Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/28/20
Area Forecast Discussion
National Weather Service Denver/Boulder CO
723 PM MST Sun Dec 27 2020
.UPDATE...
Issued at 710 PM MST Sun Dec 27 2020
Update to the overnight forecast to adjust low temps & add a
little fog and snowshowers in the northeast. For temps, felt the
overnight lows, especially east on the plains, were too cool based
on previous nights performance. Blended in a healthy dose of bias
corrected ECMWF & collaborates well with neighbors. Patchy fog - HRRR
and RAP both break out lower visibilities in northeast CO, HRRR
is more widespread and centered on the Platte River Valley and RAP
is more NE Morgan/NW Washington counties and north. Snowshowers -
radar picking up some returns, probably very light and mostly
aloft, across northern Weld County. HRRR identifies this feature
and moves it ESE across northeast CO. No accumulations expected
with this light precip, but worthy of a mention and based coverage
and movement on the HRRR forecast.
&&
.SHORT TERM...(This evening through Monday)
Issued at 230 PM MST Sun Dec 27 2020
Currently, there is significant upper level cloudiness moving
into the CWA in the strong flow aloft. There are still some minor
light snow showers falling mainly along the divide right now.
Winds over the plains are all over the place, but most locations
have some form of northerly component to them. Models keep a 110-
135 knot west-southwesterly jet maximum over the CWA tonight and
Monday. The QG Omega fields show synoptic scale subsidence over
the CWA most of tonight. Upward vertical velocity is progged all
day Monday for the forecast area. Easterly upslope low level
winds are progged over the plains after 06z tonight. The plains
have southeasterly winds in place most of Monday. Moisture is
lacking in the lower and mid levels of the CWA this evening.
However moisture begins to significantly increase after 06z
overnight. By 15Z Monday morning, deep moisture is in place over
all the CWA. This continues all of Monday. Models show measurable
snow, in varying amounts, over all the plains on Monday. There is
the jet and the associated synoptic scale energy along with
plenty of moisture and an easterly component to the low level
winds. However, the better snowfalls for the plains generally
entail a decent new cold airmass with accompany north-
northeasterly upslope flow. Will refrain from any highlights for
plains at this time. For the high mountains, advisory criteria
amounts look okay. The progged mountain top wind direction is not
great for orographic enhancement in out mountains but all the
other ingredients are there. Zone 31 could get into "warning"
criteria amounts eventually on Tuesday. The southeasterlies
getting into South and North Parks will increase amounts there as
well. Those two zones will be included in the Advisory. For
temerptures, Monday`s highs are colder than today, about 2-4 C
over the plains.
.LONG TERM...(Monday night through Sunday)
Issued at 221 PM MST Sun Dec 27 2020
Storm system described in the short term period will continue to
produce widespread snowfall across the mountains and plains Monday
evening and Monday night. Upper low will continue to drift
eastward Monday night, and will begin to interact with a piece of
northern stream energy. Southwesterly flow aloft under the left
exit region of a strong upper jet streak combined with cyclonic
vorticity advection should contribute to widespread (but modest)
synoptic scale lift... near -10 to -25 mb/hr. Much of the flow at
low to mid-levels will initially be southeasterly, before
transitioning to southwesterly by Tuesday morning. This should
favor southwest facing aspects in addition to portions of South
Park, which could see a decent period of upslope flow early
Monday night. The position of the upper jet should also favor some
banded snowfall, especially with fairly steep (-6 to -8 C/km)
lapse rates over the higher terrain. Recent CAMs have shown banded
potential quite well, though there is some disagreement on the
positioning of the band. Models seem to like the Park Range and
into North Park, with additional band(s) setting up somewhere in
the central mountains.
Meanwhile, on the plains... continued isentropic ascent should
produce light snow through Monday night, especially across far
northern and northeast Colorado. The Denver metro area looks a
little quieter, though a few of the snow bands could make it into
the metro area. A few inches of snow will be possible from Fort
Collins-Greeley- Akron and north, with an inch or so to the
south. There still is a good amount of uncertainty if a sustained
period of snow can make it into the Denver metro. Recent CAMs are
a little more enthused with bringing slightly higher totals to the
I-25 corridor, and our current forecast is a nudge up from the
previous. Expect some minor tweaks as we draw closer to the event.
By Tuesday, the upper trough axis slides east of our area. While
moisture will be a little more limited behind the front, northwest
flow and sufficient moisture should allow for northwest favored
mountain aspects to see continued snow showers. Low-level flow
becomes northerly over the plains, and there could be some
continued light snow, especially over the Palmer Divide, through
much of the morning and afternoon. System should finally be
completely out of the are by Tuesday evening. Storm total snowfall
amounts are tricky given that banded snow is expected. Over most
of the mountains, 6 to 12 inches of snow is likely, with higher
totals possible if prolonged banding can set up. We`ve issued
Winter Weather Advisories across the higher terrain and North/South
Parks, though it wouldn`t be surprising to see one of these zones
upgraded in the future. On the plains, a few inches of snow
possible along and north of Fort Collins to Akron, where
advisories may be needed eventually. Further south, an inch or
two is expected for the Denver metro and points to the south and
east.
Forecast becomes more complicated beyond Tuesday. Wednesday looks
quiet as drier northwest flow dominates. Guidance then suggests
another trough embedded in northwesterly flow will track into the
Rockies by Thursday/Friday, with additional chances for light snow
across the mountains. Overall the pattern looks relatively cool
into the weekend, before ridging begins to build back into the
region by Saturday and Sunday.
&&
.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 450 PM MST Sun Dec 27 2020
VFR overnight, then deteriorating conditions Monday morning with
snow moving into the region. Gusty west to northwest wind at the
beginning of the TAF period will lessen by 02z with a shift around
to the east and then southeast after 04z. By 13z cigs lower to
mvfr, with north to northeast wind direction all TAF sites
becoming east around mid day. Forecast for CIG to drop to IFR
after 17z, with IFR vis possible as well in the afternoon during
best chances for snow, roughly 12-23z. Conditions to improve after
00z with snow tapering off to scattered snow showers with VFR vis
and MVFR cigs.
&&
.BOU WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 8 AM Monday to noon MST Tuesday for
COZ030-031-033-034-037.
&&
$$
UPDATE...Hanson
SHORT TERM...RJK
LONG TERM....Hiris
AVIATION...Hanson
Area Forecast Discussion
National Weather Service Grand Forks ND
944 PM CST Sun Dec 27 2020
.UPDATE...
Issued at 943 PM CST Sun Dec 27 2020
Reports of light snow continue in northeast ND with light returns
on radar spreading south generally in line with timing from last
update. No reports of accumulations yet, but visibility
restrictions3-6sm continue where snow is falling. Minor
adjustments to near term, otherwise forecast is on track for very
light snow and potential for dusting.
UPDATE Issued at 731 PM CST Sun Dec 27 2020
Light snow is being reported across the northern RRV and southern
Manitoba as the next cold front drops south into the region, with
visibilities as low as 2 miles supported by webcams. RAP analysis
doesn`t show any organized elevated frontogenesis, but PV anomaly
upstream along trough axis(apparent on WV)is digging southeast
and good synoptic ascent coupled with CAA is likely supporting
snow development. Latest signal in CAMs for light amounts matches
trends from obs and supports a high coverage/low amount type
event. Currently expected a brief period of very light accumulating
snow (less than 0.5") and this should quickly transition south
thorugh the RRV this evening/overnight. Winds aren`t expected to
be high and impacts will mainly be a result of dusting of snow and
brief visibility restrictions.
&&
.SHORT TERM...(This evening through Monday night)
Issued at 224 PM CST Sun Dec 27 2020
Concern for this period will be minor impacts associated with
frontal passage tonight. Weak ridging into this evening with the
next upper level wave set to propagate through the region as upper
level flow becomes northerly. Guidance indicates a sharp low level
baroclinic zone will propagate through the region with a colder
airmass for Monday. Along with the frontal passage anticipate on
and off light snow (low level Fgen) along with gusty winds. With
that said, limited moisture should lead to only a dusting of snow
(maybe a few tenths of an inch), and winds aloft within the mixed
are only ~20 knots, limiting potential impacts. Snow squall
parameter is also very low. A few area may touch -25F for wind
chills Monday morning (especially NW MN), but only for a brief
period.
Monday into Monday night, expect ridging will lead to quiet
weather (although colder than previous days).
.LONG TERM...(Tuesday through Sunday)
Issued at 224 PM CST Sun Dec 27 2020
Impacts during this period still revolve around a Tue into Wed
system and center on snow amts and wind potential over primarily
southeast ND and west central MN.
Confidence continues to increase regarding the Colorado system
taking a more south and easterly track in its evolution Tue into
Wed. Ensemble solutions still exhibit small variability within the
solution envelope. This is also reflected by the suite of
deterministic solutions taking on a low track farther east of our
forecast area, over a Iowa/Wisconsin tract. Isentropically induced
forcing well ahead of the low pressure center looks to be the
primary snow generating mechanism over most of the area from Tue
afternoon into the night. Wind induced impacts, at least for now,
appear to be on the low side, more aligned with drifting as opposed
to blowing. Maximum wind speeds would most likely be attained over
areas after the snow has fallen, further reducing impacts to low
levels.
Snowfall amounts for our area still the main question mark, but
again would turn out the lowest from Devils Lake through the
northern Valley, with increasing amounts for southeast North Dakota
and west central Minnesota. High pressure building over us Wed will
then bring near seasonal temps our way for couple of days, followed
by a weekend warmup.
&&
.AVIATION...(For the 00Z TAFS through 00Z Monday afternoon)
Issued at 545 PM CST Sun Dec 27 2020
VFR conditions are prevailing across much of eastern ND and
northwest MN, while the next cold front is slowly moving out of
Canada and will pass north to south through the evening/overnight
period. This will bring a period of MVFR conditions and the
potential for very light snow. Confidence is higher in the
ceilings tonight and lower in the snow coverage/location, so I
held off on introducing -SN to TAFs at this time. Where snow
develops visibility may decrease to the 3-6sm range. VFR should
return after 11-15Z Monday. West-northwest winds should remain
10kt or less through much of the TAF period, eventually shifting
to the southwest later in the afternoon Monday. KDVL has
potential for winds during the day Monday to occasionally gust
15-20kt.
&&
.FGF WATCHES/WARNINGS/ADVISORIES...
ND...None.
MN...None.
$$
UPDATE...DJR
SHORT TERM...TG
LONG TERM...WJB
AVIATION...DJR
Area Forecast Discussion
National Weather Service Wichita KS
542 PM CST Sun Dec 27 2020
.SHORT TERM...(This evening through Tuesday night)
Issued at 320 PM CST Sun Dec 27 2020
Brisk winds are anticipated through this evening in the wake of
the cold front that moved through the forecast area this afternoon.
Prefer the NAM/WRF for surface temps tonight, with a reinforcing surge
of colder air expected to spill into the area for tonight through Mon,
dropping high temperatures on Mon into the lower 30s for most areas.
Monday night-Wed: Things get a little more uncertain and tricky for
Monday night through Tue. Surface temperatures will be the tricky
part Monday night, as operational models traditionally have trouble
handling shallow cold airmasses. Surface temperatures for Mon night
through early Tue will be critical, as warm advection begins over
the top of the shallow air, as a pair potent shortwaves merge over
the plains after moving across the Rockies for Tue. Operational
models are fairly similar on how the overall pattern will evolve, as
the northern stream shortwave and the southern stream shortwave come
into phase as a large sweeping shortwave for Tue through Wed. But
the big challenge will be the surface temperature profile and sharp
baroclinic zone expected to be across KS for late Mon or early Tue,
ahead of the shortwave, as warm advection/moisture transport gets
going late Mon night for increasing precipitation chances. Latest
NAM/WRF and RAP are the coldest, keeping most locations well below
freezing for much of Mon night into Tue morning, with a prominent warm
nose aloft. This setup would suggest a lot messier winter precipitation
forecast for late Monday night into early Tue, with potential for a mix
of light snow and freezing rain in central KS, and possibly a wintry
mix of freezing rain/sleet, further south. Latest model trends suggest
most of the precip will hold off until close to daybreak on Tue. Prefer
to use the NAM/WRF and RAP for surface temps into Tue morning as well.
Which suggests surface temps may be slower to rise above freezing, as
moisture transport and precipitation begins Tue morning. With the
colder surface profile, NAM bufkit soundings suggest a chance of
freezing rain and possibly some sleet for Tue morning, possibly lasting
until mid morning on Tue. Current QPF forecasts suggest some light
ice accumulations will be possible Tue morning, mainly on elevated
surfaces, with around a tenth to a quarter of an inch glazing possible.
So think some winter headlines will eventually be needed, but will defer
to later shifts and let them evaluate the surface temp pattern.
Depending how cold the surface airmass gets, could see the freezing rain
chances extend as far south as Wichita Metro area, impacting the Tue
morning commute.
GFS continues to be is the warmest with surface temps, and scours out
the shallow cold air the fastest, which would lead to less freezing rain
for early Tue. So stay tuned.
For the daytime on Tue, still lots of uncertainty, as the colder
NAM/WRF would suggest a possible lingering freezing rain event for much
of Tue morning, for central and portions of south central KS. While the GFS
and ECMWF suggests a transition to a cold rain event for most
locations by Tue afternoon. Even with the colder NAM/WRF think the
below freezing surface temps will eventually scour out by Tue afternoon,
with the wintry precip transitioning to a cold rain.
The main shortwave moves out into the plains for the daytime on Tue,
with a rainy afternoon and evening for most locations, as the
shortwave moves rapidly across KS. As the shortwave moves across,
another shot of colder air moves into the area, as a cold front,
sweeps most of the rain showers into eastern KS by Tue night.
Ketcham
.LONG TERM...(Wednesday through Sunday)
Issued at 320 PM CST Sun Dec 27 2020
Rain showers transition back to snow or a wintry mix before coming
to an end in the wake of the cold front Wednesday, as better
moisture continues to push east of the area. Thereafter, a southern
stream low is progged to develop over southwest Texas on Thursday
and quickly intensify into a wrapped up system by early Friday as
it moves northeast ward into the Middle Mississippi Valley
region. GFS/EC solutions would suggest southeast KS maybe close
enough to land some deformation zone snow fall from the northwest
edge of the departing system on Friday. Another shortwave on the
heels of the system will help keep it more progressive in nature.
Mean northwest flow/marginal ridging returns for the weekend
bringing dry conditions along with a warming trend to the area. The
slow warming trend Wed through Sunday will bring temperatures into
the upper 40s to near 50 by Sunday.
&&
.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 542 PM CST Sun Dec 27 2020
VFR conditions expected to prevail thru the forecast valid period
with surface high pressure ridge across the area. VFR stratocu
bases/cigs across northeast Kansas this evening could clip the
KCNU terminal, though higher confidence they will stay just north
of the area. Otherwise, scattered high clouds tonight will thicken
and lower to altostratus on Monday in advance of the approaching
storm system moving into the southwest CONUS. Persistent lift
and saturation down to some stratocu VFR cigs are possible in
central Kansas by late Monday afternoon. Surface winds will be
light and gradually veer from a northerly to easterly component.
KED
&&
.PRELIMINARY POINT TEMPS/POPS...
Wichita-KICT 26 39 30 46 / 0 10 50 90
Hutchinson 24 37 29 43 / 0 10 40 90
Newton 24 37 28 44 / 0 0 40 90
ElDorado 25 38 29 45 / 0 0 50 90
Winfield-KWLD 27 40 31 48 / 0 10 60 90
Russell 22 36 26 39 / 0 10 40 90
Great Bend 23 36 27 41 / 0 10 30 90
Salina 23 36 28 41 / 0 0 40 90
McPherson 23 36 28 43 / 0 0 40 90
Coffeyville 28 40 32 50 / 0 0 40 70
Chanute 25 39 31 48 / 0 0 40 80
Iola 25 38 30 46 / 0 0 30 80
Parsons-KPPF 26 40 31 50 / 0 0 40 70
&&
.ICT WATCHES/WARNINGS/ADVISORIES...None.
&&
$$
SHORT TERM...BDK/KMB
LONG TERM...KMB
AVIATION...KED
Area Forecast Discussion
National Weather Service San Francisco Bay Area
440 PM PST Sun Dec 27 2020
.SYNOPSIS...Next system will move inland across Central Coast
tonight through tomorrow morning, bringing widespread rain to
primarily the southern portion of the forecast area, starting
this evening and continue through at least Monday morning. The
highest rainfall totals are expected from Monterey Bay southward.
Dry weather returns late Monday through Wednesday morning. Then a
brief shot of rain is possible Wednesday afternoon through early
Thursday.
&&
.DISCUSSION...As of 02:00 PM PST Sunday...GOES-West Imagery
depicts a clearing over much of the California and Oregon coast
this afternoon owing to a dry slot ahead of the approaching storm
system to our west. Winds have remained light across much of the
CWA, but have observed slightly milder temps across the Central
Coast due to SW winds that have been mixing down over the last few
hours. As for the interior and the Bay Area, are still observing
east winds for the most part. This flow will continue to play out
through the overnight and into tomorrow morning as the cut-off low
continues to track towards the Central Coast. Currently, not
seeing a substantial SFO-SBA gradient, but that is expected to
change over the next few hours.
The most notable shift in the short-term, hi-res storm track was
observed overnight last night and into this morning, with the vort
max associated with the storm system taking a more southerly
path, traveling just south of the Monterey Peninsula and
continuing its SE track through tonight and tomorrow morning.
While some rain bands may begin to roll in as early as 7PM,
currently looking like most will begin tracking over the immediate
coast after 8PM tonight. This track will result in much less
orographically-induced precipitation across the Santa Cruz
Mountains, and as such have adjusted pops down to 0.50-0.75 for
some of the highest elevations. North of the Mountains, not
expecting any rainfall totals to exceed a quarter of an inch
except for a few isolated coastal ranges in Marin County and
extreme southern Santa Clara County. In fact, most of the Bay Area
will be lucky to receive a tenth of rain through the period. The
highest rainfall totals will be confined to the Central Coast,
where Monterey Bay and the Salinas Valley can expect anywhere from
0.25-0.35 and portions of coastal Santa Cruz County may get as
much as 0.5 by daybreak tomorrow. CAPE values and other
stability parameters suggest there is also the potential for some
isolated thunderstorms along our southern waters that would then
move into the Big Sur Coastline.
Primary concern will be rainfall totals along Highway 1 south of
the Monterey Peninsula, where orographic influence and the
continuation of the moisture flow will result in some of the
highest rainfall rates to occur between 10PM to 4AM across much of
the Santa Lucia Range, including the Dolan Fire Burn Scar. Some
concerns over debris flow do exist at this time, but thankfully
the most prominent rainfall rates look to be short lived as per
the latest HRRR and WRF-ARW runs. Overall, expecting between
1-2 of rain over Southern Monterey County, with some isolated
pockets getting as much as 2.5 of rain by the time the system
exits the region. Most rain will clear out across the Bay Area
early on and just before daybreak, while the Monterey Bay and
Salinas Valley may not clear out until mid-morning in some spots,
and southern Monterey County may not clear out until late-
morning/early afternoon. Have already observed offshore lightning
this morning and afternoon along the incoming FROPA, but given the
current instability parameters depicting the most conducive
environment confined to southern Monterey County and the coast,
only have added a slight chance of isolated t-storms for that
domain during the overnight. It should also be noted that 850mb
temps are expected to be at/near freezing across much of the
region, and as such are expecting snow levels to fall to just
below 4000 ft across the CWA. Given the amount of moisture
overhead across the Central Coast and the height of the Santa
Lucia Range, cannot rule out the possibility for some snow to fall
over some of the highest peaks. Whether enough of it accumulates
is much less certain given soil temperatures, but the possibility
for some minor (albeit short-lived) accumulations are there. Other
potential pockets of potential snow are the highest peaks of the
Diablo Range, but again not expecting any substantial
accumulations there either. Overall this system will bring some
beneficial rains to an area of the CWA that has thus far missed
out on receiving the same amount of rainfall as locations in the
North Bay have over the last few weeks.
A brief dry trend is expected following the passage of this cut-
off low as an upper-level ridge attempts to build back over the
West Coast. The dry conditions will result in clearer skies and
some chillier overnight conditions Tuesday night and into
Wednesday morning, with many locations in the interior (e.g. North
Bay and Salinas Valley) once again flirting with min temps near
freezing. It should be noted that despite the precip gains
expected from tonights storm system, we are still running
below-average in terms of precipitation totals by this time in the
winter season over much of the Central Coast, Santa Cruz
Mountains, and South Bay. And while substantial gains have been
made in recent weeks across some of the most critically-behind
portions of the Bay Area (e.g. the North Bay Interior Valleys),
they have not been sufficient enough to get the Bay Area and
Central Coast down to the abnormally dry or none categories on the
US Drought Monitor. Nonetheless, the synoptic pattern for the
next few days, and really for the next couple of weeks, across the
Pacific is somewhat promising; a continuation of upper-level
zonal (east-west) flow is expected to continue between Japan and
the West Coast. This setup will continue to transfer jet streaks
across the Ocean, introducing more energy that will help to break
down upper-level ridges that attempt to develop over California.
As such, this upcoming ridge during the first half of the week
will be short-lived as two shortwaves stream from the Northwest
Pacific towards the West Coast. Because of the breakdown of the
initial ridge, moisture is once again looking to move into
Northern and potentially Central California by Wednesday
afternoon. Long-range models are suggesting IVT values over 250
kg/m/s entering Northern California by mid-week and then a second
time during the weekend. This is consistent with not only the
continuation of a negative PNA forecast through the early part of
January but also with the long-range ensemble members that have
continually picked up on this extended wet trend over the past
several days. Some caveats to keep in mind however: the initial
shortwave currently is progged to move north over the PAC NW,
limiting how much moisture makes it down to the Bay Area. For the
most part, current runs limit rain chances to northern Napa and
Sonoma counties. Moreover, totals range around a few hundredths of
an inch to upwards of almost a quarter of an inch along the
mountains. Farther south, not looking incredibly promising. The
second caveat is that a ridge will once again develop following
the passage of the shortwave and depending on the timing of its
breakdown, may not get much moisture to trickle down to the Bay
Area then either. As such, attempting to remain hopeful that we
will be able to get another round of precipitation sometime mid-
week next week and then again over the first weekend of 2021, but
given the current outlook, it seems like most of next weeks
precip will be limited in its scope and at best confined to the
North Bay.
&&
.AVIATION...As of 04:40 PM PST Sunday...For the 00z TAFs.
Satellite imagery shows high clouds beginning to move over land
with the initial rain band about 80 mi offshore, moving ENE.
Expecting rain to arrive at Monterey Bay terminals around 03z-
04z, and SF Bay terminals between 05z-06z. VFR conditions
presently will degrade to MVFR as cigs lower and steady rain
arrives. The North Bay will mainly see showery activity, but rain
is expected at KSFO, East Bay terminals and points southward.
Heaviest rain will be in the overnight hours near the Monterey Bay
where IFR conditions are likely. As the front approaches, winds
will increase out of the south and become gusty, mainly around
Monterey Bay. Rain will taper off after 15z Monday with spotty
showers continuing through 18z.
Vicinity of KSFO...VFR conditions expected through early evening
before lower cigs arrive after 03z. Light to occasional moderate
rain will lower vis to MVFR. Showery conditions continue
overnight and into the morning hours, keeping MVFR until late
morning or early afternoon. Light winds will become breezy
offshore overnight with occasional gusts possible. Conditions
should improve to VFR by Monday early afternoon.
SFO Bridge Approach...Similar to KSFO, chance of reduced vis and
lower cigs on approach after 05z during heaviest rainfall.
Monterey Bay Terminals... VFR through early evening before arrival
of lower cigs and rain 03z-04z. MVFR with IFR in heavier rain
bands this evening and overnight. Winds have been occasionally
gusty at KMRY around 15-20 kt, but lighter elsewhere. More
widespread gusty conditions expected this evening and overnight
with the approaching storm. Showers continue into Monday morning,
but tapering off after 16z, with mostly VFR for the afternoon.
&&
.MARINE...as of 02:22 PM PST Sunday...Winds have switched to the
south-southeast as a cold front moves southward over the coastal
waters. Winds will increase as the center of the low pressure
passes over the waters Sunday evening into Monday morning, then
switch to the northwest behind it. Rain is likely as it passes.
Lighting has been observed on its leading edge, and a slight
chance of thunderstorms over the southern waters will continue
through Monday morning. Dry conditions are expected to return by
Monday night, along with gradually diminishing winds out of the
northwest. A moderate period northwest swell will prevail with a
longer period swell arriving midweek.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.Tngt...SCA...Pt Arena to Pt Reyes 0-10 nm
SCA...Pt Reyes to Pigeon Pt 0-10 nm
SCA...Pigeon Pt to Pt Pinos 0-10 nm
SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
SCA...Pt Arena to Pigeon Pt 10-60 nm
SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
SCA...Mry Bay from 7 PM
&&
$$
PUBLIC FORECAST: Diaz
AVIATION: Lorber
MARINE: DK
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Area Forecast Discussion
National Weather Service Riverton WY
453 PM MST Sun Dec 27 2020
.SHORT TERM...This evening through Monday night
Snow has mostly ended across the western mountains this morning. A
few flurries may linger into the early part of this afternoon, but
otherwise, skiers should have a good day to be out on the slopes and
enjoy the fresh foot of powder from last night. The remnants of this
system, in the form of convective snow bands, continue to move east
across areas east of the Divide this morning. These bands, are
dropping anywhere from a trace to an inch or two of snow. Models
have not done a good job with forecasting these snow bands, and due
to their convective nature, they are particularly challenging to
forecast. Kept general PoPs across Natrona, Washakie, and Hot
Springs County through the afternoon to account for the uncertainty.
After sunset, the NAM and HRRR are hinting at a pretty decent fog
signature for the Wind River Basin, as well as northern Johnson
County later in the night, through Monday morning. Included patchy
fog in the grids for these areas for now. The other thing to mention
for the overnight period will be a localized weak cold front moving
westward across Sweetwater County. The front is expected to reach
Rock Springs around 09Z. This will shift winds to the east and drop
temperatures ahead of Monday`s system.
Models continue to disagree on the track of the southern low for
Monday`s system. Have continued more along the lines of the
NAM/ECMWF which continue to bring more precipitation into the Wind
River basin for Monday night with the low track across southeastern
WY. Moisture input from the Gulf and the Southeast flow within the
system, are solid indications that southern and eastern portions of
the Wind River Basin will see at least a little snow. Again, the
banded nature will bring a good bit of uncertainty to the snow
totals, but generally looking at around an inch in the Riverton area
towards 3 inches in the Lander Foothills. The greatest snow fall
will likely be from South Pass to Muddy Gap, where 2 to 4 inches is
not out of the question. Sweetwater County and southern Lincoln
County likely will see 1 to 3 inches and Natrona County and the
Casper area will see 1 to 2. Other areas east of the Divide will see
less than an inch, especially towards the north and west.
A couple other areas of uncertainty on snow totals will be the
timing of the dry air and also the timing of the cold front.
Starting with the cold front, Sweetwater County should stay in the
low 20s Tuesday, so any precipitation there will be all snow. The
question is the Casper area and eastern Johnson County. These areas
may see diurnal heating and temperatures into the low 30s on
Tuesday. Precip for these areas begins around 00Z for the Casper
area and 03Z for eastern Johnson County. By this time, temperatures
should cool off enough for precipitation to be all snow, so we are
not expecting any period of freezing rain or mixed precip.
The timing of the dry air also is a bit tricky, as it depends on the
track of the northern low. The closer to WY the low tracks, the
quicker the entrainment of dry air into the region, but also the
more likely the chance for wrap around snow showers behind the
system. These items will be on the Tuesday timeframe as the storm
moves eastward, so will leave the in depth discussion for the long
term forecast.
.LONG TERM...Tuesday through Sunday
Snow will be ongoing by 12Z Tuesday, along and east of a Buffalo to
Rock Springs line. Snow activity will be ending across this area
through the morning hours, as the storm system continues to exit
over the Plains. The gradient will tighten across the Cowboy State
Tuesday night, as the storm strengthens. Gusty north-northwest winds
will be likely through the day Wednesday, especially northern
portions of Johnson County and the Bighorn Basin. A ridge will build
over the western CONUS Tuesday and begin to fold over the Northern
Rockies through the day Wednesday.
Another storm system will be moving onshore over the PACNW Wednesday
afternoon. This storm will bring the next round of snow to western
WY by Thursday morning. Most of the energy with this system will be
diving southward toward the Desert Southwest and begin to support a
separate system developing over southern TX. However, the midlevel
circulation is progged to move over central WY during the day
Thursday. This, in addition to the trough, could bring light snow to
much of the forecast area. Snow will quickly end once again
across the area late Thursday night, as another ridge builds over
the PACNW. This ridge will transition eastward over the region
Friday night and exit over the Plains by Saturday morning. This
will set up a more zonal pattern, bringing snow back to far
western portions due to orographic/upslope flow. Breezy to windy
conditions will be possible across the Wind Corridor Saturday as
well.
&&
.AVIATION...For the 00Z TAFS through 00Z Tuesday
Issued at 452 PM MST Sun Dec 27 2020
West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals.
VFR conditions will prevail across the area through the TAF period.
A cold front will reach KRKS after 09Z, turning winds easterly. BKN-
OVC clouds at FL 150-200 will increase from the south after 09Z as
well, reaching KBPI/KPNA by 15Z. Snow showers will develop at KRKS
after 18Z, with snow likely by 00Z Tuesday. These showers will reach
KBPI around 00Z as well.
East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals.
VFR conditions expected through the TAF period. VCFG will be a
possibility between 05Z and 10Z at KRIW and KLND. The aforementioned
BKN-OVC cloud shield will move over these terminals between 12Z and
15Z, which should aid in lifting the fog. Snow showers will be
possible at KLND and KCPR by 00Z Tuesday.
Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.
&&
.FIRE WEATHER...
West of the Divide, snow showers have ended and remnant snow bands
have moved east. Winds will remain light through Monday with
increasing northwest winds on Tuesday, and the area should remain
dry until the next system moves into the region Wednesday. Across
the south, snow, associated with the next storm system, will move
into the area early Monday afternoon and continue through Tuesday
morning. Snow totals are not expected to be more than 1 to 3 inches.
East of the Divide, snow bands will move out of the region this
afternoon and evening. Snow moves into the region Tuesday afternoon
with the next cold front. A dusting to 3 inches of snow are expected
with lesser amounts toward the north and west through Tuesday
morning. Smoke dispersal across the entire region will remain poor
due to light winds. Gusty northwest winds will increase Tuesday
afternoon, especially from Buffalo to Casper.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...NONE.
&&
$$
SHORT TERM...Fisher
LONG TERM...LaVoie
AVIATION...LaVoie
FIRE WEATHER...Fisher
Area Forecast Discussion
National Weather Service San Diego CA
200 PM PST Sun Dec 27 2020
.SYNOPSIS...
Mostly cloudy and cooler conditions will prevail this afternoon
ahead of an approaching Pacific storm system. Rain with mountain
snow will begin late tonight and continue at times through Monday
night. The rain showers will be briefly heavy, and snowfall will be
locally heavy in the mountains late tonight through Monday night
with difficult travel conditions. Gusty winds will occur from the
mountains into the deserts and also at times along the coast Monday
into Monday night. The storm will move east Tuesday, followed by
gusty, dry, northerly winds Tuesday night and Wednesday, with
possible strong northeast winds Friday. Temperatures will mostly be
below normal until around the New Year when they will recover to
near normal.
&&
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...
Mostly thick high clouds continued moving over the region, and the
clouds played a big role in temperatures mostly only being around 60
west of the mountains, though cooling aloft played a role as well.
The winter storm system will bring precipitation starting late
tonight, mostly after midnight, with the first wave a little ahead
of what was previously expected, mostly before sunrise Monday. Thus,
widespread precipitation should begin by sunrise Monday, then taper
a bit before another wave and great instability occurs Monday
afternoon, and then the trough axis Monday evening. Best
available precipitable water amounts continue to be around 0.75"
both very early Monday morning and Monday evening, though no
atmospheric river is expected as IVT values barely top 250 kg/m/s
and briefly. Some of the showers may be heavy as they move rapidly
from west to east, with a slight chance of thunderstorms. HRRR and
local WRF both show maximum hourly precipitation rates remaining
mostly at/below 0.40" per hour, and that combined with the low snow
levels, will keep the debris flow risk relatively low, but
definitely not zero, in burn areas. Above 4500 feet in the
mountains, snow will be falling and heavy accumulations are likely
into Monday night above about 5000 feet, with over a foot likely in
some locations at/above 7000 feet. Gusty southwest winds will
occur at times late tonight through Monday evening, with local gusts
in the mountains and desert slopes around 55 MPH, and even a few
gusts over 25 MPH could occur at the coast at times. A Winter Storm
Warning is in effect for this storm. For details on expected
rainfall, please see the Hydrology discussion below.
A transient ridge will build over the West in the wake of the storm,
turning our winds back offshore late Tuesday into Wednesday. These
winds will be dry and gusty, but relatively cool, keeping temps a
bit below average. Another, weaker trough will dig over the West
later in the week, passing by to the NE. Winds will turn briefly
back onshore around Thursday before turning back offshore New
Years Day when Santa Ana conditions will be possible. Temperatures
should increase a bit starting Friday under the ridging, with no
strong signals after that as we will be under a more progressive
weather pattern with fewer strong ridges and troughs. Dry weather
will prevail Tuesday through Saturday, with small precipitation
chances early the following week when troughs pass by to the north
in that progressive west-to-east pattern.
&&
.AVIATION...
272130Z...Coast/Valleys...BKN clouds at/above 10,000 ft MSL will
prevail through this afternoon. Low clouds will begin to spread back
inland after 00Z Monday, with bases around 1500-2500 feet MSL.
Isolated showers could begin as early as 06Z Monday, becoming more
widespread overnight and into Monday morning. Lower vis is possible
during heavier showers.
Deserts/Mountains...Increasing clouds at/above 10,000 ft MSL and
unrestricted vis through this evening. Increasing mid and some low-
level clouds tonight, with mountain snow expected and vis
restrictions affecting obscured coastal slopes.
&&
.MARINE...
An approaching storm system will bring scattered showers starting
early tonight, with periods of rain through Monday night.
West winds associated with this system will arrive early Monday
morning, with gusts on Monday afternoon that will near 30 knots
across the Outer Waters, and up to 25 knots for the Inner Waters.
Combined seas 7 to 12 feet are expected, highest across the Outer
Waters late Monday and through Tuesday. In addition, there will be a
chance of thunderstorms on Monday, along with the possibility of a
few waterspouts. A Small Craft Advisory is in effect for all of the
coastal waters beginning at 3 AM Monday and lasting through noon
Tuesday. Conditions will improve by Wednesday.
&&
.BEACHES...
Building west swell due to an approaching storm system will raise
surf across area beaches today, with widespread elevated surf
conditions likely, and a few sets to 7 feet possible in San Diego
County. Surf will increase on Monday, peaking between Monday
afternoon and Tuesday, with sets to 8 feet. In southern San Diego
County, sets may reach 9 to 10 feet Monday and Tuesday. There will
also be a chance of thunderstorms with lightning at the beaches on
Monday. A High Surf Advisory is in effect through 10 AM Wednesday.
&&
.HYDROLOGY...
The Pacific storm is slightly faster than earlier expected, so
precipitation will likely become widespread between around 1 AM and
4 AM Monday. Precipitation will generally be widespread and showery
through the day Monday and Monday evening. There is sufficient
instability and cold air for low topped thunderstorms. Rainfall
rates could be briefly heavy, up to around 0.40 inch per hour, and
some showers will contain small hail. The more steady rainfall and
heavier accumulations will be on the coastal mountain slopes where
an extended period of nearly saturated, upslope flow will add up
over time. The showers/tstorms should be moving along at 25-30 MPH.
This will limit rainfall accumulation in any one spot.
Given the dry conditions, the modest atmospheric moisture
availability, the showery nature of this event, and the speed at
which the showers will be moving, significant runoff will be
limited. Those areas most susceptible to minor flooding will be
urban, and with poor drainage, and where heavy showers repeat or
train over the same area. Mainstem river and small stream flooding
is not expected. Even the coastal slope areas, where storm totals
could reach two inches or more, should be able to absorb much of
this over time. The stark exception of course is fresh burn scars
which are much more susceptible to runoff and debris flows if
rainfall rates become heavy, which there is a small chance of and
will be monitored.
This system will remain progressive, limiting overall precipitation
totals. Latest storm total precipitation expected....0.60-1.20 inch
west of the mountains...1.00 to 2.50 inches mountains...and 0.15 to
0.35 deserts. Locally higher amounts are possible in spots,
especially the southwest facing mountain slopes.
With snow-levels down to 4000-4500 FT, snowfall should be a major
impact for travelers in the mountains. Even the higher mountain
passes could get icy for a time. For now 3-6 inches of snow looks to
be widespread above 4500 FT, with areas of 8-12 inches or more above
5500 FT, and 12+ inches at/above 7000 FT, highest in San Bernardino
County and the San Jacinto Mountains.
&&
.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.
&&
.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Winter Storm Warning from 5 AM Monday to 4 AM PST Tuesday for
Riverside County Mountains-San Diego County Mountains.
Winter Storm Warning from 1 AM Monday to midnight PST Monday
night for San Bernardino County Mountains.
High Surf Advisory until 10 AM PST Wednesday for Orange County
Coastal Areas-San Diego County Coastal Areas.
PZ...Small Craft Advisory from 3 AM Monday to noon PST Tuesday for
Coastal Waters from San Mateo Point to the Mexican Border
and out to 30 nm-Waters from San Mateo point to the Mexican
Border Extending 30 to 60 nm out including San Clemente
Island.
&&
$$
PUBLIC/HYDROLOGY...Maxwell
AVIATION/MARINE/BEACHES...Connolly