Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 231951
AFDCYS

Area Forecast Discussion
National Weather Service Cheyenne WY
1251 PM MST Thu Nov 23 2017

.UPDATE...
Issued at 1243 PM MST Thu Nov 23 2017

Winds at wind prone locations have been slowly increasing and with
gradients from the surface and aloft strengthening, we have
decided to upgrade now to a high wind warning for our wind prone
locations. Also, upgraded the high wind watch areas to a high wind
warning for later tonight based on 12Z model guidance.

&&

.SHORT TERM...(Today - Friday)
Issued at 334 AM MST Thu Nov 23 2017

We are looking at a very nice Thanksgiving across all of southeast
WY and the western NE Panhandle. The combination of high-amplitude
ridging aloft and low-level downslope along/E of the Laramie Range
should support widespread record highs today. H7 temps are progged
to climb to +6C to +8C over the high plains today, a difference of
at least +3C from yesterday which recorded widespread highs in the
65-75 degree range. The latest MOS guidance suggests daytime highs
only a degree or two F warmer than yesterday, and that really does
not make much sense in this pattern. We increased highs around 5 F
warmer than the warmest guidance for the plains, mainly because of
the strong chinook.

High wind potential increases by 00z as a strong short-wave tracks
across the northern high plains and resultant pressure falls begin
to spread across eastern MT into the western Dakotas. 700 hpa CAG-
CPR gradients will climb to 60-65 meters per both the GFS/NAM with
a broad swath of 55-65 knot flow expanding across the area between
00z and 12z Friday in the H7-H8 layer. H7 flow peaks around 12z in
the central/southern Laramie Range with a bulls-eye of over 70 kts
from Horse Creek to Bordeaux per the GFS. The flow aloft, combined
w/ a strong pressure gradient along the spine of the Laramie Range
is supporting high numbers from our in-house prob guidance for BRX
between 00z and 15z. These factors, along w/ potential for extreme
subsidence w/ a coupled upper jet overnight across the CWA results
in high confidence in 65+ MPH wind gusts in the wind corridors, so
we upgraded the High Wind Watch to a Warning, and bumped the start
time to 00z.

We also have some concerns about the potential for winds to spread
into the I-25 corridor in southeast Wyoming, and possibly westward
into the Laramie Valley as well. GFS soundings for DGW show a very
weak, almost negligible inversion at 09z with 60+ knot flow at 800
mb. Omega fields are also showing notable downward momentum during
that time, so believe lower elevations of Converse county may very
well get in on the action as well, before the cold front kills low
level gradients between 12z-15z. Areas further south should have a
window of opportunity between 12z and 19z as downward omega inches
into the I-25 corridor and low-level lapse rates steepen. In-house
probs for Cheyenne are slightly less than ideal, but still support
high confidence in (at least) 50-55 MPH wind gusts. Soundings from
both the GFS and NAM suggest 50-55 knots on top of the mixed layer
over CYS/LAR between 8 AM and Noon Friday. We went w/ the issuance
of a High Wind Watch for these areas. In general, winds are likely
to decrease from north to south through the day w/the cold frontal
passage, becoming sub-warning for all areas by afternoon. A couple
rain showers may accompany the front on Friday as well.

.LONG TERM...(Friday Night - Wednesday)
Issued at 334 AM MST Thu Nov 23 2017

Highly amplified upper air pattern returns to start the weekend
with strong ridging building over the Rockies through Sunday.
Southeast Wyoming will flirt with 580 dm heights at H5 and +8 C
temps at H7 by Sunday which should result in near record temps
once again especially over the plains. Still quite a spread
between the operational GFS and EC regarding the next strong upper
trough progged to cross the area from the west early next week.
GFS is much faster with the trough, keeping it an open wave while
driving a strong Pacific cold front through the area on Monday.
The EC is much slower and stronger, actually closing off the mid
level low over southern CO for a time on Tuesday. Don`t see much
support for the EC solution in the ensemble guidance and think the
pattern probably favors the more quickly ejecting and northern
system given the stronger jet energy downwind of the trough axis.
Should have a quick shot of mountain snow with the potential for
yet another wind event on Monday, this time more of a bora type
event with the strong cold frontal passage. Ridging builds in once
again behind this system for mid week, keeping us in a warm and
dry pattern.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1205 PM MST Thu Nov 23 2017

VFR conditions to prevail thru the period. Main aviation concern
will be periodic strong winds anticipated mainly at SE Wyoming
terminals today & tonight and across the entire region on Friday.
Wind gusts of 30-40 knots can be expected, with a periodic gust
upwards of 50 knots at KLAR and KCYS after 09Z.


&&

.FIRE WEATHER...
Issued at 334 AM MST Thu Nov 23 2017

No fire weather concerns. Despite periods of very windy conditions
over the next several days, RH values are likely to remain outside
of critical thresholds. Today will be the warmest/driest day, with
widespread highs in the 70s and minimum RHs near 20-25 percent.

&&

.CYS WATCHES/WARNINGS/ADVISORIES...
WY...High Wind Warning until noon MST Friday for WYZ106-110-116-117.

     High Wind Warning from 2 AM to noon MST Friday for WYZ101-107-
     115-118.

NE...None.
&&

$$

UPDATE...RUBIN
SHORT TERM...CLH
LONG TERM...DEL
AVIATION...CAH
FIRE WEATHER...CLH



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