Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 230844

Area Forecast Discussion
National Weather Service Riverton WY
244 AM MDT Mon Apr 23 2018

.SHORT TERM...Monday through Wednesday night
Issued at 230 AM MDT Mon Apr 23 2018

The cold front that has been talked about for the past couple of
days is now moving through the state with a well defined band of
rain and snow trailing behind it. This band of precipitation will
move into western Wyoming early this morning and continue
progressing across the state through the day. The models are also
persistent in keeping the heaviest precipitation across northern
Wyoming. The big conundrum this morning continues to be expect
impacts with snow in the mountains and exactly the amounts and
impacts of any snow in the lower elevations. The models have trended
slightly warmer this morning. Nevertheless, we still expect the rain
to change to snow above around 5000 feet today. As a result, the
main effects would be in the Cody Foothills, especially along and
west of Highway 120 as well as in the Big Horn Mountains. There is
the potential for a period of moderate to even heavy snow, possibly
even some thundersnow with some locations. However, with a bulk of the
snow in the lower elevations falling during the daylight hours it
will be fighting the high late April sun angle. As a result, snow
may have trouble sticking to paved surfaces unless the snow is
heavy. As a result, we will stick to the special weather statement
for now. Effects will be less further south although showers will be
possible along with spotty thunderstorms. It will also be breezy to
windy across much of the area with the strongest winds in northern
Johnson County with the favored northwest flow/cold advection
pattern. However, 700 millibar winds are not that strong so high
winds are not expected. Most of the steady precipitation should move
out by sunset but a few showers may linger through the night.
Temperatures will definitely be a shock after the mild weekend with
some areas 30 degrees cooler than yesterday and combined with the
gusty wind it may feel 40 degrees cooler.

Tuesday looks much quieter. Other than a few morning flurries, the
day will feature decreasing clouds through the day. Temperatures
will remain cool, but with much less wind and more sunshine than
Monday it will feel much warmer. Wednesday at this point looks much
warmer with many areas rising to around 70. There will be a bit of a
breeze as well and that could lead to elevated fire conditions in
the windier spots Wednesday afternoon. Following that, a cold front
will drop down from the north for Wednesday night. The models,
especially the European may be overdoing the QPF though given the
source of the system is Continental Canada. For now, we stuck with
slight POPS and light QPF.

.LONG TERM...Thursday through Monday
Issued at 230 AM MDT Mon Apr 23 2018

Any lingering rain or snow showers Thursday morning east of the
continental divide will end by late morning. Otherwise dry over
the region Thursday with decreasing clouds east of the divide.
High temps a little cooler but seasonal. Friday looks dry for most
of the region. Isolated late day and evening showers and storms
over and near the western mountains. Warmer day on Friday. Forecast
problem Saturday through Monday is the track of an upper low
pressure center along the U.S. west coast. The GFS model swings
this system through Idaho Saturday and into Western Montana
Saturday night, then into Canada for Sunday. The ECMWF model takes
the upper low across California Saturday night and Sunday, then
into Utah Monday. The Canadian model takes the upper low through
California Saturday and into Nevada Saturday night and Sunday,
then into Utah Monday. GFSenembles seem to be closer to the
ECMWF/Canadian solutions versus the operational 00Z GFS model run.
It seems tough for this upper low to barrel into the upper ridge
and destroy it. It is more likely to weaken and slide up and
around the ridge or undercut it. Low confidence on the outcome.
Have went with a chance of showers and storms in the west Saturday
afternoon and night with a little better coverage Sunday afternoon
and night. Little or no shower/thunderstorm activity east of the
divide both days. Next Monday could be more unsettled depending on
where this upper level low tracks. High temps Saturday and Sunday
will be on the warm side, then cooler highs next Monday.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday)

A cold front with an accompanying band of precipitation will move
across areas west of the Divide from 09z-14z, and across north
central Wyoming from 14z-18z.  Snow levels will be between 7-8kft as
the band of precipitation moves across the west with prevailing MVFR
conditions at KJAC, brief MVFR conditions possible further south at
KBPI-KPNA.  More widespread IFR conditions are expected to develop
vicinity and north of a KCOD-KPOY line 14-18z with snow levels
lowering to near 5kft.  MVFR ceilings will spread further south to
KWRL with lower ceilings eventually spreading south into KCPR and
KLND Monday evening with areas of -SN.  Ceilings will gradually
improve into VFR category across NW Wyoming Monday afternoon/evening.


Issued at 230 AM MDT Mon Apr 23 2018

Much cooler temperatures and higher humidity will move into Wyoming
today following the passage of a cold front. Rain and snow will
spread across northern Wyoming with snow levels possibility lowering
to around 5000 feet. Precipitation will likely be more showery
elsewhere. Isolated thunderstorms will be possible as well. A gusty
north to northwest wind will develop today with strong winds
possible over the northern Big Horn Basin and Johnson County. Mixing
and smoke dispersal will range from poor in portions of northwestern
Wyoming to very good in southern Wyoming.





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