Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 131724
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1024 AM MST Mon Nov 13 2017

.SHORT TERM...Monday through Wednesday night

Dry weather will continue through Monday evening. In addition,
expect a mild day across most areas with temperatures running
above normal. The main concern today will be wind. Outer Drive is
already seeing wind gusts to around 45 mph, but this is isolated.
At this point, mid level winds do not look like they will be
strong enough for high wind highlights at this time. The main
areas of concern will be from the Red Desert through Casper. The
wind will be enough for some enhanced fire concerns however. At
this time, relative humidity looks to remain above critical levels
so no red flag warning though.

The weather will become more active tonight. Across the Absarokas
and Cody Foothills, 700 millibar winds will increase to 50 to as
high as 65 knots. With a tightening pressure gradient and from
downward forcing from the right front quadrant of a jet streak
moving into Montana, high winds look to be a good possibility,
especially around Clark and possibly along the Chief Joseph Highway.
For now, we will continue the High Wind Watch. These winds should
begin to decrease on Tuesday morning as cold advection begins. In
addition, the front will spread some snow into the western
mountains. The models continue to trend weaker with the system,
certainly under advisory level amounts.

On Tuesday, some mainly light snow will continue out west. The
concern then shifts to the possibility of high winds across the wind
corridor from the Red Desert through Natrona County. At this time,
700 millibar winds are borderline. In addition, the strongest will
be at night when mixing will be the least. MOS guidance is also
below warning levels. So for now, we will forgo any watches for this
area. There could be some warning level gusts at places like Camp
Creek or Fales Rock, but since population there is almost non
existent, impacts would be minimal. It will be a blustery day
though. Any fire concerns should ease a bit with higher relative
humidity. Any snow out west should end by evening as the system
moves away to the east.

High pressure will then bring dry weather for around 24 hours from
Tuesday night through Wednesday. But with the fast moving
progressive pattern continuing, another system will move into the
west on Wednesday night. It will move than likely be later at
night since the models tend to rush systems in too quickly this
far out. At this time, this one looks to have a bit more moisture
and upper level support to work with. However, this has been the
situation with the past couple of systems and the trend has been
weaker each time. We will have to wait and see. Meanwhile, expect
dry weather to continue East of the Divide with above normal
temperatures and a gusty breeze continuing in the favored
locations like the Lee of the Absarokas and from Rock Springs
through Casper. There is also the chance of another high wind
event in the Lee of the Absarokas Wednesday night, but still time
to watch this.

.LONG TERM...Thursday through Monday

Overview...A strong Pacific cold front will move across Wyoming
Thursday night.  A mild, strengthening southwest flow will precede
the front with the potential for high winds along the east slopes of
the Divide, and from South Pass to Casper from Wednesday night into
Thursday.  Precipitation will increase across the west through the
day Thursday with snow levels around 7000 feet.  The frontal passage
could bring a period of moderate to locally heavy snow, along with
strong wind Thursday night.  Gusty west-northwest wind and rain/snow
showers will accompany the front east of the Divide Thursday night-
Friday morning with some light snow accumulations possible Friday
morning.  A ridge of high pressure will build in over the weekend
bringing mainly dry weather with a gradual warming trend,
unseasonably mild temperatures expected Sunday and Monday.

Discussion...Our main weather maker Thursday and Friday is an upper
low pressure system sliding south across the Gulf of Alaska early
this morning.  This system will `reload` the persistent trough along
the NW coast by Tuesday making up the eastern flank of a Rex Block
centered over the central Pacific.  The main forecast problem has
been the differences in model timing (GFS fast-ECMWF slow) with
bringing this system and its cold front across the area in the
Thursday-Friday timeframe.  The model trends over the last couple of
days has been to slow the system down, and with overall upper air
pattern remaining unchanged, the slower solutions have been
verifying as of late.  The ECMWF operational is in good agreement
with its ensemble mean this morning, so confidence in the slower
solution is increasing.

The ECMWF keeps most of the area in the warm sector through Thursday
afternoon ahead of the cold front, with the front quickly blasting
across the area Thursday night.  This scenario would not bring more
widespread precipitation into the west until Thursday afternoon,
with impacts mainly at pass level, rain or a rain/snow mix in the
valleys.  More widespread ascent and cooling aloft will occur
Thursday evening when the heaviest bands of snow will move across
the west, with this event looking like winter weather advisory
material (6"-12" mtns, 3"-6" western valleys) over a 12-18 hour
period.  Similar to the last few systems, some light snow
accumulations will likely accompany/follow the frontal passage
across the Big Horn Basin to Johnson County Friday morning, as well
as along the I-80 corridor.  Snow will taper off, become more
showery, west to east Friday as H5 -32C cold pool translates the
area. The overall picture from the ensemble means and GFS 5-wave for
the weekend into next week is a strengthening ridge over the western
U.S. as Rex Block pattern over the Pacific retrogrades slightly, and
trough amplifies near 140W.  This pattern looks to persist into the
Thanksgiving weekend, with the 8-14 day (Nov 20-26) outlook showing
good chances of above normal temperatures.  Pacific moisture moving
over the top of the ridge will bring some chances of light
precipitation to the west beginning early next week, mainly dry
conditions expected across central areas.

&&

.AVIATION...18Z/Monday Issuance

Conditions will be VFR at all terminals except KJAC through 18Z/Tue.
Northern stream shortwave will bring light snow to northwest Wyoming
late tonight with the best chance of MVFR at KJAC coming between 09Z
and 18Z/Tue. For now, have trended forecast in that direction with
VCSH and a period of prevailing -SN. Ahead of this shortwave, wind
will increase along the east slopes of the Absaroka and Wind River
ranges. LLWS a good bet in the vicinity of KCOD and KLND. Surface
southwest wind 15-25kts will also increase at KCPR early Tuesday
morning with furthering strengthening through the morning. The
shortwave will drag a cold front across the forecast area during the
later morning and early afternoon with gusty west-northwest wind in
its wake. This will favor higher speeds at favored sites like KBPI,
KPNA, and KRIW, and KRKS just prior to or around 18Z/Tue.

&&

.FIRE WEATHER...

...ELEVATED FIRE CONDITIONS THIS AFTERNOON THE RED DESERT THROUGH
NATRONA COUNTY...

Today will be another warm and dry day across western and central
Wyoming. A gusty wind will develop in the lee of the Absarokas and
Cody Foothills as well as across the southwestern wind corridor from
the Red Desert through Natrona County. Relative humidity may fall
into the upper teens this afternoon at times. Although critical fire
conditions are not expected, elevated fire weather is possible this
afternoon. Some light snow will spread into the west late tonight.
High wind be possible in the Lee of the Absarokas tonight.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from this afternoon through late tonight for
WYZ002-003.

&&

$$

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



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