Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 132332
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
432 PM MST Fri Jan 13 2017

.UPDATE...
Issued at 416 PM MST Fri Jan 13 2017

Updated forecast for zones along and west of I-25 for some light
snow showers or snow flurries this evening with KCYS radar loop
showing light showers developing from southwest to northeast along
the I80 corridor from near Rawlins to just north of Cheyenne.
Once skies begin to clear later this evening...will have to
closely monitor the intensity of expected fog across the valleys
of Carbon and Albany counties tonight. May need a Dense Fog
Advisory into early Saturday morning for these locations.

&&

.SHORT TERM...(This evening through Sunday night)
Issued at 315 PM MST Fri Jan 13 2017

Forecast concerns deal with possible dense fog for portions of the
forecast area tonight into Saturday. Then increasing snow chances
Sunday evening.

Currently...High pressure over northwestern Wyoming this afternoon
resulting in a weak northeast flow over much of the CWFA. We did
have dense fog develop west of the Laramie Range this morning that
persisted until early this afternoon that we had to issue a Dense
Fog Advisory on. For the most part, fog has dissipated. Upper low
on water vapor imagery off the coast of southern California this
afternoon. Guidance still struggling on what this low is going to
do over the next few days.

For tonight...Not seeing a big change in conditions from what
happened this morning. Crossover temperatures overnight easily met
at Rawlins and Laramie as we are forecasting mid teens for lows at
both locations. Afternoon Tds at peak heating 18 at Laramie and 21
at Rawlins, so both locations easily cross that threshold. Both
NAM and GFS forecast soundings showing strong surface inversions
at both locations. Went ahead and added areas of dense fog into
the forecast for Carbon and Albany Counties overnight into
Saturday morning.

Next concern will be late Sunday. Still a lot of uncertainty on
exact track of that southern California low. GFS tracks the low
through southern New Mexico Sunday. Latest ECMWF beginning to
take a similar track. So confidence remains low on snow chances
for Wyoming and the southern Nebraska Panhandle. The ECMWF still
has the furthest west track for the low, tracking north northeast
into western Kansas. GFS tracks the low into northern Texas Sunday
night/Monday morning. Did maintain some chance PoPs for the I-80
Corridor from Sidney to Cheyenne. Hopefully over the next 24
hours, we will get a better idea on where this low will track.

.LONG TERM...(Monday through Friday)
Issued at 315 PM MST Fri Jan 13 2017

Monday...The models continue to differ on the positioning of the
closed upper trough moving across the Central Plains states. The GFS
tracks the low from the Texas Panhandle in the morning to Missouri
by evening, whereas the ECMWF tracks the low to north central Kansas
by evening. The NAM tracks the low to south central Kansas by
evening. Thus, with its further north track, the ECMWF is the only
model bringing precipitation to our far southeast counties. Will
continue to match with our neighbors, painting a chance of snow
primarily south of a Laramie to Alliance line.

Tuesday...A dry and warmer day with north northwest flow aloft and
limited low and mid level moisture.

Wednesday...The flow aloft backs to more zonal, west to east, thus
allowing the surface lee trough to strengthen and aiding in the
warming trend. With 700 mb temperatures progged near 2 Celsius, high
temperatures in the upper 30s to near 50 degrees expected. Chance of
orographic snows over the Snowy and Sierra Madre Ranges in moist
zonal flow aloft.

Thursday...Models show the upper flow backing further to southwest
and with enough low and mid level moisture in the flow, expect
orographic snows to develop mainly over the Sierra Madres and lesser
so in the snow shadowed Snowy Range. Dry elsewhere. Continued
relatively mild based on progged 700 mb temperatures.

Friday...Cooler temperatures in the wake of a shortwave and
associated cold front passage. ECMWF is faster with the shortwave
movement versus the GFS, though in either case, will paint a chance
of snow in the afternoon across our counties.

&&

.AVIATION...(For the 00Z TAFS through Saturday evening)

VFR conditions expected across western Nebraska tonight and
Saturday. The onset and intensity of fog will be the main aviation
concern this evening through tonight for KRWL and KLAR...with some
light snow showers near KCYS this evening. Pretty high confidence
of IFR to LIFR conditions around Laramie and Rawlins tonight with
some potential of periods of dense fog tonight.

&&

.FIRE WEATHER...
Issued at 315 PM MST Fri Jan 13 2017

Minimal fire weather concerns as snowpack is in place for much of
the mountains and cold temperatures keep afternoon humidities
high. Winter weather and cold temperatures to continue through the
weekend. The next chance for snow will be Sunday evening into
Monday as a low tracks across eastern Colorado.

&&

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

$$

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



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