Area Forecast Discussion
Issued by NWS Riverton, WY

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980
FXUS65 KRIW 260601
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1201 AM MDT Wed Apr 26 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 06Z TAFS through 06Z Wednesday night)
Issued at 1141 PM MDT Tue Apr 25 2017

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

A band of MVFR/IFR snow showers vicinity KJAC will move east
across the Continental Divide 9-12z with an overall break,
prevailing VFR conditions through early morning. The next Pacific
storm system will spread scattered rain and snow showers into the
western Wyoming 12z-18z Wednesday with areas of MVFR ceilings and
increasing mountain obscurations. Valley rain and mountain snow
showers will become more numerous Wednesday afternoon with a few
thunderstorms possible. Snow levels will lower to near valley
floors Wednesday evening as showers reach peak coverage with MVFR
conditions becoming widespread, areas of IFR conditions also
pushing into vicinity KRKS by 02z Thursday. These conditions will
persist across the area through 12z Thursday.


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

VFR conditions will prevail through Wednesday evening. Widely
scattered rain and snow showers will move off the Absaroka range
into vicinity KCOD-KWRL 9z-15z Wednesday before moving further
east and dissipating. The next Pacific system and associated cold
front will spread scattered showers and isolated thunderstorms off
the mountains into central Wyoming late Wednesday afternoon and
evening. Rain and snow showers will become more numerous along
with widespread MVFR/local IFR conditions vicinity and north of a
KRIW-KCPR line 03z-12z Thursday.

&&

.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...
Winter Weather Advisory from noon Wednesday to noon MDT Thursday
for WYZ001-002-012-024.

Winter Weather Advisory from 6 PM Wednesday to noon MDT Thursday
for WYZ013-023.

&&

$$

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



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