Area Forecast Discussion
Issued by NWS Riverton, WY

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285
FXUS65 KRIW 272058
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
258 PM MDT MON JUN 27 2016

.SHORT TERM...Tonight through Thursday

Water vapor imagery as of 19Z shows a weak wave across north-central
Wyoming pushing southeast. LAPS data shows weak instability
beginning to develop over the higher terrain as well as into lower
elevations of Johnson and Natrona Counties. Cumulus are developing
over the northern mountains...as well as the Laramie Range...while
the rest of the area remains cloud free. Could see a few
thunderstorms over the northern mountains /Absaroka & Bighorns/ as
well as Casper Mountain. Any activity should dissipate rather
quickly as it tries to push off the mountains...and should be gone
shortly after sunset.

A weak mid-level disturbance over the northern Great Basin will top
the upper ridge tonight and Tuesday. This feature could result in
high based...mainly virga showers developing late tonight over the
far west/northwest. This wave will push across western Wyoming in
the morning and across central Wyoming midday to mid-afternoon. As
this feature pushes east, it will encounter a better moisture
profile and instability. Showers and thunderstorms are expected to
develop east of the Divide between 15Z and 19Z, probably peaking
between 18Z and 23Z. The NAM and GFS both show the best instability
occurring across Johnson and Natrona Counties...with the peak
instability occurring between 17Z and 21Z with a marked decrease in
CAPE values late in the afternoon. The NAM goes from around 2500
j/kg at 18Z to around 500 j/kg by 00Z...with the GFS showing the
same trend, but with less overall instability as it only has
dewpoints in the mid 40s while the NAM as mid 50s to possibly near
60. Strong to possibly severe thunderstorms are expected east of the
Divide with an emphasis over Johnson and Natrona Counties. Have kept
severe wording in those areas with plenty of wind shear and
instability for large hail and damaging winds. If the lifted
condensation level is low enough over the east, not out of the
question we could see an isolated tornado. Farther west...a more
inverted V-Sounding will result in strong winds and some small hail
being the main hazards.

Pondered a fire weather watch for zone 280 as models show the
potential of a mix of wet and dry thunderstorms especially over far
western portions of zone 280. Any new fire starts could be hard
to control as winds could be very gusty and erratic. In the wake
of the thunderstorm activity models show a drying boundary layer
late in the afternoon, but current thinking is thunderstorms over
the north will send a cool, relatively moist boundary
south/southwest across the area late in the afternoon/early
evening. The overnight shift can take one more look to see if the
trend is for dry or wet thunderstorms.

The low-level easterly/northeast moist flow looks to become better
established Wednesday east of the Divide. The best instability
remains over Johnson and Natrona Counties where strong to possibly
severe storms are once again possible...but overall severe
parameters are weaker on Wednesday than what is expected Tuesday.
Across the southwest and west...models are showing a bit more
moisture and thus have added in some afternoon/evening mountain
convection.

By Thursday...models continue to indicate some mid/high level
monsoonal moisture beginning to overspread the region. Isolated
to locally scattered thunderstorms are expected across much of the
area, with an emphasis over the east where better low-level
moisture will remain. One wild card could be considerable high
level cloudiness associated with the monsoonal moisture could
limit insolation and the resultant instability.

.LONG TERM...Thursday Night through Independence Day

...Hot, Windy, and Very Dry Conditions with Elevated to Extreme
Fire Danger Possible on Independence Day...

Synopsis...An increase in cloud cover from high level monsoonal
moisture will bring slightly cooler daytime temperatures and slight
chances of showers and thunderstorms to most of the area Thursday
and Friday. A drier westerly flow will shift most of this moisture
south and east over the weekend with warmer temperatures and
isolated thunderstorm coverage. Very warm, dry and windy conditions
are expected ahead of an approaching cold front on Sunday and
especially the Fourth of July, that will result in elevated to
extreme fire danger.

Discussion...ECMWF and GFS are in good overall agreement through the
extended period across the western U.S. with the most noticeable
spread involving upper low diving out of the Gulf of Alaska into the
northern Rockies early next week. These differences may come into
play further down the road, but will not significantly impact the
forecast through the Fourth of July.

The extended period begins with ridge roughly along 110W from the
Four Corners area into Canada, flanked by upper lows over the Great
Lakes region and over the Gulf of Alaska. Return flow around
surface high pressure over the northern plains will continue to
bring low level moisture, CAPE into the east half of Wyoming
Thursday and Friday while mid and high level monsoonal moisture
drift north into the south and west. Lower levels remain dry
across the west Thursday with PWATs expected to increase to 0.75-
1.00 inch across the entire area Friday. Best chance of
thunderstorms and favorable shear profiles for stronger
thunderstorms will be across Natrona and Johnson counties on
Thursday. The `monsoonal plume` will likely be mostly high level
moisture on Friday that could act to suppress convection - a lot of
virga with a few thunderstorms mainly forming on cloud/clear air
boundaries.

Gulf of Alaska low begins to push into western Canada and the NW
U.S. over the weekend with drier west southwest flow over the area;
isolated thunderstorms lingering over the elevated terrain and along
a retreating surface trough across Natrona/Johnson counties.

Strong upper low from the Gulf of Alaska pushes east across the U.S.
Canadian border on Independence Day, putting most of Wyoming in the
warm sector ahead of an approaching cold front. A similar scenario
to last Friday, June 24th. Hot, windy, and very dry conditions are
expected across most of the area with extreme fire danger where
fuels are critically dry. Have nudged temperatures up a degree or
two over the lower elevations of the east, but this may still be
underdone. Seems that these pre-frontal wind events always boost
temperatures a few extra degrees. Wind gusts the afternoon of the
4th could be 30-40 mph with temperatures hovering around 90 and
perhaps into the mid 90s. Coupled with humidities of 10-15 percent,
the fire danger will certainly be elevated.

&&

.AVIATION.../00Z ISSUANCE/

Terminals to remain VFR with less than 12kts of wind through
18Z/Tuesday. Shortwave will carve slightly south and impact eastern
terminals after that time. Convection is most likely to impact
KCOD, KCPR, and to a lesser extent KWRL, KRIW, and KLND. Best
instability and higher dew points will remain along and north of a
line from KCOD and KBYG. Strongest convection will form over the
Bighorn Mountains and roll east-southeast during the early
afternoon. All indications are that this will be an earlier show
with 18Z-22Z/Tue the best timeframe for convection. 250mb jet
streak could enhance convection during that time. Could be small
hail and brief heavy rain over the far north and east of the
Bighorns. Main hazard will be gusty outflow wind the farther south
you go where temperature-dew point spreads could be 60F.

&&

.FIRE WEATHER...

...Elevated fire weather conditions over south and southwest Tuesday
and Wednesday...

A combination of a couple weak upper level systems and a quasi-
stationary frontal boundary east of the Divide will result in
isolate to locally scattered thunderstorms along and east of the
Divide both days. Could see some very isolated activity west of the
Divide, but activity would be mainly dry with gusty winds. Also
across the south and southwest fire zones, the combination of very
low RH, breezy westerly winds and the chance of dry thunderstorms
will enhance fire weather conditions. The storms east of the Divide
will be more wet especially across Johnson and Natrona County where
more low level moisture will be present. Some storms in those areas
could be severe with large hail, damaging winds and a plethora of
lightning. Some monsoonal moisture spreads northward across the area
Thursday and Friday with isolated to locally scattered thunderstorms
across the much of the area. The thunderstorms will be more dry over
the west and trend more wet east.

Hot, windy with low to very low RH are possible Sunday into the
Fourth of July.

&&

.RIW Watches/Warnings/Advisories...None.

&&

$$

SHORT TERM...WM
LONG TERM...AEM
AVIATION...CNJ
FIRE WEATHER...WM



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