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Area forecast discussion 
National Weather Service Cheyenne Wyoming
434 am MDT Sat Apr 25 2015

Short term...(today through monday)
issued at 410 am MDT Sat Apr 25 2015

Main forecast concern for the weekend will be the Pacific storm
system forecast to become a closed low near The Four Corners
region on Sunday. Confidence is high regarding the overall
pattern...but there is limited confidence on snow level...liquid
precipitation amounts...and thus snow amounts over the higher
terrain of southeast Wyoming.

Current watervapor satellite loop shows the Pacific trough digging
southward into California and Nevada early this morning. Models
all show the location of this trough axis over the Great Basin by
this afternoon...with a developing surface low pressure center
over Utah. This will result in upslope easterly winds this
afternoon as a cool front slowly develops across the forecast
area and moves into Colorado...becoming a stationary front. Low level
instability will result in rain shower activity becoming more
widespread through the afternoon...with thunderstorms eventually
developing by this evening. There is a low risk of severe thunderstorms
across the High Plains this afternoon and this evening...but
conditions will be marginal at best. Added hail and gusty wind
wording to the forecast across western Nebraska for now. Current
models show the potential of a weak squall line forming near the
Wyoming/Nebraska border late this afternoon with individual cells
moving northeast into this evening. This activity will transition
to moderate to heavy rainfall across the High Plains tonight as
the upper level low begins to close off from the mean flow aloft.
Models all indicate a period of strong upper level divergence
tonight due to a left exit/right entrance jet couplet aloft. All
models place this over the eastern Wyoming plains and into western
Nebraska sometime between 06z tonight to 18z Sunday morning. There
is some concern of a widespread area of heavy rainfall and an
nocturnal thunderstorms developing over the High Plains and moving
northward slowly through the night. Kept high pop near 100 percent
tonight and into Sunday morning. Thankfully...snow levels will be
high and above 8500 feet. However...some model runs including the
sref are showing respectable quantitative precipitation forecast amounts over the eastern plains.
Area of precipitation should shift westward over Wyoming on

For Sunday night...models show a deformation band shifting east a
bit as the main upper level low nudges eastward over New Mexico
and western Texas. Confidence is increasing that this system
should remain well south of the forecast area instead of giving US
a more prolonged precipitation event. But will have to closely monitor
this since these systems historically give last minute surprises.
Snowlevels lower to near 6500 feet Sunday night and early Monday
morning. Thankfully...precipitation rates do not appear very high with
the deformation band as the main upper level low is too far south.
Regardless...will have to watch snowfall amounts for the i80
corridor from Cheyenne westward to Rawlins...and especially the
i80 Summit. Even though temperature profiles indicate rain as high
as not convinced given the results of the last storm
to impact the region. Lowered the rain/snow line down to around
6000 feet for late Sunday night....with 3 to 5 inches of snow
across the i80 Summit. Winter weather advisories may be needed for
portions of the i80 corridor for late season impacts.

Long term...(monday night through friday)
issued at 333 am MDT Sat Apr 25 2015

A period of warm dry weather is on the horizon through the long
term forecast. A few showers may linger Monday night into Tuesday
with relatively cool air in place but westerly flow aloft will
kick in Wednesday as an upper level ridge expands across the
western Continental U.S.. daytime temperatures will warm into the 60s and
70s across the area by the middle of next week and it is looking
like 80s are a reasonable bet into the Nebraska Panhandle for
later portions of the week. Other than a few instability showers
possible over the mountains the next chance for more significant
precipitation will hold off until at least Friday if not until
next weekend.


Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 410 am MDT Sat Apr 25 2015

VFR across the area this morning but conditions will deteriorate
late today and into tonight as an upper level trough moves into
The Rockies. Scattered showers and thunderstorms will become
numerous by this evening with snow developing overnight across the
mountains of southeast Wyoming with snow levels dropping down to
around 8000 feet late tonight. Along and west of the Laramie range
the main concern will be reduces visibilities due to showers and
thunderstorms and of course snow in the mountains with mountains
becoming obscured...while east of the Laramie range and into the
plains low stratus...moderate rain and fog will be the main
aviation impact late tonight through at least Sunday morning.


Fire weather...
issued at 321 am MDT Sat Apr 25 2015

No fire weather concerns through early next week as a period of
unsettled weather will return to southeast Wyoming and western
Nebraska this weekend. Periods of rain showers and high elevation
snow are expected into Monday with isolated thunderstorms possible
Saturday and Sunday. Humidities will trend much higher this
weekend with recoveries near 90 percent...and daytime humidities
between 60 to 80 percent.


Cys watches/warnings/advisories...


Short term...tjt
long term...small
fire weather...tjt

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