Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 201619
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1019 AM MDT Sat Apr 20 2024

.KEY MESSAGES...

- Dry today, with warmer temperatures, though still below normal.

- Much warmer Sunday, with highs above normal. Elevated fire
  weather concerns and precipitation chances (15% to 60%)
  across northern Wyoming. Elevated fire weather concerns return
  Monday.

- Next major system looks to arrive towards the end of the
  workweek, though there is a lot of uncertainty in the details.

&&

.DISCUSSION...
Issued at 125 AM MDT Sat Apr 20 2024

A brief period of ridging is the weather story for today into the
start of tomorrow. Today, as the high pressure begins to move across
the area, temperatures warm. Highs will generally be in the upper
40s to around 60 degrees, the warmer of those in the Sweetwater
County area. For the most part (not including Sweetwater County),
these are only slightly below normal for this time of year. However,
with ample sunshine and a late April sun, it should feel fairly
pleasant outside. The ridging also acts to stabilize the air and
divert moisture away, resulting in less than 10% chance of
precipitation for the area; the northern Bighorn Mountains are the
exception, with about a 30% chance. Winds gusting to around 30mph
occur from the Rock Springs to Muddy Gap area early this morning,
decreasing by sunrise.

The ridge axis passes over Sunday. Temperatures climb to above
normal, with basin locations having nearly 100% chance for
temperatures in the 60s, with a 60% to 70% chance to hit the 70
degree mark. Jackson Hole is cooler, with temperatures more in the
mid 50s. This has been a fairly consistent forecast the past couple
days. One limiting factor may be increasing clouds ahead of the next
weather system.

Speaking of, a trough extending down from an upper low in Canada
sweeps through the state beginning Sunday afternoon. Models have
trended drier compared to yesterday. Precipitation chances pick up
the first couple hours of the afternoon for the mountains of
northwest Wyoming. Chances spread eastward through the afternoon as
the trough does. Precipitation chances have dropped for the lower
elevations, such as the Bighorn Basin, as a result of the drier
models. North of a Cody to Greybull line has the the best chances
(15%-30%) Sunday afternoon and evening. Northern mountains (YNP,
Absarokas, and Bighorns) have roughly 15% to 60% chances, the higher
of which for the Bighorns. All-in-all, not looking at any impacts
from this system when it comes to precipitation.

The other aspect to this trough is the wind. The HREF gusts have
come down a little, showing gusts more on the order of 20 to 30mph
across western Wyoming and the Wind Corridor. Gusts to 25mph are
possible elsewhere Sunday afternoon, with about a 50% chance for
gusts above 25mph. There is some uncertainty for some central
Wyoming locations, with the HREF showing a spread of 10 to 25mph
gusts in the Wind River Basin mid-afternoon.

This time of year, relative humidities (RH) also are on the mind.
The minimum RH deterministic forecast has values around 20% for most
of the lower elevations east of the Divide, as well as Sweetwater
County. There is currently a 20% to 40% chance RHs fall below
20% Sunday. These values, combined with the wind gust
potential, lead to elevated fire weather concerns for Sunday.

The main energy of the trough exits the area for Monday, but a
tight pressure gradient remains. There could be some lingering
showers over the northern mountains (30%-40%), which come to an
end by Monday night. RHs look to be as dry, or drier than
Sunday. Wind gusts also pick up again in the afternoon,
especially for locations along and west of the Divide. Elevated
fire weather concerns once again return for Monday.

Fairly zonal flow Monday through at least the first half of
Wednesday keeps precipitation chances low. Could be some isolated
(30%) showers in the mountains during this time. A more potent
system looks to be in the making for the end of the work week.
Confidence is very low at this time on details, with the GFS and
ECMWF having effectively different synoptic patterns at this time.
Am confident in saying a pattern change is coming though, with
cooler and wetter conditions for next weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1013 AM MDT Sat Apr 20 2024

VFR conditions, dry and light (11 kt or less) winds will be in
place through the TAF period. Wind gusts up to 20 kt will occur
at KBPI/KPNA and KRKS this afternoon. Mid to high level clouds
will develop/pass over the forecast area today, becoming mostly
clear by this evening. high clouds will return from the west
late tonight into Sunday morning.

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

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Wittmann
AVIATION...LaVoie


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