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fxus65 kriw 212148 

Area forecast discussion
National Weather Service Riverton Wyoming
348 PM MDT Sat Sep 21 2019

Short term...(tonight and sunday)
issued at 243 PM MDT Sat Sep 21 2019

Satellite imagery shows a distinct shortwave over northwest
Wyoming this afternoon. Widely scattered mountain Snow, Valley
rain showers are expected to continue for the rest of the
afternoon and spread east into central and eastern sections of the
forecast area by late afternoon and evening. Some of this activity
could linger to around midnight with shower activity ending from
west to east as the shortwave slowly exits the region.

In wake of this system, skies are expected to clear late tonight
with the exception across the far west. The clear skies and light
winds will allow for good radiational cooling tonight. Some areas
could see their first freeze tonight. Elected to only upgrade
Sweetwater County, and leave out the Wind River basin tonight.
Temperatures across the Wind River basin look to have similar
temps as last night, while Sweetwater County look to be a tad
cooler with a longer period of clear skies. Either way, a
widespread frost is expected in both locations. Patchy frost could
also occur along the Big Horn river basin and along the North
Platte river in Casper. Would not be surprised to see the Casper
Airport hit freezing, but again widespread freeze not expected
across Natrona County. Also fog will be a concern especially
where the significant rain occurred across the north and west
Friday. Over the far western valleys, have warmed overnight lows
as a stratus deck should linger through the night with most
confidence in Jackson Hole.

On Sunday, warmer temperatures and dry conditions are expected as
a transitory ridge pulls across The Rockies ahead of a trough.

Long term...(sunday night through saturday)
issued at 243 PM MDT Sat Sep 21 2019

The trough is expected to split as it pushes across the
intermountain west with the southern part of the trough becoming
closed off across Southern California/Desert Southwest, while the
weaker northern stream pulls across Montana. Cannot rule some very
isolated showers over the northwest, but this system will be dry.
Northwest flow is expected Tuesday and Wednesday with a weak wave
and associated cold front clipping the area Tuesday night. There
will be some chances of precipiation mainly across the North, East
of The Divide. Ahead of this system Tuesday night, above average
temperatures are expected Monday and Tuesday. Could see elevated
fire weather conditions in the Evanston to Casper corridor.
However, wind and relative humidity are not expected to reach
critical fire weather thresholds on the same day. Relative humidity will be the
issue Monday while some wind on Tuesday.

The main focus turns to a trough sliding southeast along the
British Columbia coast Wednesday, and then pulling into the
Pacific northwest Thursday, before becoming an established
longwave trough over the west Friday into early next week.
Although models are in good agreement with the general pattern,
there are significant differences in details with timing of the
main cold front and waves impacting the forecast area. For now,
the timing of the main cold front looks to be Thursday night into
Friday with widespread precipitation possible in the Thursday
night to Saturday time frame.


Aviation...(for the 00z tafs through 00z monday)
issued at 1150 am MDT Sat Sep 21 2019

VFR conditions will prevail at most terminal sites tonight. Rain
showers and mountain snow showers will continua over much of the
area through 04z this evening. Areas of MVFR ceilings will occur
near storms in the west and north with local IFR conditions in areas
of heavier precipitation. An isolated thunderstorms cannot be ruled
out this afternoon through sunset. Kjac has a chance for IFR
conditions Sunday morning but confidence is low so went with scattered and
vcfg in the tafs for now.


Fire weather...
issued at 243 PM MDT Sat Sep 21 2019

After a widespread valley Rain/Mountain snow event across the west
and north, dry conditions are expected Sunday and Monday with a
warming trend. Slightly elevated fire weather conditions Sunday
and Monday from Evanston to Casper. On Monday the relative
humidity looks to meet criteria, but winds should be relatively
light. On Tuesday the wind are expected to be stronger, but
relative humidity will be higher above critical fire weather

The next storm system with the potential for widespread
precipitation will be Thursday and Friday.


Riw watches/warnings/advisories...
freeze warning from 2 am to 9 am MDT Sunday for wyz028>030.



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