Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 201757

Area Forecast Discussion
National Weather Service Riverton WY
1057 AM MST Fri Jan 20 2017

.SHORT TERM...Today through Sunday night

Imagery shows weak south to southwest flow aloft ahead of large
broadening/digging negatively tilted wrn CONUS trof with an also
weak (weaker than previously thought as it also expands) embedded
SW/vort max starting to move into/across srn/cntrl WY. Isolated to
widely scattered snow showers currently exist over the western
half of WY with no real focus yet. The SFC has general low P
across the FA with the H/L gradient from sw to ne.

As the digging and expanding trof spreads across the western CONUS
with the main jet and upper level forcing digging well to the south
over the Desert Southwest, the weak embedded shortwave will finish
lifting across Wyoming today. This will bring some chances for snow
east of the Divide this morning as the tail end of an exiting
weak jet continues to move north. Most favorable position of the jet
and associated forcing is actually happening now and will only get
weaker from now on. All areas, east and west of the Divide look to
keep snowfall totals through this evening below Advisory level for
the most part.

West of the Divide, snowfall totals through this morning generally
look to range from 1 to 5 inches in the mountains and up to an inch
across the valleys. East of the Divide, snowfall will range from a
trace to 2 inches over most lower elevations with 1 to 4 in the
mountains. Highest amounts over the lower elevations should occur
across the northern Big Horn Basin and possibly over northern
Johnson county with a weak frontal push later today. Additionally,
over the Bighorn mountains anywhere from 1 to 6 inches could fall by
Saturday morning, with lowest amounts across the south and highest
over the north.

Saturday morning through most of Saturday night, the broad but still
digging, negatively tilted trof will cover the FA with a SW ridge
building into the swrn CONUS and the jet still well to the south
over the swrn CONUS. Forcing and instability across the FA will
continue limited/modest until at least Sunday afternoon. Then,
increasing moisture from the EPAC, but Sierra wrung out, will arrive
across the wrn/swrn FA along with the nose of a strong jet. This
will greatly increase precip chances across west/southwest WY with a
much better chance for significant snowfall through the end of the
forecast period (and beyond). Some sort of highlights will likely
be needed by Sunday night over this region.


Synopsis...A large trough will move onto the west coast on Sunday
and across the the Rockies Monday and Tuesday.  Snow showers will
increase across the west and south Sunday and Sunday night, and
spread into central Wyoming on Monday.  Snow will taper off from
west to east on Tuesday and Tuesday night.  High pressure will build
in Wednesday and Thursday with dry and cool conditions prevailing.

Discussion...Synoptic pattern on Sunday morning to feature upper low
upper ridge over the Rockies flanked by upper lows over the
southeast U.S. and along the northwest coast.  19/00z ECMWF and GFS
runs in good overall agreement with progression of longwave trough
across the western U.S. into the plains Monday and Tuesday, with
upper low lifting out of the central Rockies across Kansas/Nebraska
Monday night and Tuesday.  Within this longwave trough lie the
differences, with ECMWF having an overall further southward
placement of individual features and precipitation than GFS as upper
trough moves across the Continental Divide.  So far, the southward
solutions have been verifying better in this split flow regime.
Thus, expectations are that more significant snow amounts and
impacts will stay to our south Sunday and Monday.  Surface lee
cyclogenesis is forecast over eastern Colorado on Monday with low
level northerly flow developing east of the Divide through Tuesday,
favoring the Casper area, north facing slopes, with possible
moderate snow amounts.

GFS more progressive than ECMWF folding upper ridge into the
northern Rockies Wednesday and Thursday.  Forecast maintains some
low PoPs in the mountains due to the uncertainty, but upper ridge
will eventually prevail over the western U.S. by the latter part of
the week with dry weather expected into the last weekend in January.


West of the divide:

Low clouds and fog will finally clear out by early this afternoon
across the area. Flurries will remain a possibility near the
mountains with the mountains likely remaining obscured. Will
continue with borderline MVFR ceilings with light snow expected
between 09z-10z time frame from JAC, extending eastward through
the end of the taf period.

East of the divide:

A band of mainly light snow will continue to shift northward out
of the area through the eastern sections with MVFR ceilings and
visibilities expected at least through 19z. Low end VFR to
borderline MVFR ceilings are expected through the rest of the
afternoon, especially at CPR and in Johnson county. Mountains
will likely remain obscured through the afternoon.


Fire Danger low for all of WY through the next week. Below seasonal
to seasonal temperatures, a good existing snow pack, and increasing
chances for measurable snowfall over the next week will keep fire
danger very low. Heaviest snowfall of the period will begin by
Sunday night west of the Divide, expanding east of the Divide by
Monday. Smoke dispersion fair to good across the forecast area this
afternoon, except for the Basins which will remain poor. Fair to
good west of the Divide Saturday afternoon, poor to fair east of
the Divide.




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