Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 131809

Area Forecast Discussion
National Weather Service Cheyenne WY
1209 PM MDT Wed Sep 13 2017

.SHORT TERM...(Today through Thursday night)
Issued at 238 AM MDT Wed Sep 13 2017

Current water vapor imagery shows a disturbance moving over the
forecast area early this morning. Showers and t-storms developed
earlier in the evening from this wave, but have since diminished now
that the disturbance is moving east of the mountains. The remainder
of tonight should be remain a bit breezy in a few locations along
and west of I-25 in the wake of the wave with just some high clouds
moving overhead.

Moisture evident on water vapor imagery to our west will trek over
the forecast area over the next couple of days while the upper ridge
weakens. For today, a vigorous shortwave looks to move east through
the area around peak heating and flow through the column will be
zonal. This pattern is conducive to breezy westerly winds at the sfc
and convective development through the afternoon. The breezy west
winds will promote a dry boundary layer, especially just to the
lee of the Laramie Range where downslope flow will enhance drying.
But sufficient moisture in the mid and upper levels will aid in
developing showers/storms from west to east as the wave
approaches. SBCAPE progs are around 500 J/kg or less for most of
the area with the exception of the northern Panhandle where they
could peak around 1500 J/kg. So all in all, not anticipating
strong activity. Model QPF is up to one tenth of an inch of
rainfall with much of the activity. However, forecast sounding
profiles are the inverted-V type, so gusty winds will certainly be
a high likelihood with both showers and thunderstorms.
Temperatures will remain on the hot side with the thermal ridge
moving across southeast WY/western NE, once again approaching
record values for this day.

Chances for convection on Thursday will be higher due to a stronger
upper level shortwave trough approaching from southern California
during the day and advecting up even more deep layer moisture. In
addition to the upper trough, a cold front will be pulled down from
the north with perhaps a secondary sfc trough developing just to the
lee of the Laramie Range ahead of the main front. As a result,
expect a better chance for showers and storms over a broader portion
of the CWA. Activity will most likely come in two rounds, one with
the heating of the day and a second through Thursday night as the
shortwave moves overhead. Again, expect at least light to moderate
rainfall with this activity along with gusty downdraft winds.
Temperatures will be much cooler across the board due to the
influence of the cold front and breakdown of the upper level ridge.
Looking at highs in the 70s out west and 80s across the plains.

.LONG TERM...(Friday through Tuesday)
Issued at 238 AM MDT Wed Sep 13 2017

Transition to much cooler weather will be underway on Friday into
Saturday. Lead shortwave, the remnants of the upper low currently
off the CA coast, will traverse the region on Friday ahead of a
relatively deep upper trough over the northern Rockies. At the
surface, a cold front will bisect the region from SW to NE on
Friday before sinking quickly southward by late Friday night/early
Saturday. Should see pretty good coverage of showers with a few
thunderstorms on Friday with the best concentration being over
northern and western portions of the CWA, closer to the best upper
diffluence and low level frontal zone. Areas immediately east of
the Laramie Range may be in a relative minimum in precipitation
due to low level westerly flow and downsloping effects. Decent
agreement among the various model solutions in sinking the front
south through the area Friday night with the main upper trough
swinging through sometime on Saturday. Cool air aloft and
convergence along the front should maintain the threat for at
least scattered showers Friday night and into Saturday. Snow
levels look to drop to around 10 kft Friday night so looks like
the highest elevations in the Snowy and Sierra Madre Ranges may be
able to pick up a couple of inches of wet early season snow.

Ensembles are bullish in digging a deep upper trough into the
Pacific Northwest Sunday through Tuesday. This places our area in
deep W to SW flow on the anticyclonic side of the jet favoring
warm and dry weather with breezy conditions developing west of the
Laramie Range during peak afternoon mixing.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1206 PM MDT Wed Sep 13 2017

VFR conditions through the taf period. Isolated storms will be
possible this afternoon over much of the area. Gusty and erratic
outflow winds will be the main aviation hazard. West to northwest
winds will gust to 20-25 kts through 00Z at the southeast WY


Issued at 238 AM MDT Wed Sep 13 2017

Very warm temperatures for this time of year are expected again
today, with readings in the 70s/80s across the high country and
80s/90s east over the Plains. Resultant humidity values will drop to
around 15-20 percent mainly along and east of I-25. Westerly winds
will be breezy across southeast Wyoming through mid afternoon, but
gusts look to remain around 20 mph. Therefore will maintain Elevated
Fire Weather conditions in the forecast, mainly for the plains of
southeast Wyoming. Will see at least isolated showers and storms
over the district as well, with rainfall expected to be up to one
tenth of an inch. A series of Pacific storm systems will then impact
the area Thursday and on through the weekend bringing cooler
conditions and higher humidities, along with better chances for
wetting rains.




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