Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 252319
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
519 PM MDT Tue Apr 25 2017

.UPDATE...
Issued at 517 PM MDT Tue Apr 25 2017

Winter Storm Warning for NE Johnson, and Winter Weather Advisory
for the Bighorn Mountains have been cancelled. Snow has ended or
become light across this area. No Winter Weather Highlights remain
in effect.

&&

.SHORT TERM...(This evening through Friday)
Issued at 245 PM MDT Tue Apr 25 2017

With a large scale low pressure trough extending from off of
the west coast east into the Mississippi Valley, a series
of short wave troughs in northwest flow will bring periods
of widespread showers with isolated afternoon and evening
thunderstorms through Friday with continued cooling with
periods of rain mixed with snow changing to snow in the
lower elevations east of the Continental Divide.  Some
areas will see 3 to 6 inches of snow in the big Horn, Wind
River basins and across Eastern Johnson and Natrona counties
with rapid melting. The Absaroka, Wind River and Bighorn
Mountains could see 5 to 10 inches of snow each day.

.LONG TERM...(Friday night through Tuesday)
Issued at 245 PM MDT Tue Apr 25 2017

Beginning of fcst, the large trof and associated cold front that had
been affecting the FA since Thursday will be active, continuing with
widespread areas of snowfall across WY...but beginning to taper off
from north to south overnight as the primary trof axis moves east.
Lingering snow showers will still exist east of the Divide and
across southern WY by Saturday morning with afternoon convective
snow or rain/snow showers possible across the west Saturday
afternoon...ending later Saturday evening.

Overall, the MR models in fairly good agreement until Tuesday when
there is some divergence in placement and trajectory of a modest
shortwave trof moving into the region through continuing NW flow
aloft. ATTM, the EURO has flipped completely from the previous run
in the latter portion of the forecast period Monday night/Tuesday).
Whereas before it was far and away more robust and further south
with developing the sw trof than the shallower and more northerly
biased GFS (at least until they got to the cntrl Plains Wednesday),
it now has hardly any development of a shortwave trof
...and virtually no SFC development. Accordingly, precip generated
by the EURO is very light and diffuse Monday night through Tuesday
while the GFS is now "the Heavy."

Summing up Sunday through Tuesday, taking into the account the mdl
differences noted above in the latter periods: ( Well) Below
seasonal temperatures through the forecast period under SW ridging
aloft (and dry) ahead of the next approaching upper level
disturbance that will begin to encroach on the nwrn FA Sunday night,
bringing chances for light shower activity across nwrn WY before
associated lee side SFC troffing and weak frontogenesis takes place
Monday with increasing precip chances across the entire FA...but
especially the northern forecast zones (before the front heads south
and out tuesday night/Wednesday).

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday afternoon)
Issued at 245 PM MDT Tue Apr 25 2017

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

Isolated to Scattered MVFR/IFR conditions in lower CIGs with
occasional VIS restrictions in light snow showers will remain
vicinity and north of a KAFO-KPNA-KDUB line through 03z. Lingering
higher elevations clouds and light showers possible through
Wednesday morning across the Western mountains as a weak ridge of
high pressure builds in this evening/overnight. Improving conditions
elsewhere...until later Wednesday afternoon as clouds and increasing
precipitation chances move into far western WY ahead of the next
weather maker due to impact Western WY by Wednesday night.

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

The upper level and surface low pressure system that has impacted
much of the area east of the Divide today will sink to the south
along with with the band of precipitation and widespread MFVR/IFR
conditions in lower CIGs and VIS restrictions this evening. A
relatively cold northerly flow also continue to push south behind
the exiting low pressure system. Snow mixed with rain or all rain
this afternoon may return back to wet snow early this evening south
of a KDUB-KWRL-KBYG line with drier air and improving conditions
gradually taking over from the north and west later this evening.


&&

.FIRE WEATHER...
Issued AT 245 PM MDT Tue Apr 25 2017

With a large scale low pressure trough extending from off of
the west coast east into the Mississippi Valley, a series
of short wave troughs in northwest flow will bring periods
of widespread showers with isolated afternoon and evening
thunderstorms through Friday with continued cooling with
periods of rain mixed with snow changing to snow in the
lower elevations east of the Continental Divide.  Some
areas will see 3 to 6 inches of snow in the big Horn, Wind
River basins and across Eastern Johnson and Natrona counties
with rapid melting. The Absaroka, Wind River and Bighorn
Mountains could see 5 to 10 inches of snow each day.
Overall smoke dispersal will be poor with low clouds, cold
temperatures and precipitation.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

UPDATE...Meunier
SHORT TERM...Baker
LONG TERM...Braun
AVIATION...Braun
FIRE WEATHER...Baker



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