Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 191759
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1159 AM MDT Tue Sep 19 2017

.SHORT TERM...(Today through Thursday)
Issued at 325 AM MDT Tue Sep 19 2017

Current forecast through Thursday looks on track, so did not make
any major changes to the previous forecast. Current 300 AM obs
show the strong cold front stretching northeast from just west of
Rawlins to Casper and northward towards Gillette across eastern
Wyoming. Temperatures drop into the 50`s and 40`s behind this
front with thick cloud cover. Expect these similar conditions to
move into Carbon, Albany, and Converse county sometime early this
morning as this front moves eastward slowly. Winds are already
increasing across southeast Wyoming and even around Chadron with
gusts between 45 to 50 mph observed in some of these areas. Will
have to closely monitor wind gusts approaching High Wind criteria
(gusts over 58 MPH) late this morning through this afternoon.
Model sounding have trended higher with winds speeds at the top of
the boundary layer with values now around 55 knots. Not confident
enough at this time to go with any High Wind headlines due to
uncertainty with timing and location. High temperatures are likely
occurring right now from Rawlins towards Douglas, with
temperatures expected to lower through the day behind the cold
front. Further east, high temperatures in the upper 70`s to mid
80`s are still expected ahead of the front. Expect winds to pick
up from west to east today with gusts up to 55 MPH not out of the
question. However, cloud cover may inhibit some mixing through the
day. Once the upper level trough moves into the area by this
afternoon, precipitation in the form of scattered/numerous rain
showers will develop and quickly move northeast along and just
west of the I-25 corridor. Kept POP between 40 to 60 percent from
the Sierra Madre Range northeast towards the Douglas area for rain
showers. Rain is expected to change over to snow above 9000 feet
with some snow possible down as low as 8000 feet. One to three
inches of wet snow is expected across the higher peaks by late
tonight. Little or no precipitation is anticipated along and east
of the I-25 corridor into western Nebraska due to dry downslope
winds.

For Wednesday, although winds tonight will keep temperatures a
little warmer than if it winds were calm, it will still get quite
chilly with readings in the 30`s west of I-25. Wind chills will
likely be in the 20`s around sunrise Wednesday for the high
valleys of southeast Wyoming west of I-25. Models then show the
upper level trough quickly lifting northeast. Expect moderating
temperatures Wednesday afternoon with readings near average for
this time of the year. It will be dry with gusty winds through the
day. The next Pacific storm system, which will likely impact the
region this weekend, will dig southward into the Great Basin
region for Thursday. Upper level winds will gradually back into
the south or southeast ahead of this system with gusts winds
continuing for areas along and west of the Laramie Range Thursday
afternoon. Kept POP below 10 percent as all the precipitation
associated with the jet and the next strong cold front will be
northern and western Wyoming. High temperatures will continue to
trend warmer on Thursday with highs in the low to mid 70`s along
and west of I-25, and in the 80`s east of I-25 into western
Nebraska.

.LONG TERM...(Thursday Night through Monday)
Issued at 249 AM MDT Tue Sep 19 2017

A significant upper-level trough will encompass much of the western
US through the extended forecast period. This feature will dig from
the Pacific Northwest into the Four Corners region by 12z Friday w/
moist southwest flow aloft becoming well-established across the CWA
through the weekend. The models are in strong agreement w/ a highly
difluent pattern through the weekend, and a 100-120 knot 250 mb jet
bisecting the CWA through at least 00z Monday. This will definitely
result in a cool and unsettled weather pattern for all of southeast
WY and the western NE Panhandle. The biggest change in the guidance
over the last 24 hours has been more of the CWA remaining under the
southwest flow, thus resulting in warmer thermal profiles. Even so,
the GFS/ECMWF bring the 700 mb 0 deg C line roughly along and north
west of a line from Rawlins to Lusk. This may combine w/ impressive
upper dynamics to produce snow in the mountains of southeast WY, as
well as some of our lower elevation areas around 6000 feet. At this
time, would not expect this to be a significant winter event, but a
close eye will need to be kept over the next few days as this storm
system will have enough cold air to work with. Timing is key. Model
guidance shows very little progression of the upper-lvl trough thru
Tuesday, so high temperatures are expected to remain well below our
consensus model guidance.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1154 AM MDT Tue Sep 19 2017

Mostly vfr conditions through the taf period. Rain showers will be
possible through the afternoon for sites to the west of Laramie
Range (RWL/LAR), with brief MVFR cigs at RWL. A cold front will
push across the region through 00Z with winds becoming west to
northwest. Gusts to 30-35 kts will be common behind the front
through 03Z.

&&

.FIRE WEATHER...
Issued at 249 AM MDT Tue Sep 19 2017

A strong cold front is currently moving into southeast Wyoming
early this morning. This front is expected to bring strong gusty
winds to the region, including western Nebraska, through this
afternoon. Ahead of the front, dry downslope winds will result
in low humidities and critical Fire Weather conditions where winds
will frequently gust over 30 MPH. Issued a Red Flag Warning for
zones 309 and 310 across SE Wyoming, and zones 312 and 313 for
western Nebraska. Areas further north and west of these zones will
likely see the cooler air arriving earlier in the day with higher
humidities in addition to some rain shower activity. Zone 309 is
marginal in terms of humidity and timing of the front, but with
gusts as strong as 50 MPH, decided it was best to include the
southern Laramie Range in the warning. The only concern is the mid
to high cloud cover pushing across the area, which may result in
slightly higher humidities than the current forecast suggests.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...Red Flag Warning until 7 PM MDT this evening for WYZ309-310.

NE...Red Flag Warning until 7 PM MDT this evening for NEZ312-313.

&&

$$

SHORT TERM...TJT
LONG TERM...CLH
AVIATION...ZF
FIRE WEATHER...TJT



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