate river flooding continues across central New
York and northeast Pennsylvania this weekend. Otherwise, light
scattered lake effect snow showers will continue through tonight
before a dry day Sunday. A light mix of rain and snow is likely
on Monday. Colder air and lake effect snow returns behind this
system for Monday night and Tuesday. The pattern will shift back
to milder and wet conditions toward New Year`s Eve.

&&

.NEAR TERM /THROUGH SUNDAY NIGHT/...
930 PM Update...

Increased PoP, QPF and snow totals across Northern Oneida County
through the overnight hours. Lake effect band is starting to dip
south and graze Northern Oneida County. Latest CAM guidance,
especially the HRRR and NAMNest are quite aggressive now in
drifting the band into Northern Oneida over the next few hours.
If the band can stay organized for a couple hours, then a quick
2 to 4 inches will be possible across far the far northern
points of the county. Will continue to monitor Radar and update
as needed over the next couple hours as the lake effect band
progresses.

525 PM Update...

Weak shortwave pushing through Central NY combined with some
lake enhancement is making light snow more widespread than just
flurries this evening. Have increased snow chances from
Tompkins County on eastward to Otsego and Delaware Counties. Any
snow will be quite light, but some areas could see a half inch
of accumulation over the next couple hours.

330 PM Update...
We remain under a broad upper trough this afternoon, with
another shortwave rippling through the larger scale flow.
This has allowed chances for flurries and even a stray light
snow shower to spread across many of our central NY zones,
though for the most part most of the snow this afternoon
continues to stay to our west and northwest as southwesterly
flow is leading to persistent lake bands off of Erie and
Ontario. With fairly dry soundings this afternoon as the
aforementioned shortwave passes through, we really shouldn`t see
much by way off snow accumulations apart from perhaps northern
Oneida county, which continues to be grazed by those lake effect
snow showers. Otherwise, temperatures are peaking in the lower
to mid 20s, a couple degrees cooler than originally expected for
some spots, with lower level clouds in place.

Tonight, low level flow will shift more to the west after the
shortwave trough passes through. This will allow the lake band
to shift a little southward, with better chances for more
persistent snow across northern Oneida county. Still thinking
that we may be able to see another 1 to 3 inches by early
Sunday, so will continue to highlight this in our HWO.

Weak ridging into early Sunday and a drier airmass will allow
for a brief return to dry weather. More sunshine is expected,
and southwesterly flow should help to advect in some warmer air.
Temperatures starting in the teens to lower 20s peak in the low
to mid 30s with southerly winds at around 5 to 10 mph.

Clouds will be thickening and lowering into Sunday night as
another shortwave trough starts to dig into the Lower Midwest.
As this starts to move northeast through Lower Michigan,
chances for snow showers work back in late Sunday night into
early Monday morning. Overnight lows will be in the lower to mid
30s, but temperatures aloft will be warming. So, we will have
to watch for the potential for more of a wintry mix. That said,
most of the precipitation holds off until during the daytime
Monday as this system grazes us and moves into Ontario. So,
minimal QPF is expected as it is.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
400 pm update...
This period will feature a low pressure system passing to our north
on Monday, bringing a mixture of light rain and some wet snow. Our
area will then see a lake effect snow showers Monday evening through
Tuesday. High pressure builds over the region Tuesday night bringing
dry weather, light winds and seasonably cold temperatures.


Low pressure system will pass across south-central ontario monday
morning, with a cold front extending south across western NY and NW
PA. Our area will initially be in the warm sector of this weather
system and this warm air should be able to mix down to the surface
for a time during the day Monday...allowing temperatures to warm
into the upper 30s to lower 40s. As the short wave pushes across the
eastern Great Lakes with an associated surface cold front there
should be enough large scale lift and a modest amt of deep
moisture...PWATs around 0.5 inch...to trigger scattered light
rain/snow showers. The rain/snow line will likely be elevation
based, with locations above 1500 ft seeing mostly snow through the
day and the lower elevations with mostly rain from late morning into
the mid afternoon. Snow amounts through the afternoon in the higher
elevations of the Finger Lakes may reach 0.5 to 1 inch...while
little to no accumulations are expected in the lower elevations. The
other concern with this system early Monday will be the gusty south
winds. Most areas will have sustained winds around 10 to 20 mph and
gusts up to 30 mph...especially across the Finger Lakes where
downsloping will enhance wind speeds.

The cold air should begin to make a strong push into the area from
the west during the late afternoon Monday. This will allow any
lingering rain showers to change to snow showers, and likely become
enhanced with possibly some lake effect bands off Lake Erie pushing
into Central NY and a band off of Ontario getting into Oneida County
Monday night. The flow pattern will be generally westerly with the
air mass cold enough (-16C 850mb) to produce steep low level lapse
rates and enough lift in the dendrite zone for LES off the Great
Lakes. The inland extent will be a challenge. Model guidance is
hinting at a potential upstream lake connection, which would be
conducive for lake bands to extend well into central NY. So, some
areas of central NY could conceivably see another 1-3 inches Monday
night through Tuesday. The flow will veer to the northwest late
Monday night and during the day Tuesday, which will drop the Lake
Ontario band to the south. Conditions remain favorable through most
of the day as the 850mb ridge axis stays to the west until Tuesday
night. So, snow amounts could be more than 3 inches in parts of
Oneida County by the time the lake snow winds down late Tuesday
afternoon/evening. Much colder Monday night and Tuesday, with lows
in the upper 10s to mid-20s and highs mainly in the 20s.

420 am update...
Main concerns in the short term are focused on the next cold
front moving through Monday with a light rain/snow mix during
the day become all snow Monday night into Tuesday.

Weakly amplified short wave moving across the Great Lakes
Sunday night will draw up a relatively warm and dry air mass
along the Appalachians into the ern Great Lakes and Northeast.
This warmer air will be short-lived and mainly act to keep
Sunday night/Mon morning temperatures steady in the 20s and 30s.
This warm air should be able to mix down to the surface for a
time during the day Monday and allow temperatures to warm into
the mid 30s to lower 40s. As the short wave pushes across the
eastern Great Lakes with an associated surface cold front there
should be enough large scale lift and a modest amt of deep
moisture...PWATs around 0.5 inch...to trigger scattered light
rain/snow showers. The rain/snow line will likely be elevation
based, with locations above 1500 ft seeing mostly snow through
the day and the lower elevations with mostly rain from late
morning into the mid afternoon. Snow amounts through the
afternoon in the higher elevations of the Finger Lakes may reach
1-2 inches...while little to no accumulations are expected
lower down. The other concern with this system early Monday will
be the gusty south winds. Most areas will have sustained winds
around 10 to 20 mph and gusts up to 30 mph...especially across
the Finger Lakes where downsloping will enhance wind speeds.

The cold air should begin to make a strong push into the area
from the west during the late afternoon Monday. This will allow
any lingering rain to change to snow, and likely become enhanced
with possibly some lake effect bands off Lake Erie pushing into
central NY and a band off of Ontario getting into Oneida County
Monday night. The flow pattern will be generally westerly with
the air mass cold enough to produce steep low level lapse rates
and enough lift in the dendrite zone for LES off the Great
Lakes. The inland extent will be a challenge. Model guidance is
hinting at a potential upstream lake connection, which would be
conducive for lake bands to extend well into central NY. So,
some areas of central NY could conceivably see another 2-3
inches Mon night through Tuesday. The flow will veer to the
northwest during the day Tuesday which will drop the Lake
Ontario band to the south. Conditions remain favorable through
most of the day as the 850mb ridge axis keeps to the west until
Tuesday night. So, snow amounts could be more than 3 inches in
parts of Oneida County by the time the lake snow winds down late
Tuesday afternoon/evening.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
400 pm update...
Not a lot of change to this portion of the forecast with this
update. Most guidance does seem to be trending a little slower and
therefore warmer with the Thursday/Friday system...so the official
forecast followed this trend.

The large scale synoptic pattern across the eastern third of the
CONUS will become amplified once again Wednesday as a ridge builds
across the east coast in response to a deepening trough in the
central US. This ridge will allow a warm air mass to overspread the
entire region with 850mb temperatures in the +5 to +7 deg C range.
The air mass will also be relatively dry as well, so weather
conditions will be quiet Wednesday and Wed night...possibly into
parts of Thursday as well. There are some model differences with
respect to the timing and location of the precipitation associated
with the incoming storm system. There may be enough cold air in
place Thursday morning to produce some light wintry mix before all
the precip turns to rain Thursday afternoon...however this is
looking more and more unlikely.

At this time there appears to be a period of steady (possibly heavy
at times) rain Thursday evening through Friday morning associated
with a ribbon of deep moisture from the Gulf of Mexico. This
moisture will become entrained into the broad area of large scale
lift on the front edge of the incoming upper trough. There still
remain a number of uncertainities with respect to the timing and
location of the axis of heaviest rain, but the GFS, ECMWF and CMC
ensembles are all pointing toward the same general solution of more
than an 1 inch of rain across a broad area of the mid Atlantic into
southern New England. So, confidence continues to increase in the
potential for moderate rain during this time, but the details are
yet to be resolved and will likely not be complete until some time
early next week. It should be noted that the GFS and some of its
ensemble members develop a strong, more well defined area of low
pressure along the cold front, bringing it north through our
area...a few of the members are actually showing cold enough air
filtering in from the north for changeover to snow later Thursday
night into Friday...but this seems like an outlier solution at this
time. There are some meteorological similarities with this system
next week to the heavy rain event we just experienced a couple days
ago, but there are some antecedent differences that will also need
to be taken into account. This storm system, and conditions on the
ground/in the rivers, will need to be monitored closely as we
approach late next week.

There could be some weak lake effect snow showers or flurries behind
this departing system Friday night...otherwise high pressure builds
in for a dry and quiet start to the first weekend of the New Year.

420 am update...
The large scale synoptic pattern across the eastern third of
the CONUS will become amplified once again beginning Tuesday
night into Wednesday as a ridge builds across the east coast in
response to a deepening trough in the central US. This ridge
will allow a warm air mass to overspread the entire region with
850mb temperatures in the +5 to +7 deg C range. The air mass
will also be relatively dry as well, so weather conditions will
be quiet Wednesday and Wed night...possibly into parts of
Thursday as well. There are some model differences with respect
to the timing and location of the precipitation associated with
the incoming storm system. There may be enough cold air in place
Thursday morning to produce some light snow showers before all
the precip turns to rain Thursday afternoon.

At this time there appears to be a period of heavy rain
Thursday and Thurs night associated with a ribbon of deep
moisture from the Gulf of Mexico. This moisture will become
entrained into the broad area of large scale lift on the front
edge of the incoming upper trough. There still remain a number
of uncertainities with respect to the timing and location of the
axis of heaviest rain, but the GEFS, ECMWF and CMC ensembles
are all pointing toward the same general solution of more than
an 1 inch of rain across some broad location of the mid Atlantic
into New England. So, confidence continues to increase in the
potential for heavy rain during this time, but the details are
yet to be resolved and will likely not be complete until some
time early next week. There are some meteorological similarities
with this system next week to the heavy rain event we just
experienced a couple days ago, but there are some antecedent
differences that will also need to be taken into account. This
storm system, and conditions on the ground/in the rivers, will
need to be monitored closely as we approach late next week.

&&

.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
Mostly MVFR to VFR conditions expected through tonight, then VFR
everywhere tomorrow as high pressure builds into the region.
There will be some lingering lake effect clouds/snow showers
tonight with RME and SYR having the best chance of seeing any
light snow showers, but may not see any restrictions from the
snow. Keeping an eye on the lake effect band north of the area
in Lewis County, as some model guidance has it pivoting into RME
later tonight. However, confidence is too low at this time to
put IFR restrictions with snow, but do think that at least MVFR
ceiling will be possible at RME through the overnight hours. If
the band does shift south, then will handle with amendments.

Outlook...

Sunday night...Mainly VFR and there is a chance for low level
wind shear developing overnight Sunday.

Monday...MVFR or IFR restrictions possible with a rain-snow mix.

Monday night through Tuesday...Restrictions possible mainly
toward SYR and RME with lake effect snow.

Wednesday...VFR expected.

Thursday...Restrictions possible in rain showers.

&&

.HYDROLOGY...
We continue to deal with rivers running high; with several points in
the Susquehanna and Delaware Basins at or above minor flood stage.
At this time, Vestal & Waverly are still above moderate flood stage,
but will continue to fall, getting into the minor category by this
evening. Otherwise, expecting subfreezing temperatures and mainly
dry weather...which will allow water levels to continue to fall.

Be sure to heed any flood warnings that remain in effect...and be
weary of flood waters at night. Remember, never cross a flooded
road. Turn around, don`t drown.

All points along the Susquehanna and the East and West Branches of
the Delaware have already crested and will continue to fall through
the evening/overnight hours. It appear most, if not all river points
in our area will be back down below minor flood stage before 7 AM
Sunday.

Please stay up to date with the latest river forecasts and levels at
this webpage:
https:/water.weather.gov/ahps2/forecasts.php?wfo=bgm

&&

.BGM WATCHES/WARNINGS/ADVISORIES...
PA...None.
NY...None.

&&

$$
SYNOPSIS...HLC
NEAR TERM...HLC/MPK
SHORT TERM...BJT
LONG TERM...BJT
AVIATION...MPK
HYDROLOGY...



Area Forecast Discussion

National Weather Service Des Moines IA

 526 PM CST Sat Dec 26 2020

...Updated Aviation Discussion...

.DISCUSSION.../Tonight through Saturday/
Issued at 320 PM CST Sat Dec 26 2020

Synoptic Overview:

Axis of weak ridge is moving through the High Plains this afternoon,
leaving Iowa in an area of weak AVA. Although weak, subsidence has
been just enough to keep skies mostly clear across the forecast area
this morning, and weak surface high pressure has developed as well.
This has been keeping winds light. Satellite imagery does show large
area of stratus over Minnesota, which is currently on the backside
of the cyclonic flow. Further west, a short-wave pulse is moving
through the Sierra Nevada Region, and will be on track to move
through Sunday. Another short-wave trough is developing well north
of the region in Canada. Impacts to sensible weather from these
features will be minimal. The main forecast story will be a massive
PV anomaly moving across the CONUS, impacting Iowa through the
middle of this week. Substantial accumulating precipitation is
likely with this event. Long term guidance solutions continue to
demonstrate a large spread with regards to snowfall amounts, track,
and even timing. However, given the large areal extent of forcing,
some part of the forecast area is expected to encounter issues in
the middle of this week.

Today through Tuesday Morning:

Thermal ridge will continue to progress eastward across the central
Plains this afternoon but will deamplify as the short-wave trough
trails behind it, as well as another wave dropping south from
Canada. For today though, the approach of the weak axis will turn
low-level winds southerly, providing weak WAA. Across southern Iowa,
this will boost temperatures into the upper 30s, perhaps even lower
40s. For the rest of the forecast area, temperatures will be in the
lower to mid 30s. The stratiform cloud cover currently over
Minnesota has had a slow southward track, but has not entered Iowa.
It is possible for some of this to reach the far north later in the
afternoon, but RAP soundings continue to show a very dry profile
that it must contend with. During the overnight hours, the ridge
axis will pass eastward through the area, and short-wave trough will
move across the Plains. Surface lee cyclogenesis will begin
overnight, and will propagate east rather swiftly arriving in the
forecast area 12-14z Sunday morning. The center will pass along the
IA-MO border, leaving areas north of Highway 30 in a deformation
zone. The convergence is not all to impressive, overall forcing is
weak, and atmosphere does not have much moisture at the moment. As a
result, only light snow showers are expected for areas mainly north
of Hwy. 30. Accumulations will struggle to make it to half of an
inch. Overall impacts if any will be minimal. For Monday, another
weak thermal ridge becomes wedged between deepening trough over the
western CONUS, and short-wave dropping south out of Canada. Surface
high pressure develops over the Northern Plains in association with
the ridge, and is pushed southeastward toward the area by the short-
wave to the north. The flow will be predominantly northwesterly
Monday, which will keep temperatures on the cooler side. Tuesday,
the short-wave dropping south out of Canada quickly lifts back to
the northeast. As the trough over the western CONUS intensifies, the
ridge amplifies Tuesday morning and the axis moves eastward across
the area. Behind the ridge axis at the surface, lee cyclogenesis
begins and surface flow ahead of it turns southerly. This will begin
to advect warmer and somewhat moist air back into the region.
Convergence will steadily increase heading into Tuesday afternoon.
Then, the activity begins.

Large PV Anomaly:

For the past 4-5 days, there has been a lot of talk about the a
strong system moving across the CONUS during the middle of this
week. Long term guidance continues to depict a deep trough moving
through, dragging a strong surface cyclone with it. There is still a
lot of discrepancy between guidance. At 12z, the deterministic GFS
is still considerably faster with the system moving through the
area, being done with any activity by late Wednesday Night.
Meanwhile, the ECMWF remains much slower, keeping Iowa in the
deformation zone through much of all day Tuesday and Wednesday, and
even into Thursday morning. Diving into the ensemble world, there is
still quite a bit of spread. Each member paints a heavy band of snow
developing, with some members having this line from Council Bluffs
to Estherville, and others from NW Missouri through Ottumwa. This
kind of divergence in these solutions is main factor in the
uncertainty with this forecast. Several solutions still try to
depict insanely high snow amounts. With low confidence in the track
and timing of the storm, will refrain from mentioning any specific
snow totals, regardless of how troublesome a particular map may
look. Instead, let`s discuss what will need to be analyzed closely
over the next 3-4 days with this system. First, is the track. This
system most certainly will have a strong frontogenetical band
develop somewhere, and expect there to be plenty of lift through the
deformation zone. Where exactly that sets up, hard to determine at
this time. Secondly, it looks like quite a bit of moisture will wrap
around this system, meaning there will be plenty of latent heat
processes and other microscale processes ongoing. This will impact
the distribution of snow, rain, sleet, and maybe freezing rain. If
anything heads toward liquid rain, this would greatly eat into any
high snowfall accumulation. Third, this is a big synoptic event.
Tracking the warm and cold conveyor belts via water vapor imagery
will be necessary with this event. Typically in these highly dynamic
systems, a big dry slot can develop. This can either eat moisture
entirely, or, make determining precipitation type extremely
difficult to pinpoint, changing the distribution of how QPF is used.
For now, be prepared for high impact Winter Weather Tuesday-
Wednesday, and maybe into Thursday. In the coming days, as the
models get a better handle on the strong PV anomaly, these features
within the deformation should begin to show their hand. Once there
is better consensus on the timing and track, then a discussion about
snow amounts and the heaviest bands can start. But until then, can
only speak generally about the strength of this system.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 524 PM CST Sat Dec 26 2020

VFR conditions currently prevail across central Iowa. A stratus
deck with mvfr/ifr cigs slides south through the area Sunday
morning into early afternoon and resides through the end of the
TAF period. Some -sn is possible as well, but the spatial extent
remains in the question. Have included mention at KMCW and KALO
where confidence is current highest, and may need to expand
further south in the next update.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

DISCUSSION...Krull
AVIATION...Martin




Area Forecast Discussion

National Weather Service Gray ME

 1017 PM EST Sat Dec 26 2020

.SYNOPSIS...
An upper trough will cross the area today producing clouds along
with scattered snow showers across the higher terrain. High pressure
will gradually build into the region for the second half of the
weekend. A weak wave crosses to the north late Monday with light
showers possible. The next weather system to keep an eye on has
heavy rain potential and approaches from the west around the
New Year.

&&

.NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
1015 PM Update...No changes to the going forecast. The 1-3" snow
band across the north has developed as forecast and will remain
in place or drift south before weakening overnight.

630 PM Update...Just a few minor adjustments to the going
forecast at this hour based on latest trends in observational
data as well as latest mesoscale model output. Have adjusted
cloud cover based on satellite imagery, with much of NH covered
in clouds except SE. Have also sharpened up PoPs for the evening
and overnight hours, mainly across northern NH. A sagging cold
front picks up some Great Lakes moisture and likely produces
accumulating snow showers in Coos and northern Grafton Counties.
Followed a blend of the last few hours of the HRRR guidance on
that.

Previously...


Impacts: None anticipated.

Current satellite imagery shows clouds across much of the northeast
U.S. except for portions of southern New England and coastal ME. An
area of snow showers moved across northern portions of the region
today, providing at least a trace of new snow for some and possibly
very light accumulations for others.

Those showers were triggered by a shortwave passing through the
region, and it continues to push north and east into eastern ME. A
second shortwave and a weak surface cold front will push through the
region tonight, allowing some light snow showers to develop over the
mountains and far northern zones with very light accumulations
possible. Outside of those areas, clouds will gradually give way to
mostly clear skies as the night progresses. Lows will range from the
lower teens north to the upper teens to lower 20s south.

&&

.SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
Impacts: None anticipated.

Weak high pressure builds in behind the cold front and should
provide a mostly sunny and dry Sunday. Clouds will begin to stream
into the region in the afternoon as weak warm air advection begins
in New England ahead of a developing system over the Great Lakes.
Highs will range from the mid 20s north to the mid to upper 30s
south. Increasing clouds will be the story Sunday night as warm air
advection continues. Most of the region will be under mostly cloudy
to overcast skies by sunrise Monday, but any precipitation should
hold off until later Monday morning. Lows Sunday night will range
from near 20 north to the mid to upper 20s south and along the coast.

&&

.LONG TERM /MONDAY THROUGH SATURDAY/...
At the start of the next work week, the synoptic pattern will
feature troughing anchored north over Hudson Bay with a shortwave
traveling over the Great Lakes region to the maritimes... and a
Pacific low entering California. Starting on Monday, cyclogenesis
will be underway over Canada`s capital region. Locally, warm
advection will be well underway as this wave passes to the
northwest. Thus Monday will fairly warm, albeit under increasing
cloud cover with highs in the 30s and 40s. A few rain/snow showers
are possible over the higher terrain in warm advection, and then
again as the cold front crosses later that evening/overnight but
forcing won`t be particularly strong. A much cooler airmass invades
behind this wave with h850 temperatures running around -15 to -20C
by Tuesday, yielding highs in the 20s to near freezing with clearing
skies, a northwest breeze, and upslope showers ending early.
Will have to keep an eye on the timing and strength of this for
snow squall potential.

By mid-week, aforementioned Pacific low pressure carves out a deep
trough over the Rockies with a see-sawing ridge building ahead of
it. This ridge pushes to the East Coast with surface high
pressure building near New England Wednesday and Thursday. This
will be a quiet and dry couple day period with temperatures
rebounding above normal by Thursday as a southwest return flow
works in behind departing high pressure. Meanwhile, a double-
barrel low sets up upstream with the aforementioned trough
being kicked northeast by another incoming wave of Pacific
energy. This action sparks cyclogenesis along a frontal boundary
stretching from Ontario to the Yucatan... when and where this
occurs is the main question going forward in terms of storm
track and potential impacts to New England. Model solutions are
widely variable at this time with some solutions sliding the
system south of the area... whereas other solutions take it
north, down the Saint Lawrence. What ensembles favor regardless
of eventual storm track however is an anomalously warm airmass
building and the formation of an atmospheric river taking aim
some where along the East Coast. Not a great sign for winter
lovers. This wave eventually crosses by late Saturday with a
cooler airmass returning in its wake.

&&

.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
Short Term /Through Sunday Night/...VFR conditions expected through
the near and short term. Clouds will scatter out later this
evening into tonight as high pressure builds into the region.
This will also allow winds to diminish from the occasionally
gusty winds today. HIE could see some light snow showers this
evening or tonight, but confidence is low; no precipitation is
expected elsewhere through the period.

Long Term...Mainly VFR through the long range period. A passing
wave may bring periods of MVFR ceilings with a few rain or snow
showers. MVFR ceilings may linger into Tuesday over the
mountains.

&&

.MARINE...
Short Term /Through Sunday Night/...SCA remains in effect for the
waters and it has been extended to 12Z Sunday morning. The bays
will likely be able to come down before that, but with gusty
winds and high seas still out there, decided to leave them in
the SCA for now. Conditions should improve Sunday with weak high
pressure building over the waters and remain below SCA criteria
into Sunday night as winds begin to increase ahead of an
approaching cold front.

Long Term...A passing wave on Monday may bring seas to SCA
thresholds over the outer waters with winds possibly reaching
25+ kts over the waters and bays. Offshore winds behind the
system on Tuesday may be fairly brisk, to gale force. High
pressure crosses mid-week for a quiet period over the waters.
Another storm system approaches around the New Year which will
likely bring another period of active seas.

&&

.GYX WATCHES/WARNINGS/ADVISORIES...
ME...None.
NH...None.
MARINE...Small Craft Advisory until 7 AM EST Sunday for ANZ150>154.

&&

$$
SYNOPSIS...
NEAR TERM...Ekster/Watson
SHORT TERM...Watson
LONG TERM...Casey
AVIATION...
MARINE...




Area Forecast Discussion

National Weather Service Northern Indiana

 624 PM EST Sat Dec 26 2020

.SYNOPSIS...
Issued at 240 PM EST Sat Dec 26 2020

Strong warm air advection will continue through Sunday allowing for
temperatures to rise into the 40s Sunday. Otherwise increasing
clouds Sunday ahead of the next system which will bring a light
mix of rain and snow Sunday night.

&&

.SHORT TERM...(Tonight through Sunday)
Issued at 240 PM EST Sat Dec 26 2020

Ongoing warm advection will escalate this period in advance of
upstream wave disturbance over ID this aftn amplifying into the
central plains early Sun. While downstream mid-high cloud shield
expected, will still make a run for low 40s in the aftn. Thereafter
delayed moisture return into this sys tempers a more potent precip
response with primarily some light rain through the frontal scale
circulation Sunday evening followed by some muted lake response
primarily confined to srn MI tied to nwd confined low level thermal
trough progression Monday.

&&

.LONG TERM...(Monday through Friday)
Issued at 240 PM EST Sat Dec 26 2020

Brief but vigorous push of renewed cold advection swings in during
the day Monday as upper wave accelerates newd into QB Canada.

Thereafter substantial mid level ridging to commence across the ern
US in response to sig deep layer troughing through the plains as
competing strong nrn/srn stream wave disturbances intertwine and
eventually phase into a major storm through the cntrl US midweek.
This deep sys then lifts out ne through the lakes/OH valley with a
likely potpourri of precip types and substantial wind late period.

While local area initially remains firmly within broad warm sector
with rain late Wed - late Thu, following intense post frontal cold
surge supports a period of vigorous snow showers Fri-Sat and
potential swath of substantial synoptic snow inbetween pending
trailing frontal wave development late Thu-Fri.  Thereafter some
likelihood exits for another substantial lake snow event pending
track of leading upper trough sys and resultant flow trajectories
that develop in wake of this intense sys.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday Evening)
Issued at 623 PM EST Sat Dec 26 2020

A sharp cutoff of stratus clouds just south of SBN is expected to
continue for a couple hours into this TAF issuance before winds back
more westerly or southwesterly. Dry warm advection will continue
through Saturday on the back side of a high pressure system and
ahead of a low pressure system approaching from the west. The dry
air is expected to limit the intensity of the incoming rainfall and
it may wait until the this TAF period is over. The GLAMP25 and
ADJLAV are bringing MVFR CIGs into the area later in this period
with the entrance of that low pressure system, but will side with
the CONShort and RAP outlook of just above MVFR threshold for this
issuance. Will leave to future shifts to nail down timing and
intensity of lowered CIGs during the later Sunday time period. LLWS
also looks to come in during the evening hours tomorrow, but mixing
heights may preclude shear if they can last beyond 3z.

&&

.IWX WATCHES/WARNINGS/ADVISORIES...
IN...NONE.
MI...NONE.
OH...NONE.
LM...NONE.
&&

$$

SYNOPSIS...T
SHORT TERM...T
LONG TERM...T
AVIATION...Roller


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Area Forecast Discussion

National Weather Service Baltimore MD/Washington DC

 944 PM EST Sat Dec 26 2020

.SYNOPSIS...
High pressure will shift to our south tonight and to the east on
Sunday. A cold front will cross the region Monday. Another high
pressure will build across the region Tuesday and Wednesday
before departing east Thursday. A potent storm system may then
cross the region around New Years.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of mid evening, an upper-level trough axis was pivoting across
New England. At the surface, high pressure was centered near Mobile,
Alabama. A weak area of low pressure was situated along a stalled
cold front just north of Lake Ontario. Locally, a weak surface
trough was situated NNE-SSW, roughly along the I-81 corridor, and
a secondary area of high pressure was developing just southeast
of Baltimore, Maryland.

Subtle moisture advection beneath a stubborn subsidence inversion
around 4500 feet above the ground continues to result in an
equally stubborn thin layer of stratocumulus clouds across much
of northern Maryland, portions of eastern West Virginia, and
far northern Virginia. Guidance outside of the LAV doesn`t
really show these clouds explicitly (the RAP does, but appears
too aggressive too far south). Model soundings in BUFKIT show
the subsidence inversion weakening by late this evening, though
the 00Z KIAD RAOB showed more moisture between 4000 and 5000
feet above ground than any available guidance. There may also be
another slight uptick in moisture around daybreak, per the
latest high resolution guidance.

Basically, we took what was currently on satellite and placed
that into the gridded forecast database at hour 0, then
interpolated to the somewhat more aggressive/cloudier guidance
over the next 6-12 hours. This also required a slight nudge
upward in overnight low temperatures where cloud cover may hold
tough, but if clouds clear even briefly, this adjustment may
have been in the wrong direction.

Otherwise, cold high pressure will continue building eastward to
our south, but likely far enough south to allow some radiational
cooling to take place where the sky clears overnight. Thus,
tonight may in fact turn out to be a bit colder than Friday night,
when wind and clouds kept temperatures a little elevated.

The high pressure will continue to shift eastward Sunday allowing
southwesterly flow to develop. This should allow warm air advection
to take place. Sunshine should be more abundant with the ridge moving
overhead, so expect warmer temperatures.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/...
Southwest flow increases ahead of the next front Sunday night,
keeping temps elevated compared to tonight. Clouds will start to
increase as well. Some upslope rain/snow showers may affect the
Allegheny Front late at night, but rest of the region will stay
dry.

Front moves across the region Monday, mostly during the
afternoon. No precip expected east of the Allegheny Front, but
some upslope rain/snow showers, becoming all snow after frontal
passage and lingering into the night, could put down a small
accumulation along the Allegheny Front. Highs should be rather
mild, with 50s expected in much of the area, before the next
cold surge puts lows back in the 20s Monday night with a brisk
wind.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
High pressure builds in during the day on Tuesday, leading to
dry and cold conditions. We`ll continue that trend into
Wednesday as high pressure begins to exit the area by early
Wednesday morning. Highs will be in the upper 30s to low 40s for
the start of the extended period.

A potent low pressure system with a cold front will be tracking
eastward from the Midwest by Wednesday night. A few showers
ahead of the system is possible for Wednesday night before the
main impacts begin to be felt Thursday. A negatively tilted cold
front will be making its way into the area by early Thursday
afternoon. Latest GFS run is a bit faster with this system by
about 6 hours than the ECMWF but nonetheless, impacts expected
to be felt by Thursday afternoon. Essentially a potential
amplified repeat of what was experienced Christmas Eve is
possible for this system. Many global ensemble members are
hinting at an inch plus across most of the area. With an already
saturated ground and new additional precip expected, flooding
threats will likely be the main concern for this system.
Convective threat possibilities are non-zero at this time given
the setup with some GEFS members hinting at a HSLC scenario that
could pose a problem across much of the area.

Similar to the Christmas Eve system, a blast of cold air from a
deep digging trough over the middle part of the country could
allow for wintry precip to follow with the exiting of this
system late Friday night into early Saturday morning. At this
point, temperatures should not be quite as cold behind the front
as the previous system. Expect Thursday and most of Friday to
be wet and cold across the area. Will continue to monitor any
trends with this system with it being 5-6 days out.

&&

.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
VFR stratocu holding tough near/N of a line from KMRB-KDCA, with a
gradual erosion forecast overnight.

VFR is then forecast through Monday night. High pressure will
provide lighter winds through Sunday. Southwest winds increase
Monday then becomes northwest at night behind another cold
front, but little if any precip expected with said front.

VFR conditions expected Tuesday and Wednesday ahead of a potent
low pressure system moving in Thursday afternoon through
Friday, leading to sub-VFR conditions likely across most of the
area.

&&

.MARINE...
A period of northwesterly wind gusts of 15 to 20 knots are
expected through the first part of the overnight over the upper
part of the Chesapeake Bay, with a few gusts in the same range
(but more sporadic) possible over the lower tidal Potomac River
and adjacent Chesapeake Bay/Tangier Sound waters.

Otherwise, a light wind should dominate with high pressure
building to the south through Sunday. Increasing southwest flow
Sunday night into Monday ahead of the next cold front may
require SCAs.


SCAs expected Tuesday with high pressure just to our west, then
winds will gradually calm as the high pressure centers Tuesday
night into Wednesday before exiting. Expect SCAs or potential
Gales to be issued for Thursday and Friday with a strong cold
front passing through.

&&

.HYDROLOGY...
River levels are generally coming down with the exception of the
Susquehanna. The ultimate height here will depend on dam
releases from Conowingo but currently expect a peak by tomorrow
into the minor stage. The other remaining flood warnings should
end by tomorrow. While we will have a few days to dry out, the
soils remain saturated, and with another significant rain storm
expected late in the coming week, another round of flooding is
increasingly possible.

&&

.TIDES/COASTAL FLOODING...
Tides should generally stay below action with northwest flow
through tonight. However, Havre de Grace and the gauges near DC
may reach action or even minor depending how the high water
coming down the Susquehanna and Potomac affect these areas. As
winds turn southwest ahead of the next front by Monday, water
levels may increase again with some minor flooding possible.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 2 AM EST Sunday for ANZ530>532-
     538>540.

&&

$$
SYNOPSIS...RCM
NEAR TERM...RCM/DHOF
SHORT TERM...RCM
LONG TERM...ADM
AVIATION...RCM/DHOF/ADM
MARINE...RCM/DHOF/ADM
HYDROLOGY...RCM
TIDES/COASTAL FLOODING...RCM