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000
FXUS65 KCYS 220508
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1108 PM MDT Thu Sep 21 2017

.UPDATE...
Issued at 729 PM MDT Thu Sep 21 2017

Winds are dropping off as the boundary layer decouples most
locations. With diminishing winds and humidity rising with cooler
temps, have allowed the Red Flag Warning to expire on time. No
other changes at this time.

&&

.SHORT TERM...(This evening through Saturday)
Issued at 248 PM MDT Thu Sep 21 2017

Main forecast concern through the aftn will continue to be the
critical fire weather conditions across lower elevations of
southeast Wyoming. Deep southwesterly flow and very dry midlvls is
causing dewpoints to drop well below the consensus guidance, with
the HRRR really the model to capture the very low RH values (8-12
percent). The Red Flag Warning is in good shape with even some
extreme Red Flag conditions to the west of the Laramie Range where
gusts to 30-40 kt are occurring. Highly amplified upper trough
will remain rather stationary on Friday with the axis across the
Great Basin. Temps will be around 10-15 degrees cooler on Friday
over the CWA, ending fire weather concerns. With increasing large
scale ascent ahead of the trough during the aftn/evening,
scattered showers and isolated thunder will be possible over
mainly southeast Wyoming. The models continue to show excellent
upper level divergence and overrunning over the llvl front by
Saturday afternoon/evening. Raised PoPs to above 60 percent on
Saturday aftn with the good model agreement. Wet bulb zero heights
through 00Z Sunday stay mostly above 11000-11500 ft, so only the
higher peaks of the Sierra Madre and Snowy Ranges will see any
chance of snow throu8gh 00Z Sunday.

.LONG TERM...(Saturday night through Thursday)
Issued at 248 PM MDT Thu Sep 21 2017

Closed mid/upper level low will be located over southern Utah
early Saturday night. To the east a broad area of upper diffluence
combined with a low level frontal zone over our Panhandle
counties will contribute to widespread moderate rainfall
especially over the plains. Best forcing for ascent will push
north and east of the area by Sunday morning with some drying
aloft noted on forecast soundings. With the low level NE flow and
weak overrunning, Sunday should be a cloudy and cool day with
little temperature recovery continued the trend of undercutting
MOS guidance a bit.

Models have come into a bit more agreement in the mid level low
weakening and moving to the north and east directly over central
Wyoming Sunday night through Monday.  This track is not typically a
good one for low elevation snow in SE Wyoming.  Models have
continued the trend of keeping the colder air at bay to the west
until the precipitation rates begin tapering off late Sunday night
and Monday morning.  So while a period of accumulating snowfall in
much of the Snowy and Sierra Madre ranges is likely Sunday into
Sunday night, accumulating snowfall in the lower elevation is
becoming less likely.  Still expect some rain/snow mix or maybe a
brief period of all snow for the higher valleys west of the Laramie
Range late Sunday night into early Monday morning but the steadier
precipitation should be ending by that time and boundary layer
temperatures would be marginal for much accumulation.

Tuesday through Thursday look basically dry and seasonable as the
upper trough pushes east and some ridging rebuilds along the west
coast.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday evening)
Issued at 1108 PM MDT Thu Sep 21 2017

VFR will prevail. Clouds will increase and lower through the day
Friday in advance of a Pacific storm system. Light winds overnight
will become breezy from a southerly direction Friday, with gusts
of 25-35 kt west of the Laramie Range. Scattered SHRA and a few TSRA
will develop west of the Laramie Range early Friday afternoon,
spreading east across the plains late Friday afternoon and evening.

&&

.FIRE WEATHER...
Issued at 248 PM MDT Thu Sep 21 2017

It will continue to be a warm, very dry, and breezy afternoon over
much of southeast Wyoming.  A Red Flag Warning remains in effect
through the early evening for Fire Weather Zones 304, 306, 308, 309,
and 310.  Fire weather concerns will be minimal after this
afternoon.  Cooler temperatures on Friday will cause RH values to
stay mostly above 25 percent.  It will turn much cooler over the
weekend with good chances for widespread precipitation.

&&

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

$$

UPDATE...JG
SHORT TERM...ZF
LONG TERM...DEL
AVIATION...MAJ
FIRE WEATHER...ZF