Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190017
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
617 PM MDT Thu May 18 2017

.UPDATE...
Issued at 615 PM MDT Thu May 18 2017

Winter Storm Warning for the eastern Bighorn Range has been
allowed to expire at 6 pm. The Winter Weather Advisory for
northeast Johnson County was cancelled; accumulating snowfall is
expected to remain to the south through tonight.

UPDATE Issued at 401 PM MDT Thu May 18 2017

Flood Watch for eastern Johnson County has been cancelled.
Precipitation amounts have been light across most of this area today,
reducing the threat of flooding.

&&

.SHORT TERM...(This evening through Friday)
Issued at 211 PM MDT Thu May 18 2017

Well developed baroclinic leaf showing up over Wyoming/Nebraska/Co
attm. Best dynamics with this ongoing widespread potent Spring
storm should be with us through about 03z this evening. The center
of the main low appears to be over Central Colorado. The precip
will gradually diminish from northwest to southeast tonight as QG
forcing gradually slides off to the southeast and wrap around
isentropic lift diminishes by around 06z tonight. However, upslope
favored areas like the East Slope of the Wind River Mountains and
adjacent Lander Foothills should see ongoing snowfall for much of
the night tonight as the wedge of northeasterly upslope flow has
deepened and strengthened. This is not the most ideal direction
for Casper/Casper Mtn to get upslope snowfall, they favor a more
northerly flow. Therefore, the significant snowfall from Lander
to Casper could shut off around the same time tonight even though
Casper would be closer to the exiting dynamics. If you believe the
WRF, it bulls eyes the East Slope of the WND RVR Mtns with 1.75
inches of precip today through Friday taking the upslope precip
pattern into account. Going snow amounts of 10 to 22 inches looks
reasonable with Hobbs Park already having received 17 inches. Let
the Absaroka Mtns expire with the activity having moved southeast
of there by 18z this afternoon. Also decided to let cancel the
warning for the west slope of the Big Horn Mountains given the
fact that the web cams were not showing much in the way of
snowfall accumulation, with the higher elevation SNOTELS receiving
most of the snow, as this area has been getting drying downslope
winds. Will let the rest of the hilites ride as is for tonight
for now. Accumulating snowfall levels have been hovering around
5200 feet or so and will likely continue to descend to most areas
other than the lowest areas in the Big Horn Basin this evening as
h7 temps tank to -8c east of the divide.

On Friday, snowfall will continue to diminish as the storm center
continues to pull away with the center drifting to northeast New
Mexico with snow showers continuing as the asct trough lingers
ovhd. Even in the far west will see lingering showers where the
most unstable air will be. The snow showers will be turning to
rain showers at the lower elevations Friday. Temperatures will
warm somewhat but still on the cool side with temperatures a good
10 to 15 degrees milder in the central zones.

.LONG TERM...(Friday night through Thursday)
Issued at 211 PM MDT Thu May 18 2017

The slow-moving upper low responsible for our mid-march weather
in mid-May will begin to track northeast Friday night onto the
Northern Plains. Best dynamics will shift east of the forecast
area Friday evening. Any lingering precipitation would mainly be
confined to the central and Bighorn mountains with perhaps some
evening sprinkles along the I-25 corridor. Clouds will clear from
northwest- to-southeast overnight. There could be the potential
for some early Saturday morning fog in Fremont County given the
available low-level moisture if clouds clear prior to sunrise.
Northwest flow aloft will bring another shortwave into the
northwest Saturday afternoon as conditions improve across the
east. This shortwave will shear as it drops south across Wyoming
leaving enough instability to work with the available moisture and
initiate weak convection late Saturday into the overnight hours.
Have increased the chance of showers across almost all of western
and southern Wyoming Sunday as this wave is slow to clear the
state. Accordingly, adjustments were made to temper daytime high
temperatures.

The departing upper low will help to carve a broad trough across the
Northern Plains to begin the work week. Medium-range forecast models
are in agreement in bringing a quick-hitting shortwave south across
Wyoming Monday and Monday night in northerly flow aloft. Have
increased precipitation chances across our northern and eastern zones
and bumped northwest wind at favored locations in the northern Big
Horn Basin and northern Johnson County. Upper flow will back to the
northwest Tuesday as this shortwave digs into the mid-Mississippi
Valley and a ridge builds along the west coast. For the most part
conditions look dry for Tuesday and Wednesday with temperatures
finally returning to more seasonal readings. Model solutions begin
to diverge to end next week and confidence is low in how the
pattern may evolve. At this range, have opted to try and blend by
trending toward climatology and blended/WPC solutions.

&&

.AVIATION...(For the 00Z TAFS through 00Z Friday afternoon)
Issued at 211 PM MDT Thu May 18 2017

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS Terminals

Upper low over Colorado will continue to rotate moisture north
and west across Wyoming through tonight. This will mainly continue
to impact KRKS as moisture heads westward across southern Wyoming.
LIFR/IFR conditions anticipated through 06Z/Fri at KRKS before
conditions begin to improve to MVFR. Gusty northeast wind of
20-30kts will diminish Thursday evening with lingering 10-20kt
winds possible through about 09Z/Fri. Other terminals will see
generally VFR conditions with only some VCSH and MVFR at times
through this evening. Gusty 10-20kt winds at KBPI, KPNA, and KJAC
should decrease between 01Z-04Z/Fri. For Friday, lingering light snow
and MVFR conditions expected at KRKS until around 16Z. After that
time, should see low end VFR conditions through the afternoon as
the moisture gradually pushes east. The other three terminals
will remain VFR with winds less than 12kts. Mountain obscurations
will be frequent across southwest Wyoming through 12Z/Fri, with
mountain top obscurations remaining until after 18Z/Fri. There
could be occasional mountain top obscurations across northwest
Wyoming through about 06Z/Fri.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL Terminals

Messy forecast through at least tonight as moisture continues to
rotate across Wyoming on the north side of an upper low over
Colorado. Primary impacts will be focused on KCPR, KLND, and KRIW
through at least Thursday evening. Moisture and easterly, upslope
flow will keep conditions LIFR/IFR through at least 06Z/Fri with
those conditions lingering longer at KCPR. Moisture will gradually
pull away to the east Friday morning with conditions at all three
terminals showing improvement between 15Z-18Z/Fri. However, it
still looks like widespread VFR conditions in central Wyoming will
hold off until after 18Z/Fri. Forecast is tricky across the Big
Horn Basin including the KCOD and KWRL terminals. For the most
part, conditions should be dry as easterly fetch will be
downsloping, especially at KWRL. The concern focuses on the
possible low clouds IFR/MVFR ceilings later tonight. Am not as
confident with this scenario playing out and may need to make
last minute adjustments to these terminals to account for improved
conditions at KCOD and KWRL overnight. Regardless, expect VFR to
prevail after 16Z/Fri. As for obscurations, mountains will be
obscured through at least 09Z/Fri in central Wyoming. Mountain
top obscurations will be frequent throughout the period.


&&

.FIRE WEATHER...
Issued AT 211 PM MDT Thu May 18 2017

Fire danger low across the forecast area through the rest of the
week as all fuels are currently in green-up and a late season
winter storm pushes through the forecast area...continuing into
the Friday. Precip will continue east of the Divide through
tonight...with snow levels reaching the lowest valley/basin floor
levels at times. More significantly, copious amounts of
precipitation is expected to fall with this weather system.
Northeast winds will gradually diminish through this evening but
will continue at speeds of 20 to 30 mph through early this evening
in Sweetwater County before diminishing by mid evening. Smoke
dispersion fair to good afternoons through the rest of this week.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 6 AM MDT Friday for WYZ015-019-022.

Winter Weather Advisory until 6 AM MDT Friday for WYZ007-011-017-
018-020-030.

&&

$$

UPDATE...Meunier
SHORT TERM...Lipson
LONG TERM...Jones
AVIATION...Jones
FIRE WEATHER...Lipson


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