Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 120400
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1000 PM MDT Tue Aug 11 2020

.SHORT TERM...(This evening through Wednesday night)
Issued at 140 PM MDT Tue Aug 11 2020

With the mid-level flow more zonal, or west-southwest, a weak
leeside surface low is developing this afternoon from central into
east-central WY. Mid-level moisture continues today from southwest
WY ENE across the southern half of the state as well. With weak
CAPE, LIs at or just below zero, and moderate lapse rates,
convection is firing from around South Pass eastward toward
Natrona County. Expecting convection to develop ENE from the
Uintas into Sweetwater County with isolated thunderstorms
elsewhere across southern WY. Dry lightning is a serious concern
with these storms due to high cloud bases and little precipitation
reaching the surface.  With surface flow increasing today and
humidities continuing to be low, a red flag warning is in effect
through 8 PM for much of the area. The northern half of the area
will continue with mostly clear skies and increased wind as well.

Models are still showing a weak disturbance moving from NW Nevada
toward western WY overnight, bringing slight instability and
cloudiness into western WY around sunrise. Light showers and
isolated thunderstorms are expected from mid morning onward from
west-central WY northeast toward the Bighorns. Humidities
southeast of this line will again be low with breezy conditions to
continue critical fire weather conditions. The areas of light
showers and thunderstorms across much of northwest WY should
contain dry lightning again. As a result, have issued a red flag
warning for Wednesday afternoon for much of the area. Land
agencies have reported quite a bit of issues with dry lightning
the last few days. A short-wave trough will move from eastern
Washington into Montana Wednesday evening. A weak boundary will
thus bring increase moisture and some showers to far northern WY
after midnight Wednesday into Thursday. As the surface pressures
increase behind the system, a boundary with northerly flow will
push into the Bighorn Basin early Thursday morning. The mid- and
upper-level height gradients will tighten somewhat early Thursday
to bring 30+ knot flow over the area.

.LONG TERM...(Thursday through Tuesday)
Issued at 128 PM MDT Tue Aug 11 2020

Dry southwest flow looks to reestablish itself on Thursday. With the
southwest flow and a shortwave tracking to the north tightening the
pressure gradient, a worsening in fire weather once is expected. I
would expect to see a good coverage of Fire Weather highlights
across much of the area for Thursday afternoon. As for the chance of
convection, the only chance would be in the far north which will be
in proximity to the shortwave. At this time, we left the forecast
mostly dry since at this point and showers would be very isolated.

The base of the upper trough will be to the east of the forecast
area by Friday morning. In the wake of this feature, a dry cold
front looks to be quasi-stationary along the Divide. Critical fire
weather conditions look likely south of this front, while central
areas look more elevated than critical.

A subtropical ridge slowly builds across the southern Rockies this
weekend then pulls into the Great Basin early next week. The main
impact will be for lighter winds this weekend which will lower fire
weather concerns, but not eliminate them. The GFS shows enough winds
aloft to result in near critical fire weather thresholds being met
over the southwest. Dry and warm conditions over the weekend with
temps 5 to 10 degrees above average west, and slightly above east.

As the subtropical ridge pulls into the Great Basin, this could
result in northwest flow developing over the area which could result
in diurnal convection. However kept a mainly dry forecast for now.
Also would expect a slight warming trend especially over the
southwest part of the forecast area closer to the thermal ridge.

&&

.AVIATION...(For the 06Z TAFS through 06Z Thursday)
Issued at 1000 PM MDT Tue Aug 11 2020

Expect VFR conditions to prevail at all terminals through the
period. Winds may gust to around 25-30 knots during the afternoon
for any location. Any location may have a shower in the area during
the afternoon hours. Some terminals may need an upgrade to VCTS.

&&

.FIRE WEATHER...
Issued AT 145 PM MDT Tue Aug 11 2020

Red Flag Warning continues through 8 PM today for most of the area
due to very warm, dry, and breezy to windy conditions. Dry
lightning is a concern as well across the southern half of the
state with thunderstorms today. A red flag warning is now in
effect for Wednesday as well. From southwest WY to Natrona County
and south, warm temps, wind, and low RH will continue Wednesday
afternoon. From west-central WY ENE toward the Bighorns and
Johnson County, the primary concern will be breezy wind with dry
lightning from isolated to scattered thunderstorms. Thursday and
Friday look to be mostly dry and windy again for continued
elevated fire weather conditions. Some showers are possible
Thursday across far northern WY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Red Flag Warning from noon to 8 PM MDT Wednesday for WYZ275>283-
285>289-300-414>416.

&&

$$

SHORT TERM...McDonald
LONG TERM...Hulme
AVIATION...Clayton/AR
FIRE WEATHER...McDonald


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