154
FXUS65 KRIW 172320
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
520 PM MDT Thu Jul 17 2025

.KEY MESSAGES...

- Hot, dry, and breezy weather will create elevated to critical
  fire weather conditions across much of the area today. A Red
  Flag Warning is in effect from Noon to 8PM today across much
  of southern and central Wyoming.

- Isolated to scattered virga shower and dry thunderstorm
  chances (up to 40%) exist across southern Wyoming this
  afternoon, with gusty outflows 40 to 50 mph being the primary
  concern.

- More widespread afternoon and evening shower and thunderstorm
  potential Friday and Saturday. The focus will remain over
  western and southwestern WY both days.

- Remaining hot, with limited shower and thunderstorm chances
  Sunday into early next week. Elevated fire weather conditions
  will thus remain a concern through the weekend and much of
  next week.

&&

.UPDATE...
Issued at 115 PM MDT Thu Jul 17 2025

As expected, convection has begun to initiate along the Unitas and
is drifting across northeast UT, northern CO, and southern WY as of
this writing (1230PM). Not much is expected out of this convection,
but outflow or dry microburst winds 40 to 50 mph will be a concern
through around 7PM this evening with any stronger storms that may
develop. Dewpoint depressions of around 50 degrees with inverted-V
soundings will be the primary driver of the strong wind concern
through the afternoon.

The other concern this afternoon is Critical Fire Weather
conditions. RHs 10 to 15% across much of the region along with wind
sustained 10 to 15 mph with gusts 25 to 30 mph will be common across
much of the area today. Red Flag Warnings continue until 8PM MDT
this evening for much of southwest and central WY. Given latest
forecast guidance, also added fire zone 286 (Absaroka Mtns) to the
Red Flag Warning.

For Friday, models remain on track for more widespread convection,
but focus remains across the west and southwest initially. Models do
continue to prog convection pushing east of the Divide late in the
afternoon. Severe chances are low, with gusty winds being the
primary concern.

&&

.DISCUSSION...
Issued at 258 AM MDT Thu Jul 17 2025

A weak surface boundary extending roughly in a line from the Teton
Mountains to Rock Springs is evident on 08Z surface observations.
Dewpoints west of the boundary are in the low 30s whereas east of
the boundary, dewpoints in the 50s are common. This boundary is
forecast to wash out by mid-morning as the daytime heating
begins and abundant dry air filters in from the west and
encompassing the entire forecast area by midday today. This dry
air, mostly sunny skies, and deep mixing will allow a return of
seasonal to slightly above normal high temperatures today.

A somewhat tight pressure gradient and deep mixing allowing 20-25kt
700mb winds to mix to the surface will create breezy conditions (25-
30 mph gusts) across much of southern and central Wyoming today.
These conditions, combined with relative humidity values
dropping below 15%, will create critical fire weather conditions
across much of the aforementioned locations this afternoon and early
evening. Thus, a Red Flag Warning has been issued for fire zones
across Natrona, Fremont, Sweetwater, Sublette, and portions of
Lincoln Counties today from Noon to 8PM.

There is also mostly virga shower and dry thunderstorm potential (up
to 40% chance) across far southern Wyoming this afternoon. A weak
disturbance embedded within zonal flow aloft will trek across
southern Wyoming and provide support aloft for convection to fire
off the Unita Mountains and trek east-notheast. Given dewpoint
depressions of 40-50 degrees, any convection will largely be in the
form of virga or dry thunderstorms that will be capable of 40-50 mph
gusty outflows.

By Friday, there is consensus among model guidance in west-southwest
flow aloft as an upper-level trough begins to develop over the
Pacific Northwest and an upper low circulates over southern
California. Increased moisture aloft, thanks to the southern
California low, along with an embedded shortwave across Wyoming will
provide support for shower and thunderstorms along a weak front
Friday afternoon and evening. Convection at this time looks to
initiate off higher elevations of western Wyoming and push east and
becoming more isolated with the low-levels remaining relatively dry.
A similar set up is forecast Saturday afternoon with convection
initiating off higher elevations and trekking east with a weak front.

There is consensus that the Pacific Northwest trough remains and
leaves the Intermountain West in dry west-southwestly flow Sunday
through the first half of next week. Dry and hot weather looks
favorable during this timeframe. Although weak disturbances aloft
pass across the region each afternoon, low shower and thunderstorm
chances remain confined to higher elevations of northern
Wyoming.

&&

.AVIATION /00Z TAFS THROUGH 00Z FRIDAY/...
Issued at 519 PM MDT Thu Jul 17 2025

VFR conditions expected for all TAF sites for the entirety of the
period. Winds will diminish towards sunset after 02-03Z due to
radiational cooling before picking back up again after 18Z for areas
west of the Divide up to 18-20kts. Mid to upper level cloud coverage
will increase with a minor disturbance pushing across the zonal flow
aloft. Storm chances increase at RKS/BPI/PNA after 19Z and JAC after
20Z before making it over the Divide to RIW/LND. Any coverage for
COD/WRL/CPR will be after the end of the period into Friday evening
but waning in coverage losing daytime heating. Gusty outflow up to
35kts are the main concern with any of these storms nearby the
aforementioned TAF sites.

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

&&

.FIRE WEATHER...
Issued at 115 PM MDT Thu Jul 17 2025

Dry and breezy westerly flow will create elevated to critical fire
weather conditions across much of the area this afternoon and
evening. Confidence is high enough that southern and central Wyoming
(Fire Zones 277, 278, 279, 280, 283, 286, 288, and 289) will see
minimum relative humidity values below 15% and wind gusts between 25-
35mph. Thus, a Red Flag Warning continues for the aforementioned
fire zones through 8PM today. Note that 286 is a new addition to the
previous Red Flag Warnings issued this morning. Latest forecast
guidance indicates that RHs will be a bit lower than previously
thought, and thus warranted inclusion to the Red Flag Warnings.
Isolated mostly virga showers and dry thunderstorms begin to develop
along the Uinta Mountains and trek east-northeast as expected. Any
convection will be capable of 40-50 mph gusty and erratic outflows
across Uinta, Sweetwater, and southern portions of Lincoln and
Sublette Counties. These outflows will exacerbate the critical fire
weather conditions already in place across those zones.

Friday looks to be a bit cooler and critical RHs are not anticipated
with lighter winds occurring as well. However, elevated fire weather
conditions will return for the weekend and continue through mid-
week. Winds look to be the limiting factor for fire weather concerns
over the next week.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning until 8 PM MDT this evening for WYZ277>280-
283-286-288-289.

&&

$$

UPDATE...Hensley
DISCUSSION...Straub
AVIATION...Straub
FIRE WEATHER...Gerhardt/Hensley