Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 281131

Area Forecast Discussion
National Weather Service Riverton WY
531 AM MDT Wed Jul 28 2021

.SHORT TERM...Today through Thursday

As expected, there were a few thunderstorms in the west yesterday
but nothing really widespread. Radar does show some isolated showers
out there early this morning but nothing widespread. As for today,
the reasoning in the forecast from yesterday still looks pretty good
and we made few changes to continuity. Satellite imagery is showing
a good area of rain moving northward into Idaho from Utah. However,
much of this would miss the area to the west. With the increase in
precipitable water, still expected to exceed an inch across much of
the west, coverage of showers and thunderstorms should increase
today in the afternoon. There are disagreements on the placement of
the heaviest rain though. For now, we have gone with high chance
POPs in the afternoon and evening and then tapered them further
east. The main threat will be heavy rain with the precipitable water
values approaching twice the seasonal averages.

Meanwhile, the models continue to indicate mainly dry conditions
East of the Divide, except near the divide through most of the day.
There will be more cloud cover though. And this should keep
temperatures about 5 to 10 degrees cooler than yesterday, although
still above normal for most locations. The smallest difference will
be further east, which will have more sun in the afternoon. And as
for the chance of convection, it is small. However, it is not zero
with the monsoonal moisture slowly deepening. As a result, we have
gone with isolated showers and storms in the evening and even
lingered some into the night time hours. However, many areas will
not see one. And one more thing to consider is fire weather. There
will be a northerly push on wind in areas favored, mainly Johnson
and Big Horn Counties. However, dew points will be on the rise
through the day. So, we will go with a fire statement rather than a
warning for the aforementioned counties.

Convective activity should increase across the entire area through
most of the area on Thursday. There will plenty of moisture
available with precipitable waters again approaching twice the
seasonal averages. But the models are all over the place in
pinpointing the area with the greatest coverage. The European is by
far the wettest and keeps the most convection across northern
Wyoming. The American Models are again drier. For now we went with
higher POPs across the north with again everyone having at least
isolated coverage. However, the truth is the heaviest will come with
some areas of moisture that are hard to nail down more than 12 hours
out so stay tuned. As for stronger storms, the Storm Prediction
Center does have a marginal risk roughly from Riverton and Worland
on to the east. There is some shear and plenty of moisture. I am
concerned though that the expected cloud cover may limit instability
somewhat like it has for a couple of previous events and not lead to
many strong thunderstorms. We will watch it though.

.LONG TERM...Thursday night through Tuesday...

The ridge continues to wobble in place across the central US,
keeping western WY in the favorable position to receive more
monsoonal moisture. This brings an extended period of active
weather across the region, with showers and thunderstorms
bubbling up late each morning, and continuing into the evening.
Precipitable water values still forecast to remain at or above 1
inch through the weekend, so ample moisture is available for any
thunderstorms to produce copious amounts of rainfall. Thursday and
Friday, with the ridge centered to the southeast, any storms that
do form will get pushed to the west and northwest. The more
critical days look to be Saturday and Sunday, as the ridge shifts
back west and elongates, stretching north/south from Montana to
the Four Corners. This will minimize the steering flow across the
state, with any showers more prone to stay in place, increasing
the flooding threat. Models do pick up on a elongated wave
breaking through the ridge Monday and Tuesday, bringing some
elevated instability to the already unstable atmosphere, so shower
and thunderstorm activity will continue those day as well,
although the possibility of drier atmosphere returning with the
trough are noted in the extended models.


.AVIATION...(For the 12Z TAFS through 12Z Thursday morning)
Issued at 519 AM MDT Wed Jul 28 2021

Isolated shower activity continues to drift northward across
western WY this morning, and may briefly lower vis at KJAC and
KRKS through midmorning. Showers and thunderstorms will become
more numerous this afternoon in the vicinity of
KJAC/KBPI/KPNA/KRKS, with briefly lowered cigs and variably gusty
winds around any storms. Showers will likely continue for these
areas overnight tonight.



The chance of showers and thunderstorms will increase through the
day across western Wyoming as moisture moves northward from Utah and
Colorado. There will be a chance of some wetting storms. In areas
East of the Divide, today will be a somewhat cooler but still warm
day. A stray late day or nighttime shower or thunderstorm is
possible as well, but most of the daylight hours should be rain
free. Gusty winds across Big Horn and Johnson Counties will bring
elevated fire weather this afternoon. Mixing and smoke dispersal will
range from fair to good in the afternoon. The chance of showers and




SHORT TERM...Hattings
LONG TERM...Straub
FIRE WEATHER...Hattings is the U.S. government's official web portal to all federal, state and local government web resources and services.