Area Forecast Discussion
Issued by NWS Riverton, WY

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074
FXUS65 KRIW 180526
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1126 PM MDT Tue Apr 17 2018

.SHORT TERM...(This evening through Friday)
Issued at 130 PM MDT Tue Apr 17 2018

Imagery shows ridge/trough/ridge/trough pattern across the CONUS. A
rather strong trough is currently moving across WY with its
associated jet (100kt +/-) moving over the southern Rockies then up
through the western Central and Northern Plains. The trough`s axis
cuts southeast from north central MT, through WY and on into the
Four Corners region. The surface shows a developing low pressure
center over east central CO with an inverted trough up through
central/northern WY. Nearly stationary fronts extend around this
low,  one from central WY down through western CO...the other from
central WY through northeastern/eastern CO.

The low in eastern CO continues to strengthen and move into the
Central Plains with the slow moving front pulling completely out of
the forecast area by this evening. Heavier rain/snow/rain-snow mixes
will taper off later this afternoon leaving areas/patchy but
decreasing showers behind the front. SPSs are in place supporting
todays activity. Still, brief periods of heavy snow with visibility
less than a mile will be common from time to time. Additionally,
accumulations under the stronger portion of the deformation band
aloft will see up to 3 inches of snow on grassy areas through the
rest of the afternoon (including some of the Wind River Basin).
Later tonight, ridging will begin to overtake WY, continuing through
Thursday morning. Warming/drying trend both Wednesday and Thursday
with very dry air infiltrating portions of south-central WY by
Thursday along with moderately gusty south winds.

Thursday afternoon through Friday, rapidly developing cyclogenesis
will be taking place, starting over the Great Basin, moving
toward/across the Four Corners region by Friday morning, then
continuing eastward toward the Central/Southern Plains and starting
a period of instant occlusion cyclogenesis that goes beyond this
forecast period. As the trend over the last several days has slowed
slightly and taken this system further south, there is also a delay
in tapping into Gulf moisture and instability...so much so that it
will take longer for moisture to work its way back into the
southern/southeastern/central CWA...if at all. Other than isolated
to widely scattered rain/snow showers and/or some gusty north or
northeast winds, the overall impact should be low.

.LONG TERM...Friday night through Tuesday night
Issued at 130 PM MDT Tue Apr 17 2018

An upper low over the Four Corners, will continue its eastward
trek toward the Central Plains. Eastern and southern portions of
the forecast area will remain on the northern fringe of this
system, with precipitation ending Friday night.

A warming trend with mainly dry conditions is expected over the
weekend. Cannot rule out an isolated mountain shower over the
north. Temperatures look to be 5 to 10 degrees above average.

Both the ECMWF and GFS are showing a weak wave pulling across the
northern Rockies Monday and Tuesday, which could give the area
some valley rain/high mountain snow.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1125 PM MDT Tue Apr 17 2018

West of the Continental Divide...KJAC, KBPI, KPNA, and KRKS

Cold drying high pressure will move over the west through
06Z Thursday with clearing and decreasing wind.  VFR conditions
will prevail at the terminal sites, with clearing skies after
06Z Wednesday. Patchy valley radition fog may form in Jacskon
Hole, the Star Valley and the Upper Green river basin from
09Z to 16Z Wednesday with local ifr ceilings and visibilities
reduced in fog. At this time have vicinty FG in KJAC and KPNA
Tafs.  Some high clouds will move over the area 00Z to 06Z
Thursday in increasing southwest flow aloft.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

Cold drying high pressure will move over the area from the
Continental divide extending east through 06Z Thursday with
clearing and decreasing wind.  VFR conditions will prevail at
the terminal sites, with clearing skies after 06Z Wednesday.
Patchy valley radition fog may form in the Bighorn and Wind River
basins along and near rivers and lakes from 09z to 16z Wednesday
with local ifr ceilings and visibilities reduced in fog. At this
time have vicinty FG in KLND and KRIW Tafs. Some high clouds will
move over the area 00Z to 06Z Thursday in increasing southwest
flow aloft.

&&

.FIRE WEATHER...
Issued AT 130 PM MDT Tue Apr 17 2018

A strong upper level disturbance and associated cold front continue
across Wyoming today, and will clear out of the forecast area from
northwest to southeast by this evening. Light to moderate rain or
snow with occasional bursts of heavier precipitation will occur
into/through this afternoon with a few light instability rain or
snow showers following the heavier snowfall into this evening.
Blustery north to northwest wind will continue into the early
evening period with gusts 35 to 50 mph at times. Then, it will be
dry and warmer Wednesday and Thursday with low relative humidities
making their way back into Wyoming from the south.

&&

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

&&

$$

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



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