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Area forecast discussion 
National Weather Service Cheyenne Wyoming
644 PM MDT Thursday Aug 28 2014

issued at 640 PM MDT Thursday Aug 28 2014

Gridded forecasts and zone forecasts have been updated to remove
all mention of precipitation for the rest of the night based on
WSR-88D radar loops from Cheyenne...Denver...Grand Junction...
Riverton and Rapid City. Also decreased cloud cover based on
visible and infrared satellite imagery loops. Temperatures...
dewpoints and winds also adjusted for the rest of the night based
on current trends.


Short term...(this evening through Saturday night)
issued at 313 PM MDT Thursday Aug 28 2014

Light to moderate rain has been ongoing for much of the day across
southeast Wyoming and the western NE Panhandle. This is occurring on the
back side of a vigorous midlvl shortwave currently exiting the area
to the east over NE. Widespread clouds/precipitation has wreaked havoc on the
forecast today with temperatures only in the 50s at lar and cys. Afternoon radar and
satellite imagery shows this activity has decreased in coverage as
of 20z...and believe this trend should continue through the early
evening hours as better upper-level support shifts to the east and is
replaced by subsidence beneath a shortwave ridge axis. Visible Sat
indicates skies are clearing over our western expect a
quiet overnight period for the most part with more clouds over the
Panhandle. Rain should end completely by 03-06z Friday. Went a little
warmer for lows tonight farther east where radiational cooling will
be less.

Warming trend ensues for Friday with broad upper-level ridging in place
over the County Warning Area. European model (ecmwf) 700 mb temperatures increase to 10-12 degree c on it
should be quite a bit warmer with dry/subsiding conditions in place
across the area and no major inhibiting factors for some decent surface
heating. A quick shot of midlvl moisture with a weak wave traversing
the flow should produce isolated showers and storms...generally in
the afternoon. GFS instability parameters are unimpressive with lifted indices only
around -1 to -2c and less than 500 j/kg SBCAPE. Convection on Friday
should be garden variety. Sat will be the warmest day of the period
ahead of a more significant shortwave moving across the Pacific northwest. Middle
level lapse rates around 8-9 degree c/km suggest better instability on
Sat...better over the Panhandle where surface convective available potential energy may approach
1000 j/kg. Increased probability of precipitation for Sat afternoon and evening with excellent model
agreement between the GFS/NAM/ECMWF with increasing 700-500 millibar
qg ascent. Good convergence along the cold front will help generate
convection as well. Cold front blasts across the County Warning Area on Sat afternoon and
early evening...enhancing precipitation chances and setting the stage for a cool
start to the extended period. It will be breezy as well with 35 kts
of flow at 700 mb...especially along and west of the Laramie range on
Sat afternoon.

Long term...(sunday through thursday)
issued at 313 PM MDT Thursday Aug 28 2014

A relatively cool and unsettled end of the weekend expected as an
upper trough and associated surface front moves through. The European model (ecmwf) is
6-12 hours quicker with frontal passage moving it through at the end of the
previous period. Comparatively...the GFS holds it back until later
Sunday. In either case...expect at least some convective
development Sunday...with best coverage during the late afternoon
and over the northern Nebraska Panhandle. Trough axis will clear the
area Sunday night and when combined with nighttime stabilization
will bring an end to shower/thunderstorm chances by nightfall.
Another fast moving shortwave will track across the area again on
Monday. There may be just enough moisture/instability to initiate
an isolated shower/storm late Monday afternoon or early Monday
evening east of a Chadron to Alliance line. A much drier air mass
will advect in Tuesday-Thursday allowing a low level thermal ridge to amplify.
The result will be for dry and warmer conditions. Sunday will be
the coolest day of the period in the cool Post-frontal air mass.
Forecasted 700 mb temperatures only reach around 8c...which will keep afternoon
temperatures in the 60s and 70s. 700 mb temperatures will return to 12-16c for Monday-
Thursday...bringing high temperatures back well into the 70s west and 80s to
near 90s east by Wed-Thu.


Aviation...(for the 00z tafs through 00z Friday afternoon)
issued at 455 PM MDT Thursday Aug 28 2014

Low pressure that brought the widespread rain is moving off to the
east. Still some showers for an hour or so out in the Panhandle
that should end early this evening. Westerly winds will bring
downsloping conditions Friday...which will give the area VFR


Fire weather...
issued at 313 PM MDT Thursday Aug 28 2014

No major fire weather concerns over the next few days. The weather
system responsible for the recent rainfall will move to the east of
the area way to a gradual warming trend on Friday and
Sat. Minimum relative humidity values will be in the upper teens or
low 20s for western zones. Afternoon showers and thunderstorms may
produce gusty and erratic winds through the rest of the week...but
in general strong storms are not expected. A strong cold front will
move from west to east across the area on Sat way to
breezy conditions with wind gusts up to 40 miles per hour over parts of Carbon
County. Cooler temperatures will follow for sun.


Cys watches/warnings/advisories...


short term...hammer
long term...Hahn
fire weather...hammer

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