Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 250925

Area Forecast Discussion
National Weather Service Cheyenne WY
325 AM MDT THU AUG 25 2016

.SHORT TERM...(Today through Saturday)
Issued at 325 AM MDT Thu Aug 25 2016

A mid/upper trough axis stretched NE-SW from Hudson Bay Canada
through North Dakota, Montana, Idaho and Oregon. Another trough
extended N-S from eastern Colorado through southern New Mexico.
A ridge was centered over the southeast CONUS. Gulf and Pacific
moisture streaming northward over a stalled surface boundary along
the Colorado Front Range to the Wyoming border and northern Utah
was producing widely scattered rain showers and a few thunderstorms
across central/eastern Wyoming into northwest Nebraska. Prevailing
surface winds were light with temperatures in the mid 40s-mid 50s.
Patchy fog recently formed over the south Laramie Range/foothills
where T/Td spreads are less than 2 deg. This fog will linger
through the early morning hours.

The short range model guidance consensus progs the mid/upper trough
axis to the west to gradually shift east across the northern/central
Rockies and plains during the next 24 to 36 hours. Several weak
shortwaves will pivot through the base of the trough, providing
the necessary lift for scattered to locally numerous convection.
Isentropic lift and marginal instability today will lead to elevated
thunderstorms during the afternoon/evening. High temperatures
today will average about 10 degrees below normal with mid 60s-mid

On Friday, a cold pool aloft will spread east across the plains
during the afternoon. Resultant CAPEs approach 1000 j/kg east of
the I-25 corridor. Bulk shear values are less than 20 kt, so
organized intense convection does not appear likely at this time.
Cannot rule out a few strong thunderstorms producing hail up to
one inch and strong/gusty winds across the high plains. High
temperatures will be a tad milder Friday, but still below normal.
Shower/thunderstorm coverage decreases markedly Friday night with
passage of the trough and influx of drier/more stable air.

More seasonal temperatures are forecast Saturday under a dry
westerly flow aloft. A lee surface trough develops over the high
plains in the afternoon, which will generate breezy west-southwest
winds along and west of the Laramie Range. There may be enough
moisture and orographic lift to squeeze out a few afternoon showers
or thunderstorms over the southeast Wyoming mountains.

.LONG TERM...(Saturday night through Wednesday)
Issued at 325 AM MDT Thu Aug 25 2016

A dry and seasonally warm period upcoming as pattern deamplification
occurs thru the middle of next week. High temperatures will climb
well into the 80s thru the period as progd 700mb temperatures warm
to 13-15C. Despite minimum relative humidities that will fall to
near critical thresholds across southeast Wyoming each afternoon,
the absence of coupled flow will hold afternoon wind gusts precluding
widespread fire weather concerns. This should change sometime
Wednesday or Thursday as the flow backs to the southwesterly
coupling low and mid-level flow. This along with deep mixing should
be enough for at least enhance fire concerns. Moisture streaming
ahead of the backing SW flow will yield a return of thunderstorm
chances too, mainly over the mountains on Wednesday and perhaps
everywhere west of the Laramie Range for Thursday.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1225 AM MDT Thu Aug 25 2016

Initial aviation concern will be the potential of low stratus
and/or fog developing KRWL-KLAR-KCYS by sunrise. Confidence is
highest of fog occurring at KLAR so trended the TAF into the MVFR
category 11-16z. Will continue to monitor trends for possible
inclusion later at KCYS. Otherwise, will need to watch once again
for the development of isolated thunderstorms at KLAR and KCYS
after 21z. Given uncertainty in timing and spatial extent of
convective activity...VCTS mention will suffice. Otherwise, expect
VFR conditions to prevail at other SE WY and W Nebraska Panhandle
terminals thru the period.


Issued at 230 AM MDT Thu Aug 25 2016

A relatively cool and moist air mass will prevail across the
districts today through friday. weak upper level disturbances will
provide the lift for periodic shower and thunderstorm activity,
with decent chances for wetting rainfall. Warmer, breezy and drier
weather will return this weekend, with high fire danger for areas
west of the Laramie Range.


.CYS Watches/Warnings/Advisories...


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