Area Forecast Discussion
Issued by NWS Cheyenne, WY

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FXUS65 KCYS 251910 AAB

Area Forecast Discussion...UPDATED
National Weather Service Cheyenne WY
110 PM MDT THU AUG 25 2016

Issued at 945 AM MDT Thu Aug 25 2016

Low stratus has become quite widespread across the area early this
morning. Forecast soundings show ceilings rising over the next few
hours, but profiles remain very moist and suggest continued broken
to overcast sky cover through the afternoon w/precipitation likely
increasing in coverage. Main change with the morning update was to
lower high temperatures a bit, and adjust precipitation chances to
better match radar observations and recent HRRR output.


.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 18Z TAFS through 18Z Friday afternoon)
Issued at 110 PM MDT Thu Aug 25 2016

Active period for scattered showers and isolated thunderstorms
through the overnight period expected. HRRR, GFS and NAM develop
buildups and showers with isolated thunderstorms over the southern
Wyoming mountains this afternoon and move that activity east
northeast across the region through early Friday morning. Expect
scattered coverage overnight. Brought in some lower MVFR ceilings
and visibilities at Laramie and Cheyenne toward morning due to
upslope moist boundary layer flow.


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...


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