925
FXUS65 KRIW 192318
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
518 PM MDT Tue Aug 19 2025

.KEY MESSAGES...

- High pressure today brings well above normal temperatures and
  very dry conditions with relatively light winds. There is a
  small chance (10-15%) chance of showers and thunderstorms over
  the Bighorn mountains this afternoon. Smoke continues in the
  Bighorn Basin from area fires.

- Wednesday will see near record high temperatures as high
  pressure continues, with some locations east of the divide
  hitting 100 degrees. Northwest WY will have a slight chance
  (10-20%) chance of showers in the morning, with isolated
  showers and thunderstorms in the afternoon.

- A cool front early Thursday morning will bring more seasonable
  temperatures Thursday and Friday, with increasing chances of
  showers and thunderstorms. Far west-central WY though will
  remain very warm and dry with gusty west winds in the
  afternoon.

&&

.UPDATE...
Issued at 116 PM MDT Tue Aug 19 2025

Upper-level high pressure continues to dominate western and
central Wyoming today, bringing mostly clear skies, relatively
light winds, and well above normal temperatures. Areas of smoke
are problematic for northwest WY (moving northeast) and in the
Bighorn Basin (due to the Red Canyon Fire and others). While
most of the forecast area is quite dry with relative humidity
values dropping below 15-20 percent (and even lower than 10%),
enough mid-level moisture remains over parts of Johnson and
Natrona Counties for cloud development, and even some light
showers on the east side of the Bighorns. These conditions will
continue through sunset with winds decreasing overnight. Models
are indicating a small area of enhancement moving from east-
central Idaho into northwest WY early Wednesday morning,
increasing clouds and possibly bringing some light showers to
the Tetons and SW YNP, then moving toward the northern Absarokas
mid-morning. While the rest of the forecast area remains mostly
clear through the day, northwest WY should continue with partly
cloudy skies and a small chance of showers and thunderstorms
through the afternoon as the weak shortwave trough moves into
south-central MT. Winds across western and northwest WY will be
breezy and possibly gusty due to outflow, but they are not
expected to be strong. Afternoon temperatures will be 10-13
degrees above normal on Wednesday, with some locations east of
the divide hitting 100 deg or higher. Some reprieve will come
very early Thursday morning as the surface pressure leeside low
moves southward and allows a cool front to move south out of
Montana first into northern WY late Wednesday night, then into
central WY around midnight. Thursday and Friday should be
notably cooler with more cloud cover and higher moisture, though
parts of west-central and NW WY should continue to be warm and
dry.

&&

.DISCUSSION...
Issued at 130 AM MDT Tue Aug 19 2025

WV imagery continues to show the southwest flow aloft across the
CWA as it continues to weaken and become more broad due to the
building higher heights centered near the Four Corner Region.
Mainly clear on IR with nothing on radar of consequence.

With increasing convergence aloft, the aforementioned ridge
continues to build in across the CWA giving way to hotter than
average temperatures and dry conditions Tuesday afternoon. Near
record temperatures for some parts east of the Divide but better
chances Wednesday afternoon with the strengthening high. Weak
winds and mainly clear skies continue much of these days. Some
weak chances (<15%) across northwest portions of the CWA to
include the Absarokas and Yellowstone for Wednesday afternoon
and early evening with a subtle mid level shortwave around the
aforementioned high. However, confidence low on anything
significant if anything at all at this time. The weak flow
aloft will not see that strong of outflows but some lightning
may be of concern with the ongoing fire weather threat currently
with active fires.

Otherwise, expect a pattern change come Thursday as the high
weakens with decreasing convergence aloft retreating some to the
south allowing for monsoonal moisture to increase into the CWA
from the southwest. Isolated thunderstorms increase in the
afternoon and evening hours with the upper level trough
skirting to the north of the CWA through Montana and southern
Canada. Thunderstorm coverage increases Friday afternoon and
even over the weekend, especially east of the Divide becoming
more widely scattered. As always, frequent lightning and gusty
outflows the main concern this time of year for fire weather.
The aforementioned higher heights near the Four Corner Region
looks to build back up early next week that will give way to a
dryer and warmer than average end to the month of August.

&&

.AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 518 PM MDT Tue Aug 19 2025

VFR conditions and mostly clear skies will prevail through the TAF
period. Smoke from the Red Canyon Wildfire in the southern Bighorn
Basin may lead to reduced VIS at KWRL and KCOD, down to MVFR is
possible (20%). Wind will remain light, less than 10 knots, across
the area through Wednesday morning. wind will increase Wednesday
afternoon to about 10-15 knots, with gusts around 20 knots at most
terminals. A very slight chance (10%) of an isolated rain shower at
KCOD Wednesday afternoon into Wednesday evening, otherwise it will
remain dry at all terminals.

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...
Air Quality Alert until 1 PM MDT Wednesday for WYZ003>008.

&&

$$

UPDATE...McDonald
DISCUSSION...Lowe
AVIATION...Rowe