Forecast Discussions mentioning any of
"ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 12/13/21
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
819 PM CST Sun Dec 12 2021
.UPDATE...
Issued at 819 PM CST Sun Dec 12 2021
No significant changes to going forecast this evening.
Relatively quiet weather continues this evening, with breezy
southwest winds being the main feature. Despite diurnal loss of
deeper mixing, low level jet of 45 kt, sampled in recent AMDAR
aircraft ascent sounding out of KORD at about 1400 feet AGL,
continues to produce sporadic surface gusts of 25-30 mph per
regional surface obs. These breezy southwest winds will persist
much of the night before gradually diminishing early Monday
morning, helping to maintain overnight temps in the 30-35 degree
range, which are just a little below our average daytime highs
for this time of year. Otherwise, a little patchy cirrus moving
across the region this evening will move east of the area
overnight, making for partly cloudy to mostly clear skies.
Did tweak sky cover up just a bit this evening for the cirrus and
tweaked hourly temp trend slightly this evening, but otherwise
the going forecast is right on track and no other changes are
warranted.
Ratzer
&&
.SHORT TERM...
Issued at 210 PM CST Sun Dec 12 2021
Through Monday night...
There are no significant weather impact concerns as we continue
the unseasonably mild pattern to kick off the work week. Dry and
mostly clear to partly cloudy skies (from passing cirrus) will
prevail on the northwest periphery of expansive ~1030 mb high
pressure centered over the southeast US.
A strong 40-50 kt low level jet will be overhead tonight, yielding
occasional gustiness from the south-southwest up to 20-30 mph,
particularly over the city and north shore to Lake County IL
suburbs. Lows will be 10-15F above normal in the lower-mid 30s
except upper 30s for the Chicago shore. Monday will feature much
lighter southwest winds with the pressure gradient slackening.
Still dry air mass and warming temps aloft should enable highs to
reach a few degrees higher than today in the upper 40s to lower 50s.
Light southerly to calm winds on Monday night and forecast lows
near crossover values (upper 20s-lower 30s) outside of Chicago
may set the stage for some patchy ground fog for portions of the
interior. Guidance is mixed on the signal and there may be
sufficient coverage of cirrus overhead to limit the threat, so
will refrain from formal fog mention in the grids and let
subsequent shifts take a look at this potential.
Castro
&&
.LONG TERM...
Issued at 314 PM CST Sun Dec 12 2021
Tuesday through Sunday...
The period starts on a warming trend with a ridge of high pressure
centered over the mid-Atlantic states. The clockwise motion around
the high will keep southerly flow across the area which will
advect in warm-moist air from the Gulf of Mexico. Temperatures on
Tuesday are forecast to warm into the lower 50s which is roughly
10 to 15 degrees above normal for mid December. Even warmer
temperatures are expected on Wednesday with temperatures
approaching record highs in the mid-60s.
At the same time, a low pressure system is expected to develop
along the eastern Rockies Tuesday night which is projected to
track northeast across Nebraska, Iowa, Minnesota, and Wisconsin
throughout the day on Wednesday. As the low crosses the plains a
warm front will advance across the area Wednesday afternoon which
will bring with it the chance for rain showers Wednesday morning
through the afternoon. Guidance suggests that coverage of showers
with the warm front will be sparse with the highest chances
expected along and east of I-57. A cold front will then push into
the area late Wednesday afternoon and evening which will bring
more widespread rain showers across the area from west to east.
Forecast soundings do show some marginal instability with MU CAPE
values around 250 to 500 J/kg Wednesday evening which could
develop a few thunderstorms along the front.
Behind the front winds will veer westerly behind the front and
increase in speed Wednesday night due to a tightening pressure
gradient and strong cold air advection in the wake of the low.
Intense pressure rises on the order of 15 mb/6 hr are expected to
pass through the northern Illinois which will generate strong
winds potentially gusting upwards of 40 to 45 mph Wednesday night
into Thursday morning. Winds will slowly begin to ease Thursday
afternoon and then back towards the southeast by Friday morning.
Temperatures behind the system will be more typical for December
with highs near 40 degrees on Friday. Colder temperatures are
expected for the weekend with highs in the upper 20s and lower 30s
Saturday with slightly warmer highs in the low to mid-30s on
Sunday. Another weak system is expected to pass through Friday
night into Saturday which will bring us the chance for some light
snow showers.
Yack
&&
.AVIATION...
For the 00Z TAFs...
VFR conditions will continue with passing upper-level cirrus
through the period. A 45-50kt LLJ developing early this evening
will promote LLWS through the night at all sites. SW gusts to
around 25 knots are expected through the early morning hours, but
may be more sporadic instead of frequent. Otherwise, SW winds of
10-15 knots will persist through Monday before beginning to veer S
Monday evening.
Kluber
&&
.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters until 6 AM Monday.
&&
$$
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Area Forecast Discussion
National Weather Service Phoenix AZ
458 PM MST Sun Dec 12 2021
.UPDATE...
Updated Aviation discussion.
&&
.SYNOPSIS...
Quiet weather will persist to finish the weekend. Slightly warmer
afternoon temperatures are expected over southeast California
through Tuesday. A strong low pressure system and cold front
will move through California and Arizona later Tuesday into
Wednesday. This weather system will bring very good chances for
rain, but the greatest impact will likely be strong winds, with
the greatest impacts on Tuesday night. Cooler, tranquil weather
resumes on Wednesday to finish out the work week with some rural
areas dipping to or below freezing in the mornings.
&&
.DISCUSSION...
It was another cold morning across the area with many lower
desert locales falling into the 30s. Aircraft soundings this
morning out of Phoenix show much of the lower atmosphere has
warmed since yesterday, so expect highs this afternoon several
degrees warmer than yesterday, with lower deserts topping out in
the upper-60s to low-70s. Aircraft soundings also show the wind
profile is becoming more deeply southwesterly. This is evident on
satellite as well, with the synoptic flow pattern becoming more
amplified with troughing strengthening along the west coast.
Monday is expected to be similarly warm to today, but feature
increasing clouds within the deep southwesterly flow. Greatest
increase in cloud coverage and thickness is expected during the
afternoon through Monday night as a disturbance, currently between
Mexico and Hawaii, is expected to work its way up through the
local area. Ensemble guidance continue to show a good moisture
flux with this wave (IVT: 200-300 kg/ms) as well and some virga
and light showers will be possible Tuesday morning. HREF keeps the
best chance for these showers mainly across the southeastern half
of Arizona, with a 10-15% chance in Phoenix. Overall QPF amounts
are expected to be minimal (<0.10").
The shortwave Monday night/Tuesday morning will be sliding
through just ahead of the more significant trough digging down the
west coast that has been on the radar, pun not intended, for
several days now. Confidence in the synoptic scale timing and
evolution of the next trough remains very high. Shortly after the
trough axis moves inland Tuesday evening the trough is expected to
become negatively tilted with a 110+ kt 500 mb jet kicking round
the base of the trough. After becoming negatively tilted the
trough will likely traverse the area very quickly, possibly as
quickly as 6 hours between 11 PM Tue to 5 AM MST Wed.
The impending trough Tue/Wed is already driving an atmospheric
river event in California and will still likely have a moderate
IVT (300-500 kg/ms) when the trough reaches and progresses through
the Desert Southwest. This will support great chances for
precipitation across the CWA late-Tue/early-Wed, with the best
chances and greatest QPF focused in the higher terrain of central
Arizona and southern California with orographic enhancements.
PoPs remain high across the local CWA with NBM probabilities for
greater than 0.10" of precipitation upwards of 50% or higher
across most of the CWA. Given how quickly this trough is expected
to traverse the area, rain may only fall for a few hours, which
will limit greater QPF chances. Probabilities for >0.25" are still
upwards of 50% to the north and east of Phoenix, but drop off
quickly to the south and west and the probability of >0.50" is
only up to 25% in the higher terrain and less than 10% for lower
elevations. So, while it will be nice to see some more rain, it
will likely be short-lived and not excessive.
The greatest expected impact with this next system continues to be
the wind threat. As mentioned, a 110+ kt 500 mb jet is expected
aloft, but flow will also be strong down to at least the top of
the PBL, with model soundings showing 850 mb winds increasing
up to 50 kts or higher Tuesday evening/night. Initially, these
winds may not mix fully down to the surface, except at elevation,
but a strong cold front is expected to race across the region
Tuesday evening/night that could briefly mix strong winds to the
surface. A wind advisory has been hoisted for southeast CA and
southwest AZ zones, where confidence is currently highest, but
there is potential for expansion further east in the following
forecast updates.
Confidence in the potential for thunderstorms is quite low, given
very minimal forecast CAPE, but strong dynamic and frontogenetic
forcing, with a strongly sheared profile, may be enough to
overcome this. Regardless, the highly sheared and strong forcing
regime is supportive of QLCS development along the cold front.
Recent UA WRFs are now hinting at this potential now as well.
Further details will be resolved over the next couple of days, but
there looks to be potential for at least a low-end severe wind
threat.
In addition to the winds with the front, snow levels are expected
to crash rapidly post front. Southerly warm air advection ahead of
the front will likely push snow levels up over 8000 ft only to
fall to 3000-4000 ft 4-5 hours post-front. Rapid dry air
advection will also follow the front and passage of the trough
axis. So, chances for frozen precipitation is pretty low, but
areas like Joshua Tree NP and the high terrain of southern Gila
county could still see a brief 1-3 hour changeover to snow. Any
accumulations will be light.
As previously mentioned this will be a fast moving system,
expected to eject through the Four Corners by late Wednesday
morning. In its wake, temperatures will hover around seasonal
averages as a general trough pattern persists. The coldest morning
is expected to be Thursday as most lower desert locales are
expected to fall into the 30s with many rural communities
potentially flirting with freezing.
&&
.AVIATION...Updated at 2358Z
South-Central Arizona including KPHX, KIWA, KSDL, KDVT and
Southeast California/Southwest Arizona including KIPL and KBLH:
The approach of a Pacific weather system will lead to slow
strengthening of southwest winds aloft beginning tonight and
increasing cirrus during the day Monday. Surface winds will remain
light through the day Monday. Downvalley directions will be
favored tonight and Monday morning with south and southeast
directions favored in the afternoon.
&&
.FIRE WEATHER...
Tuesday through Saturday:
We begin with increasing RH values and scattered rain showers
Tuesday. A period of windy weather and locally heavy rain showers
is expected Tuesday night. Later Wednesday and beyond, much drier
and cooler air returns. Min RH values each afternoon will range
from 25% in our western zones to 35-40% in our eastern zones Wed-
Sat.
&&
.PSR WATCHES/WARNINGS/ADVISORIES...
AZ...Wind Advisory from 5 PM Tuesday to 1 AM MST Wednesday for AZZ530-
531-533.
CA...Wind Advisory from 2 PM to 10 PM PST Tuesday for CAZ560-561-
563>570.
High Wind Watch from Tuesday afternoon through Tuesday evening
for CAZ562.
&&
$$
DISCUSSION...Benedict
AVIATION...AJ
FIRE WEATHER...Heil
Area Forecast Discussion
National Weather Service Great Falls MT
730 PM MST Sun Dec 12 2021
.SYNOPSIS...
Afternoon high temperatures will remain near to above seasonal
averages through Tuesday, before cooling below average for the
remainder of the week. Accumulating snow can be expected along
the Continental Divide and over the central and southwest Montana
mountains through Tuesday. By Tuesday night, an approaching
trough will bring some lower elevation snow, followed by
occasional hit and miss snow showers through the remainder of the
work week.
&&
.UPDATE...
An upstream upper level trof located along the 140th meridian,
pumps moisture from The Pacific Ocean, eastward to The Great
Divide. Most precipitation falls along and west of The Divide,
with some moisture forecast across the mountains of central and
southwest Montana. At the surface, strong southeasterly/southerly
winds continue across southwest Montana. A Winter Weather Advisory
remains in place for the southwest. Please, see details, below.
The forecast is performing well; therefore, no updates will be
made, at this time.
- Fogleman
&&
.AVIATION...
516 PM MST Sun Dec 12 2021 (13/00Z TAF period)
KCTB, KHVR, KGTF & LWT:
VFR conditions prevail for the duration of this TAF forecast period.
Until about 13/05Z, southwesterly/northwesterly winds gust to 25KT.
Then, shift easterly/southeasterly and become light to breezy. Low
level wind shear conditions are included in the KGTF and KLWT TAFs,
from 13/16Z to 13/23Z.
KHLN, KBZN & KEKS:
This TAF period begins with predominant VFR conditions. Expect
periods when conditions degrade into the MVFR range in snow and
blowing snow or mist, and with lowering ceilings. Mountain
obscuration is forecast. Winds remain light throughout this period
at KHLN. KBZN begins the TAF period with southwesterly winds gusting
to 33KT, and by 13/06Z, winds are light and southeasterly. With
mostly south southeasterly/southerly winds, KEKS is forecast to
experience wind gusts to 36KT throughout this TAF forecast period.
Funneled winds through north-south oriented passes impact light
aircraft aviation ops. Low level wind shear conditions are included
as part of all of these TAFs.
- Fogleman
Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.
&&
.PREV DISCUSSION...
/ISSUED 516 PM MST Sun Dec 12 2021/
Forecast models and satellite imagery are currently depicting an
upper level closed low off the British Columbia/Pacific Northwest
coast. Moist southwest flow aloft prevails across the
Intermountain West into the Northern Rockies with precipitation
mainly affecting the Continental Divide, portions of southwest
Montana, and a few of the central Montana island ranges. The
offshore low is expected to evolve into an open trough and dig as
far south as Baja California by Tuesday afternoon. This system
then moves rapidly eastward with the northern branch of the upper
trough sweeping across Montana Tuesday night. This will bring
widespread accumulating snowfall to southwest Montana with
southern portions of central Montana possibly receiving 1 to 3
inches of accumulating snowfall at lower elevations and 3 to 6
inches likely in the mountains.
For the remainder of this afternoon into this evening, gusty
winds will continue to affect portions of Madison County. As
mentioned in the morning update discussion, I extended the ending
time for the High Wind Warning through 7 PM MST. Additionally,
with the continued flow of Pacific moisture, the Winter Weather
Advisory currently in effect for Gallatin, Madison, and Beaverhead
Counties will remain in place through Tuesday morning. As noted
above, the upper trough moves across the state Tuesday night and
Wednesday with precipitation becoming widespread for the southern
half of the NWS Great Falls forecast area. At this time, I
anticipate that another round of winter weather highlights will be
needed for this snow event. mpj
Wednesday night through next Sunday...An unsettled northwesterly
flow aloft will help maintain slightly cooler than average
temperatures and increased snow shower chances going for much for
the week. Thursday night looks to be the best time to see the most
widespread light snow chances as a fast moving trough passes
through the region. Longer range ensembles are in decent agreement
in bringing slightly milder temperatures, and increased surface
winds for the first half of next weekend, followed by another cool
down and mostly mountain snow shower activity. -RCG
&&
.PRELIMINARY POINT TEMPS/POPS...
GTF 23 42 28 41 / 20 10 10 20
CTB 14 33 15 26 / 0 10 20 20
HLN 25 44 30 42 / 40 10 20 40
BZN 28 47 31 45 / 30 10 30 50
WYS 23 34 24 33 / 70 80 80 90
DLN 26 42 29 39 / 20 30 50 60
HVR 20 37 20 33 / 10 10 10 10
LWT 22 42 28 44 / 20 0 10 10
&&
.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 AM MST Tuesday Beaverhead...
Gallatin...Madison.
&&
$$
http://www.weather.gov/greatfalls