Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 160915

Area Forecast Discussion
National Weather Service Cheyenne WY
315 AM MDT Tue May 16 2017

.SHORT TERM...(Today through Wednesday Night)
Issued at 315 AM MDT Tue May 16 2017

Today...Southwest flow aloft continues with the next shortwave
trough aloft moving across Colorado this afternoon, helping to
generate isolated to scattered afternoon and evening showers and
thunderstorms across our counties. A few storms may be strong to
severe along and east of Interstate 25 based on projected shear and
instability. Another relatively mild day expected with warming
limited by cloud cover.

Tonight...Rather potent shortwave trough aloft moves into western
Nebraska, helping to generate scattered to numerous showers and
isolated thunderstorms, with the greatest pops across western
Nebraska. A few strong thunderstorms possible this evening mainly
over western Nebraska. Boundary layer progs suggest areas of fog
developing late east of a Douglas to Kimball line.

Wednesday...Next cold front moves across our counties and with
diffluence aloft ahead of the next closed low aloft over western
Wyoming, expect scattered to numerous showers and isolated
thunderstorms. Significantly cooler with cloud cover and in the wake
of the cold front.

Wednesday night...The upper low drifts slowly south over northern
Utah and northwest Colorado, with best lift across southeast Wyoming
and western Nebraska, helping to generate scattered to numerous
showers with snow levels lowering to near 6000 feet by morning.

.LONG TERM...(Thursday through Monday)
Issued at 315 AM MDT Tue May 16 2017

Main attention paid to slow-moving, unseasonably cold area of low
pressure that will spin across the Rockies Thursday and Friday.
Latest satellite pix show this low diving southeast into the
PacNW. Given this feature`s Alaskan origin, it will have plenty of
cold air to tap into. 00z models are consistent in this low
spinning upstream over Utah to begin Thursday. H7 temperatures are
progd to be in the -2 to -6C range 12z on Thursday, yielding
snow-levels that will be at valley floors west of the Laramie
range and 6000-7000ft on the ern plains. So, confidence is highest
that areas along and west of the Laramie Range will be seeing
snow on Thursday. Most of the ern plains should see mainly rain on
Thursday, with the exception in elevations of 5500-6500ft precip
will be mixed. Models are spitting out fairly healthy QPF totals
on Thursday of generally 0.5-0.75" with deep llvl upslope flow in
place. These may be a bit optimistic especially if convection to
the SE blocks best moisture influx. At any rate, this should yield
some light accumulations mainly to grassy surfaces 6000-7000 feet,
with perhaps several inches ovr the I-80 Summit. Although
temperatures will be colder at Laramie longer, the valley often
struggles due to shadowing in northeast flow. Temperatures will
feel more like a February day that late May, with highs only
reaching the 30s at Cheyenne and Laramie with 40s generally

Snow-levels are progd to fall a bit further, perhaps to as low as
5000 feet Thursday night. Would not expect any accumulations below
this elevation for this event. Upper low spins into Colorado
Thursday night, yielding perhaps 0.25-0.5" of add`l precipitation,
much of which should fall as snow above 5000 feet. Low pressure
lingers Friday morning before finally lifting into the plains by
Friday afternoon. Too early to feel confident with snow amounts,
but regardless, with late-season impacts expected to travel and
leafed vegetation, will likely need some sort of winter highlight
as the event nears, especially for the mountains. Temperatures
Friday will change little from Thursday.

Will see a brief respite from moisture chances Saturday before
very active northwest flow sends a series of shortwaves into the
area for late Sunday and Monday. Temperatures moderate through the
weekend, with highs reaching the 50s on Saturday and 60s for
Sunday and Monday.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday Morning)
Issued at 315 AM MDT Tue May 16 2017

Expect mainly VFR conditions to prevail through the period, with
the exception of MVFR CIGS in stratus at Chadron from 12-17Z.
Another round of thunderstorms can be expected potentially
impacting terminals east of the Laramie Range. For now, have
trended TAF`s towards VCTS given uncertainty in timing and
coverage. This activity should dissipate once again later tonight.


Issued at 315 AM MDT Tue May 16 2017

Minimal concerns based on expected humidities, and expected
rainfall and snowfall over the next several days into the weekend.





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