Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181741 AAA
AFDRIW

Area Forecast Discussion...Update
National Weather Service Riverton WY
1141 AM MDT Sat Mar 18 2017

.SHORT TERM...Saturday through Monday night

Mainly dry conditions will dominate the area with a ridge of high
pressure in control of the weather. It will also be a warm day with
many areas cracking the 70s. A few of the warmer spots may even
crack the 80 degree mark. However, we do have come concerns. First,
as a trough and front approach from the west, the pressure gradient
will tighten across the area and increase the wind in areas favored
by southwesterly flow, a.k.a. Rock Springs through Casper and the
southern Big Horn basin. With the warm temperatures, humidity will
also be quite low. With fuels critical in some areas, critical fire
conditions are likely. So, we will upgrade the Fire Weather Watches
to Red Flag Warnings. In addition, out west with snow melt
continuing there could be some flooding. We will keep the flood
watch as is.

Precipitation will spread into the west tonight. At this time,
almost all should stay away until around 9 pm with majority moving
in after midnight. There are some decent jet dynamics with this
system so there could be a heavier burst of precipitation with the
models zeroing in on the Tetons. However, moisture is rather limited
and the models have really backed off on QPF. In addition, it will
be rather warm at the onset with snow levels starting at around 9000
feet. The could lower to 7500 feet by morning but by then the best
push of moisture will move away to the east. Areas East of the
Divide will remain dry Saturday night.

At this point Sunday looks to be a somewhat cooler and cloudier day
although temperatures are expected to remain above normal. A frontal
boundary will be located across northern Wyoming as well. The models
have shifted a but further north it and also trended a bit drier. As
a result we trimmed POPS a bit and confined them to the western and
northern portions of the area. Showers will continue out west but at
this point nothing heavy is expected except for possible in
Yellowstone. We also can`t rule out some thunder but at this time I
am not plastering thunder over a whole area for what will likely be
only a few lightning strikes.Gusty southwest winds will also
continue in the favored areas.

On Sunday night and Monday a disturbance will move through the area
with a bit of right rear quadrant upper level disturbance. It is
similar to the other systems in that it is a quick mover and does
not have a ton of moisture to work with. In addition, the models are
not agreeing on it`s placement and that makes pinpointing any rain
or snow showers problematic. For now we made only minor tweaks.
Temperatures will be tricky as well since a lot depends on where
clouds and sky cover will set up. Again, we made few changes until
there is more consistency. Another disturbance will move into the
west for Monday night. As of now, most of the models favor a
northern track with NAM especially bullish with the QPF. It looks
overdone however. Again, we made few changes until the models come
into better agreement.

.LONG TERM...Tuesday through Saturday

A transitory ridge pulls across the Rockies Tuesday ahead of a
deep trough off the West Coast. This ridge will bring mainly dry
and warmer conditions to the forecast area. By Tuesday night, a
disturbance in the southwest flow ahead of the main trough should
result in at least a quick shot of precipitation over western
Wyoming. Then models show the trough splitting as it shifts east
across the Intermountain West and Rockies Wednesday through
Thursday night, but with several waves in the overall trough
making it quite chaotic. Overall, models are farther north with
the main energy, and about 12+ hours slower with cold front now
projected to slide through the area Thursday. The best chances of
precipitation appear to be late Wednesday night through Thursday
evening as the cold front, and the 700mb circulation/baroclinic
zone pulls across the area. Ahead of the front, valleys will be
rain, with the change over to snow between 7500 and 9000 feet.
Then snow levels possibly lower to valley floors Thursday into
Thursday night, even east of the Divide as colder air works into
the area. Precipitation should exit the area by Friday morning as
shortwave ridge builds in wake of this storm system. Another
trough and associated cold front could impact the area late
Saturday and Sunday.

Elevated to extreme fire weather conditions are possible Tuesday,
and more likely Wednesday in the lower elevations east of the
Divide.

Low-elevations snowmelt will continue to be an issue west of the
Divide Tuesday/Wednesday, but should slow as cooler air works into
the area Wednesday night/Thursday.

&&

.AVIATION.../18Z ISSUANCE/

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

VFR conditions will prevail through 00z Sunday.  Increasing
southwest flow aloft will result in areas of LLWS today vicinity
KJAC.  Surface south to southwest winds at 15-25 knots prevail
across the area this afternoon.

A Pacific cold front will move east across the area 03z-09z Sunday.
Scattered showers and isolated thunderstorms will develop ahead and
along the front this evening, mostly concentrated from southeast
Idaho and along and north of vicinity KAFO.  Rain and snow showers
will become more numerous with increasing mountain obscurations 06z-
12z Sunday with snow levels lowering to near 8KFT.   Most of the
activity will remain along and north of a vicinity KAFO-KPNA line.
Expect areas of LLWS ahead of and with the frontal passage through
12z Sunday.


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

VFR conditions will prevail through the period.  South to southwest
winds will increase across the area this afternoon along with
areas of LLWS.  Winds will remain gusty tonight with a
Pacific cold front moving east across the area 08z=12z Sunday.  Most
precipitation with this front will remain along and west of the
Continental Divide with mainly some passing mid and high clouds
along the front across central Wyoming.

&&

.FIRE WEATHER...

...RED FLAG WARNING FOR ZONES 280...281 AND 282...

Fire danger will increase today as a combination of gusty
southwest winds, warm temperatures and relative humidity falling
into the teens in some areas. Critical fire conditions are possible
in portions of Hot Springs, Washakie, Natrona and Johnson Counties
so have issued a Red Flag Warning for the above zones. All areas
will have a mainly dry day today. A system moving in from the west
will spread some rain and snow into the west tonight. Fire danger
should ease Sunday with somewhat cooler temperatures and higher
humidity.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Flood Watch from noon MDT today through late Sunday night for
WYZ013-023-027.

Red Flag Warning from noon today to 7 PM MDT this evening for
WYZ280>282.

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...Meunier/Lipson
FIRE WEATHER...Hattings


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