Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 171122

Area Forecast Discussion
National Weather Service Cheyenne WY
522 AM MDT Wed May 17 2017

.SHORT TERM...(Today through Thursday night)
Issued at 330 AM MDT Wed May 17 2017

Today...Temperature and dewpoints are close early this morning
across western Nebraska and also across far eastern Wyoming and if
we see some clearing of the high clouds, will likely see areas of
fog developing through the early morning hours.

Rain chances will increase through the daylight hours as diffluence
aloft increases ahead of the approaching upper low over southern
Idaho. Highest shower chances this afternoon along a surface trough
axis from Douglas to Cheyenne with isolated thunderstorms as well
due to instability. Cooler today with more cloud cover versus

Tonight...The upper low remains nearly stationary over northern Utah
with a mid level low forming over northern Colorado producing deep
moist upslope over our counties, and especially over southeast
Wyoming. Models indicate widespread rain developing in this warm
advection pattern aloft, quite impressive looking at the low and mid
level theta-e advection. Our confidence in model track has increased
enough to warrant upgrading the winter storm watches to warnings for
our mountains and nearby foothills. Although the Sierra Madres will
be somewhat rain shadowed, we expect enough precipitation there to
warrant a winter storm warning, especially with time of the season
considerations. Snow levels lowering enough to include our western
Laramie county foothills in the winter storm warning.

Thursday...The upper low slowly pivots across western Colorado with
deep and moist upslope continuing across our counties. All models
show plenty of QPF with over a half of inch in 6 hour accumulations.
With warm air advection aloft and dynamics, expect widespread rain
with snow over the mountains. NAM shows about 3.6 inches of liquid
falling over Cheyenne through Friday, not unreasonable when compared
to the GFS and other models.

Thursday night...Models indicate a nearly stationary powerhouse
upper low over northern Colorado with another shortwave trough aloft
rotating westward over our counties, producing more widespread rain,
and mountain snow. Snow levels may lower enough to allow rain to
change over to snow at lower elevations such as Laramie, Wheatland
and Cheyenne, assuming Laramie is not rain shadowed too much by the
Laramie Range. Thus, we felt it was prudent to issue a winter storm
watch for our neighboring lower elevations near the winter storm
warning, especially considering impacts of how late this storm is in
the season.

.LONG TERM...(Friday through Tuesday)
Issued at 330 AM MDT Wed May 17 2017

Widespread precip ongoing Friday as the ECMWF and GFS have upper
low across northeast Colorado. 700mb temperatures pretty cold. GFS
forecasting these temperatures anywhere from -3 to -6C over
southeast Wyoming. ECMWF a little warmer with the low slightly
further north. It has temperatures from 0 to -6C. Snow should be
fairly widespread in both of these scenarios as morning lows below
freezing across southeast Wyoming.

Upper low finally pulls off to the northeast into north central
Nebraska Friday evening. Should see precip ending from west to
east through the day.

Fairly dry day Saturday with departing low. Do not see a warm up
for the weekend as we stay in northwesterly flow. Clipper system
Sunday could bring another round of showers to the area. Best
chances out west.

Pretty potent system forecast for Tuesday on the GFS with
widespread precipitation east of the Laramie Range. ECMWF has this
system further east and our CWFA pretty much dry. Did lean towards
the GFS and kept chance PoPs going for much of the area.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 520 AM MDT Wed May 17 2017

Of concern this morning is the northern Nebraska Panhandle where
northeast upsloping winds are established. Supported by latest
HRRR forecast, so did introduce lower conditions up there. Will
need to watch KAIA as well. More widespread IFR/LIFR tonight after
03Z or so as low approaches southwestern Wyoming and low level
winds turn more upslope. Rain changes to snow out west of the
Laramie Range by late evening.


Issued at 330 AM MDT Wed May 17 2017

No concerns due to precipitation and humidities over
the next several days and continuing into the weekend.


WY...Winter Storm Warning from midnight tonight to 3 PM MDT Friday
     for WYZ103-110-112-114-116-117.

     Winter Storm Watch from Thursday evening through Friday
     afternoon for WYZ105>107-115-118.



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