798
FXUS65 KRIW 291747
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1147 AM MDT Sat Jun 29 2024

.KEY MESSAGES...

- Seasonal temperatures today, with a slight breeze this
  afternoon. Isolated showers and storms over Sweetwater County
  this afternoon and evening.

- Hot and windy Sunday. Elevated to critical fire weather conditions.

- Weather system late Sunday into Monday, bringing rain chances
  and cooler temperatures.

&&

.DISCUSSION...
Issued at 217 AM MDT Sat Jun 29 2024

A seasonal day today. Temperatures warm a bit, sitting around normal
for the end of June. Highs will generally be in the 80s for the
lower elevations. This is due to a ridge building in over the area
today. This high pressure also keeps a mostly clear sky. High
pressure centered over the southern U.S. pushes just a little
moisture into the area this afternoon. High-resolution models are
indicating this is enough to trigger a few showers and thunderstorms
over Sweetwater County to maybe far southern Natrona County this
afternoon and evening. Chances are 15% to 30%, or very isolated. The
main concern with any of these would be gusty outflows, given model
soundings showing the classic inverted-v signature. Outflow gusts 40
to 50 mph would not be surprising for any stronger convection. These
rain showers and thunderstorms should dwindle through the evening,
ending by around midnight. As for fire weather concerns today, they
will be less than yesterday. The ridge moving over should lessen
winds. Of note is Sweetwater County, where relative humidities (RH)
around 15% during they afternoon may combine with the aforementioned
gusty outflows, creating localized elevated fire weather conditions.

Sunday becomes more complex. The focus will be on fire weather
concerns. An incoming trough over the Pacific Northwest tightens the
pressure gradient. Wind gusts 20 to 30 mph look common across the
area. For the Wind Corridor, (from roughly Sweetwater County through
Natrona County) gusts 30 to 40 mph as favored southwest flow occurs.
Very dry air will also be in place. Afternoon minimum RHs around 10%
to 15% occur across much of the area, with RHs 20% to 25% for
northwest Wyoming. These elements bring elevated fire weather
conditions to most of the area Sunday afternoon. A Fire Weather
Watch has been issued for the lower elevations of Natrona County,
where fuels are deemed critical. Now for the complexity.

The projected synoptic pattern of models has added some complexity
into Sunday afternoon and evening with recent runs. That high over
the southern U.S. is projected to push moisture into the area,
namely across the Wind Corridor (approximately that stretch of
area). This will determine just how low afternoon RHs get. Minimum
RHs have trended up since last`s shift forecast. Have dropped them
slightly due to the uncertainty of how quickly and how north this
moisture makes it. At the same time, the trough from the Pacific
Northwest begins making its way into northwest Wyoming. This will
act to push that moisture further east, stopping/slowing its
northward extent. These two factors add a bit of uncertainty to the
forecast when it comes to RH and precipitation (next paragraph) for
the area from roughly Rock Springs northeast around Casper.

Now for precipitation Sunday. Starting with the approaching trough
from the northwest, it brings rain chances to Yellowstone starting
around the middle afternoon as the cold front advances. Rain chances
spread southeastwards through Sunday night as the cold front
continues. Most rain chances occur for the northern half of the area
(roughly an Afton to Casper line, with lesser chances in the
basins). The other rain chances come from moisture advecting from
the high to our south. Isolated to scattered (15% to 50%) rain
chances occur over Sweetwater County Sunday afternoon. The best
chances remain west of the Continental Divide, though some isolated
chances also occur for southern Fremont County and into Natrona
County. As explained in the paragraph above, there is some
uncertainty when it comes to how far this moisture advances. Latest
model trends are indicating these rain chances though.

Past Sunday (Monday and Tuesday), a broad trough over the area keeps
(relatively) cooler temperatures over the area with continued rain
chances, mainly over the northern half of the area. Some model
uncertainty begins Wednesday with how high pressure builds over the
western U.S. As of now, there is potential for showers and storms on
the 4th, but mainly across the northern half of the area.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1146 AM MDT Sat Jun 29 2024

VFR conditions will prevail through the period. There is a low
chance (less than 20%) of a shower or thunderstorm near KRKS this
afternoon and early evening, but confidence is not high enough to
keep mention of this in the TAF for now. Otherwise, wind will
generally remain on the lighter through this evening. The exception
will be at KCPR which will see a gusty easterly wind from around 0Z
to 06Z. Then, southwesterly wind will begin increasing at most
terminals late Sunday morning.

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 217 AM MDT Sat Jun 29 2024

Winds will be lighter today, with slightly higher afternoon RHs,
generally 15% to 25%. Some afternoon and evening convection in
Sweetwater County may bring localized elevated fire conditions with
outflow gusts up to around 50 mph possible. Widespread elevated fire
weather conditions Sunday, with a Fire Weather Watch in effect for
zone 280. In general, RHs about 10% to 18% occur across most of the
area, with afternoon gusts 20 to 40 mph. Afternoon convection is
also notably possible (15% to 50%) across zones 279, 289, 280, and
300. See the above discussion section, as there are some points of
uncertainty in the forecast.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Sunday afternoon through Sunday
evening for WYZ280.

&&

$$

DISCUSSION...Wittmann
AVIATION...Myers
FIRE WEATHER...Wittmann