055
FXUS65 KRIW 212255
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
455 PM MDT Sat Sep 21 2024

.KEY MESSAGES...

- Cooler today. Areas of frost are possible tonight, mainly for
  low-lying areas or cold prone locations.

- A little warmer Sunday, with no precipitation chances.

- A quick moving system Monday brings a north wind and cooler temperatures.
  Little to no precipitation chances.

- A warming trend Tuesday through the end of the week, with no
  precipitation chances.

&&

.DISCUSSION...
Issued at 1241 PM MDT Sat Sep 21 2024

North to northeast flow dominates much of the area today in the wake
of a cold front that moved through last evening and overnight.
Temperatures are noticeably cooler as a result, with highs expected
to only be in the 60s for much of the area. The main synoptic
feature over the next 24 hours is a low south of Wyoming, tracking
eastwards. Its main impacts will keep south and southeast of the
CWA, however, a tighter pressure gradient brings some stronger
northeast winds for the Wind Corridor (Sweetwater County up to
around Casper). Afternoon gusts of 25 to 35 mph is forecast for this
area. These winds decrease this evening.

Precipitation chances have really trended down over the last couple
days. Recent runs of the NBM, NAM, and hi-res models keep
precipitation out of the area. Would not rule out a stray shower
over far southeast Sweetwater County late this afternoon or evening,
but little to no liquid is expected, even if something does form. Of
interest is recent hi-res models producing some showers over the
Wyoming and Salt River Ranges between 6pm and 11pm MDT, perhaps
in response to a little moisture advection from the southwest.
Low confidence in this recent trend, but like with Sweetwater
County, not expecting much, if any, measurable liquid.

With a mostly clear sky and cooler temperatures in place, there is
discussion on frost tonight/early Sunday morning. The lower
elevations east of the Continental Divide, as well as most of
Sweetwater County, have yet to have a growing season ending freeze.
It does not look like this will occur tonight. That being said,
widespread temperatures in the the mid-to-upper 30s occur by sunrise
Sunday morning. For those in cold prone locations, such as low-lying
areas or near rivers, localized freezing temperatures (and frost)
are a real possibility. One location of note is Natrona County.
Models have been consistent in showing there being a chance of
slightly more widespread freezing temperatures (generally 28 to 32
degrees). Casper-town looks to remain just above freeze, but
locations west of town, such as the airport and towards the town of
Natrona, temperatures are looking near or just below freezing. A
Special Weather Statement (SPS) has been issued for lower elevations
east of the Divide and for Sweetwater County to cover these around
freezing temperatures. Impact-wise, it may be a good idea to protect
any gardens or vegetation tonight, even if temperatures are not
quite forecasted to be freezing, as there is a chance (even if a
small one) that frost forms.

As the low tracks east Sunday, some slightly warmer air filters in.
Highs climb a few degrees, to seasonal values. Winds should remain
relatively light, with no precipitation expected.

A very quick trough moves southward through Wyoming Monday. Models
have trended drier with this weak system. The recent NAM run only
produces precipitation in the Bighorn Mountains. Elsewhere, the
moisture is likely to just produce some clouds. Will have to see how
hi-res models resolve this system once we get into range. Otherwise,
a weak cold front returns cooler temperatures and a north wind
for locations east of the Divide.

Past this quick system, the GFS and ECMWF build in high pressure,
keeping the jet stream to our north. This should trend temperatures
up, while keeping precipitation chances near zero. We could see the
return of some 80 degree temperatures towards the end of next
week.

&&

.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 447 PM MDT Sat Sep 21 2024

With a cutoff low to the south and a recent cold front moving
through the forecast area east of the Divide, gusty northeast
winds have developed this afternoon from north-central WY into
southern WY. Gusts are expected to be 25-35 kts across eastern
Sweetwater County and KRKS through 22/03Z, with other areas
possibly seeing 12-20 kt gusts as well. Winds will then
decrease after sunset and be relatively light overnight and into
Sunday. KBPI and KPNA may see 10-15 kt southeast wind this
evening as a surge pushes into the Upper Green River Basin. All
TAF sites are expected to have VFR conditions through 23/00Z.
Some cumulus has developed over the mountain areas this
afternoon, and some high clouds are expanding as a result of
the cutoff low slowly moving northeast today. There is a very
low chance of isolated convection over far southern WY and the
southern end of the Wyoming Range by the start of the TAF period,
but expecting showers and storms to stay to the south and
southeast as the low moves east-northeast overnight and Sunday.
Sunday afternoon will see clearing skies from west to east
under weak high pressure, although clouds will then increase
over northwestern then northern WY Sunday evening into Monday
morning due to a weak system moving through MT. Another weak
cold front will then move north to south east of the Divide on
Monday with very low chances of precipitation.

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

&&

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

&&

$$

DISCUSSION...Wittmann
AVIATION...McDonald/LaVoie