Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210540
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1040 PM MST Thu Feb 20 2020

.SHORT TERM...(This evening through Friday night)
Issued at 210 PM MST Thu Feb 20 2020

Forecast issue through the next 36 hours continues to be cold
temperatures, with most basins still expected to fall below zero
again tonight. Models have been underestimating actual lows the
past couple nights, so have undercut tonight`s forecast a bit,
with Green River basin still reaching -15 to -20, Wind River Basin
reaching -5 to -10, Star Valley around -5, and the Cokeville/Bear
River Basin around -15. Higher elevations of western and central
WY should remain just above freezing, remaining above the cold air
inversions.

Friday, the H5 ridge remains in place across the state, with broad
westerly winds becoming notable for the northern mountain ranges.
Winds will not be strong, but it will be enough that good
downsloping conditions will be noted for western Fremont County,
most of the Big Horn Basin, and Johnson County. Natrona County
will also benefit from increasing southwesterly sfc winds. All
these areas will see considerable warming Friday, with highs in
the mid 30s to lower 40s. The cold pool across Sublette and
Sweetwater Counties will hold firm, likely keeping temps only in
the teens all day. The remainder of the Wind River basin will
depend on how much of the westerly flow will push or mix out the
cold pool.

.LONG TERM...(Saturday through Thursday)
Issued at 210 PM MST Thu Feb 20 2020

Mid-level flow will stay westerly on Saturday morning ahead of a
trough that will pass over the Four Corners region Saturday night.
This should keep most of the precipitation well south of the area.
However, it`s becoming more likely that some light snow will
reach southern Sweetwater County Saturday night into Sunday as
recent model trends continue to shift the low slightly northward.
Otherwise, most areas will stay dry through Sunday.

A stronger wave with Pacific moisture will approach from the
northwest Sunday night. Both the GFS and ECMWF models have trended
later with the arrival of this system, but nonetheless snow will
start first across the northwest mountains and spread south and
east through Monday. Some snow will spread east of the Divide on
Monday and Tuesday as the associated cold front moves across the
state. While this event still does not look significant as far as
snow totals, the pattern will favor keeping widespread snow
chances Monday through Tuesday night. Weekend temperatures will
moderate somewhat, but cold and gusty conditions will return with
the cold front on Monday. Another thing to watch will be the
potential for stronger winds on Tuesday as a progged 300mb 100+kt
jet passes over the area, though it`s not yet clear how much this
will translate to the surface. A ridge over the western CONUS will
keep drier northwest flow across Wyoming on Wednesday and
Thursday, with only slight snow chances across the mountains.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1026 PM MST Thu Feb 20 2020

The best chance for morning fog will be across the Wind River
Basin, but cannot rule out patchy fog in other basins/valleys.
Have gone with persistence from last night, and prevailed fog
(above minimums for now) to develop at KRIW around 10Z and
dissipate around 17Z while keeping VCFG at KBPI and KPNA.

Otherwise, VFR conditions with mainly lee enhanced high
cloudiness and light winds. The exception to this will at KCPR
where windy southwest wind will be observed much of the TAF
period.

A decent amount of mountain turbulence is possible especially
along the Continental Divide.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 210 PM MST Thu Feb 20 2020

Dry weather through Saturday, with no precip expected. Humidity
values will be fairly low for all higher elevations through the
weekend. Winds will remain light except for the wind corridor for
zones 280 and 289. Basin inversion across 278 will remain steady
into Saturday, with low mixing and poor dispersal. Light west wind
may start to mix up the cold pool across 283, with better mixing
heights and dispersal over the weekend.

&&

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

$$

SHORT TERM...Straub
LONG TERM...Myers
AVIATION...Murrell
FIRE WEATHER...Straub



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