NWS Riverton, WY wrote its last area forecast discussion before the first event at Sep 9, 11:02 AM MST, 2 hours 17 minutes earlier. Its key messages:
Area forecast discussion, Sep 9, 11:02 AM MST, 2 hours 17 minutes before
Area Forecast Discussion
National Weather Service Riverton WY
1202 PM MDT Mon Sep 9 2024
.KEY MESSAGES...
- A breezy afternoon today, with gusts of 25 to 30 mph.
Elevated to critical fire weather this afternoon due to the
breezy winds, warm temperatures, and low relative humidities.
- Showers and thunderstorms possible (15% to 40%) today, mainly
along and east of the Continental Divide into Sweetwater
County. Gusty winds up to around 50 mph are the main hazard.
- A larger weather system impacts the area Wednesday through
Friday, with widespread precipitation chances (including
mountain snow), cooler temperatures, and windy conditions.
&&
.DISCUSSION...
Issued at 1153 AM MDT Mon Sep 9 2024
It will be a more active day today as a lead shortwave ahead of a
developing longwave trough flattens the ridge as it currently moves
over the Cowboy State. Winds will increase late this morning over
the western mountains, as the trough continues eastward. The sfc
pressure gradient will also tighten a bit across the CWA irt the
trough, leading to slightly stronger winds this afternoon. Showers
and thunderstorms will become possible after 18Z, along a line
extending from Lovell southward to Rock Springs/Green River. This
line will push eastward with the trough, making it to far eastern
portions of the forecast area and ending by this evening. Strong
outflow boundaries are likely this afternoon, due to large inverted-
V soundings. Expect wind gusts up to 40 mph again, but stronger
gusts of 50 to 55 mph will be possible as well due to dewpoint
depressions close to 50 degrees. Little to no rainfall is expected
as well due to the dry boundary layer. Fire conditions are still
expected to be elevated to critical today, as afternoon relative
humidities (RH) of 10 to 20 percent occur. Red Flag Warnings remain
in effect for portions of the CWA for this afternoon, as a result of
these conditions.
Conditions remain largely unchanged for Tuesday, with mostly dry
conditions and above normal temperatures across the forecast area.
The main difference will be another shortwave trough/mid-level upper
low moving over southwest portions from northern UT. This will
significantly increase clouds across the forecast area, with mostly
cloudy to overcast skies over the western half. Isolated to
scattered showers and thunderstorms will be possible over central
and southern portions of the CWA as a result, with the better
chances occurring over sections of Lincoln, Sublette and western
Sweetwater counties. A wetting rain could occur with the storms in
these areas, as moisture advection occurs with the low.
Winds begin to increase Wednesday, as the aforementioned longwave
trough begins to deepen along the West Coast. 700mb winds of 20-30kt
will be common, which will easily mix down to the surface during the
afternoon. The main area to be impacted will be the Wind Corridor
(from Rock Springs to Casper), but also areas susceptible to
southwest winds. Red Flag Warnings will likely be needed, despite RH
values staying in the 18-25% range. Afternoon and evening showers
and thunderstorms will be possible again across western portions, as
an embedded shortwave moves over eastern ID into southern MT. The
associated cold front looks to reach far western Wyoming between 11Z
and 16Z Thursday, with 700mb temperatures dropping below 5C through
the rest of the day. This will result in snow levels dropping to
around 10kft. Temperatures will be much cooler, especially across
the west, as a result. Surface winds will also be a bit stronger, as
a result of the tightening pressure gradient associated with the
cold front. Red Flag Warnings will likely be needed again,
especially across Johnson/Natrona counties. Another round of showers
occurs as the main low approaches the Cowboy State, bringing the
first round of snow to the western mountains above 9000-9500 ft.
700mb temperatures could drop to -2C to 0C over northwestern
portions Thursday night, as the upper low tracks over MT. On Friday,
after the front passes through the entire area, highs will be in the
upper 50s and 60s west of the Divide, with upper 60s to middle 70s
east. Conditions are expected to remain dry Friday through the
weekend, with temperatures rebounding each day during that time as
well.
&&
.AVIATION /18Z TAFS THROUGH 18Z TUESDAY/...
Issued at 1149 AM MDT Mon Sep 9 2024
VFR to prevail at all terminals through the forecast period with the
primary aviation hazard being gusty outflow wind from afternoon
convection. The tail end of a shortwave in Montana is tracking east
across the forecast area at midday Monday. Convection is already
firing over the southern Absaroka Range along this boundary.
Convection becomes more widespread along the Continental Divide and
in the downstream basins through 21Z/Monday and continues east
through 01Z/Tuesday. A dry sub-cloud layer favors frequent outflow
wind gusts of 35-45kts along and east of a KWRL-KRIW-KRKS line
during the late afternoon. Convection diminishes in intensity and
exits the area by 02Z/Tuesday leaving decreasing cloud cover
overnight. Wind speeds trend to less than 10kts at all
terminals between 02Z-06Z/Tuesday. KCPR could see 10-18kt
southwest surface wind after 14Z/Tuesday.
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 122 AM MDT Mon Sep 9 2024
Red Flag Warnings are in effect for Zones 276, 280, 281, 283, 286,
287, and 415 from noon to 8pm MDT today. High temperatures are
roughly 5 to 10 degrees above average. West winds this afternoon
increase to around 25 mph for most locations, which should decrease
shortly after sunset. Afternoon RHs will be 10 to 20 percent as
well. Winds will be the marginal factor today, with moderate
confidence of greater than 25 mph and 3 or more hours of these
winds.
Showers, and the occasional thunderstorm, are also possible today,
mainly for Sweetwater County and locations east of the Continental
Divide. High-resolution models are showing a push of these
chances from west to east, starting around 1pm MDT, and exiting
Natrona County around 8pm MDT. Not expecting much, if any,
measurable liquid from these. However, there is potential for
gusts up to around 50 mph from outflows.
Looking ahead, a cold front will push through Thursday and Friday,
from west to east. Gusty winds are expected Wednesday through Friday
as a result. There will also be rain/mountain snow chances during
this period, with the best chances (80%) currently over northwest
Wyoming, with decreasing chances as you travel southeast.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
Air Quality Alert until 1 PM MDT Tuesday for WYZ001-002.
Red Flag Warning until 8 PM MDT this evening for WYZ276-280-
281-283-286-287-415.
Air Quality Alert until 1 PM MDT Tuesday for WYZ012>016.
Air Quality Alert until 1 PM MDT this afternoon for WYZ025.
&&
DISCUSSION...LaVoie
AVIATION...CNJ
FIRE WEATHER...Wittmann