Area Forecast Discussion
Issued by NWS Riverton, WY

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622
FXUS65 KRIW 191739
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1139 AM MDT Mon Jun 19 2017

.SHORT TERM...Monday through Wednesday night

High pressure will build across the area today and bring a quiet
start to the workweek. With thicknesses starting to rise,
temperatures will be quite warm today. And with the ridge axis close
to the area, winds should not be as strong as yesterday, although a
breeze will continue to blow across the favored areas. A couple of
the models are still doing the Bighorn Mountain rain forest effect
for today. It was doing it today as well and nothing happened. With
a drier atmosphere and even higher heights, all areas should be dry
today. The dry weather will last through Tuesday morning.

On Tuesday afternoon and evening, some returning mid and high
moisture and a shortwave moving across the state will bring the
return of the chance of thunderstorms, especially to the northern
two thirds of the state. Given the large temperature-dew point
spreads and inverted V soundings, strong wind gusts will be the main
threat. This could also be a so called "Little Green Blob" day where
20 dBZ showers produce 50 mph plus wind gusts with no notice. This
will also be the warmest day with many of the lower elevation areas
East of the Divide climbing to around or over the 90 degree mark.
As for the first day of summer, it arrives at 10:24 pm on Tuesday
with the summer solstice.

Wednesday, the first day of summer,  at this point looks to be
another warm to hot day although a couple of degrees cooler than on
Tuesday.  Flow at this point looks to be largely zonal. The models
have backed off on the chance of precipitation as well with any
thunderstorms confined mainly to the northern mountains. And right
now, even these look to be few and far between.

.LONG TERM...Thursday through Monday

An initial cold front should be along the Divide by Thursday
morning, with cooler conditions especially over northern Wyoming
than what occurs Wednesday. However, airmass remains fairly stable
with limited moisture and any convection Thursday should be
confined to the northern mountains. Models show a stronger cold
front sliding south across the forecast area Friday/Friday night
with 700mb temperatures possibly dipping to between -2 and
-4C across northern Wyoming. Models only show some weak instability,
 but strength of the shortwave trough and cold front is enough to
 warrant widespread slight/low chance PoPs along and the east of
 the Divide. Areas west of the Divide especially over the far
 southwest look to be dry. The main concern west of the Divide on
 Thursday and Friday will be dry and windy conditions with
 possible red flag conditions if fuels were critical.

In wake of the system Friday, a mainly dry northwest flow is
expected Saturday with unseasonably cool temperatures with
isolated convection confined to the mountains along and east of
the Divide. The ECMWF is slightly slower with some precip
lingering into Saturday morning, but have kept the lower
elevations dry for now Saturday.

Global models than differ significantly on how quickly
temperatures rebound early next week with the GFS backing the flow
to the west with 700mb temperatures warming in the +12 to +15C
range by Monday while the ECMWF and Canadian models keep the
northwest flow with much cooler temperatures. Have gone with a
compromise for now.

&&

.AVIATION...18Z Issuance

VFR conditions will prevail across the region through Tuesday.
However, moisture in the flow around an upper ridge over the
Desert Southwest will allow for some gusty high based showers and
thunderstorms to develop by Tuesday afternoon and evening,
especially in and near the mountains.

&&

.FIRE WEATHER...

Mainly dry conditions will dominate today with high pressure over
the area. Expect warm temperatures with less wind than yesterday.
Relative humidity may fall to around 15 percent in portions of
Sweetwater County, but should remain above critical levels
elsewhere. Mixing and smoke dispersal will range from fair in the
basins to good to very good in the normally windier areas. Mainly
dry conditions will continue into Tuesday morning with breezy
conditions and a chance of thunderstorms returning Tuesday
afternoon.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...PS
FIRE WEATHER...Hattings



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