Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 162025
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
225 PM MDT Mon Apr 16 2018

.SHORT TERM...(This evening through Wednesday night)
Issued at 225 PM MDT Mon Apr 16 2018

Early afternoon water vapor imagery depicted a deep upper trough over
the western CONUS and high amplitude upper ridge centered over the
Great Plains. Southwest flow aloft prevailed over the CWA, with a
swath of mid/high clouds spreading northeast across the central/southern
Rockies and adjacent plains. It was mild with temperatures in the
60s-low 70s southeast WY and 40s-50s western NE. Southwest winds were
rather breezy west of the Laramie Range with gusts to 45 mph, and
southerly 15-30 mph elsewhere.

Main forecast concerns during the next 24-36 hours will be the potent
Pacific upper trough/cold front as it translates east through the CWA.
The low/mid level gradients will be increasing tonight, and
especially Tuesday with the potential for bora-wind event across
southern/southeast WY. Southwest winds across Carbon County this
evening will ramp up overnight tonight, with gusts of 55-65 mph
possible by Tuesday morning. Both the GFS and NAM CAG-CPR 700/850mb
gradients peak in the 60-95m range between 12Z and 18Z Tuesday.
Model soundings indicate 55-65kt winds at 700-750mb, with strong
post-frontal subsidence progressing west to east, affecting I-80
from Arlington-Cheyenne and I-25 from Wheatland-CO border. The 12Z
GFS sustained surface winds at Laramie and Cheyenne are around 35
kt between 15Z and 21Z Tuesday. Went ahead and issued High Wind
Watches for WY zones 110-115-116 from 3 AM to 9 AM, and 9 AM to 3
PM for WY zones 106-117-118. A couple of mitigating factors to
deep layer mixing will be increasing cloud cover and
precipitation. The next shift can evaluate the 00Z models before
making any timing adjustments or warning decision. Temperatures
will peak in the 40s and 50s through early afternoon, then fall
thereafter.

Scattered to numerous rain showers, with high elevation snow showers,
will spread west to east through the day Tuesday. Cannot rule out
isolated afternoon thunder given the steep mid level lapse rates
and 200-300 j/kg SBCAPE. Showers will change to all snow Tuesday
evening before ending. Upwards of a third-inch QPF is forecast,
with snow levels falling from 7500 to 4500 ft Tuesday evening. The
higher mountains can expect 3 to 6 inches of snow, 1 to 2 inches
valley floors of Carbon County, and little or no snow elsewhere.

West to northwest winds will subside Tuesday night, and remain
breezy through Wednesday. Temperatures moderate Wednesday with
dry northwesterly flow aloft. Upper ridge to the west shifts east
across the CWA Wednesday night with lighter winds expected.

.LONG TERM...(Thursday through Monday)
Issued at 225 PM MDT Mon Apr 16 2018

Main focus this period will be on an upper low that will move from
southern California Thursday eastward across the 4 corners region
Friday, on to the southern plains Saturday. This system should bring
scattered rain showers and higher elevation snow showers to the CWA
initially late Thursday night into friday, then more widespread
rain/snow later Friday into Saturday. This system is not as cold
as the last one, so main problem continues to be the snow levels.
Accumulating snow still appear to be confined mainly to the mtns,
with rain more likely over lower elevations. Pretty decent pcpn
amounts still looking a good bet for much of the CWA by the time
it ends late Saturday. Sunday and Monday looking mild and mainly
dry, though some showers could return to parts of the CWA later
Monday in association with another disturbance. Temperatures should
stay pretty close to seasonal averages, with Friday the coolest day.

&&

.AVIATION...(For the 18Z TAFS through 18Z Tuesday morning)
Issued at 1118 AM MDT Mon Apr 16 2018

VFR predominating over the area through 09Z, then some hint that
there could be some MVFR cigs over parts of the Panhandle after 09Z.
Otherwise, some gusty winds affecting the area through this evening
with gusts over 30kts mainly from KRWL to KCYS.

&&

.FIRE WEATHER...
Issued at 225 PM MDT Mon Apr 16 2018

Mild and dry conditions will continue this afternoon with gusty southwest
winds west of the Laramie Range. Fuel moistures are non-critical with
green-up underway and remnant high terrain snowpack. A strong cold
front will bring scattered showers, isolated afternoon thunderstorms,
and strong westerly winds Tuesday. Rain changes to snow from west to
east late Tuesday and Tuesday night as colder air infiltrates the
districts. Wednesday through Thursday afternoon will be dry with
moderating temperatures and breezy winds. A more potent Pacific storm
system will impact the region Thursday night through early Saturday
with widespread locally heavy rain and snow.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Watch from late tonight through Tuesday morning for
     WYZ110-115-116.

     High Wind Watch from Tuesday morning through Tuesday afternoon
     for WYZ106-117-118.

NE...None.
&&

$$

SHORT TERM...MAJ
LONG TERM...RE
AVIATION...RE
FIRE WEATHER...TJT/MAJ



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