Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 131659
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1059 AM MDT Wed Sep 13 2017

.SHORT TERM...Wednesday through Friday night

It was a rather toasty day across western and central Wyoming with
many 80s and a few 90s. However, we will now begin a transition
into fall that could be a shock to the system for many areas. This
will take one more day however, as temperatures will still be
warm today ahead of an advancing Pacific trough and cold front
that will slide across the state. The radar is quiet right now but
all areas will likely have a chance of showers and thunderstorms
this afternoon and evening. However, much like many summer days,
most areas will be rain free most of the time. Temperatures will
be warm, although cooler than yesterday. This will especially be
the case across areas further west which will see clouds come in
sooner than places like Casper. In addition, with a gusty breeze
developing ahead of the front, elevated fire conditions look
possible in the eastern zones of the County Warning Area.

Thursday at this point looks to be the transition day. A deep trough
will develop southeastward toward the area and drag through a
cold front that will bring the coolest air so far this season.
Some of the models have come in a bit slower with the system
though, so many areas, especially East of the Divide, should have
a dry start to the day and possibly into the afternoon. Showers
and thunderstorms will develop across the west and then progress
east. With precipitable water values rising close to an inch,
there should be a good coverage of showers in the afternoon and
especially at night. And this is when the cooler air will move
into the area behind the front. This will likely lead to some
decent overrunning and rain across much of the area. In addition,
with the cooler air, something else of the white and cold kind
will likely fall over the mountains. A guess anybody? Of course, I
am talking about snow. The latest run of the models have come in
a bit warmer however. There will still be some decent snow across
the mountains but at this point lower elevations look to stay
mainly rain. As for highlights, it is too early at this time but
this could be our first winter weather issuance of the year in the
next couple of days. The models still have some differences in
placement of the best QPF, but the best snow at this point would
be across the northern Absarokas, Big Horns and East Slopes of the
Wind River Range with lesser amounts West of the Divide.

Friday looks to really be a slap in the face, especially East of
the Divide. With the trough only moving slowly through the area
things will stay unsettled. At this point, it looks like a wet day
for many areas with some snow continuing in the higher elevations.
The models also indicate possibly some better lift coming in the
afternoon with the left front quadrant of a jet streak rounding
the trough. With lifted indices falling below zero across the
south, we added the maintain of thunder as well. Many areas will
see high temperatures around 20 or more degrees below normal on
this day. Precipitation will likely linger into Friday night as
well as the trough only slowly moves across the area.

.LONG TERM...(Saturday through Wednesday)

Cold upper low/trough is passing through the area into Saturday
before improvement Saturday night. GFS is on the warm side while
the GEM/NAM and Euro are considerably colder. Considering the high
latitude and swd movement over B.C., it is likely that the colder
models may prove more right. With that said, snow levels will
likely be quite low Saturday morning. In fact, the NAM and Euro
would have it snowing in Cody for example. We have a rain and snow
mixture there right now with a morning low of 34. Snow will
continue in the mountains and foothills through at least the
morning with some decrease in precipitation as the afternoon wears
on. Another chilly day with highs only in the 40s and 50s with 30s
above 9000 feet or so. Next week looks quite active as mean
longwave trough settles in over the western CONUS. The next
potentially colder trough yet begins to affect the west as early
as Monday (with a lead shortwave in the developing sw flow
possibly impacting the west late Sunday already with some
showers/tstms). This next system is currently over Siberia and
stays at a high latitude as it moves across the far nrn
Pacific/Gulf of Alaska region this weekend. This system will
actually have better support from the longwave pattern (mean
trough in the west and mean ridge in the eastern Pacific) so with
all that said, this next system may be stronger and colder yet
(than our upcoming system). GFS is a little faster with the
initial shortwave and front with this next system but overall, the
big picture is fairly similar. Bottom line: Another cold (colder)
system with a high likelihood of more rain changing to snow out
west Tuesday through at least Wednesday (if not into Thursday) of
next week. Another strong cold front will move in from the west
Tuesday into Wednesday with falling temperatures and more cold
rain changing to snow in the western and northern mountains. More
significant mountain snow is quite possible with this system. East
of the divide, we may not see as favorable of a pattern (no strong
surface high providing upslope) for significant precipitation but
still a period of showers and thunderstorms with frontal passage
around Tuesday night/Wednesday of next week followed by
instability showers through the end of this period and possibly
beyond. Details will become more clear over the coming days but
little doubt that another cold(er) upper low will impact our area
next week!

&&

.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1115 AM MDT Wed Sep 13 2017

Expect isolated SHRA/TSRA to develop and move across portions of
the west and even to some locations east of the Divide by this
afternoon. Will continue with the current trending of having
isolated SHRA/TSRA into the KBPI-KJAC line beginning by 19z and
from KWRL-KRIW to KCPR between 21-22Z time frame this afternoon.
Most of the activity should continue until around 03Z tonight.
Expect some lingering SHRA through the rest of the forecast period
especially along and west of the Divide as a trough approaches the
forecast area. There should be increasing wind speeds into the
breezy category from KRKS-KCPR line as expected along and ahead of
the trough. Ceilings and visibilities should remain in the VFR
category through the forecast period.

&&

.FIRE WEATHER...

...ELEVATED FIRE CONDITIONS THIS AFTERNOON FOR PORTIONS OF CENTRAL WYOMING...

A Pacific cold front and upper level disturbance will move into
the west today and cross the state this afternoon and evening. It
will be a warm day before the frontal passage with the warmest
temperatures in the eastern zones. In addition, a gusty breeze
will develop ahead of the front. Relative humidity will fall into
the teens at times in the lower elevations East of the Divide.
Although critical fire conditions are not expected, elevated fire
behavior is likely this afternoon in the eastern zones. Showers
and isolated thunderstorms will be found across many areas this
afternoon and evening. A wetting rain will be possible for many
areas Thursday into Saturday with snow likely in the mountains,
especially Friday and Saturday.

&&

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

$$

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



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