Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190552
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1152 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 06Z TAFS through 06Z Friday night)
Issued at 1152 PM MDT Thu May 18 2017

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

An upper low pressure system over the central Rockies will slowly
lift into the High Plains on Friday. Areas of MVFR, local IFR
ceilings with light snow will continue to wrap west across the
Interstate 80 corridor, vicinity KRKS, through the morning while
areas further north remain under prevailing VFR conditions with
some higher mountain obscurations. Lower clouds should dissipate
vicinity KRKS and eastward late Friday morning-early afternoon.
Slight instability in the afternoon will result in scattered rain
and snow showers developing over the high terrain and moving
southeast, vicinity KJAC-KPNA-KBPI in the afternoon possibly
pushing south into vicinity KRKS Friday evening. Brief MVFR
conditions are possible with these showers. This activity should
dissipate from NW-SE 03z-09z Saturday.

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

An upper low pressure system over the central Rockies  will
slowly lift into the High Plains on Friday. Areas of MVFR light
snow with local IFR conditions will persist primarily along and
south of a KDUB-KRIW-40SM N KCPR line through 15z with upslope
snow persisting lastly vicinity KLND and the eastern Wind River
Range possibly into early afternoon. Otherwise areas of MVFR
ceilings should begin to lift and dissipate 15z-19z Friday leaving
prevailing VFR conditions in the afternoon with some higher
mountain obscurations. Clearing from northwest to southeast is
expected Friday evening and overnight with some patchy valley fog
possible early Saturday morning.

&&

.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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