Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/06/20
Area Forecast Discussion
National Weather Service Caribou ME
1018 PM EST Sat Dec 5 2020
Strong low pressure will cross the Gulf of Maine tonight, then
exit across the Maritimes Sunday through Monday. High pressure
will cross the region Tuesday, then exit across the Maritimes
.NEAR TERM /THROUGH SUNDAY/...
1010 PM Update: Wx types ovr the FA updated based on latest sfc
obs and precip types estimated from radar ref/CCs, which
indicates the transition line is traversing a little further N
and W then originally thought, currently running from
Bridgewater southwest to just west of Dover Foxcroft.
Subsequently, we adjusted the most northward xtnt of the
transition zone a little further N to about Caribou SW to alg
the ern periphery of Baxter State Park.
Snow totals will likely be reduced somewhat alg the ern
periphery of the wntr stm wrng area with a little more in sleet
In addition, far NE areas may see a reduction of intensity of
precip rates when precip goes back to all sn due dry slotting
wrapping back from the NE due to the maturity of the sfc-upper
low system entering into the occluded stage.
Orgnl Disc: Low pres moving toward Cape Cod late this afternoon
per latest sfc analysis. A coastal front extending from the low
up into the Maine coast. This coastal front is forecast by the
HRRR and NAM to move into the coast up into eastern Washington
County and then stall for a time, before drifting back off the
coast early Sunday. This will allow for colder air to work back
in to allow any rain/mixed precip to switch back over to snow.
Further n, snow overnight mixed w/a period of sleet as WAA moves
in aloft. The NAM soundings show that warm layer setting up
overnight into the early mrng hrs as far n as Caribou and
Presque Isle. Heavy banding potential is there and confidence is
high that a deformation band will set up portions of eastern
and central Maine where snfl rates of 1-2 inches per hr can be
expected. The upper level low is forecast to lift along and just
off the Maine coast. An upper front/trof will extend back from
the low across the aforementioned areas. This feature will be
the mechanism for that banding. Further s, along the Downeast
region, this deformation banding will enhance the heavy
rainfall. Temps will be a challenge overnight especially across
eastern sections of the CWA due to the proximity of the coastal
front and the warmer air trying to push northward. Therefore,
snow will be mixing with and changing to rain east of Bangor to
Lee line. N and west of that line, precip should be in the form
of snow w/the sleet mentioned above. So, we could be looking at
a foot or more of snow from parts of the Penobscot Valley up
into the St. John Valley. This would include the Maine Central
Highlands which could easily see 15" if the banding sets up just
The low will continue to intensify as it lifts to the nne
overnight into Sunday. Snow will be winding down from s to n
during the day. An interesting feature that was pointed out this
afternoon is that there could be some dry slotting working
across the NE Sunday morning which will shutdown the intensity
of the snowfall. The 12Z ECMWF and GFS show the banding to pivot
back the St. John Valley and North Woods into late morning
which will allow the snow to pile up in those areas before
winding down by the afternoon. Winds will be increasing during
the day leading to a good deal of blowing and drifting of snow,
despite the high water content. Not much drop in temps on Sunday
w/upper 20s to lower 30s north and low to mid 30s across
central and Downeast areas.
In regards to the hazards, Winter Storm Warnings remain in place.
A Winter Weather Advisory was issued to Nrn Hancock and Nrn
Washington Counties til 4 PM Sunday. A Flood Watch remains in
place for the Downeast. See Hydrology section below. A Wind
Advisory remains in place for the coast as gusts could hit 45
.SHORT TERM /SUNDAY NIGHT THROUGH TUESDAY/...
Broad low pressure will be entering the Maritimes and exiting
New Brunswick Sunday night into Monday. Expect some residual
snow showers across the north as moisture remains in the area
amid cool NW flow. These will taper early in the morning as dry
air slowly pivots in.
Clouds will remain overhead through Tuesday with high
temperatures widely in the upper 20s across the north and lower
30s Downeast. Winds will be a bit breezy, up to 25 mph at times
Monday afternoon. This may continue to blow around recent snow
accumulations for the northern half of the CWA.
.LONG TERM /TUESDAY NIGHT THROUGH SATURDAY/...
After the departure of this weekend`s storm, high pressure will
move over the region and bring a period of relatively quiet
weather. There is some discrepancy in the models whether a
trough will move in from the northwest and bring a period of
showers in the Weds/Thurs time period. Decided that a period o
showers during this time is likely, although stay tuned for
further details on timing. By Saturday the ridge starts to move
eastward, allowing another system to approach from the west.
However the models are showing a lot of disagreement on this
next system, so details are difficult to refine at the moment.
.AVIATION /03Z SUNDAY THROUGH THURSDAY/...
NEAR TERM: Mod to hvy snow will pivot across Aroostook TAF sites
this evening and overnight. This will keep ceilings and vis
LIFR. Gusty winds will also cause blowing snow. HUL will see a
period of PL mixing in with SN tonight. BGR will see a period of
heavy precip in the vicinity this evening with RNSNPL. LLWS will
move across all of northern and eastern Maine through early
Locations will trend to IFR mid-Sunday morning with some
remaining snow showers across the far north in breezy NW winds.
Sunday night: IFR to MVFR north, MVFR Downeast. Light snow
showers for Aroostook terminals. NW wind.
Monday: MVFR, possibly improving to VFR Downeast. Light NW wind.
Monday night: VFR. Light N wind.
Tuesday: VFR. Light N Wind .
Tuesday night: VFR. Light N wind becoming calm.
Wednesday: VFR with MVFR possible across the north with low
clouds and snow showers. Light SW wind.
Wednesday night: MVFR to IFR north, VFR Downeast. SW wind.
Thursday: MVFR to VFR north. VFR Downeast. Gusty W wind.
NEAR TERM: A Gale Warning stay in place through Sun. NE winds
25-30 w/gusts hitting 40+ kt later tonight as the low
intensifies as it moves across the Gulf of Maine waters. Winds
will go N later tonight and then shift to the NW on Sun as the
low pulls into the Maritimes. Winds will start to subside by
later in the day. Seas will build to a range of 12-16 ft, but
start to get knocked back by Sun morning as the winds turn more
SHORT TERM: SCA criteria continues Sunday night through Monday
morning with winds gusting to 25 kt, subsiding below 20 kt
during the afternoon. Waves will fall from 4 to 6 ft to 2 to 3
ft Monday afternoon as offshore NW winds decrease.
Looking for heavy rain w/upwards of 2.0 to 2.5 inches. This
could lead to some flooding of roads and washouts across
southeast portions of the region, especially and the Downeast
region later tonight into Sunday. A Flood Watch remains in
effect for the Hancock and Washington County areas. These areas
received 2 to 5 inches of rain earlier this week, and the soils
are saturated and streams are running high.
ME...Winter Storm Warning until 7 PM EST Sunday for MEZ002-005-006-
Winter Storm Warning until 4 PM EST Sunday for MEZ001-003-004-
Flood Watch through Sunday afternoon for MEZ016-017-029-030.
Wind Advisory until 7 AM EST Sunday for MEZ029-030.
Winter Weather Advisory until 4 PM EST Sunday for MEZ016-032.
MARINE...Gale Warning until 4 PM EST Sunday for ANZ050>052.
Area Forecast Discussion
National Weather Service Des Moines IA
538 PM CST Sat Dec 5 2020
.DISCUSSION.../Tonight through Saturday/
Issued at 200 PM CST Sat Dec 5 2020
1. Chance for light snow in northeast cwa this evening.
2. Well above normal temperatures middle of next week.
3. High uncertainty with pattern evolution late next week.
Active northwest flow will highlight the short term forecast period.
The first shortwave to cross the region is already diving south into
Minnesota this afternoon, evident on satellite as a wide swath of
stratus clouds Chances for light snow will increase in a zone of
weak thermal ascent ahead of the wave which may clip the northeast
cwa this evening. Model soundings struggle to fully saturate in the
presence of a dry airmass, but some hi-res members such as the RAP
generate enough lift and saturation within and below the DGZ to
support the idea of some light snow reaching the surface. Low end
PoPs highlight this potential with little to no accumulation or
impacts expected. A second wave dives southeast on Sunday although
with a more easterly trajectory over the Great lakes, which should
keep the area dry. However, the wave may help reinforce stratus
coverage over the area and hinder warming.
Western conus ridge slightly translate east over the first half of
next week. This will further continue our stretch of dry
conditions with above average temperatures. The warmest period
occurs as the thermal axis pushes east off the high Plains Tuesday
into Wednesday. High confidence that temperatures will easily
exceed model progs, and have continued the trend of boosting
forecast highs into the 75th to 90th percentile of the guidance
spread. Some locations cracking into the 60s cannot be ruled out
on Wednesday, especially in western Iowa.
Forecast confidence quickly evaporates late next week as the pattern
becomes much more active again. A series of shortwave troughs diving
southeast out of the Pac NW work to flatten the western conus ridge.
GFS has shown some run-to-run consistency with a much more dynamic
and amplified pattern evolution and the potential for rain/snow next
weekend. After several more benign runs the 12z Euro has also
shifted trending toward the more aggressive GFS solution. Stark
discrepancies exist between the deterministic and ensemble QPF
outputs through next weekend, so it is tough to deviate away from
the drier NBM solution just yet.
.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 534 PM CST Sat Dec 5 2020
Area of light snow showers forecast for far northeastern Iowa.
Flurries possible near MCW and ALO, and thus will state VCSH in
the TAFs for those locations, as the bulk of the activity will
remain further north and east. MVFR CIGs possible across the east.
Will need to monitor satellite trends at OTM closely for low-end
MVFR CIGs, most models paint an overcast/clear sky line just east
of OTM. For now, will keep OTM TAF at VFR, but should clouds
begin to work westward a bit, may need to put OTM in MVFR with
06z TAF issuance. Overall impact though should still be low.
Elsewhere at the other terminals, VFR conditions are forecast.
Area Forecast Discussion...UPDATED
National Weather Service Fort Worth TX
613 PM CST Sat Dec 5 2020
...New Short Term, Aviation...
.SHORT TERM... /NEW/
/Through Sunday Night/
While the radar scope will be quite active over the next 12
hours, the large amount of dry air will mean that most areas will
only see a few sprinkles. However, some model guidance has
suggested that a better risk for measurable rainfall may take
place across parts of North Texas during the pre-dawn hours on
Sunday through mid-morning.
Ascent continues to overspread portions of North and Central
Texas this evening as a strong upper low across West Texas
churns and lifts northeastward. As an aside, this upper low
actually produced about 2-6" of snow across West Texas earlier
today. NO winter weather is expected for North/Central Texas. Our
presently ascending 00 UTC weather balloon indicates a plethora
of dry air below 700mb and this dry air will likely mean that
most locations only experience sprinkles at best despite what the
radar imagery may depict. However, as the upper low ventures
closer to our region, the combination of evaporative
cooling/sublimation and strong ascent will moisten the column for
parts of the area. In fact, RAP forecast soundings near
Stephenville and western parts of the D/FW Metroplex suggest that
by 0900/1000 UTC the profile will become nearly saturated from
top to bottom. Hi-res guidance appears to hint at a mid/low-level
deformation zone along the north/northwesterly periphery of the
low. This frontogenetic band is reflected in 1 hr. probability
match mean (PMM) precipitation products from the HREF and suggests
that the best chance for measurable rainfall (very light and less
than 0.05") will be along and north of a line from Eastland to
Hillsboro to Corsicana after midnight.
The most concentrated ascent and thus 30% PoPs reside from near a
Graham to Cleburne line. This "banded" precipitation structure
will likely move northeastward toward the D/FW area around 3-4 am.
By 6-7 am, the "band" will likely be focused along the U.S. HWY
380 corridor. Again, this band will just be very light rain. I
cannot rule out a brief rumble of thunder or two nor can I rule
out reports of graupel or very small hail (which we have already
seen near Lake Whitney) given the steep lapse rates. I expect the
instances of either will be on a very isolated basis, unless we
can generate more instability. Something to monitor overnight. By
mid-morning Sunday, most of the ascent will have waned and shifted
to the east, so I expect certainly by noon onward, we`ll be
dealing with precipitation-free conditions across the entire
Once the upper low exits stage right, it`ll be a breezy and cool
day. However, sunshine will help temperatures climb into the 50s
and 60s. Tomorrow night (Sunday Night) should feature a good
radiational setup with clear skies, light winds and dry air
allowing temperatures to fall into the 30s. A light freeze is
possible for parts of western North Texas and the more rural
.LONG TERM... /Issued 153 PM CST Sat Dec 5 2020/
/Sunday Night through Saturday/
The week will start off with a deep trough over the eastern US
and a cutoff low spending some time near Baja. This will place
North and Central Texas beneath some weak ridging and nebulous
flow aloft. Generally clear skies and light winds are expected
which means low temperatures will benefit from radiational cooling
and drop into the 30s, while high temperatures will be mild and
reach the 60s and 70s.
The Baja upper low will get kicked eastward by mid to late week
and bring a cold front into the area around Thursday night. The
lift associated with the upper trough moving across and the front
will lead to a low chance of showers Thursday night and Friday.
Moisture and instability will be somewhat limited and right now
we only expect any rainfall to be less than a half inch.
Temperatures behind the front will be cool and drop back to near
Cluster analysis of all of the ensemble systems suggest guidance
has not yet settled on a solution for the upper level pattern late
in the week and next weekend. About half of the members keep
upper level troughing back to our west, while the other half are
more progressive. Meanwhile the 12z deterministic guidance has all
trended more progressive. The progressive scenario would lead to
clearing skies and dry weather by Friday afternoon and into the
weekend, but the inherent uncertainty in the forecast means low
PoPs need to be mentioned Friday night and Saturday. Any rain
would be light and of the over-running variety. However if trends
continue these may be dropped from the forecast and next weekend
may end up being another nice one.
/00 UTC TAF Cycle/
Concerns---VCSH and low end turbulence potential. Otherwise VFR.
VFR is anticipated through the entire 00 UTC TAF cycle. The radar
scope will remain illuminated through Sunday morning, but dry air
between the surface and 7,000 feet will keep most precipitation
from reaching the surface through the evening hours. Given the
potential for turbulence below (TB SEV reported near ACT around
2330 UTC) cloud base, however, have advertised a rather long
period of VCSH at all TAFs. Between 0900-1400 UTC, however,
stronger lift will spread across North TX and offers the greatest
opportunity for drops to reach the surface. Otherwise, cigs
between 7,000 and 10,000 feet AGL are advertised. Light/variable
winds will attain more westerly component through the nighttime
hours with stronger northerly winds anticipated after daybreak
.PRELIMINARY POINT TEMPS/POPS...
Dallas-Ft. Worth 43 58 37 63 38 / 30 30 0 0 0
Waco 40 60 37 64 38 / 10 10 0 0 0
Paris 39 55 35 58 35 / 10 20 0 0 0
Denton 40 59 31 65 31 / 30 30 0 0 0
McKinney 41 57 34 62 34 / 30 30 0 0 0
Dallas 44 59 39 64 40 / 30 30 0 0 0
Terrell 41 57 34 61 35 / 20 20 0 0 0
Corsicana 44 59 39 62 40 / 20 10 0 0 0
Temple 38 61 36 65 37 / 10 10 0 0 0
Mineral Wells 40 59 34 65 35 / 30 20 0 0 0
Area Forecast Discussion
National Weather Service Key West FL
953 PM EST Sat Dec 5 2020
A rope cloud feature associated with the leading edge of a shallow
front is drifting southward through Gulf waters, currently
located about 10 nm north of Smith Shoal through New Ground Rocks
Light. Patchy stratus was noted north of the boundary. Isolated
showers were present in the western Straits, and recently focusing
along the western portion of the boundary. Winds were very light
or calm, favoring a divergent offshore component from the
mainland. Temperatures were in the lower 70s throughout most of
the Keys, with 69F reported at the Key Deer Refuge RAWS toward the
north end of Big Pine Key. The evening sounding showed westerlies
had dried out the 750-450 mb layer, as a small mid-level trough
axis in the same layer has moved east to near Andros Island, just
slightly perceptable in the 10-7.3 micron difference GOES-16
imagery this evening.
The main weather issue ovenight will be the possible expansion of
the stratus field across the Gulfside/Bayside waters and
advecting southwest from the Everglades as high pressure moves
east along the northern Gulf coast. HRRR and other convective-
allowing guidance suggests some redevelopment of isolated, perhaps
scattered, showers over the Straits as the initial rope
cloud/boundary slides south of the Keys overnight. This scenario
along with the variable winds favoring north-northeast later
tonight is already accounted-for in the public forecasts; no
changes are planned this evening.
Light and variable winds this evening are expected to becoming
north to northeast at light to gentle speeds during the early
morning hours, then clocking around to southeast later Sunday. The
previous discussion from this afternoon states the weather
following Sunday night and onward well, and is repeated here: A
low pressure and strong cold front will result in a period of
showers and thunderstorms likely Sunday night into at least Monday
morning. Small Craft Advisories may be required during that time,
as southwesterly flow increases ahead of the front. After a lull
in wind speeds Monday afternoon, the front is forecast to sweep
through the Florida Keys coastal waters Monday evening, with
northwest to north fresh to strong breezes commencing. Small Craft
Advisories will likely be needed for some or all coastal waters
at that time. Northerly breezes will gradually diminish Tuesday
through Wednesday and begin to veer to the northeast to east late
in the week.
A stalling, shallow frontal boundary bringing light northerly
winds, along with rich low level moisture overtopping a
seasonably cool Gulf and Bayside will mean a significant
potential for stratus with IFR ceilings possible during the early
morning hours. KAPF (Naples Airport) and KMKY (Marco Island
Airport) had reported ceilings in the FL003-005 range for a couple
hours earlier this evening. The arrival could be as early as 05Z
at KEYW and following 06Z-07Z at KMTH. These conditions could
linger after 12Z, but at this time opted for a slow lift to low
MVFR before breaking up a few hours after sunrise. Light, mostly
north to northeast winds will become easterly midday Sunday.
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Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
840 PM CST Sat Dec 5 2020
839 PM CST
Currently monitoring a rather vigorous, but moisture-starved,
mid-level vort max moving through southeast Minnesota. Earlier
guidance was a bit too far west with the eastern extent of the
higher 850-700 hPa moisture represented by a 5-8kft cloud deck
entering northwest Illinois. Given upstream obs of snow showers
and the presence of a decent area of mid-level cloud cover over
developing low-level clouds, seeding from the mid-level deck
should result in some isolated snow showers across western
portions of the CWA (west of a Rockford to Morris to Paxton line)
during the overnight hours.
Also, inversion heights rising to around 5kft with appreciable
low-level omega profiles over and around Lake Michigan should
support isolated to scattered lake effect showers late tonight
through Sunday morning. Though 850 hPa temps will lower tonight,
delta Ts with the water temps will be marginal for lake effect and
should limit coverage and/or intensity. Synoptic low-level
E to NE flow with some minor land breeze enhancement from Lower
Michigan will likely result in an enhanced convergent mid-lake
band shifting westward through the overnight hours. Given the
overall weak flow, a kink or two in the form of a subtle
mesovortex in the band is a distinct possibility. This activity
will likely move onshore into far northwest Indiana and
central/southern Cook County by around daybreak Sunday. Thermal
profiles do support the potential for some rain to mix in, but
convective layer min temps of -8 to -9C should be sufficient to
introduce ice given the turbulent PBL.
136 PM CST
Generally low impact weather remains in the forecast, though
there are some low chances for a dusting of snow mainly lakeside,
with flurries possible at times. And while this is not the most
likely scenario, there could some light sprinkles with close to
freezing surface temperatures briefly late tonight near the lake.
The morning sounding from International Falls, MN, located close
to where tonight`s clipper system is currently located, depicts
saturation down to about 5kft and mid level lapse rates in excess
of 7 deg/km. To the immediate east of the upper low track, snow
has been falling with visibility as low as 2SM. Model agreement is
strong that this low will dig across the Mississippi River valley
and into Central Illinois. It is across this axis where better
chances for snow will be, but with the lapse rate plume moving
overhead and some mid level saturation, a few flurries cannot be
ruled out -- especially west of Chicago.
Behind this wave, a secondary surge of slightly deeper and colder
air will filter into the lower Great Lakes region. Low level flow
will remain similar to today but with an onshore component to
northeast IL at first, then across northwest Indiana. This will
lead to a slightly better, but still somewhat marginal lake effect
precipitation setup. Lake induced inversion heights will increase
to around 5,000 feet or so, and with slightly deeper cloud depths.
More marginal parameters this morning led to some spotty light
precipitation across northwest Indiana. There would anticipate
with some upper level support too, that lake effect precipitation
is likely in spots. The question is how widespread, how long, and
what precipitation type.
The lowest levels of the forecast sounding suggest ice crystal
formation will be marginal (as was the case today), such that snow
or some liquid precipitation will be possible. Overnight into
early Sunday there could be some seeding from the mid level
moisture layer though the dry layer between the mid and lower
cloud may be a bit too deep. So what`s the message here? Lake
effect precipitation is likely into northeast Illinois overnight
into Sunday morning, with a slow but gradual shift into northwest
Indiana with time. It will likely be light, spotty, not lasting
long in one spot, but better coverage than today. And it will
likely be a mix of snow/rain/and maybe freezing drizzle (for
elevated surfaces) in the morning with colder surface
temperatures. With the shorter duration, the impacts should be
low, but a light dusting in some areas cannot entirely be ruled
out. With more clouds, highs Sunday could struggle to reach 40.
The next system is still taking shape across north central Canada,
and will take southward dip through the western Great Lakes
Sunday. Any precip with this should hold off until Sunday evening.
300 PM CST
Monday through Saturday...
Most of the period will be quiet with two exceptions. First, on
Sunday evening and night, looking at potential for isolated snow
showers and marginal lake effect setup into northwest Indiana,
but impacts probably minimal at most. Then late in the work week
into early next weekend timeframe, probably our best precip chance
though confidence is low on timing. Temperatures will be near
seasonable Monday into Tuesday morning, then mild thereafter.
In the rather odd blocky, meridional mid and upper level pattern
we`ve been in thanks to anomalously and amplified western ridging
and downstream eastern troughing, another potent short-wave
disturbance is progged to track due south from western Hudson Bay
to overhead Sunday evening-night. Despite antecedent lacking
moisture in roughly 850-700 mb layer, think that there will be
enough large scale forcing (PVA and height falls) to support
isolated to possibly widely scattered snow showers. Guidance
continues to suggest a plume of steep mid-level lapse rates will
accompany the short-wave and aid development of convective snow
showers. With the above in mind and support from RAP and HREF
high-res sim-ref, added in some slight chance PoPs. Footprint of
snow showers will be small and they`ll be moving south at a good
clip, would think nothing more than a localized dusting on colder
surfaces in the offing.
Regarding the marginal lake effect setup, modest cold advection
accompanying the short-wave could allow lake induced inversion
heights to get as high as 5-6kft with some minor lake induced
CAPE. It appears that convergence could set up and favor primarily
northeast Porter County for activity, followed by loosely
organized band or bands *possibly* drifting west towards Illinois
shore early Monday before weakening. Lake remains rather mild so
modification of air mass over it could mean that if any patchy
coating of snow occurs, it might be a bit inland of the lake. For
temperatures Sunday night, expecting extensive lower cloud cover,
so temperature drop will limit temperature drop.
Monday should start partly to mostly cloudy and then northwesterly
dry advection aloft should scour out the cloud cover, with lake
induced clouds latest to erode. Forecast highs are a bit above
normal at upper 30s to around 40. Monday night will have a surface
high pressure ridge extending northeast across the area, so
assuming clouds aren`t an issue, probably the last near normal
lows of the week (low-mid 20s inland and around 30 Chicago).
Tuesday will start the warming trend though slower departure of
ridge axis and weak warm advection should keep highs primarily in
lower 40s, locally mid 40s points west and southwest.
The flattening of the western ridging will bring a surge of
unseasonably mild warm advection for the time of year in downslope
west-northwest flow from higher terrain in western US. Again
assuming no significant cloud cover issues, Wednesday will likely
reach or get a few degrees above 50F in most of the area. Most
recent guidance did speed up the passage of a weak cold front to
Wednesday night with winds flopping over to easterly by Thursday.
It will remain quite mild aloft, so especially for areas south of
I-80, 50+F highs appear probable again, with slightly cooler
conditions north (including rare for December minor lake cooling).
Have been noting that medium-long range guidance has been
indicating ejection of short-wave energy from a western trough. If
another wave can dig in behind lead energy and phase, that will
increase chances of a stronger system affecting the region.
Overall, confidence in evolution is low so low NBM PoPs appear
reasonable, but note that the ECMWF and GFS suites did speed up
timing of possible system to mainly Friday-Friday night for
primarily rain, possibly changing to snow before ending.
For the 00Z TAFs...
Aviation weather concerns this period relate mainly to MVFR cigs
and the potential for periodic chances for light snow showers
(possibly mixed with some rain).
VFR cigs prevail at the Chicago-area terminals early this evening,
but anticipate a gradual expansion to high-end MVFR through the
overnight hours as flow aloft switches a bit more northeasterly
and helps to push some of the lake-effect stratus inland again.
Regional TDWRs are picking up a smattering of light echoes just
east of GYY where some off-and-on flurries or drizzle will be
Monitoring a compact but strong disturbance rotating southward
across Minnesota which is expected to swing in overhead later
tonight. Forcing with this system is strong, but the depth of
lower-level moisture remains in question, as does the exact track
of the upper low even at this short range. Given surface
observations indicating cigs around 5-7 kft across western
Wisconsin, thinking is that guidance is a bit too dry in the low-
mid levels at this time. As a result, have added in a brief PROB30
for -SHSN at RFD overnight as the core of this upper-level feature
This feature will also result in slightly deepening inversion
heights over the lake on Sunday morning with the potential of
lake-effect precipitation increasing a bit. Still uncertain as to
the orientation/location of any lake convergence region, but
latest guidance is trending westward a bit, which would bring
chances into the MDW and GYY sites. Some question as well
regarding the potential for ice nucleation, so for now have
introduced some TEMPO groups for VFR -SHSNRA Sunday morning, even
though surface wetbulb temperatures would be cold enough for all
snow. Will continue to monitor guidance trends this evening.
Otherwise, high-MVFR cigs look to prevail tomorrow with light
surface winds which may oscillate between north-northwest and
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Area Forecast Discussion
National Weather Service Medford OR
222 PM PST Sat Dec 5 2020
.SHORT TERM...Through Tuesday Morning...One light rain band from
the approaching front made its way onshore a few hours ago and no
rain was reported with it. A second, more concentrated band, is
beginning to move onshore now. A few embedded showers are showing
convective activity with 40 dBZ returns on radar. No lightning has
been reported yet though. The threat for offshore and coastal
lightning should cease around 7 PM PST, as the cold pool moves
inland and the upper level trough shears apart.
The front has come in at a more southerly approach than suggested
by earlier model runs. The most recent runs of the HRRR show the
850 mb and surface flow from the south and this indicates stronger
winds in the Shasta valley, near Weed, than previously forecast.
Gusts are not expected to exceed advisory criteria, but could
approach 30 mph.
Snow levels will remain mostly above 5000 feet, dropping to
around 4500 feet as the showers taper off Sunday morning. The high
Cascades will get one to three inches of snow from this system.
The showers will end Sunday morning as the upper low departs,
moving south into Nevada. High pressure will build in behind this
system and is likely to be the most significant front until the
day eight through ten timeframe.
Fog/freezing is expected in west side valleys tomorrow night into
Monday morning and could develop to be quite dense, possibly
warranting Dense Fog/Dense Freezing Fog Advisories. There is
uncertainty as to which valleys the fog will lift and erode away
enough for the valley floors to see sunshine. It is very possible
none of the west side valleys fully clear. Some hi-res models also
suggest patchy fog in the Klamath Basin and Klamath Marsh. This
is possible with less snow and more liquid and mixed phase
precipitation with this front. However, confidence is very low and
so elected to exclude it from the forecast for now.
More fog is expected Tuesday morning. The ridge won`t have much
time to build as another system approaches. -Miles
.LONG TERM...Tuesday through Saturday...The long term begins with a
large cut-off low to the south of the western coast of Baja, and
broad, flat ridging over the western US. Upstream, a blocking ridge
will reside over the Bering Strait, or in that general vicinity.
Being features that models have been known to have frequent
difficulties with, the cut-off low and the blocking high appear to
be the wrenches in the model inner workings, keeping the GFS and
ECMWF suites from coming to any consistent solution. Given the wide
range of solutions in the ensemble members, have trended the
forecast towards a blend of the operational models, which
surprisingly enough, have fair agreement between them.
The ridge overhead Tuesday should shift east under pressure from a
weak ridge expected to move onshore late Tuesday into early
Wednesday morning. The front is likely to wash out as it moves
onshore, leaving chances for light precipitation confined to the
coast, the coastal mountains, and the Umpqua Basin. This front
appears to be the sacrifice necessary to allow a second trough to
dig into the region form the northwest on Thursday. Precipitation
will be a bit more widespread with this event, although still mostly
confined to areas along and west of the Cascades. Despite the lack of
precipitation, temperatures will be noticeably cooler after passage,
dropping to values more typical of early December.
Heading into next weekend, the overall pattern appears to become
more progressive, with the cut-off low opening up and re-joining the
westerlies, and numerous shortwaves lined up and headed our way out
of the eastern Pacific. The one caveat is the persistent blocking
ridge mentioned above, still holding position somewhere between
Siberia and western Alaska, and still keeping the upstream pattern,
and therefore the downstream details, uncertain. The operational EC
and GFS both resolve a significant frontal passage next weekend,
something more typical of our December storms, but confidence is
low, and the timing and/or strength of this system will likely
change over the coming forecast cycles. -BPN
.AVIATION...For the 06/18Z TAFs...Most of the lower level clouds
have cleared up this morning. Upper level ceilings will build
ahead of a cold front. This cold front will bring rain to locations
west of the Cascades during the late afternoon and evening hours.
Look for widespread MVFR ceilings and terrain obscuration once the
cold front moves through the region. Spotty LIFR and IFR
visibilities are possible in the valleys Saturday night.
.MARINE...Updated 200 PM PST Saturday 05 Dec 2020...Seas will become
high, steep and wind wave dominated this afternoon. South winds
will approach and may briefly reach gale force during the afternoon
with the strongest winds expected north of Port Orford. The
combination of swell and wind driven seas this afternoon and evening
will result in a very steep and hazardous warning level seas beyond
20 nm from shore and north of Cape Blanco with conditions hazardous
to small craft expected for all other areas.
Behind the front, high pressure briefly builds Sunday with seas
remaining high and steep into Monday morning due to west swell.
Another heavy, very long period swell builds early Monday with seas
likely becoming very high and very steep Monday afternoon into
Monday night. Guidance has been consistent in showing seas of 17 to
20 ft at 18 to 21 seconds Monday afternoon into Tuesday morning,
likely peaking Monday evening. This will lead to dangerous bar
crossings as well as hazardous surf conditions. Seas are expected to
remain high and steep, but trend lower Tuesday into Wednesday with
the next cold front moving through on Tuesday night. Additional
fronts are possible late in the week with seas likely to remain high
and steep into the week. /BR-y/Smith
OR...High Surf Advisory from 10 AM Monday to 4 PM PST Tuesday for
Pacific Coastal Waters...Small Craft Advisory until 10 AM PST Monday for
Hazardous Seas Warning until 10 PM PST this evening for PZZ370.
Area Forecast Discussion
National Weather Service Mobile AL
918 PM CST Sat Dec 5 2020
.NEAR TERM /Now Through Monday/...An upper low currently ejecting
from the southern Rockies out over the South Plains will open
into a positively tilted shortwave trough as it continues eastward
over the lower Mississippi River Valley through Sunday. Ahead of
this feature, synoptic ascent improves beneath the right entrance
region of a strengthening upper level jet that extends from the
Texas coastline northeast over the Deep South and Southeastern
states. With significant surface cyclogenesis lacking, low-level
winds remain out of the north and we don`t see the typical onshore
flow and moisture return ahead of this system. Thus, vertical
moisture profiles remain quite dry, which will play a large
factor in who sees rainfall from this system on Sunday and how
Per the 00Z LIX sounding, seeing a deep layer of dry air extending
from the surface up to around 600 mb (which matches well with the
00Z HRRR analysis soundings). It`s therefore no surprise that
surface obs are reporting little to no rain beneath the stratiform
shield over SE LA right now (as seen on the KMOB WSR-88D). It`s
going to take some time for the dry air near the surface to
moisten via sub-cloud evaporation alone, likely all of tonight and
through Sunday morning. However, by Sunday afternoon and evening,
we should begin seeing instances of light rain across primarily
the southern half of our local area as the low levels finally
moisten enough for rain to reach the ground. Maintained 30-50%
PoPs across these areas during that time to reflect this, with
lower chances (~20%) farther inland. Accumulations will be light
and likely measured on the order of hundredths of an inch. Rain
tapers off from west to east Sunday night as the upper shortwave
pushes east of the area and the forcing departs with it. Highs on
Sunday remain mostly in the 50s (some locations could see temps
right around 60) due to the thick cloud cover and sub-cloud
evaporative cooling. Lows Sunday night dip into the upper 30s to
low 40s. A dry, deep-layer northwesterly flow then sets up Monday,
with clear skies and temps holding steady in the 50s. /49
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