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wy discuss


Office: CYS

FXUS65 KCYS 211611
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
911 AM MST Wed Feb 21 2018

.UPDATE...
Issued at 910 AM MST Wed Feb 21 2018

Minor update to expire wind chill advy and make minor changes to
temps/winds.

&&

.SHORT TERM...(Today through Friday)
Issued at 340 AM MST Wed Feb 21 2018

Pretty quiet this morning compared to what we have been seeing
over the past week weatherwise. Clear skies over southeast WYoming
over into southern Idaho this morning. Pretty cold this morning,
Alliance down to -23, Chadron -13 and Scottsbluff -7 at 2AM this
morning. Winds are pretty light this morning, but any wind at all
will cause windchills to plummet. Because winds are light, will
keep the current Windchill Advisories going as they area. If winds
pick up any more, will then consider warnings for some areas.

Pretty nice day today as clear to mostly clear skies continue.
Temperatures are going to be a challenge with widespread snow pack
and light winds. But 700mb temperatures forecast to warm about 4C
over yesterday, so would expect slightly warmer temperatures over
yesterday. Generally followed guidance on temperatures today. Sure
the day shift will have to make an update or two based on observed
trends.

Start to see the effects of a low pressure system moving into
Colorado Thursday as guidance begins to paints some light QPF over
our western mountains. Did keep chance PoPs going for the SNowy
and Sierra Madre Ranges for much of the day Thursday. Not much of
a warm up seen for Thursday as 700mb temperatures remain around
-14 to -16C as well as increased cloud cover, so did not mess too
much with guidance highs.

Best chances in the short term for more snow appears to be late
Friday as upper level diffluence ahead of upper low over eastern
Utah begins to move into the area. Best chances will be out west
across Carbon and Albany Counties and the mountains.

.LONG TERM...(Friday night through Tuesday)
Issued at 340 AM MST Wed Feb 21 2018

Friday night/Saturday...Shortwave trough aloft moves eastward across
Colorado, producing a chance of snow across portions of our
counties, mainly in the evening, with decreasing chances after
midnight. Mostly dry and continued cold on Saturday in the wake of
the cold front and with 700 mb temperatures near -15 Celsius.

Sunday...Next fast moving shortwave trough aloft moves across our
counties, producing orographic snows over our Snowy and Sierra Madre
Ranges, with dry conditions elsewhere. Slightly warmer based on
thickness trends.

Monday...Windy and a bit warmer as the flow aloft turns westerly
with strengthening flow aloft and near the surface, and with 700 mb
temperatures rising to -8 Celsius.

Tuesday...Next Canadian cold front enters our counties and along
with a shortwave trough aloft, will see a chance of snow, mainly in
the afternoon over our Wyoming counties.

&&

.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 448 AM MST Wed Feb 21 2018

VFR, except for occasional IFR at Alliance through 15Z today, and
occasional MVFR at Rawlins through 15Z today.


&&

.FIRE WEATHER...
Issued at 300 AM MST Wed Feb 21 2018

Minimal fire weather concerns over the next several days with cold
temperatures, light winds and a widespread snowpack. Next chance
for snow looking like Thursday into Friday as a low pressure
system tracks across Colorado.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...None.
NE...None.
&&

$$

UPDATE...RE
SHORT TERM...GCC
LONG TERM...RUBIN
AVIATION...RUBIN
FIRE WEATHER...GCC



Office: RIW FXUS65 KRIW 210827 AFDRIW Area Forecast Discussion National Weather Service Riverton WY 127 AM MST Wed Feb 21 2018 .SHORT TERM...(Today through Friday night) Issued at 126 AM MST Wed Feb 21 2018 Imagery continues to show a large deep trough across most of the CONUS with some amplifying ridging over the East Coast. An upstream trough axis cuts through northwest MT-western WY-through OR and into/through northern California. One portion of the northern branch of the jet continues to dive down the West Coast, with the downstream piece extending to the northeast from the TX Panhandle across the Great Lakes. The surface has a large area of post frontal high pressure enveloping the Plains/High Plains and northern Rockies including Wyoming. Today, extremely cold temperatures solidly in place across all of Wyoming under the post frontal high pressure. With all the new fallen snow and H7 temps running -20 to -25 deg C under clear skies, morning low temperatures will range from -5 to -30 deg F while only rising into the upper single digits and teens during the day. May be the coldest morning of the week. The coldest air looks to be trapped in the forecast area's Basins and across Yellowstone NP. With the extreme cold temperatures aloft still in place, a few isolated light snow showers continue occasionally across the west. Tonight through Thursday night, the main upper level trough continues expanding across the western CONUS as the West Coast jet and associated embedded shortwave dig deep down along the coast. This general troughiness will prevail through most of the rest of the forecast period with the cold air aloft providing for modest lapse rates at times and continued small chances for light snow showers remaining in the forecast across the western zones. Additionally, by Thursday, the approach of the western trough with an embedded weak shortwave running out ahead of the trough will allow for the development a surface clipper type front to push south east of the Divide and into south central and southern Wyoming. Meanwhile, low pressure development across the Great Basin will push southerly winds and a possible weak warm front into southern Wyoming. These conditions will provide some modest convergence across portions of southern/central Wyoming Thursday night and the impetus for the areas of light snow to develop and continue through the day Friday. There will also be some potential for gusty west to southwest winds to return to the mountains and foothills along and east of the Divide later Friday. The Clark area may get occasional high wind gusts by Friday night...although it looks localized. Finally, temperatures will remain well below seasonal through the end of the forecast period...and even longer! .LONG TERM...(Saturday through Wednesday) Issued at 126 AM MST Wed Feb 21 2018 A longwave trough will remain in place across the western U.S. for the extended. Temperatures are expected to below average to much below average as a result. A shortwave trough is expected to dive southeast out of southwest Canada across the Rockies in the Saturday/Saturday night time frame. This should give a period of light to moderate snowfall mainly across far western Wyoming, while areas east of the Divide should remain mainly dry. Isolated areas over the far Western mountain could approach advisory amounts. Also breezy to windy southwest winds in the wind corridor from RKS-CPR could result in a decent amount of blowing snow from the recent snowfall. Medium range models then show a transitory shortwave ridging Sunday. Despite this ridging, far western area could continue to get light snow/flurries on Sunday. Models show a strong trough digging into the West Monday and Tuesday, and the possibly pushing east into the Rockies Wednesday. Global models have significant differences on how this trough will evolve, but consensus is to have a deep trough developing over the Western U.S. Looks like the west would be impacted on Monday with snow spilling east of the Divide Tuesday and Wednesday as the associated cold front pulls south across the area in the Tuesday time frame. Models are showing a signal of isentropic lift over the cold front on Tuesday/Wednesday time frame. This system has the potential for some significant snowfall depending upon how everything evolves. && .AVIATION...(For the 12Z TAFS through 12Z Thursday morning) Issued at 104 AM MST Wed Feb 21 2018 West of the Continental Divide...KJAC, KBPI, KPNA and KRKS Some lingering light snow through 12Z with local MVFR conditions. Otherwise VFR conditions will prevail after 12Z through 06Z Wed. Snow showers will occur in the western mountains through 03Z Wed with some mountain top obscuration. Will have VCSH in the western terminal sites in the afternoon into early evening. East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL Lingering light snow in places through 14Z with local MVFR conditions. After 14Z VFR conditions will prevail over the area. Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for the latest information on icing and turbulence forecasts. && .FIRE WEATHER... Issued AT 126 AM MST Wed Feb 21 2018 Extremely cold conditions continue across the region in the wake of the exiting cold front. Winds are relatively quiet except for near Rattlesnake, Green and Casper Mountains where moderately gusty winds are keeping temperatures elevated a bit. Still, most locations will see brutally cold morning lows of 5 to 30 below zero. Smoke dispersion will be poor to fair (east of the Divide) to good (west of the divide) both today and Thursday. && .RIW WATCHES/WARNINGS/ADVISORIES... Wind Chill Warning until 10 AM MST this morning for WYZ019-020. && $$ SHORT TERM...Braun LONG TERM...Murrell AVIATION...Ross FIRE WEATHER...Braun