Area Forecast Discussion
Issued by NWS Riverton, WY

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120
FXUS65 KRIW 140450
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
950 PM MST Tue Feb 13 2018

.SHORT TERM...(This evening through Friday)
Issued at 120 PM MST Tue Feb 13 2018

Inversions are holding strong in the snow-covered basins and we may
have to deal with those until the front comes through Thursday. Some
of the guidance is still trying to bring a nw wind down to the
Riverton area tonight but we`ll see. We`ve been surprised a couple
times this winter with apparently strong inversions giving way to
a nighttime breeze. If not, another chilly night, especially down
low. Lowered highs the next couple days for the Wind River and
Big Horn Basins. Wind will be cranking up across the wind corridor
and Cody Foothills tonight. In fact, parts of Cody have already
gusted over 40 mph today. Outer Dr. in Casper is already gusting
near 50 mph. Will upgrade both to warnings (Cody after 00z and
wind corridor after 03z.). Winds will gradually increase into
Wednesday across the wind corridor and quickly this evening in the
Cody Foothills. Cross- sections and upstream soundings are still
showing mountain wave potential with near mountain top inversion,
36-40kt flow into the barrier and a calculated froude near 1. On
top of that, the thermal gradient looks pretty strong in the
14-16C range at H7. So both mountain wave and gap flow will be in
play. The favored gap flow areas could be quite windy, especially
Canyon Rd west of Clark where I`m sure winds in the 80-90kt range
will be occurring. Other favored areas impacting traffic and more
people could easily be in the 60+ range with some 70 mph plus
gusts possible. In the wind corridor, gusts on Outer Dr could
start hitting high winds early this evening and then increase into
Wednesday morning. Other areas may take longer to reach near high
or high wind but they will be increasing through the night and
impacting travel. Snow starts to develop in the nw mountains
Wednesday afternoon with some steady snow Wed night before gradually
tapering off Thursday. East of the divide, the cold front will
move quickly south from the early morning hours Thursday into the
midday timeframe. Snow will spread into the nrn zones Thursday
morning with mainly sub-advisory amounts. The far nrn parts,
especially nrn Johnson will still have to be watched but for now
going 2-3 inches. Lesser amounts as you go south and southwest
through Central WY. Clearing Thu night and chilly. Friday we see
another quick hitting shortwave in the nw flow bringing another
bout of snow to the nrn/nwrn mountains with increasing w-sw flow
ahead of it. Any snow that occurs east of the divide with this
system will probably hold off until Friday evening.

.LONG TERM...(Friday night through Tuesday)
Issued at 120 PM MST Tue Feb 13 2018

Mid- to upper-level NW flow will continue to stream moisture and
weak energy toward Wyoming Friday night into Saturday, as a
shortwave trough moves from southern Alberta through eastern MT.  NW
mountains should see several inches of snow as a result, and a quick
moving weak front will push the winds to northerly across the
northern Big Horn Basin and northern Johnson county early Saturday
morning.  All model guidance is then suggesting more zonal westerly
flow Sunday, with continued moist upslope flow from the Alpine-
Jackson area northward.  YNP and the Tetons look to gain the most
from the pattern.

From Sunday night onward, the models though are in quite a bit of
disagreement.  Consensus is for more widespread snow Sunday night
into Monday, with the ECMWF being the most aggressive.  All models
do however bring down another cold front from the north east of the
divide on Monday morning. Tuesday then could either be a cold post-
frontal regime as the upper level low slides southward (ECMWF), or
widespread precipitation and strong northerly flow across the
eastern 2/3 of the state (GFS).

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday evening)
Issued at 345 PM MST Tue Feb 13 2018

VFR conditions to prevail through the forecast period at all
terminal sites except KJAC. However, gusty (sometimes strong) west
to southwest winds and/or low level wind shear will continue to be
possible along and east of the Divide through Wednesday. As the next
upper level disturbance approaches Wyoming this afternoon, there
will be an increase in clouds west of the Divide with convective
snow showers possible later this afternoon and evening especially
over the mountains. Mountain obscuration will become more frequent
and the KJAC terminal will begin to see MVFR conditions in lower
CIGs and possible reduced VIS due to the showers. The other terminal
sites west of the Divide will also see an increase cloud and better
proximity to showers by the end of the forecast, but should remain
above MVFR status for the most part.

&&

.FIRE WEATHER...
Issued AT 120 PM MST Tue Feb 13 2018

Winds will be on the increase again across the Cody Foothills
today into Wednesday and also across the Wind Corridor from
Jeffrey City to Casper. The lee of the mountain ranges and also
the area from Dubois to near Riverton will also likely see
increasing wind today and tonight. The light snow will return to
the northwest Wednesday and spread east of the divide Wednesday
night into Thursday morning with a gusty north to northeast wind.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from 8 PM this evening to 5 PM MST Wednesday
for WYZ019-020.

High Wind Warning from 5 PM this afternoon to 11 AM MST
Wednesday for WYZ003.

&&

$$

SHORT TERM...Skrbac
LONG TERM...McDonald
AVIATION...Braun
FIRE WEATHER...Skrbac



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