Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 051750 AAA

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1050 AM MST Tue Dec 5 2017

.SHORT TERM...Today through Thursday night
Issued at 210 AM MST Tue Dec 5 2017

Low clouds continue across the west, as well as over the Bighorns
and Casper Mountains. Otherwise, skies are clear this morning
across the Cowboy State. Mid to high level clouds from the north will
infiltrate over the area today. A massive ridge of high pressure
will be in place over the western CONUS this week, putting WY
under northerly flow. A cold front will sweep through the state
Tuesday night into Wednesday, bringing a reinforcing shot of cold
air. Light snow will be possible in the Bighorns today and
Wednesday. Temperatures and fog will be the main forecast
challenges through the period. Inversions, along with the fresh
snow cover, could keep temperatures from moderating. Fog formation
will also be possible under these inversions. Depending on how
long this snow stays on the ground, fog may be possible into

.LONG TERM...Friday through Tuesday
Issued at 210 AM MST Tue Dec 5 2017

GFS and ECMWF models are similar Friday and Saturday with dry
weather and moderating temperatures, especially the mixed areas
and higher elevations as 700MB temps warm. The basins will not see
much mixing both days, and with inversions in place, temps will
not warm up to their full potential. The deeper snow covered
areas from the past storm will also hinder the warming. Have
adjusted high and low temps downward in these areas. The mixed
areas will be very mild Fri and Sat as well as those places with
no snow cover. Model differences show up Sunday night into Monday
now. ECMWF model bring down a weather system and cold front
Sunday night into Mon. The GFS model is much weaker with this
system and less cooling. With this high amplitude ridge to our
west, any system that rides over the top of it must be watched for
potential digging as it heads toward the mean trough to our east.
Sunday high temps will still be milder in the mixed areas and
higher elevations with the basin inversions holding. Will keep the
forecast dry Sunday night and Mon with the cold front passage. I
have brought in some cooler temps east of the divide. Some basin
inversions may actually break in the central basins due to the
cold air advection resulting in mixing. Next Tue appears to be
dry with warming 700MB temps and thus back to the temperature
scenario mentioned above for Friday and Saturday.


.AVIATION...For the 18Z TAFS through 18Z Wednesday Issued at 210
AM MST Tue Dec 5 2017

A stratocumulus deck will continue to obscure the far western
mountains through this afternoon with the higher peaks poking
through the deck. The cloud deck will hover around 2000 feet agl.
Otherwise, VFR conditions with variable mid-high level cloudiness
are expected. Breezy west to northwest wind will occur at most
locations with winds becoming more northerly east of the Divide
late in the afternoon into the early evening as a weak frontal
boundary pushes south across the area. LLWS will be a concern
through the morning for terminals on the lee side of mountain
ranges especially for KCOD.


Issued AT 210 AM MST Tue Dec 5 2017

Temperature inversions will be developing the next few mornings and
continue the remainder of the week. The northern mountains could
see a few flurries at times the next couple days, especially the
Bighorns. Otherwise, expect seasonably cold December weather. A
cold front will push south Tuesday night with gusty north wind
east of the divide. Inversion areas will have poor mixing most of
the week.





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