Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 172033
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
233 PM MDT Tue Apr 17 2018

.SHORT TERM...(This evening through Thursday night)
Issued at 230 PM MDT Tue Apr 17 2018

Tonight...As predicted by earlier shifts, winds have increased
significantly after cold frontal passage with the cold front having
moved eastward across almost all of our counties. As of 1 to 2 pm,
we are seeing wind gusts in excess of 60 mph across much of Laramie
County, and expect this trend to continue into the early evening
hours across far southeast Wyoming and the southern Nebraska
Panhandle where High Wind Warnings remain in effect. With the
shortwave passage, will see isolated to scattered showers through
the early evening hours across mainly western Nebraska and near the
mountains of southeast Wyoming.

Wednesday...More tranquil day with less wind as the gradients relax
with a shortwave ridge aloft over our counties. Cooler in the wake
of the cold front, though still seasonable with maximum temperatures
in the upper 40s and 50s with 700 mb temperatures near -5 Celsius.

Wednesday night...Ridge aloft moves east of our counties with
southwest flow aloft developing. Quiet night with minimal cloud
cover and low level southeast winds from surface high pressure over
the Central Plains states.

Thursday...Warmer day as southwest flow aloft strengthens ahead of
the approaching upper low over southern Nevada. With warming 700 mb
temperatures, expect highs to climb into the mid 50s to lower 60s at
lower elevations.

Thursday night...The powerhouse upper trough moves near the Four
Corners, and with the low and mid levels moistening up, will see
isolated to scattered showers develop, mainly of the liquid variety
with temperatures remaining above freezing at lower elevations.

.LONG TERM...(Friday through Tuesday)
Issued at 230 PM MDT Tue Apr 17 2018

The main forecast concern/highlight in the extended period remains
the potential for a significant spring storm affecting the Central
Rocky Mountain region from Thu night through early Sat. The EC and
GFS remain in excellent agreement w/ a vigorous upper low tracking
across the Four Corners at 12z Fri. We have seen some changes over
the last 24 hours with regard to the track, as all guidance is now
quite a bit further south. Still quite a bit of uncertainty though
with respect to how much this system will dig. Standard deviations
per the GEFS to the west & east of the 500 mb low track would help
to confirm the possibility of continued variability. We have still
been looking at a trend toward warmer thermal profiles as well, so
snow levels are still a major question. We expect to have somewhat
of a better idea by Wednesday AM as the system approaches the west
coast and becomes better sampled by those networks. Until then, we
do not have a lot of confidence in a high-impact winter event, but
we do acknowledge the possibility despite recent trends in a less-
favorable direction for our CWA. Climatology says this is the type
of system we need to watch closely.

700mb temperatures 0 to 2C Friday lower to -2 to -4C Saturday.
However, snow levels only lower to 6500 feet by Saturday morning.
Precipitation diminishes west to east Saturday afternoon. Milder
and dry weather returns Sunday with light winds. The next Pacific
trough and front approaches early Monday, and slowly exits Tuesday,
accompanied by scattered showers, with isolated t-storms Monday.
It will be mild Monday with highs in the 50s and 60s, then cool
slightly Tuesday.

&&

.AVIATION.209..(For the 18Z TAFS through 18Z Wednesday morning)
Issued at 1130 AM MDT Tue Apr 17 2018

VFR CIGS, except MVFR-local IFR CIGS/VIS in snow showers at KRWL,
KCDR and KAIA this afternoon and evening. Strong winds and mountain
wave turbulence will be primary aviation hazards across the region
through the afternoon and evening. Wind gusts of 45-55 kt expected
through 03Z, diminishing thereafter. Skies become mostly clear
overnight tonight.

&&

.FIRE WEATHER...
Issued at 230 PM MDT Tue Apr 17 2018

Minimal concerns, especially Friday and beyond, based on expected
non-critical winds and humidities.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until 4 PM MDT this afternoon for WYZ106-
     115>117.

     High Wind Warning until 9 PM MDT this evening for WYZ107-108-118-
     119.

NE...High Wind Warning until 9 PM MDT this evening for NEZ019>021-054-
     055.

&&

$$

SHORT TERM...RUBIN
LONG TERM...CLH/MAJ
AVIATION...MAJ
FIRE WEATHER...RUBIN



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