Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 210920

Area Forecast Discussion
National Weather Service Cheyenne WY
320 AM MDT Thu Sep 21 2017

.SHORT TERM...(Today through Saturday)
Issued at 315 AM MDT Thu Sep 21 2017

It will be a pleasant, but breezy day across much of the area today
as a significant upper-level trough continues to dig into the Great
Basin. The main weather concern for today will be the potential for
critical or near-critical fire weather conditions for areas along/w
of the Laramie Range including FWZs 308/309. We expect strong lower
level mixing once again today, so RH values may fall to around 15 %
by early afternoon along with wind gusts around 30-35 MPH. However,
like Wednesday, the models show a considerable moisture increase in
the mid-levels through the day. The 00z GFS suggests H7-H5 dewpoint
values climbing from -20C at 15z to around -10C at 00z. Once again,
it seems this may limit the duration of Red Flag conditions to less
than the necessary 3 hours. Forecast soundings from the GFS and NAM
both maintain RH values above 15 percent. Not comfortable w/letting
the Fire Weather Watch go since it will be close, but confidence is
too low to warrant an upgrade based on what happened Wednesday with
a similar scenario further north and east.

Our attention then shifts to the aforementioned upper-level trough,
and likely major changes to sensible weather across the area by Fri
and Sat. We know two things for sure: It will turn colder as models
strongly agree that H7 temperatures will fall from 12-14C on Thu to
around 2-8C by late Sat. Widespread precipitation is also likely as
flow aloft becomes strongly difluent w/a 100+ knot H25 jet blasting
northeast across the CWA. Precipitation type will be a challenge. A
dprog/dt of the models over the past 24 hours reveals a significant
increase in H7 temperatures at 18z Sat (i.e. the cold air is moving
in slower than earlier runs), such that not even the mountains have
H7 temperatures under about +4C through 00z Sun. This is at least 5
deg C warmer than we were looking at yesterday at this time. We had
no choice but to cut back on snowfall amounts based on this, but at
least a few inches of snow appears quite likely over the high peaks
with most accumulations occurring at night. Otherwise, strong upper
dynamics, combined with cold northeast upslope will yield increased
coverage of low clouds and rain, especially on Saturday which could
be a miserable day for many with highs struggling to get out of the
40s. Forecast high temperatures remain near 5 F below consensus MOS
guidance values.

.LONG TERM...(Saturday night through Wednesday)
Issued at 315 AM MDT Thu Sep 21 2017

00Z medium range models/ensembles in decent agreement progging the
closed mid/upper level low over UT/NV to gradually weaken as it
meanders eastward across WY/CO Monday, opening up over the northern
High Plains Tuesday. More shortwave energy drops south and forms
another closed low over the Four Corners Wednesday.

Models have been trending slower and slightly warmer at 700mb with
this Pacific storm system. Consensus 700mb temperatures fall to -2
to -4C Sunday west of I-25. Snow levels lower to 7500 ft. Snow
accumulations at this time will range from 2 to 4 inches above 7500
ft Saturday night and Sunday. Rain showers are likely for the lower
valleys and plains. High temperatures Sunday will range from the
30s and 40s west of I-25 to the 50s east. Scattered rain and high
elevation snow showers wil continue Monday, ending Monday evening
as main lift shifts northeast of the CWA. High temperatures Monday
will be similar to Sunday. Tuesday and Wednesday will be mainly
dry and milder under a west-northwest flow aloft, with highs in
the 50s and 60s east of the Laramie Range.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 315 AM MDT Thu Sep 21 2017

VFR. Gusty southerly winds at Chadron will slowly diminish through
the morning. Elsewhere, southerly winds around 10 kt will increase
to 10-20 kt with gusts 25-40 kt west of the Laramie Range.


Issued at 315 AM MDT Thu Sep 21 2017

A Fire Weather Watch remains in effect for FWZ 308/309 through this
afternoon. Near-critical fire weather conditions are likely with RH
values near 15 percent and wind gusts up to 35 MPH. RH values could
increase some through the afternoon due to a slight increase in low
and mid level moisture, so in general expect RH values around 17-19
percent today. It will turn much cooler for Friday-Sunday with good
chances for widespread precipitation.


WY...Fire Weather Watch from 11 AM MDT this morning through this
     evening for WYZ308-309.



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