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564
FXUS65 KCYS 251013 CCA
AFDCYS

Area Forecast Discussion...CORRECTED
National Weather Service Cheyenne WY
318 AM MDT Mon Sep 25 2017

.SHORT TERM...(Today through Wednesday)
Issued at 315 AM MDT Mon Sep 25 2017

Overnight GOES-R water vapor loop showed the upper low spinning over
western WY, with a trough axis extending to the Desert Southwest.
Surface front from the western lake Superior through central IA/KS
to a low pressure center over northeast NM has made slow eastward
progress, with scattered elevated convection spreading north across
the northern and central Great Plains. Most of the shower activity
from Sunday evening has decreased in coverage and shifted into
northeast WY. A review of the mountain SNOTEL sites indicated snow
accumulations from 2 to 9 inches, heaviest above 10000 ft over
the Snowy Range. With temperatures below freezing west of the
Laramie Range, black ice will be a concern for the early morning
commute along I-80.

Good model/ensemble consensus through the short term with the
northeast track of the upper low from western WY into western
ND Tuesday morning, weakening further as it crosses into Manitoba
Canada by Tuesday evening. The flow aloft will become more
westerly by Wednesday.

With the influence of the upper trough today, unseasonably cool
temperatures and scattered showers will persist. Morning snow showers
will occur above 8000 feet elevation, with little additional
accumulation expected. High temperatures will be in the 30s
mountains, with 40s to mid 50s lower elevations. Showers and
clouds will decrease from west to east as the upper trough lifts
northeast of the CWA. Overnight lows will be chilly with 20s and
30s.

Tuesday and Wednesday are trending warmer and dry as a weak surface
ridge builds over the region. High temperatures Tuesday in the
50s and 60s will rise a few degrees Wednesday. A prevailing weak
pressure gradient will maintain light winds.

.LONG TERM...(Wednesday night through Sunday)
Issued at 300 AM MDT Mon Sep 25 2017

A weakening upper low around the 4-corners area Weds night should
spread some showers over southern parts of the CWA early Thursday
as it drifts slowly northeast. Scattered showers should persist
through Friday as the low meanders over Wyoming. Mainly dry on
Saturday as the low is absorbed into a larger upper trough that will
move over the northern Rockys. This trough will amplify as it moves
into the northern and central plains Sunday with scattered showers
expected mainly over northern parts of the CWA Sat night into Sunday.
Temperatures will be mild through the period, generally close to
seasonal averages.

&&

.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 300 AM MDT Mon Sep 25 2017

IFR conditions this morning affecting most areas east of the mtns with
low cigs and patchy fog. VFR looking more likely for KRWL and KLAR.
Should see some improvement to MVFR east of the mtns late this morning
and afternoon, persisting through tonight.

&&

.FIRE WEATHER...
Issued at 230 AM MDT Mon Sep 25 2017

The cool and unsettled weather pattern will continue today as a upper
level trough slowly exits to the northeast. Isolated to scattered
showers are forecast, with morning snow showers for elevations above
8000 feet. A westerly flow aloft and weak surface ridge of high
pressure will bring warmer and drier conditions to the districts
Tuesday and Wednesday. Winds will be light through much of this week.

&&

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

$$

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