Scientific Forecaster Discussion
fxus65 kriw 160540
Area forecast discussion
National Weather Service Riverton Wyoming
1140 PM MDT sun Sep 15 2019
Short term...this afternoon through Monday
Summer holds on for another day across the cowboy state this
afternoon. Dry, hot and gusty conditions will continue across the
area, leading to elevated to critical fire conditions.
Temperatures will be well above normal, staying just a few degrees
shy from setting any records. Gusty southwest winds this afternoon
will quickly subside this evening by sunset. High cirrus clouds
will increase from Sweetwater to Natrona counties later this
afternoon, into the evening hours. This will be a precursor of
increasing moisture tonight. As a result, milder temperatures are
expected. Cold-prone locations will still drop to near freezing
temperatures, due to mainly clear skies.
The weather pattern becomes more active Monday, as winds increase
across the area, along with chances for precipitation. This will
be due to an approaching cold front, that is associated with the
storm system that has been discussed in the long term for the last
few days. As mentioned earlier, moisture will increase from the
south tonight, raising dewpoints into the upper 30s and 40s. This
will push any fire weather concerns to the northern zones, where
relative humidity values will be critically low. This will mainly affect Johnson
County and eastern portions of the Bighorn Basin. Temperatures
will be well above normal in these areas, with upper 80s and lower
90s. Showers and thunderstorms will develop across areas west of
The Divide Monday afternoon. No major threats are expected with
these storms at this time. Cape values will range between 200 to
600 j/kg and lifted indexes around minus 2. Storm motion will be a
bit fast (given the overall wind field of 15-20 mph), so wind
gusts of 40 to 50 mph will be possible. Dewpoint depressions do
not look large enough for severe wind gusts to occur.
Long term...Monday night through Sunday
Longwave trough still expected to sweep eastward across the Pacific
northwest early in the week, as the overall pattern begins to buckle
significantly. Monday night will see the shower activity across
western Wyoming continue through Tuesday morning. Tuesday the beginning
piece of the 500 mb trough races across WY, with a much better chance
for rainfall pushing eastward across the rest of the forecast area. Colder
temperatures will be sweeping into the state as well, with highs
Tuesday 10 to 20 degrees lower than Monday. Wednesday will be a
drier day in the wake of he first trough, before the second trough
slowly meanders across The Rockies. This will keep temperatures
below normal into the upcoming weekend, with a chance of rain
continuing each day.
Aviation...(for the 06z tafs through 06z Monday night)
issued at 1116 PM MDT sun Sep 15 2019
The main aviation concerns...
Some low-level wind shear from the southwest through Monday
morning over the west.
Isolated to widely scattered showers and thunderstorms over the
west and southwest Monday afternoon and early evening. Some of
this activity could also spread to areas just east of The Divide.
The main hazard from this activity will be strong, gusty winds.
The convection is expected to end in most areas during the early
evening (exception for rks), before returning out west after 06z
Low-level wind shear from the southwest looks to be more
widespread Monday night especially in areas that decouple.
VFR conditions are expected through the period except very brief
MVFR/IFR associated with any convection. Breezy south to southwest
wind at most terminals Monday afternoon.
Red flag warnings remain in effect for zones 277, 279, 280 and 300
through 7 PM this evening. Gusty winds and critically low relative humidity
values will continue this afternoon, before improving after
sunset. Moisture will increase across the area starting this
evening and continuing tonight. Temperatures will be milder
tonight, compared to the last several nights. A cold front will
approach the area Monday, bringing gusty winds to the area once
again. Temperatures will not be as warm Monday afternoon, for all
but Johnson County and eastern portions of the Bighorn Basin.
Conditions in the latter areas will have near critical relative humidity values
and temperatures in the upper 80s to lower 90s. Showers and
thunderstorms will occur across areas west of The Divide Monday
afternoon. Chances for wetting rain will be low.