Area Forecast Discussion
Issued by NWS Cheyenne, WY

Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS65 KCYS 251147 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Cheyenne WY
547 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 12Z TAFS through 12Z Friday morning)
Issued at 535 AM MDT Thu Aug 25 2016

The first question is just how long MVFR CIGS continue at KLAR,
KRWL this morning. Latest forecast information stratus will lift
by around 16z and both sites. 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. Additionally,
have added BR mention into KCYS after 06z tonight given favorable
moist, upslope regime.


Issued at 230 AM MDT Thu Aug 25 2016...Corrected

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...


FIRE WEATHER...MAJ is the U.S. government's official web portal to all federal, state and local government web resources and services.