Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 211807 AAA
AFDRIW

Area Forecast Discussion...Update
National Weather Service Riverton WY
1206 PM MDT Tue Mar 21 2017

.SHORT TERM...(Today through Thursday night)
Issued at 147 AM MDT Tue Mar 21 2017

Imagery shows ridging across the wrn CONUS with an upstream
trof/closed low centered offshore nrn California/OR with a modest
jet moving through the trof`s base across central California.
Exiting, now down stream, strong jet over the nrn Plains and across
the Great Lakes. At the SFC, post frontal high P across the northern
High Plains and Northern Plains with lower P to the south and
southwest of WY and a nearly stationary front draped across
southwestern and southern WY then down along the Front Range of CO.
Scattered light rain or snow showers mainly east of the Divide
heading quickly to the east northeast...leaving patchy areas of
BR/FG.

Through Wednesday night: Modest ridge axis slowly translates ewd and
over the Plains` states with just enough upper level flow/forcing to
allow for isolated to locally scattered afternoon/evening showers,
including isolated thunder, to occur. By Wednesday night as the
upstream trof deepens and approaches the western coastline, both a
strong jet as well as increasing sub-tropical moisture start to make
their way through the Great Basin and into WY from the southwest.
POP chances will increase as well as the probable QPF values with
the southwest quad of WY receiving the better moisture...both liquid
and frozen. Up to a couple of melting inches could be possible by
Thursday morning...particularly over the east end of the Wind River
Mountains.

Thursday and Thursday night: The deepening and closing trof/low
swings into/through the Great Basin and portions of the Desert SW
with what looks to be setting up as a classic Four Corners Low and
strong cyclogenesis proceeding across ern CO by Thursday afternoon.
Strong low to mid-level frontogenesis will also take place to the
north of this area over WY roughly from southwest to northeast. This
frontal zone will then drift to the southeast through Thursday
afternoon and overnight. While the entire FA will be privy to
precip...convective with thunder Thursday afternoon then more
stratiform/upslope by evening and overnight...best moisture
convergence through the cyclone`s maturation will come together
Thursday night across eastern WY...possibly making it back into
portions of eastern Natrona county. Snow levels will be tricky and
borderline at times below 7.5-8k ft and will often have to wait
for the cold frontal genesis/passage later Thursday afternoon and
night before snow appears east of the Divide down to the Basin
floor. The Jackson Valley may remain protected from the colder air
all through this portion of the forecast, getting little if any
snowfall from decent moisture. Highlights will also be tricky and
may best fall into Advisory or SPS territory. Another 12 to 24 hrs
of model runs should aid in honing the forecast.

.LONG TERM...(Friday through Monday)
Issued at 147 AM MDT Tue Mar 21 2017

Synopsis...A series of Pacific storms will continue to track
across the southwest U.S., and across the southern and central
Rockies through the period. The first storm system will exit into
the central Plains Friday with rain and snow showers tapering off
across central and southern areas in the morning. The next storm
system will spread valley rain and mountain snow showers into the
west half of the area on Saturday, exiting into central High
Plains on Sunday with scattered rain and snow showers lingering
across the area through Sunday night. The third in this series of
storm systems will move into the Four Corners region on Monday,
with the next round of rain and snow showers spreading across the
area into Tuesday. Overall, expect a wet pattern with temperatures
near or slightly below normal over the weekend into next week.

Discussion...Synoptic pattern at the outset Friday morning
features first in a series of Pacific upper lows ejecting out of
the central Rockies into the central High Plains, the second
Pacific trough upstream off the west coast, and trough #3 crossing
the dateline in the central Pacific. 21/00z ECMWF is slightly
faster than 21/00z GFS with first upper low lifting across the
Central Plains on Friday, so ECMWF also moves precipitation on NW
quadrant of upper low out of eastern Wyoming a bit quicker than
GFS Friday morning. Forecast goes on slow side moving rain and
snow out due to lingering upslope flow, especially in the Casper
area.

ECMWF maintains a quicker pace than GFS with upper lows #2 and #3
with both models showing about a 48 hour spacing between systems.
ECMWF`s quicker pace with upper low #2 pushing across the Four
Corners region 00z Sunday would spread chances of mountain snow
and valley rain showers into central Wyoming during the day
Saturday while slower GFS keeps chances of precipitation east of
the Divide. Compromised on this and spread some chances of showers
into the central basins Saturday afternoon/evening. ECMWF shows
some ridging and drying over Wyoming Sunday night between system
#2 over the central Plains while GFS shows more of a broad trough
carved out from the west coast to the Central Plains. Confidence
at this stage is low on timing, but believe there will continue to
be some break/clearing between systems. With plenty of cloud cover
and showers expected through the period, high temperatures were
weighted toward cool side of model guidance.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 147 AM MDT Tue Mar 21 2017

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

Scattered showers will develop after 20Z through 12Z Wed. Snow
showers will occur in the higher elevations with some mountain
obscuration at times, especially between 08z and 14z and mainly
affecting KJAC. Local MVFR conditions will occur near the mountains.

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

Low clouds will occur in the north with MVFR/IFR ceilings. The low
clouds will retreat north and dissipate between 20z and 23z this
afternoon. Then VFR conditions are to prevail through 12Z Wed.
Isolated to scattered showers will develop after 21Z and continue
through 12Z Wed.

&&

.FIRE WEATHER...
Issued AT 147 AM MDT Tue Mar 21 2017

Fire danger will be low through the weekend. The exception to this
rule will occur on Wednesday which now shows signs of having
elevated fire danger across portions of central WY as gusty
southwest winds combine with lowering relative humidities and a few
chances for gusty isolated thunderstorms in the afternoon/evening.
Thursday also shows signs for embedded thunderstorms, however the
day on the whole will have increasing moisture spreading across WY
along with increasing chances for precipitation...both liquid and
frozen.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Flood Watch through this afternoon for WYZ013-023-025>027.

&&

$$

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



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