Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 221113

Area Forecast Discussion
National Weather Service Cheyenne WY
413 AM MST Wed Nov 22 2017

.SHORT TERM...(Today and tonight)
Issued at 224 AM MST Wed Nov 22 2017

Main concern in the short term is high wind potential in the wind
prone areas through mid morning. Latest satellite imagery had
shown some weak mountain wave activity near Arlington earlier
tonight which may have been responsible for the wind gusts near
criteria late Tuesday evening. This activity has since died down
and winds throughout the wind prone areas are only gusting between
40 and 45 mph. NAM has backed off on the gradients it was showing
last night while the GFS remains bullish on developing 50 kt of
H75 flow over the S. Laramie Range by around 12z. Think we are
going to be depending on mountain wave related subsidence to get
winds to criteria this morning. Forecast soundings are ambiguous
as to the potential for this as some of the higher resolution
models are showing the inversion a bit too low. A look upstream at
the Riverton sounding from 00z Wed shows an inversion around 700
mb though which would promote an enhancement of winds over the
Laramie Range. For now will leave the headlines in place but think
the event will be marginal at best.

Otherwise it should be partly cloudy today with high clouds
streaming overhead and temps much warmer than yesterday as heights
rise aloft. H7 temps top out around 2-4C and with downslope
westerly flow should see highs into the 60s over the plains with
50s west of the Laramie Range. Clearing and mild tonight with
overnight lows mainly in the upper 30s to around 40.

.LONG TERM...(Thursday through Tuesday)
Issued at 224 AM MST Wed Nov 22 2017

Thanksgiving Day should be dry and quite warm w/ significant upper
level ridging in place across the western/central US. 500 millibar
heights are progged to rise to near 580 decameters on Thursday per
both the GFS/ECMWF, which is impressive for late November. 700 hpa
temperatures are expected to rise to at least +4 deg C across most
of the CWA, with +6 to +8 deg C along/E of the Laramie Range. This
is a good signal for record warmth over the plains given low-level
westerly/downslope flow. We gave the nod to the warmer MET for the
high temperatures on Thursday, but it is entirely possible that we
are still too conservative depending on cloud cover. 70s should be
common east of I-25. Strong low-level gradients and mid-day mixing
will support windy conditions for much of the area, but think wind
gusts will stay under warning criteria during the day on Thu.

However, this changes Thu night through Fri morning, when the GFS/
NAM suggest an H7/H85 CAG-CPR gradient climbing to 55-65 meters by
06z and continuing through 18z Fri. Meanwhile, the models continue
to suggest a vigorous/130+ knot 250 millibar jet over northwest WY
at 12z Fri. This suggests intense/widespread subsidence as well as
notable mountain wave activity. The GFS/ECMWF both show 700-800 mb
flow in the 65-75 knot range, so there is definitely potential for
very strong winds in the wind corridors and possibly some adjacent
zones including Laramie/Cheyenne. A cold front should sweep across
the area late Friday AM, weakening winds by 18z. However, any slow
down of the fropa timing could lead to a more widespread high wind
episode possibly spreading into western Nebraska as well given the
expected flow aloft. Dry weather prevails.

In general, the remainder of the extended period will be breezy to
windy with significant jet energy remaining anchored over the area
for quite some time. In fact, we could be dealing w/wind headlines
through at least early next week. Medium range models are in quite
good agreement with a potent/fast moving shortwave sweeping across
the area on Mon-Tue, fairly similar to this past Monday. A notable
bora event is possible, along with a good chance for snow for some
areas along the front.


.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1039 PM MST Tue Nov 21 2017

More wind is anticipated tomorrow across the majority of the TAF
sites, due to a shortwave passing through the region. However, VFR
conditions will be the rule.


Issued at 224 AM MST Wed Nov 22 2017

Afternoon RH values will remain above 25 percent through mid week.
Winds will increase late in the week but at this time not
expecting any significant fire weather concerns.


WY...High Wind Warning until noon MST today for WYZ106-110-116-117.



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