Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 240838
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
238 AM MDT Mon Apr 24 2017

.SHORT TERM...Today through Wednesday night

With the contrast in the weather pattern...we will divide the
forecast into West and East of the Divide this morning.

West of the Divide...If you look at the calendar this morning it
will say late April. For most parts of the country this means warmer
weather. However, as we all know in Wyoming winter can linger for a
while. And this appears to be no exception for many areas West of
the Divide as there will be a couple of chances for snow. It is
mainly quiet early this morning but this will change fairly rapidly
this morning as a cold front and trough approach the area with a
decent amount of Pacific moisture. Snow will begin later this
morning and continue through tonight and into tomorrow morning.
However, the heaviest and steadiest snow should occur later this
afternoon and early this evening with the trough axis swinging
through. There looks to be decent amount of snow with the storm and
a result we did issue Winter Weather Advisories from this morning
through Tuesday morning for the mountains. As for the Valleys, snow
levels are expected to remain at around 7000 to 7500 feet today and
into this evening. Colder air will move in later this evening and
tonight but by then the best forcing will have moved out of the
area. In addition, any snow that falls during the day would be wet
and the strong late April sun should prevent most accumulation on
roads. So, we will not issue any highlights for the valleys.

The steadiest snow should end by Tuesday afternoon with some ridging
across the area. However, some showers will likely linger into the
afternoon and night time hours although nothing heavy is expected at
this point. The lull in activity is expected to continue into
Wednesday morning before the next system approaches for Wednesday
afternoon and Wednesday evening. 700 millibar temperatures look
fairly similar to Monday`s system so it could be a similar setup
with snow in the mountains and a mix in the valleys. Needless to
say, temperatures will average below normal through the period.

East of the Divide...Showers that occurred today are ending. At this
point most of the morning looks to be dry so we removed POPS for
most areas. With the cold front approaching there will once again be
the chance for afternoon showers and thunderstorms, With the cooler
temperatures today the chance of some small hail looks to increase a
bit over yesterday. However, much like Sunday, most of the day
should be rain free most of the time through the afternoon. However,
things may change in the evening for parts of the area.

Low pressure will likely develop and move across central and into
eastern Wyoming tonight. Both the NAM and the GFS are showing a good
flow moving into the Bighorn Range that will change over to snow
tonight. For now, the Winter Weather Advisory looks good. We do have
some concern that the air could cool enough to drop snow levels down
to the lower elevations late tonight and tomorrow morning in
portions of Johnson County to bring some decent snows. As a
result, we will issue a Winter Weather Advisory for northern
Johnson County. Many other areas further south may dry out
briefly with some dry slotting.

Tuesday still looks to be cool, cloudy, cruddy, cantankerous and
possibly causing a conundrum with the forecast. The best dynamics
should shift eastward. However, enough moisture will wrap around to
bring some stratiform precipitation to many areas. We do have
concern that it could get cold enough for some snow in places like
Cody with 700 millibar temperatures dropping to around minus 6.
Again, we have the problem of the sun angle during the day and with
temperatures borderline the snow could have a problem sticking to
paved surfaces during the day. We will punt for any advisories
here. There could even be some snow around places like Lander,
Riverton and Casper but here with somewhat warmer temperatures any
accumulations should be limited to grassy surfaces.

Some transitory ridging will then build in Tuesday night and shut
off the precipitation by late Tuesday night. Another system will
then move in for Wednesday but much like Sunday and Monday most of
the day should be rain free. However, temperatures will remain below
normal. Steadier precipitation may return for Wednesday night
although the models have some timing differences this far out.

.LONG TERM...(Thursday through Sunday) Issued at 211 AM MDT Mon
Apr 24 2017

Very active pattern continues through the medium range period and
beyond as mean trough remains near or just east of us and if
anything, strengthens early next week. Potential for a major
snowstorm still exists on Friday in central Wyo. The later Thursday
into Friday system appears to be a system just south of Kamchatka.
This system moves across the Pacific, rides up over the ridge and
then strengthens as it encounters favorable longwave trough over the
Inter-mountain west. Models continue to vary on location, overall
strength and timing with each run but overall idea is still the
same. System digs into the Great Basin late Thursday/Thursday night
and slows down on Friday and closes off most likely over UT. System
stalls over UT on Friday before heading towards the 4-corners area
Saturday morning. The system ahead of this one will keep the area
unsettled to start the period with front lingering around Central
Wyo on Thursday. Scattered to widespread showers likely in the mtns
and central portions. Snow in the mtns and a mixture of rain or snow
in the lower elevations with highs in the 40s generally east of the
divide and 30s in the mountains and foothills. Widespread snow is
quite possible on Friday/Friday night east of the divide (not that
pcpn ever really ended from our late Wed/Thu system) as low
strengthens and closes off over Utah. System will likely start to
move east and south away from the area on Saturday with gradually
improving conditions. Possible brief ridging Sunday before our next
digging trough moves in Monday. Euro and GEM more bullish here and
preferred as it fits the idea of mean longwave trough holding over
the area.

&&

.AVIATION...12Z Issuance

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

An area of partial clearing will occur overnight between weather
systems.  The next more powerful storm system will spread widespread
MVFR areas of IFR valley rain and mountain snow into the far west
valleys and western mountains 15z-18z Monday, mountains becoming
mostly obscured by 18z.  An associated cold front with this system
will move east across western Wyoming 21z Monday-00z Tuesday with a
line of heavier showers and a few thunderstorms possible along the
front, pushing east into vicinity KRKS.  Gusty west to northwest
surface winds and a drier airmass will follow the cold front across
southwest Wyoming Monday evening, while northwest Wyoming and the
western mountains will continue to see scattered to numerous valley
rain and mountain snow showers, snow levels expected to lower to
most valley floors including KJAC 6z-12z Tuesday.

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

Lingering rain showers with an upper level disturbance will be
mostly east of Interstate 25 by 09z.  Partial clearing, VFR
conditions will prevail through 18z Monday.   The next more powerful
Pacific storm system will push a cold front east across the area
around 00z Tuesday, with a low pressure system spinning up in the
vicinity of the Bighorn Mountains Monday night.  Scattered showers
and isolated thunderstorms will develop ahead of the cold front
mainly after 21z Monday, with an area of more widespread MVFR/IFR
valley rain and mountain snow showers expected to develop vicinity
and north of a KTHP-50SM north KCPR line 3z-12z Tuesday while gusty
west to northwest winds and a drier airmass prevail south of this
line.  A colder northerly flow on the backside of the low pressure
system will lower snow levels to near 5000 feet across the northern
Bighorn Basin including KCOD 8z-12z Tuesday.
&&

.FIRE WEATHER...

A cool and wet pattern looks to persist for much of the week. A
Pacific cold front will move across the state today. This front will
bring mountain snow and valley rain and snow to areas West of the
Divide from later this morning into early Tuesday. Meanwhile, areas
East of the Divide will have a chance of showers and thunderstorms
this afternoon although most areas should be rain free most of the
time through the afternoon. The usual gusty wind will develop from
Rock Springs through Casper. A steadier snow will develop over the
Bighorns tonight and may drop into the lower elevations of Johnson
County later tonight and possibly the Cody Foothills Tuesday
morning. Tuesday at this point looks cloudy and cool with occasional
rain and snow. Mixing and smoke dispersal will range from fair
across the northwest to good to very good in Sweetwater and Natrona
Counties.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 9 AM this morning to noon MDT
Tuesday for WYZ001-002-012-014-015-024.

Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Tuesday for WYZ008-009.

Winter Weather Advisory from midnight tonight to 6 PM MDT
Tuesday for WYZ010.

&&

$$

SHORT TERM...Hattings
LONG TERM...Skrbac
AVIATION...Meunier
FIRE WEATHER...Hattings



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