Area Forecast Discussion
Issued by NWS Riverton, WY

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022
FXUS65 KRIW 251656
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1055 AM MDT Wed Apr 25 2018

.SHORT TERM...Today through Friday night

A shortwave ridge will move across the region today, in the wake of
Tuesday`s disturbance, bringing dry conditions and clear skies to
the forecast area. Southwest winds will increase across the Wind
Corridor after sunrise this morning, in advance of a a quick and
relatively strong cold front. Red Flag conditions will be in effect
for portions of southern Natrona and much of Natrona counties by
this afternoon, as RH values drop into the lower to middle teens.
The cold front will enter northern portions late Wednesday
afternoon/evening and sweep through east of the Divide through the
overnight hours. Isolated to scattered rain and snow showers will be
possible across the Bighorn Basin and Johnson and Natrona counties
Wednesday night, with snow staying in the higher elevations. After a
chilly start Thursday morning (especially for portions of Johnson
County), a massive ridge will build over much of the Intermountain
West. This ridge will slowly progress eastward Thursday and Friday,
as a cut off low moves onshore by Friday. Moisture from this system
could begin to impact western portions Friday night.

.LONG TERM...Saturday through Wednesday

Overview...A Pacific cold front will push across the area on
Sunday. Breezy, warm and dry conditions will elevate fire weather
conditions across central Wyoming, high temperatures expected in
the 70s to near 80. Showers and a few thunderstorms will begin to
push into the west through the afternoon and evening. Breezy to
windy conditions with slightly cooler temperatures will accompany
the front on Sunday with chances of valley rain and mountain snow
showers continuing across the west. A cooler and unsettled pattern
takes shape over the western U.S. Monday through Wednesday A
cooling trend follows Monday through Wednesday with isolated to
scattered showers along with slight chances of afternoon/evening
thunderstorms. Issued AT 1217 AM MDT Wed Apr 25 2018

Discussion...Cutoff upper low off the west coast is expected to get
kicked onshore Saturday morning and shear/fragment into strong
downstream ridge over the Rockies and High Plains.  ECMWF and
Canadian are slower and further west than GFS lifting upper low NNE
across eastern Idaho on Sunday, and the slower guidance is
preferred.  For forecast purposes, chances of precipitation are
confined mostly to the west with slower scenario on Saturday with
cold frontal passage not expected until Sunday.   ECMWF shows
surface low just north of Sheridan Sunday afternoon, wrapping higher
SCAPE >= 1000j/kg into NE Wyoming. Forecast confidence is too low
for mention of possible strong/severe potential with this setup on
day 5, but will monitor at this stage for trends and consistency.
Both GFS and ECMWF show a split flow setting up by midweek with
ridge just off the west coast, and upper low digging south across
California into the SW U.S.  ECMWF has the low near Las Vegas by 00z
Tuesday, GFS races it into the Four Corners region.  The slower
solution will most likely win out again, especially with no kickers
within range upstream.  Most precipitation with this system should
remain to our south with this system through Wednesday.


&&

.AVIATION...For the 18Z TAFS through 12Z Thursday

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

VFR condition to prevail for most terminal sites, except KCOD and
perhaps KWRL, until after 06Z. KCOD, and soon after KWRL, will see
the effects of a passing cold front with a shift in the winds to the
north to northwest this evening and CIGs dropping to low VFR/MVFR
by/after 02Z this evening. The other terminal sites will follow suit
sometime after 06Z tonight. As this system will be a quick hitter,
conditions should improve from north to south starting after 06Z
Thursday across northern WY to 11Z-15Z Thursday over
central/southern WY. Mountains of the area will also be occasionally
obscured through around 15Z Thursday. Ahead of the front this
afternoon, winds may gust out of the west G20-30kt.


West of the Divide...KBPI/KJAC/KPNA/KRKS Routes

VFR conditions to prevail west of the Divide through the forecast
period. Some gusty west to northwest winds (G20-30kt) expected this
afternoon and early evening along/east of a KPNA-KBPI_KRKS line.

&&

.FIRE WEATHER...

Dry conditions and clear skies will be in place across the area
today. Light winds will also be the norm, except across the Wind
Corridor. Southwest winds will increase later this morning, from
South Pass to southern Johnson County. Expect winds of 15 to 25 mph
with gusts up to 35 mph. Minimum RH values will drop into the lower
to middle teens across southern, central and eastern portions of the
forecast area, however, critical fuels are in effect for zones 280,
283 and 289. Thus, a Red Flag Warning will be in effect for 280 and
289. Although winds will be less of a factor, zones 281, 283, 278
and 279 will have elevated fire concerns due to the low RH. A
cold front will stay east of the Divide tonight, bringing isolated
to scattered showers to northern and eastern portions. Winds will
also switch to the north and remain windy behind the front. A
strong ridge will build over the region Thursday and Friday,
keeping conditions warm and dry. Min RH values will reach critical
values again for many areas Friday, however, winds will be light.


&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Red Flag Warning from noon today to 7 PM MDT this evening for
WYZ280-289.

&&

$$

SHORT TERM...LaVoie
LONG TERM...Meunier
AVIATION...Braun
FIRE WEATHER...LaVoie



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