Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 202037
Area Forecast Discussion
National Weather Service Riverton WY
237 PM MDT WED JUL 20 2016
.SHORT TERM...Tonight Through Thursday
Showers and storms over southern Wyoming will continue into the late
evening hours with a few storms persisting through the early morning
hours before tapering off. Gusty wind remains the primary threat
with any storm activity this evening.
Dry southwest flow aloft will continue through Thursday, with
widespread dry conditions expected on Thursday. Warm temperatures
will return across the area, though will be slightly cooler than
those experienced today. A few locations may again flirt with the
century mark, but are currently not expected to reach that point.
Fire weather remains the primary topic of interest over the next few
days, though with wind weakening, the threat is not as strong.
.LONG TERM...Thursday night through Wednesday
The isolated thunderstorms of Thursday afternoon should end on
Thursday evening. Most should stick into the mountains and adjacent
foothills due to weak steering flow. The balance of the night should
Fire danger will increase again on Friday as a trough and cold front
approach from the west. This should tighten the pressure gradient
and increase southwest winds ahead of the front. In addition, with
the increased southwest flow downsloping will increase somewhat.
This will bring a dry of hot to very hot temperatures. We did raise
high temperatures a couple of degrees since the models do not
usually capture dry soil very well. With the cold front sweeping in,
the chance of thunderstorms will also increase across the area. A
this time, the best chance looks to be across the usual spots in the
mountains. The other spot will be across the east where slightly
higher dew points could surge in from the south ahead of the front.
Like several days previous, the main threat will be from gusty downburst
winds with the large temperature / dew point spreads.
The edge should be taken off the heat on Saturday following the
frontal passage, with high temperatures about 5 to 10 degrees cooler
than on Friday. In addition, it should be dry everywhere as moisture
gets shunted to the south of the area there should be very little to
no chance of thunderstorms. Relative humidity also looks relatively
low. However, with winds expected to be light to moderate critical
fire danger is not expected at this time.
From Sunday into the middle of the week, the forecast will feature
gradually building heights as a ridge builds across the area. A
shortwave coming over the top of the ridge at times. Both the GFS
and the European show it getting close enough to bring a bit of
convection to the northeast so we added some evening thunderstorms
to portions of Johnson County. For Monday and beyond, the models do
have some differences. As a result, we left continuity alone for the
most part with mainly isolated POPS over the northern mountains.
Temperatures will gradually warm through the period with highs in
the mid to upper 90s looking likely for the lower elevations,
especially on Wednesday. As for fire weather concerns, relative
humidity will remain quite low. However, at this point winds look to
remain light to moderate at this time. If short waves can get a bit
closer it could increase a bit but this is uncertain at this time.
Showers over southern Wyoming may continue to impact KRKS with
periods of MVFR conditions through 06z-09z tonight. Dry conditions
are then anticipated through 18z. A few showers will develop
Thursday between 18z and 21z, mainly over the areas west of the
Divide, and across the Big Horn Mountains. Mountain obscurations
will remain possible due to the isolated activity.
Smoke around the area will bring the opportunity for periods of MVFR
conditions during the overnight and early morning hours, especially
around KJAC. Smoke across the remainder of the area will also add to
the mountain obscurations, and make general flight paths hazy.
Gusty winds this afternoon will weaken overnight with the southwest
flow weakening over the area. Warm and dry conditions will persist
through Thursday, with areas still having the potential for elevated
Haines indices during the afternoon hours. Friday will see a
disturbance moving through the area. This system will help to cool
down temperatures, increase humidity, and potentially initiate
shower activity over portions of the area. After the exit of the
disturbance, westerly flow will move over the area, with generally
dry conditions through the early weekend.
Red Flag Warning until 9 PM MDT this evening for WYZ283-285-