Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 170558 CCA
AFDCYS

Area Forecast Discussion...CORRECTED
National Weather Service Cheyenne WY
1058 PM MST Sun Feb 16 2020

.UPDATE...
Issued at 915 PM MST Sun Feb 16 2020

We are seeing considerable drying over western portions of the CWA
this evening, likely the result of increasing low-level subsidence
with the initial cold frontal surge. Winds have increased again at
& around ARL with a few recent gusts to 55-60 MPH, and would fully
expect to see an increase in winds along the I80 corridor from VDW
to CYS over the next few hours as this subsidence helps aid in the
downward momentum transfer of 55-65 kt 700 to 800 mb flow.

Significant questions exist w/ regard to the prospects of snowfall
over northern areas tonight. The models (GFS/NAM/ECM) have been in
very good agreement with at least 1-3 inches of snow, perhaps even
more, already falling over Converse/Niobrara counties, but we have
not noted any IR enhancement at all and cloud bases remain above 5
k feet or so. High-resolution guidance suggests a few snow showers
overnight, but virtually no hints of the banding that is suggested
by the main deterministic models. We lowered PoPs slightly, but do
not want to cancel any headlines until we can confirm the snowfall
accumulation threat has ended.

&&

.SHORT TERM...(This evening through Monday)
Issued at 200 PM MST Sun Feb 16 2020

Lots going on weather wise across southeast Wyoming. Forecast
challenges range from strong winds to headline level snow in the
short term.

Water vapor imagery showing a shortwave tracking into our western
counties this afternoon on the nose of a 115kt jet max over
southwestern Wyoming into northwestern Colorado. Has been snowing
very heavily over the Snowy and Sierra Madre Ranges with well over
a foot of snow widespread. SNOTELS at Blackhall Mountain...and
Little Snake River in the Sierra Madre Ranges are reporting Snow
Water Equivalent well over an inch as of noon. Snowy Range SNOTEL
at Medicine Bow closing in on 1.5 inches of SWE. WYDOT webcams out
near Arlington this afternoon showing quite a bit of snow has
fallen with I-80 snow covered. With that area in the upward lift
of the strong jet...winds have come down...with gusts in the low
40 mph range at County Road 402 and Halleck Ridge. Needless to
say...WYDOT has I-80 closed at this time and would not expect it
to open any time soon.

Good agreement in short term models bringing the upper shortwave
into east central Wyoming this evening. HRRR and Hires mesoscale
guidance showing precip blossoming across Converse and Niobrara
Counties after 00Z as the shortwave lift moves over the area. At
the same time...snow out west begins to decrease quite a bit.

Only change with the afternoon package was to add central Laramie
County to the High Wind Warning. GFS 750 and 800mb winds over
Cheyenne this evening pick up to 50-55kts as winds shift northwest
behind the surface trough. Expecting enough subsidence behind the
shortwave to mix some of these winds down to the surface.

Some concern for the Arlington area tonight for possible blizzard
conditions with increasing winds. But given the decrease in snow
seen on mesoscale models for this evening...decided to stick with
the Winter Weather Advisory and High Wind Warning. Evening shift
will need to monitor closely. WOuld also advise to watch Converse
and Niobrara Counties as we are getting quite an increase in snow
totals up there for tonight.

Precip and winds begin to decrease Monday as shortwave exits east.
Snow ends from west to east through the afternoon Monday.

.LONG TERM...(Monday night through Sunday)
Issued at 200 PM MST Sun Feb 16 2020

Still looking at better conditions after Monday with lighter winds
and drier weather. Do have a dry shortwave that moves through the
area Wednesday that will likely bring us another shot of cold air.

Those trying to make plans late in February...we are starting to
see the GFS and ECMWF coming together on a potential winter storm
around the 24th of February. GFS further south...thus less snow
while the latest ECMWF hitting us with a classic Colorado low with
over 1 inch liquid equivalent. Stay tuned as the event draws
closer as this could be our next significant winter storm.

&&

.AVIATION...(For the 06Z TAFS through 06Z Monday evening)
Issued at 1013 PM MST Sun Feb 16 2020...CORRECTED

Mainly VFR now expected over the se Wy terminals with a drying
airmass but some MVFR vsbys at times at KRWL due to blowing snow.
Still some possibility for periods of MVFR cigs and vsbys at mainly
KCDR and KAIA later tonight into Monday morning with snow showers
passing by.  Sfc cold front will create gusty westerly winds behind
it as it moves across by Monday morning, strongest from KRWL to
KCYS where gusts over 30 kts are expected.

&&

.FIRE WEATHER...
Issued at 200 PM MST Sun Feb 16 2020

Minimal fire weather concerns as heavy mountain snow expected
through Monday afternoon. Even lower areas expected to pick up
accumulating snow...especially along and west of the Laramie
Range. A drier less windy pattern sets up after Monday for
southeast WYoming and Nebraska Panhandle...but colder temperatures
and favorable less wind should keep us from seeing critical fire
weather issues through the upcoming week.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Winter Weather Advisory until 8 PM MST Monday for WYZ101>105-109-
     113-116.

     High Wind Warning until 8 PM MST Monday for WYZ106-110-116-117.

     High Wind Warning until 8 AM MST Monday for WYZ118.

     Winter Storm Warning until 5 PM MST Monday for WYZ112-114.

     Winter Weather Advisory until 7 PM MST Monday for WYZ110.

NE...Winter Weather Advisory until 8 PM MST Monday for NEZ002-003-095.

&&

$$

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


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