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Office: CYS

FXUS65 KCYS 211726
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1026 AM MST Wed Nov 21 2018

.SHORT TERM...(Today and tonight)
Issued at 337 AM MST Wed Nov 21 2018

Similar story to last night, not much going on in the short term
period but that will quickly change beyond 24 hours out. A bit
chilly overnight tonight with lows in the teens and 20s for most.
Yesterday was quite mild with highs in the middle to upper 50s.
The same trend is expected today with highs approaching 60 for
the high plains and the mid 50s for the higher terrain. Winds
will be a bit gusty today from the west at 10 to 15 mph. isolated
gusts of 25 look possible near Bordeaux along I-25. Otherwise,
expect mostly sunny skies and mild weather.

Tonight will be a bit warmer than the precious few night with
lows in the middle 20s to low 30s for most. An upper level
shortwave trough will begin to enter the area early Thursday
morning. High clouds will develop as upper level jet energy
increases. This should keep high temperatures slightly below
wednesday but still mild. 700 mb winds will increase to near 40
knots by 12z. this will impact our high wind areas of Arlington
and Bordeaux. Winds of 40 to 50 mph with gusts 60 65 will be
likely in the morning and early afternoon. High Wind Watches were
put up for the wind prone areas. This will mark the beginning of
a prolonged High Wind event over much of southeastern Wyoming. A
Winter Weather Advisory was also issued for the Sierra Madre
mountains as precipitation chances will increase trough late
Thursday and early Friday.

.LONG TERM...(Thursday through Tuesday)
Issued at 337 AM MST Wed Nov 21 2018

...Post-Thanksgiving Travel Impacts Possible this Weekend...

Main long-term focus remains centered two hazards:

1) High Winds of 40-55mph with gusts to 65mph with even higher
localized winds upwards of 75 mph possible in the wind prone areas
Thursday through Saturday. A High Wind Watch is now in effect and
will become a High Wind Warning over the next 24-36 hours.

2) Additional Winter Weather Headlines will be likely for late
Friday through Sunday morning as mountain snows will pile up over
the weekend. A Winter Storm Watch will be likely during this time
frame as we get further into the week. Still need to monitor how
much snow and blowing snow impacts could be possible for the High
Plains.

Important messaging includes high blowover risk along interstates
and highways starting Thursday and extending into early Saturday
morning. Friday morning through the afternoon will have the
strongest winds of the high wind period, especially along I-25 near
Bordeaux. Winds will ease some into Saturday but still be 25-45 mph
and potentially create blowing snow impacts along and north of
Wheatland in the High Plains and along I-80 west of Cheyenne.

Regarding the weather pattern, H85 and H7 height gradients will
tighten substantially Friday morning with CRG-CPR and CRG-BRX
gradients upwards of 60-70 decameters. This pattern, combined with
strong subsidence on the backside of a departing H5 trough will aid
in greater momentum transfer of the strong winds down to the
surface. Will need to watch winds closely on Saturday as the High
Wind Watch might need to be extended into the afternoon.

Stronger divergence and synoptic lift will shift over Wyoming late
Friday and through the day Saturday as a briskly moving Pacific NW
shortwave trough moves over the region. Widespread mountain snow is
expected during this time and winter weather headlines will be
likely needed. The bigger questions will be how much of the
precipitation will be able to spill out across the High Plains. More
precipitation should occur across the north High Plains and into
portions of the NE Panhandle but reduced amounts could be possible
for far SE Wyoming due to downsloping winds off the Laramie Range.
Will need to keep watch on system trends. None-the-less, Saturday
will not be pleasant as temperatures will be much cooler with wind
chills in the single digits to below zero.

Lingering blowing snow could occur Sunday and possibly impede some
travel following the Holiday Weekend. Precipitation will end early
Sunday morning with dry conditions appearing likely through the
beginning of next week with a gradual warm up through mid-week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1023 AM MST Wed Nov 21 2018

VFR. Wind gusts to 25 knots at Rawlins through 00Z. Wind gusts
from 20 to 28 knots after 14Z Thursday at all sites except
Scottsbluff.

&&

.FIRE WEATHER...
Issued at 337 AM MST Wed Nov 21 2018

Fire weather concerns will be mostly low as fuels have become non
critical. Strong winds and low humidity may develop on Thursday
afternoon. The rest of the period will see non critical humidity
and strong winds. Winter weather is expected to develop late
Friday and Saturday which will keep fire weather threats low.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from Thursday afternoon through late Friday
     night for WYZ103-105-107>109-115>118.

     High Wind Watch from late Thursday night through late Friday
     night for WYZ101-102-104-113.

     High Wind Watch from late tonight through late Friday night for
     WYZ106-110.

     Winter Weather Advisory from 9 AM Thursday to 8 AM MST Friday
     for WYZ112.

NE...None.
&&

$$

SHORT TERM...AL
LONG TERM...JSA
AVIATION...RUBIN
FIRE WEATHER...AL



Office: RIW FXUS65 KRIW 211753 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 1053 AM MST Wed Nov 21 2018 .SHORT TERM...Today through Friday night Overview...A pattern change is imminent, as unsettled weather approaches for the Thanksgiving holiday. An upper level ridge will continue its eastward shift today, as a Pacific trough begins to move onshore. This trough will bring a cold front and enough Pacific moisture for moderate snow across the far west. Moist, westerly flow will continue Friday morning, as a strong storm system from AK/North Pacific moves onshore over the PACNW. This system will move across E ID/N UT/NW CO Friday night into Saturday. Heavy amounts of snow will be possible for the far west during this time. Synopsis...Morning fog will be possible again in the same locations, especially portions of the Wind River Basin. Temperatures will once again be a challenge for this area and have trended temperatures slightly higher from Tuesday. Otherwise, gusty winds will occur over the Wind Corridor again today. These winds will continue overnight tonight into Thursday. The Wind River Basin is expected to finally mix out Thursday, as mid level temperatures cool due to the cold front. This mixing will allow all areas east of the Divide to rise into the 40s to mid 50s. Snow chances will increase across the far west after sunrise, with minor accumulations during the day. The best time of accumulating snow will be Thursday night. Rain showers to a rain/snow mix will be possible across southern portions Thursday afternoon and evening. Elsewhere, the gusty winds across the Wind Corridor will continue through the day Thursday. Central and southern portions will be under the warm sector of the storm during the day Friday. Mid level temps will rise from -6C/-8C to -3C/-5C over these areas, with 45 to 50 kt winds. The wind direction is not looking favorable for high winds in our wind prone locations at this time, but it will be very windy over the I-80 corridor to areas of Natrona County. Light snow will continue over the west Friday morning, becoming moderate to heavy by the afternoon. This will continue Friday night, as the cold front moves through. Favorable QG forcing will move along and behind the cold front, adding to the heavy snow potential. Light to moderate snow will spread across southern portions Friday night, with very light snow for areas east of the Divide. The better snow chances for these latter areas will be during the day Saturday. .LONG TERM...Saturday through Wednesday Upper low along the west coast of Alaska early this morning will be our wintry weather maker on Saturday. This potent system will dive southeast across Wyoming Saturday morning and into the central High Plains Saturday afternoon. Heavy snow in the west Saturday morning will begin to taper off Saturday afternoon. A burst of heavy snow will be likely along the Interstate 80 corridor Saturday morning along with strong west winds. The combination of snow and wind may justify a Winter Storm Watch issuance for Sweetwater County within the next 12 to 24 hours. Lee cyclogenesis is expected over eastern Colorado Saturday morning with surface winds becoming north across northern and eastern Wyoming. The Bighorn Basin, Johnson and Natrona counties will be most favorable for significant snow accumulations and hazardous winter travel Saturday into Saturday evening where Winter Weather Advisories may eventually be needed. Accumulations are expected to be lighter across the Wind River Basin where mid-level NW flow should suppress heavier snow development. Strong negative vorticity advection and drying prevail west to east Saturday night, with good radiational cooling, plunging temperatures and possible fog development Sunday morning. A weak clipper in NW flow may bring some light snow showers to the northern mountains Sunday night. A series of Pacific storm systems will invade the western U.S. Tuesday and Wednesday bringing increasing chances of snow to western Wyoming, while areas east of the Continental Divide will see mainly breezy to windy and milder conditions. && .AVIATION...For the 18Z TAFS through 18Z Thursday Patchy morning fog in the valleys and basins will be the main concern in the near term. Most of the fog will dissipate by 19Z early this afternoon. Otherwise, VFR conditions with mainly SKC skies today. Light winds will occur in most areas today, except breezy southwest wind from South Pass to Casper. Tonight, an approaching storm system will spread mid-high level cloudiness from west to east, and increase winds, especially on the lee side of mountain ranges and along the Southern Wind Corridor from South Pass to Casper. Any snowfall over the far west, and the corresponding IFR/MVFR conditions, in the western valleys should hold off until after 12Z Thursday. Southwest winds will continue Thursday through 18Z from South Pass to KCPR. MVFR/IFR conditions/mtn obscuration will continue in the Far West including KJAC through 18Z Thursday. MVFR conditions will work into KPNA/KBPI before 18Z Thursday due to lowering and thickening clouds there. && .FIRE WEATHER... Dry conditions are expected through Wednesday evening across all zones with generally a mostly clear sky. Southwest winds will increase through Thanksgiving, especially from zone 279 northeast through zones 280/300. Precipitation will begin to increase from the west late tonight and Thanksgiving Day, mainly west of the Continental Divide. Light accumulations are anticipated across the west Thanksgiving Day and evening. Wind speeds will increase Thursday night and Friday, in advance of a stronger weather system anticipated for Friday and Saturday. Light snow will become more widespread over much of the region Friday night and Saturday morning. Above normal temperatures are anticipated for most areas Wednesday, continuing in areas east of the Divide Thanksgiving Day and Friday. Cooler temperatures will envelop all of central and western Wyoming for the weekend. Mixing heights will remain low today, with smoke dispersal remaining poor even in areas with gusty southwest wind. Smoke dispersal improves Thanksgiving Day, as inversions weaken and mixing heights rise in combination with the increased transport wind. && .RIW WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...LaVoie LONG TERM...Meunier AVIATION...Murrell/Lipson FIRE WEATHER...LaVoie