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Area forecast discussion
National Weather Service Riverton Wyoming
255 PM MST Mon Nov 18 2019

Short term...this afternoon through Tuesday
issued at 1245 PM MST Mon Nov 18 2019

Northwest flow will continue over the region today, as the ridge axis that
has been over the West Coast over the weekend shifts eastward. This
will be in response to a developing storm system that is expected to
move onshore over the pacnw by this evening. Dry and mild conditions
will continue across the forecast area, with gusty winds continuing
over the north mountains and S Wyoming. Temperatures will also continue to be
above normal today, rising 5 to 10 degrees above normal. The flow
aloft will become more westerly Tuesday, as the storm system begins
to dig. 700mb temperatures of +2c to +5c, along with west-SW winds,
will be more widespread across the cowboy state, leading to daytime
temperatures 15 to 20 degrees above normal. With this turn in the
wind flow and the approaching trough, there will be a slight chance
for precipitation over Yellowstone and the north end of The Tetons. Snow
levels over this area will be near 7500 feet, if any showers were to
develop. Gusty winds will continue over the north mountains and shifting
more from East Sweetwater, Natrona to S Johnson counties. SW winds 20
to 25 mph with gusts up to 35 mph will occur in these areas.
Conditions will begin to deteriorate at the beginning of the next
forecast period (tuesday night), as the associated cold front from
the storm system begins to make its way toward north and west portions of
the forecast area.

Long term...Tuesday night through Monday
issued at 100 PM MST Mon Nov 18 2019

As the shortwave trough approaches the area Tuesday night, mid-
level temperatures will steadily decline from northwest to southeast. All models
are showing and area of enhanced moisture stretching from west to
east across the middle of the state between the upper level low to
the SW and the approaching Canadian air in Montana. A deepening
surface low over eastern Colorado will also service to strengthen the
gradient over the state and increase the northerly surface winds
over the northern half of the forecast area. By Wednesday morning,
the low level moisture and cooling temperatures will begin to
increase more widespread precipitation from the Big Horn basin
southward toward the Wind River mountains. As the shortwave from
Montana moves into northern Wyoming and 700mb temperatures of -8c to-
12c move over the area Wednesday afternoon, precipitation should
be the heaviest. Good upslope flow will bring the majority of the
precipitation (snow) to an area from Lander across central Wyoming and
into the Casper area. At this time, likely to see 4 to 6 inches by
Thursday morning. Also expecting these amounts across eastern
Sweetwater County with 2-4 inches from Rock Springs into Green
River. The moderate snowfall with gusty wind of 25 to 35 mph will
create dangerous travel conditions Wednesday from central into
south-central Wyoming. Ensemble output is still giving a wide range of
total snow with this system, so confidence in these amounts is not
that high. Concern is that upper level support will race off
quickly into the plains with the surface low and moisture, even
though the SW upper low is still over Las Vegas area late
Thursday. As a result, not issuing a Winter Storm Watch at this
time and likely issuing winter weather advisories on Tuesday when
better details can be determined. Wednesday night into Thursday
morning will see precipitation decrease from north to south with
continued accumulations over Sweetwater County. NAM is fairly slow
with taking the precipitation out of the area, while the GFS
clears all bu far southern Wyoming by Thursday afternoon.

Weak northwest flow returns to the area as high pressure builds in
from the northwest. Temperatures from Thursday into Saturday will
be quite dependent on the amount of snow that remains. Some basins
could see temps remain quite cool while the higher elevation warm
up more quickly. During this time, the rks-cpr wind corridor will
continue with breezy conditions. Sunday will see increasing clouds
from the northwest as another system moves into the pacnw late
Sunday. Models are actually in quite good agreement that this
trough will move nicely over the area to bring another round of
widespread precipitation and colder temperatures starting late
Sunday night into Monday.


Aviation...for the 00z tafs through 00z Wednesday
issued at 255 PM MST Mon Nov 18 2019

VFR conditions are generally expected at all terminals through the
forecast period. The only exception could be kjac with MVFR and
showers Tuesday afternoon. Northwest mid-level flow continues over
the area for the rest of today, with thicker high clouds moving over
the area late this afternoon. The surface pressure gradient has
brought breezy west-southwest wind from SW into central Wyoming this
afternoon, with gusts 20-25 kts at some terminals. An approaching
weather system from the west-northwest will shift the flow to the west-
southwest and keep some breezy wind overnight. Clouds should
increase over western Wyoming overnight as well with some light high
elevation showers possible. Moisture should increase over west-
central and northwest Wyoming between 12z-18z Tuesday, with precipitation
developing from kjac northward after 18z/Tue. Breezy conditions
will again develop from krks to kcpr Tuesday afternoon with gusts 20
to 30 kts possible. A surface front will push into far northern Wyoming
between 03z-06z/Wed. More significant precipitation is expected over
the entire area after 12z/Wed.


Fire weather...
issued at 1245 PM MST Mon Nov 18 2019

High clouds will continue to increase across the area this
afternoon, becoming mostly cloudy to overcast tonight. Gusty winds
will continue over the northern mountains this afternoon through
Tuesday. Gusty winds over the wind corridor will subside tonight,
shifting to eastward Tuesday. Gusty southwest winds 20-25 mph
with gusts up to 30 mph will occur from zone 279 to southern
portions of 281 Tuesday. Temperatures will continue to be above
normal today and Tuesday. A cold front will move into the far
northwest Wyoming late Tuesday afternoon, before racing through Tuesday
night. The front will be accompanied by a chance for snow for
many areas Tuesday night into Wednesday, with several inches of
snow possible. The best chances of snow will be over central and
southern portions.


Riw watches/warnings/advisories...none.



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