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Area forecast discussion 
National Weather Service Cheyenne Wyoming
359 PM MDT sun Aug 2 2015

Short term...(this evening through Tuesday night)
issued at 359 PM MDT sun Aug 2 2015

Current kcys radar loop shows isolated thunderstorms and showers
east of the Laramie range this afternoon. Most of this activity
has been weak thus far...with no persistent updrafts...and have
been developing near surface outflow boundaries. Models indicate a
weak cool front slowly moving southward into the area this
afternoon...but current observations do not show any typical
signatures of frontal passage. Observations further north do indicate winds
shifting into the northeast up near Rapid the cool front
may be slower than models were showing earlier. This front will
gradually push into far eastern Wyoming and the northern Nebraska
Panhandle. This may result in thunderstorm activity lingering into
late this evening.

Attention then shifts to the southwest tonight and Monday as the
current watervapor satellite loop shows a vigorous upper level
disturbance lifting northeast across the eastern Great Basin
region. This shortwave will eventually move into Wyoming by
Monday and linger through Tuesday. There is quite a bit of
uncertainty in the track of this disturbance...since a track
further north will result in dry windy conditions which may lead
to fire weather concerns...while a track further south may result
in numerous showers and thunderstorms across most of the forecast
area with the potential of some severe thunderstorms. The GFS shows the
driest the disturbance moves clockwise around
southeast Wyoming through Tuesday. The other models are slower
with the progression of this system...with the Gem the most
aggressive showing widespread showers and thunderstorms across the
southern two thirds of the forecast area...especially south of the
Platte River valley. The new 18z NAM and sref are the
slowest...showing this disturbance still over the forecast area
through Tuesday night. Decided to focus mainly on Monday since
there is limited confidence in Tuesday it all
depends on the specific track this disturbance takes. For
Monday...pretty confident that areas west of the Laramie range
will see at least a scattered coverage of showers and thunderstorms by
the afternoon...and potentially lingering through Monday night as
well. Increased pop between 30 to 70 percent along and west of
i25...with lower values across the Nebraska Panhandle. With weak
flow aloft...some of these thunderstorms may stall or drift very slowly.
So there is a low flash flood concern. Temperatures will remain
near or slightly above normal Monday with highs in the 80s to low
90s across western Nebraska.

Long term...(wednesday through saturday)
issued at 359 PM MDT sun Aug 2 2015

An active quasi-zonal pattern will persist through the end of the
week into the weekend. Models seem to be in better agreement
today although still some slight timing differences are evident with
shortwave features moving overhead through this time. Wednesday
will see shortwave energy approach from the west...while surface winds
veer from the north to the east-southeast across the plains
through the afternoon and evening. Maintained at least slight
chance for thunder over the mountains and from the Laramie valley
eastward as monsoon moisture will accompany this wave. Could see
some activity overnight across the plains as a stronger shortwave
approaches during the overnight hours and maintains decent upslope
flow. This wave will move over Wyoming and into the
Dakotas/Nebraska by Thursday afternoon so the focus for lift and
surface convergence...thus overall better shot for
thunderstorms...will shift east into the Panhandle on Thursday.

Friday should remain mostly dry as a much drier airmass settles in
behind the shortwave...although cant rule out an isolated storm
or two in the afternoon over the higher terrain. Another plume of
monsoonal moisture will combine with a shortwave trough on
Saturday to bring increased chances for showers and thunderstorms
over much of the County Warning Area through the day.


Aviation...(for the 18z tafs through 18z Monday afternoon)
issued at 1206 PM MDT sun Aug 2 2015

VFR conditions to prevail outside of thunderstorm activity. MVFR
conditions are possible in moderate to heavy rainfall within
thunderstorms this afternoon...with the highest confidence for
kcys. Maintained thunderstorms in the vicinity elsewhere due to expected isolated coverage
but will amend to prevailing thunderstorms and rain as necessary. Winds will remain
relatively low with gusts up to 20 miles per hour at times across the
plains through the afternoon. The exception will again be in and
near thunderstorms where gusty erratic winds are likely.


Fire weather...
issued at 359 PM MDT sun Aug 2 2015

Fire weather concerns may become elevated on Tuesday after a
quiet Monday. There is uncertainty of the track of an upper level
trough and associated surface low across Wyoming on Tuesday. If
this low takes a track further north...windy conditions are
possible across most of southeast Wyoming and western Nebraska
where gusts up to 40 miles per hour are possible. In addition...downslope
winds will result in much lower relative humidities than what is
currently forecast...mainly due to low confidence. Otherwise...a
daily chance of showers and thunderstorms will continue through this
week...especially over the High Plains. Relative humidities will
remain between 15 to 25 percent...but wind speeds will be
relatively light by late this week.


Cys watches/warnings/advisories...


Short term...tjt
long term...rjm
fire weather...tjt

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