Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 251730
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1130 AM MDT Wed Apr 25 2018

.SHORT TERM...(Today and Thursday)
Issued at 310 AM MDT Wed Apr 25 2018

Main hazard focus will center on the increasing wind speeds for
the High Plains late tonight and through the day Thursday.
Sustained north winds of 20-30 mph with gusts upwards of 40 mph
will be likely behind a cold front passage. Caution is advised on
I-80 and other exposed west to east oriented roadways. Isolated
light snow and rain showers will be possible behind the front but
precipitation totals will be minimal.

A much more pleasant day is in store today vs. the well below
normal temperature day yesterday. Shortwave ridging will pivot
over WY and allow for ample sun with warming H7 temperatures and
westerly winds. These factors combined will help temperatures
rebound to above normal values in the low to upper 60s by this
afternoon.

Unfortunately, a quick moving shortwave trough currently over
BC/Alberta will dive southeast across MT/WY late tonight and into
Thursday. An attendant cold front will accompany the wave passage
as H7 temps fall to -5C to -7C Thursday. Luckily it will be a
glancing shot of colder air that will not remain for long. The
greatest dynamic lift will remain farther northeast but H85/H7
pressure gradients will increase in response to the system.
Greatest isohypse tightening will be across the High Plains of SE
WY and west NE and this will be the favored area of higher winds
behind the cold front. Bufr soundings also suggest good downward
momentum transfer of winds aloft. As a result, increased winds
over superblend model guidance from late tonight through mid-
afternoon Thursday as gradients weaken. Precipitation will be
light and isolated due to the brisk motion of the front and
associated forcing and limited moisture in place. Isolated
mountain flurries and lower elevation drizzle could occur but
accumulations will be quite light. Thursday will be a cooler day
with low clouds initially but clearing mid-day with highs in the
mid-50s for lower elevations.

.LONG TERM...(Friday through Tuesday)
Issued at 310 AM MDT Wed Apr 25 2018

A rather pleasant and tranquil weekend can be expected across much
of southeast WY and the western NE Panhandle. The models remain in
excellent agreement with high amplitude upper-level ridging taking
shape over the Rockies on Friday. This feature remains intact thru
at least Saturday night, setting the stage for what will likely be
the warmest temperatures of the year so far. The GFS and ECMWF are
in agreement with H7 temperatures climbing to +4C to +8C over much
of the CWA by 00z Saturday, and then potentially well in excess of
+6C by 00z Sunday. MOS guidance suggests widespread 70s, and hints
at the potential for some 80s as well on Saturday. This seems very
realistic with deep subsidence and dry profiles. Limited flow will
likely keep winds from becoming particularly strong, but it may be
gusty on Saturday with deep mixing expected.

The models begin to diverge somewhat after Saturday, but generally
agree in a transition to a cooler and more unsettled pattern later
in the extended period. An evolving upper trough will likely cause
some potential for periodic showers/thunderstorms on Sunday and/or
Monday; however, the GFS/ECMWF differ considerably with respect to
the evolution of the system and any specifics are difficult to get
a handle on at this time. Colder air is likely to gradually filter
into the region through early to mid next week, possibly resulting
in some potential for accumulating snow by Tue/Wed. The fragmented
nature of the mid-level energy would suggest low potential for any
organized/significant winter weather threat, but we will certainly
want to pay attention to the evolution of the system over the next
few days.

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday morning)
Issued at 1130 AM MDT Wed Apr 25 2018

VFR prevailing through the period with mainly light winds into this
evening.  A cold front will move across the area later this evening
with increasing clouds and gusty northerly winds behind it. Cigs may
lower to near MVFR levels over the plains late tonight into Thursday
morning.

&&

.FIRE WEATHER...
Issued at 644 AM MDT Wed Apr 25 2018

Elevated fire weather conditions today across the mountains as
relative humidity values fall into the low teens with near 20
percent readings across lower elevations across the High Plains.
Winds be in the 10-15 mph range with some gusts near 20-25 mph
at times. Brief near-critical fire weather conditions could be
met. However, ongoing green-up and recent moisture will reduce
fuel energy release component values.

Moisture will increase Thursday as humidity increases but wind
will also increase from the north behind a cold front passage.
Winds will gusts upwards of 30 to 40 mph Thursday. Limited
precipitation is expected with the cold front passage.

&&

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

$$

SHORT TERM...JSA
LONG TERM...CLH
AVIATION...RE
FIRE WEATHER...JSA



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