068
FXUS65 KRIW 211842
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1242 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 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 /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1102 AM MDT Sat Sep 21 2024

With a cold front having passed from north to south east of the
continental divide this morning, and a cutoff low over Arizona
moving NE to the Four Corners area, the surface pressure
gradient over much of WY will increase NE winds from northeast
to central to south-central WY this afternoon. KRKS and eastern
Sweetwater County could see gusts over 30 kts, with gusts 15-20
kts in the Bighorn Basin, Johnson County, and Wind River Basin.
Partly cloudy skies over northern WY will decrease through the
day, with clouds increasing over southern WY due to the cutoff
low. VFR conditions are expected at all TAF sites through the
next 24-30 hours. A couple of high resolution models are
indicating some weak convection over SE Sweetwater County as
well as the southern parts of the Wyoming Range late this
afternoon and early evening, but chances are quite low. Primary
impact from the low pressure system over UT/CO the next 36
hours will be the gusty NE winds and increased clouds over
southern WY through early Sunday afternoon. Skies then become
mostly clear Sunday afternoon from the west to east. Clouds will
increase over northern WY Sunday night into Monday morning due
to a weak system moving through MT.

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