Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 212141

Area Forecast Discussion
National Weather Service Riverton WY
241 PM MST Sun Jan 21 2018

.SHORT TERM...(This evening through Wednesday) Issued at 240 PM
MST Sun Jan 21 2018

Imagery shows broad flat low amplitude ridging across the eastern
1/2 of the CONUS with a rapidly developing trof moving into/through
the Rockies and into the Plains, undergoing also rapid cyclogenesis.
Baroclinicity is increasing once again as a secondary WCB starts to
feed into the cold air cyclone which is beginning to undergo the
instant occlusion process. The northern front range of CO into nern
CO/sern WY is now receiving the brunt of the heavy snow at this
time. The SFC has high pressure moving into the western CONUS behind
the exiting cold front which is now attached to the SFC cyclone
spinning up over sern CO/swrn KS/nwrn OK. Light to occasionally
moderate snow continues mainly across sern WY with only a few areas
of flurries directly to the west or northwest.

Yesterday`s/Last night`s winter system is getting pulled out of the
FA and into the High Plains of CO/cntrl and srn Plains as strong
instant occlusion cyclogenesis starts to take place. Post frontal
seasonally cold temperatures will lay in the wake of the system,
with areas under clearing skies see quite cold temperatures Monday
morning. These morning low temperatures still may not be low enough
than currently forecast...much of which will depend on
departing/returning (with the next system, including the return of
swrly near SFC flow across portions of central WY) cloud cover
within a brief period of upper level shortwave ridging. Along with
these cold temps and still lingering higher dew points across
portions of central and southern WY, expecting at least some fog
(patchy ATtM) over these areas. Later Monday, the upper levels
transition to zonal then nw flow with an embedded shortwave trof
moving into/through the flow and into the western FA Monday
afternoon/evening. Precip chances will increase accordingly, but
only under rather modest dynamics and moisture increases. This
system will quickly exit the area overnight Monday leaving most of
the FA, except for some residual light snow showers across nw WY,
without precipitation both Tuesday and Wednesday. While some of the
slightly more elevated areas will go through a bit of a warming
period by breaking the inversion to some extent, the central and
northern Basins may have a bit tougher time with fresh snow cover.
Wind and mixing will also play a part, but still have lowered fcst
highs over these areas.

.LONG TERM...(Wednesday night through Sunday) Issued at 240 PM
MST Sun Jan 21 2018

Overview...The continued trending in the forecast is heading toward
the idea that the main weather impacts will be snow spreading into
the west on Thursday and attempting to make it further east into
central sections by Friday.  Dry weather will be the rule outside of
this time period in the extended forecast periods, with the
exception of snow showers still expected in the western mountains
Saturday and Sunday. Temperatures will moderate and be milder from
west to east across the state on Wednesday and Thursday in advance
of the storm system and will trend slightly cooler through most of
next weekend behind the system.

Discussion...At the beginning of the extended forecast period by
Wednesday night, a surface ridge axis will be across the region with
moderating temps. Southwest winds will set up ahead of a developing
upper level trough off of the Pacific coast, with likely gusty and
increasing winds along the wind corridor from Rock Springs to
Casper.  The ECMWF/GFS models are in fair agreement involving moving
the trough off the west coast, progressively inland and into the
Great basin on Thursday. The associated surface cold front is
progged by the models to spread snow into western Wyoming by
Thursday afternoon and evening.

The models are still consistent in moving the upper trough through
the Central Rockies on Thursday night and out into the Plains on
Friday. This next storm system does not look to be as strong as the
one that affected the area this weekend, so will be leaning more
toward lesser snow amounts with the next system. The progressive
upper flow pattern will result in an upper ridge axis then building
into the region next weekend. Temps should remain cool Saturday in
low level northwest flow and then moderate Sunday as ridge amplifies
over the Interior West.


.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 240 PM MST Sun Jan 21 2018

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

VFR conditions will continue at all sites through the forecast
period. Have made some slight tweaks to add in mention of VCSH of
snow at KJAC and KPNA after 15Z Monday. This will account for some
scattered snow showers being possible at these locations and in the
northwest Wyoming mountains. Some borderline MVFR ceilings are
possible in the northwest mountains also after 09Z Monday.

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

VFR conditions will be the rule across the area through the forecast
period. Have tweaked wind speeds slightly upward at KCPR after 19Z
Monday to account for stronger low level winds expected. Mid level
broken clouds are expected to move across the area after 15Z Monday,
but still expected the cloud cover to be well within the VFR range.

Please see individual terminal sites for more details. Additionally,
the Aviation Weather Center will have the latest information on
icing and turbulence forecasts.


Issued AT 240 PM MST Sun Jan 21 2018

As the strong winter weather system  departs the region today and
tonight, so does the snowfall. Much of the region, except the
northern Basins, received significant snowfall over the last few
days. Additionally, much of the forecast area will see RH values
remaining relatively high for the next few days with winds also
staying comparatively quiet through Tuesday. A weak winter system
will return to the western mountains Monday bring mainly light areas
of snowfall through Monday night. Wednesday, winds will begin to
increase across the usual areas as atmospheric conditions become
more favorable. Smoke dispersion will generally be poor to fair
today, poor to fair Monday, and fair to good Tuesday.




LONG TERM...Troutman
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