Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 200816
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
216 AM MDT Thu Jul 20 2017

.SHORT TERM...Thursday through Saturday night

Here at the humble Riverton weather abode the dew point has pushed
into the 50s. That means the monsoon has arrived. We did have some
showers and thunderstorms around this afternoon but they are
weakening and should be down to only spotty showers by morning.
However, things look to get active as a shortwave and cold
front move across the state and brings another round of showers and
thunderstorms. Some storms could get strong as well. We actually
have a slight risk across eastern portions of Johnson and Natrona
Counties with a marginal risk back toward Worland. The models are
showing steep lapse rates. There is also decent shear of the speed
variety. As a result, there could be some decent size hail with the
tilted updrafts. And as always, some strong wind gusts. In addition,
with precipitable water values climbing to around an inch or over
there could be heavy rain with the storms and possibly some
localized flash flooding. The models do have some differences in
pinpointing the thunderstorms however. As a result, we left the
coverage in continuity alone for the most part. It will also be
another warm day, although some uncertainty since a lot depends on
cloud cover. These storms should end shortly after midnight for most
areas after the front passes through.

Quieter and somewhat less hot weather will move in for Friday and
Saturday. A much drier airmass will move in behind the front and
likely shut off any convection for most areas. The exception for
Friday would be along the southern border where just enough moisture
may hang on for isolated storms but even here coverage will be
sparse. There could be elevated fire danger as well with a gusty
breeze in spots. However, will hold off on any watches since the
strongest winds may be in the morning and not reach criteria when
humidity is the lowest. Saturday at this point looks mainly dry with
seasonably to somewhat above normal temperatures.


.LONG TERM...Sunday through Thursday

Overview...Dry and warmer (hot) conditions will prevail on Sunday.
Monsoonal moisture will make a return Monday through Thursday with
at least a slight chance of afternoon and evening thunderstorms each
day, with temperatures slightly cooler Tuesday through Thursday due
mainly to cloud cover and shower activity.

Discussion...GFS in better agreement with ECMWF this morning with
more progressive flow from the Gulf of Alaska across Canada.  Upper
high currently over the central Plains will shift westward over the
southern Rockies/Great Basin early next week with trough along the
west coast.  Monsoonal moisture will remain suppressed to the south
on Sunday.  Weak upper level disturbances from the west coast trough
or "ridge riders" from the southwest will begin to rotate across the
area on Monday with this pattern expected to continue through the
week. Climatologically, the last week of July is also near the
height of the SW Monsoon season for west and central Wyoming. `Tis
the season for low top convection and some heavy rainers. All
areas should have at least a slight chance of an afternoon/evening
thunderstorm with some activity likely continuing into the
morning hours.

&&

.AVIATION...12Z Issuance

Expect VFR conditions to continue through the forecast period. Have
tweaked afternoon winds at several of the TAF sites and kept in the
mention of VCTS/SHRA for most locations through the majority of the
forecast period until close to 06Z Friday.

&&

.FIRE WEATHER...

Monsoonal moisture and a passing cold front will move through the
area and bring an increased chance of showers and thunderstorms.
These storms have a better chance of bring wet with a chance of hail
and erratic winds as well. Relative humidity should remain somewhat
higher than recent days as well. For the most part, wind should
remain light to moderate outside of any shower or thunderstorm.
Mixing and smoke dispersal will generally range from good to very
good but fair in some of the Basins.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Troutman
FIRE WEATHER...Hattings



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