Area Forecast Discussion
Issued by NWS Riverton, WY

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875
FXUS65 KRIW 280848
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
248 AM MDT Tue Mar 28 2017

.SHORT TERM...Tuesday through Thursday night

Quite a difficult forecast for today as the systems is throwing some
curveballs. Precipitation is spreading across the area, especially
in the Wind River Basin in association with a jet streak moving
north across the area. Satellite imagery shows another surge of
colder cloud tops moving across border from Colorado. The models
show the area fairly well but look to be overdoing the QPF a bit.

The mashing of teeth is in regards to the precipitation type. As of
now, snow levels look to be running near 6500 feet. The webcam
around Beaver Rim is showing snow for example. In most lower
elevations, dew points remain rather high with wet bulb
temperatures above zero. There will be some further cooling
through this morning so there could be some mixing with snow or
even a changeover at times but with warm ground temperatures most
accumulation would be limited to non paved surfaces. And with the
high late March sun angle, snow would have a tough time sticking.
We gave serious thought to cancelling Winter Weather Advisories
except for the Big Horns but we let them ride at issuance. The
remainder of the day will be rather cloudy in the morning.
However, there could be some afternoon clearing the further west
you go. Most precipitation should end by dark. All in all, any
amounts in the lower elevations will likely end up low and mainly
on grassy surfaces. Another story will be the wind as the pressure
gradient tightens. Areas prone to gusty north winds like the Big
Horn Basin, Johnson County and even portions of Fremont County
will see gusty winds that will make it feel rather chilly.

A transitory ridge of high pressure will build across the area for
late tonight through Wednesday night and bring mainly dry conditions
and seasonal temperatures with light to moderate winds. All in all,
this looks like the nicest of the next three days. This means
seasonal to somewhat above normal temperatures, light to moderate
winds and a partly to mostly sunny sky.

The next shortwave then moves into the west later Wednesday night.
As a expected, the models have trended a bit slower so we cut POPS
on Wednesday night as the bulk of the precipitation looks to hold
off until Thursday morning. At this point, it looks to bring light
to moderate precipitation to areas West of the Divide. With
downsloping flow, areas east of the Divide will have a mainly dry
day, although a bit breezier. The fun may start later Thursday
night though, as an upper level low moves out of the Great Basin
and turns the flow upslope across many areas. This could bring the
s word back into the forecast for areas, although the worst would
be would be on Friday. My esteemed colleague will have more on
that.

.LONG TERM...Friday through Tuesday

The GFS and ECMWF models are still robust with liquid precipitation
in parts of central and southern WY Friday into Friday evening.
Then they hang on to lighter precip in the east Saturday morning,
then drying Saturday afternoon. Forecast 700MB temps would support
a change to snow in the Wind River Basin and long the Cody Foothills
Friday morning and then into Friday evening. Casper looks to be
rain at this point while Sweetwater county may be a mix of rain
and snow. The far west and parts of the Big Horn Basin will see
lesser precip from this storm Friday. Will see how the models
handle this in the coming days. Saturday will be dry for most
places except for the morning precip in the east zones. Model
differences come into play Sunday through Tuesday. The GFS model
is dry Sunday and Monday with a few showers making it into NW WY
by 00Z Tue. Then the GFS brings a weather system across the region
Monday night and Tuesday. Meanwhile, the ECMWF model has a weather
system moving through the area Sunday into Monday with a weaker
system on Tuesday. This would lead to a chance of precip all three
days. Each model has been consistent in their last few runs, so
deciding which way to lean is difficult that far out. For now will
go with a mostly dry day Sunday with a few showers in NW WY. Then
isolated to scattered rain and snow Monday through Tuesday. As for
temps, cool day Friday, then warmer Sat and Sun, then slightly
cooler Mon and Tue.

&&

.AVIATION...12Z ISSUANCE

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

Moisture in northeast Utah and northwest Wyoming will have
difficulty making its way too far north this morning Trends and
latest models forecasts continue to favor precipitation decreasing
after about 15Z/Tue. Moisture is likely to remain south and east of
this region as circulation with Four Corners low begins to dominate.
IFR or lower ceilings may persist at KRKS until 15Z/Tue, but don`t
expect a prolonged period of reduced visibility and precipitation.
After 15Z/Tue, conditions should gradually improve to low end VFR by
20Z/Tue with VCSH. Northerly wind will respond to tightening
pressure gradient with 20-30kt winds for a good portion of the day
at KRKS. Conditions will be generally VFR at KBPI, KPNA, and KJAC,
with any chance of rain or snow coming prior to 14Z/Tue. Confidence
is not high and expect VFR at all three terminals through the
period. Gusty northwest wind will be favored at KBPI and KPNA from
mid-morning through about 00Z/Wed. Speeds of 12-25kts will be common
at both terminals. Any mountain obscurations will diminish from
northwest-to-southeast during the late morning and afternoon. There
could be some lingering mountain top obscurations until 00Z/Wed,
primarily in the vicinity or KRKS and along the Continental Divide.

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

Lack of upslope flow and diminishing precipitation moving north from
Colorado and Utah may put a damper on what was supposed to be a damp
forecast. Late evening and early morning mid-level moisture has
produced little more than some light showers and sprinkles and a mid-
cloud deck. Best chance for precipitation and MVFR conditions, or
perhaps briefly IFR, looks to be from about 10Z-16Z/Tue. There is an
increasing possibility that KCOD and KWRL will stay mainly VFR once
this northward surge of moisture clears northern Wyoming late
Tuesday morning. Dynamics and moisture will begin to move to the
eastern half of the state after 18Z/Tue with gradually improving
conditions from west-to-east. This will leave KCPR with the best
chance of precipitation, mainly rain, during the day Tuesday. Light
rain looks most likely between 12Z-22Z/Tue, with mainly MVFR
conditions. Favored northerly wind at KCOD, KWRL, and KRIW will blow
12-25kts before diminishing around 02Z/Wed. Mountain obscurations
will be widespread until about 18Z/Tue, with mountain top
obscurations persisting along and east of the Bighorn Range through
02Z/Wed. KCPR will be the most likely location for the development
of fog late Tuesday night. VCFG would also be possible if cloud
cover can hang around until close to 00Z/Wed at KRIW and KLND. This
would keep boundary layer more moist and with clearing skies there
could be fog. Limiting factor will be downsloping wind which may
provide enough drying of the lower atmosphere to limit fog
formation.

&&

.FIRE WEATHER...

A front and upper level disturbance will move across the today and
bring some rain and snow to much of the area, with the steadiest
precipitation in the morning hours. A gusty north wind will
develop across the Bighorn Basin, Johnson County and portions of
Fremont County. Mixing and smoke dispersal will range from poor in
the west to fair to good East of the Divide. Mainly dry weather
will return late tonight through Wednesday before the next system
moves into the west later Wednesday night and Thursday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory until 6 PM MDT this evening for Wyz008-
009-019-028>030.

&&

$$

SHORT TERM...Hattings
LONG TERM...AR
AVIATION...Jones
FIRE WEATHER...Hattings



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