Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 140841

Area Forecast Discussion
National Weather Service Riverton WY
241 AM MDT Mon Aug 14 2017

.SHORT TERM...Monday through Wednesday night
Issued at 240 AM MDT Mon Aug 14 2017

More unsettled weather will be the rule for the next couple of days.
The system that brought the thunderstorms this afternoon and
evening has moved away. However, the next shortwave will approach
from the west and is already spreading showers into the northwest.
These will spread into the north through the morning with a few
thunderstorms embedded as well. Thunderstorms should become more
numerous this afternoon and evening. The next question is the
chance for strong or even severe thunderstorms. There is a decent
amount of shear and decent CAPE, especially in the mid levels.
However, there are a couple of factors against it. One, the best
upper level forcing will remain to the north over Montana. In
addition, with the showers starting earlier surface instability
may be limited. The Storm Prediction Center has kept the marginal
risk area to the east of the area as well. We could still have a
few stronger storms, but the bulk would be across far eastern
Wyoming. Temperatures will be a bit cooler than yesterday, mainly
owing to more cloud cover.

Unsettled weather will continue Tuesday as well as the last in a
series of shortwaves moves through the area. With the timing of
this system, this will be an earlier show with showers possible
across much of the area through much of the day. And in addition,
with more cloud cover this will be the coolest day. As usual, the
NAM guidance is out to lunch and way too cool. Some lower
elevations could stay in the 60s though. With less instability,
the chance of stronger storms will diminish however. There could
be a better chance of small hail however with lower wet bulb zero
levels. Most of these showers will shift eastward and be gone by
later Tuesday evening.

It looks like there will finally be a dry day on Wednesday as some
ridging builds across the area. Temperatures will also rebound.
With plenty of sunshine we went above MOS guidance but
temperatures will likely remain somewhat below seasonal averages.

.LONG TERM...Thursday through Monday
Issued at 240 AM MDT Mon Aug 14 2017

Thursday looks dry for most places. Isolated afternoon and evening
showers and storms will occur over the north as a weather
disturbance clips that area. Showers or storms may linger late
Thursday night in the northern Bighorn mountains and in Johnson
county. Friday will be dry. Saturday also looks dry at this time.
Very isolated showers or storms may occur in the far south late in
the day and in the evening. Sunday the flow aloft turns southwest
on the GFS and ECMWF models. The models increase the mid and high
level moisture Sunday afternoon and night. Thus they indicate
isolated showers and storms Sunday afternoon and night...mainly
over the west and southwest with little or no activity east of the
continental divide. Have added this precip chance to the forecast
in the west and southwest. Temps will be seasonably warm Thurs and
Fri, then a little warmer Saturday and Sunday with mid 80s to
around 90 in the basins...upper 70s to mid 80s in the west and

First look at Monday weather for the eclipse...ECMWF model is dry
with some mid and high clouds around for Monday morning and
afternoon. The GFS model is showing a potential for clouds and a
few showers in NW WY from Jackson to Dubois. Less clouds from
Crowheart to Riverton to Casper, but potentially some mid and high
level clouds passing through at times. Model timing a week out is
of low confidence most of the time. For now, partly cloudy sky
with more clouds in NW WY and very isolated showers toward midday.
Temps in the 70s to lower 80s at eclipse time. This forecast is
going to change all week as models will change with time.


.AVIATION...12Z Issuance
Issued at 240 AM MDT Mon Aug 14 2017

Upstream shortwave moving across eastern Oregon and western Idaho
early this morning within a broad trough across the Pacific
Northwest. This shortwave and an associated 70kt jet streak will
help to increase and enhance convection across northwest and
eventually northern Wyoming Monday. Other areas in the central and
southwest can expect scattered showers and isolated thunderstorms by
mid-afternoon Monday, with the strongest storms along the east
slopes of the Bighorn Mountains. Main hazards will be brief heavy
rain, small hail, and wind gusts of 35-45kts. VFR conditions will be
the rule during the forecast period. However, ceilings are expected
to lower, especially beginning Monday evening as a cold front pushes
east across the region. Ceilings at BKN040-050AGL could become
widespread as Monday evening turns to Tuesday morning. Have trended
terminal forecasts in that direction along with prevailing light
rain at several terminals by 12Z/Tue. Mountain top obscuration will
be common across northwest Wyoming Monday. These obscurations will
spread east during the evening with many mountain peaks above about
10K MSL obscured by sunrise Tuesday. Gusty west to northwest wind 10-
20kts expected across mainly the southwest terminals Monday
afternoon. After 00Z/Tue, winds at most terminals will veer to a
more northerly direction as the cold front pushes through the
region. Highest speeds and gusts will likely occur at KBPI, KPNA,
and KRIW where northwest flow is favored.


Issued at 240 AM MDT Mon Aug 14 2017

Fire conditions should remain below critical today. Over the next
two days a couple of upper level disturbances will cross the area
and keep the chance of showers and thunderstorms in the forecast.
A gusty breeze will develop across portions of southern Wyoming
this afternoon. However, relative humidity will remain above 15
percent for all areas this afternoon. Tuesday looks cool and
showery as well. Mixing and smoke dispersal will range from fair
to poor over the northern Basins to excellent over eastern
Sweetwater County. Drier and warmer weather should return by





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