Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 181030

Area Forecast Discussion
National Weather Service Cheyenne WY
430 AM MDT Tue Apr 18 2017

.SHORT TERM...(Today through Wednesday Night)
Issued at 426 AM MDT Tue Apr 18 2017

Today will start out rather quiet with general west to southwest
flow aloft.  Another shortwave trough will move into northern Utah
by late this aftn with increasing chances for showers and isolated
tstms across areas to the west of the Laramie Range.  Llvl
isentropic ascent/QG lift really improves during the evening,
between 00-09Z, especially across the northern half of the CWA. Will
have the highest PoPs through the night across the northern zones,
from Converse county eastward into the northern Panhandle, where QG
ascent is maximized.  SPC does have a marginal risk for severe
storms across portions of the Panhandle for elevated storms that
develop during the evening and early night.  Did add small hail to
the Panhandle as the NAM does show good elevated instability (MUCAPE
values of 500-1250 J/kg) and decent effective shear.  The tstm
threat should end over the eastern zones by sunrise Wednesday.

The other story with this shortwave trough passage will be the
increase in west to northwest winds across the region by late
tonight into Wednesday.  The GFS is the strongest with winds aloft
across the plains after 12Z Wed, and would suggest some high wind
potential. However, the other models appear more reasonable with
winds aloft showing 700 mb winds of 30-40 kts.  Temps will be cooler
in the post frontal airmass, with highs mostly in the 50s to low
60s.  Isolated to scattered showers will likely persist through the
afternoon across the region, however the tstm threat will be low
through the day given only weak instability.

.LONG TERM...(Thursday through Monday)
Issued at 426 AM MDT Tue Apr 18 2017

Vigorous shortwave trough will move onshore the NW US coast on
Thursday morning and dive into the central Rockies by Thursday
night. GFS continues to evolve this shortwave into a closed upper
low over northern CO by early Friday morning while the EC has a more
elongated system further north over northern and central WY. Both
models have been consistent with these solutions over the past
couple of days.  Strong SE low level flow ahead of the shortwave
will provide ample moisture for showers and afternoon thunderstorm
development along the Laramie Range on Thursday especially late in
the afternoon as better height falls approach the region. Both the
GFS and the EC indicate widespread precipitation Thursday night into
Friday especially along and E of the Laramie Range due to the
combination of lift with the upper trough in proximity and moist
E to SE upslope flow. Model soundings indicate that the best
cooling may not take place until later in the day on Friday,
especially if the EC track verifies. Think snow levels will
initially be quite high and slowly lower to between 6 and 7 kft by
late Friday. Should any snow fall during the day on Friday
accumulations may be hard to come by outside of the mountains due
to warm boundary layer and sfc temps. At this time, it looks like
more of a mountain snow event with predominantly rain for the
remainder of SE WY and western NE.

The upper low pulls away on Saturday and gives way to ridging and a
drying trend for the weekend.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 356 AM MDT Tue Apr 18 2017

VFR for all terminals through Tuesday. West winds will gust to 25
to 30kt at times Tuesday afternoon near KRWL and KLAR. An area of
showers with embedded thunderstorms are possible in the NE
panhandle Tuesday evening and early Tuesday night. A few of these
storms may produce small hail.


Issued at 426 AM MDT Tue Apr 18 2017

No fire weather concerns through the end of the week.  Minimum
humidity values will mostly be above 25 percent.  Cooler
temperatures along with a good chance for widespread wetting
precipitation will occur Thursday and Friday.




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