Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 240439
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1039 PM MDT Thu May 23 2019

.SHORT TERM...(This evening through Sunday)
Issued at 242 PM MDT Thu May 23 2019

Upper-level low center is currently over northern Utah and is
moving northeast, which will bring the 500mb and 700mb low center
right through central WY between 9pm-12am tonight. Very moist
southerly mid- and upper-level flow has combined with low-level
east to northeast flow to bring widespread precipitation to
southwest into central Wyoming today. Web cameras show
predominantly rain in the lower elevations with snow in the higher
elevations. While snow levels have fluctuated somewhat, they have
been around 5500 to 6500 feet since early morning. Strong east
winds continue across southern Wyoming with gusts of 30 to 40 mph,
though they are starting to diminish in some areas. Moderate to
heavy snowfall is expected to continue for South Pass into the
central Wind River Mountains through at least 6pm, with lighter
snow continuing through midnight. Short-term high-resolution
models continue to show an areas of moderate to heavy
precipitation developing over the central WY mountains and moving
northward into the Wind River Basin between 4pm to 7pm today as a
TROWAL from northwest Colorado works with the strong moist low-
level easterly flow. As the low moves northeast this evening,
precipitation is expected to spread to more areas east of the
continental divide. After 9pm the wind flow across southwest WY is
also expected to start shifting to the southwest due to a lee-
side surface low deepening somewhat over northeast WY. Winds are
expected to remain breezy from Sweetwater County northeast to
Natrona and southern Johnson Counties through the night, with
gusts as high as 40 to 50 mph over the central mountains due to 50
mph 700mb flow. As the flow shifts to the west-southwest behind
the low, low-level moisture will generate upslope precipitation on
the western mountains as well as the Bighorns through about noon
on Friday. With the west-southwest slightly warmer flow on Friday,
afternoon temperatures are expected to climb into the 50s,
especially east of the divide where skies will become partly
cloudy. The zonal somewhat moist westerly flow will keep showers
going in the western mountains Friday afternoon and evening.

For Saturday, another large-scale trough moves into the northwest
WY and starts turning the mid-level flow over WY more southerly.
700mb temperatures are expected to increase to about +5 C by
saturday afternoon, causing temperatures to push into the 60s with
skies clearing out over the central and eastern part of the state.
The southerly flow will become breezy from southern into central
WY. Mid-level moisture with weak instability though is expected
to move northward from Utah into western WY, keeping a chance of
showers and thunderstorms going. At this time, some localized
heavy precipitation may occur as a weak shortwave trough move
through Saturday evening. Partly cloudy to mostly clear skies
will develop early Sunday morning with the breezy southerly flow
continuing. Mostly cloudy skies with a slight chance of showers is
expected over the northwest mountains and YNP Sunday afternoon. A
cold front appears to be moving south from Montana late Sunday as
the upper-level western trough drifts down to the Las Vegas area
and sets up to bring active weather Monday into Tuesday.

.LONG TERM...(Sunday Night through Thursday)
Issued at 242 PM MDT Thu May 23 2019

Next upper low will be taking shape over southern California and
Nevada at the start of the medium range period. This low will
track east through the southern Great Basin Memorial Day and
reach southern Colorado by 12Z/Tuesday. Elongated trough
extending from the Northern Plains through the Rockies to this
upper low will traverse the forecast area Monday. A cold frontal
boundary poised near the Montana state line will push through
areas east of the Continental Divide early Monday morning.
Favorable upslope flow and increasing overrunning moisture will
lead to an increasing chance of precipitation through the day on
Memorial Day. Have again trended toward NBM solution for
precipitation chances which also correlate better with surrounding
offices. Fremont and Natrona counties will be favored in this
upslope flow for another round of precipitation Memorial Day and
into that evening. Seasonal weekend temperatures will drop back
to 10-15 degrees below normal for Monday.

The upper low will begin to pull away from the central Rockies and
onto the central Plains Tuesday. Overall trend will be for
decreasing precipitation chances from north to south during the
day Tuesday. The precipitation will linger longest over the south
and southwest forecast zones. Cool 700mb temperatures will remain
over the forecast area Tuesday. Less precipitation and perhaps
some late day sun will boost temperatures across the central and
north from what we will see Memorial Day. Northwest flow aloft
will allow for favored areas over northern Wyoming - like northern
Johnson County and the northern Bighorn Basin - to see gusty
northwest wind Tuesday. Have bumped speeds up accordingly.

The northwest flow aloft and the gusty northwest wind across the
north will persist into Wednesday. However, we should see the start
of a warming trend as 700mb temperatures warm to around +4C. Tough
to discount any ripple in this flow pattern and have nudged
precipitation chances upward Wednesday. The northwest and northern
mountains will be favored in this regime. Models diverge a little
more as we get to Thursday, but overall it appears we will see a
continued warming trend in west-southwest flow aloft, This flow will
favor afternoon convection along and west of the Continental Divide.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1016 PM MDT Thu May 23 2019

Light rain continues to spread northward across western and
central Wyoming this evening, with a gradual mixing in of -SN at
times later tonight. RIW has recently socked in with BR due to
local upslope flow. This fog should thin back out in the next
couple hours when winds shift back from the northwest. Rainfall
will gradually decrease overnight as low pressure moves north.
Rainfall should be ended for all locations by midmorning tomorrow,
but cloud cover will be slow to erode, but will at least lift
above 3000 by noon. Winds Friday will be from the west and
southwest through the day, with strong gusts at CPR.

&&

.FIRE WEATHER...
Issued AT 242 PM MDT Thu May 23 2019

Wet and cool conditions will persist through Thursday evening before
the pattern trends warmer and drier Friday through Sunday. The
exception will be the Great Basin forecast zones which will see
continued precipitation through Friday. Winds will become more west
to southwest Friday with gusty 25 to 35 mph wind from Sweetwater to
Natrona counties. Saturday and Sunday will see afternoon showers and
isolated thunderstorms, particularly across areas along and west
of the Continental Divide. After seasonal temperatures over the
weekend, a cold front Sunday night will drop daytime highs into
the 50s for Memorial Day and Tuesday. This cold front in
combination with another western U.S. weather system will provide
for more wet weather Memorial Day on into Tuesday. Smoke dispersal
will be at its best Friday given the gusty southwest wind. The
weekend will see much improved smoke dispersal over what was seen
this week.

&&

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

$$

SHORT TERM...McDonald
LONG TERM...CNJ
AVIATION...Straub
FIRE WEATHER...CNJ


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