Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 160900

Area Forecast Discussion
National Weather Service Riverton WY
300 AM MDT Wed Aug 16 2017

.SHORT TERM...Wednesday through Friday night

We look to be entering a drier period as we head into the latter
part of the week. A couple of leftover showers are ending with most
of the morning excepted to be dry. A bit of moisture is still moving
through the flow and it could be just enough to bring isolated
thunderstorms to the northern mountains and adjacent foothills.
However, the emphasis is on the word ISOLATED as a majority of the
area will see nothing. In addition, there will be more sunshine
today with temperatures 5 to 10 degrees warmer than yesterday.

A warming trend will continue through the end of the workweek as
well. One small concern is a bit of energy dropping in from the west
that may brush by northern Johnson County and may bring an isolated
storm or two. The models are split of this solution however so we
kept things dry for now. Friday looks dry and warm for all areas
with a good deal of sunshine. Friday looks quite warm in many areas
with some readings in the 90s possibly returning to the normally
warmer locations such as Greybull and Worland.

.LONG TERM...Saturday through Wednesday

Overview...For Monday, Eclipse Day, a weak cold front is expected
to push into northern Wyoming in the morning with some high clouds
along and south of the front.  Slight instability over the western
mountains may result in cumulus buildups by late morning, and isolated
to widely scattered showers and thunderstorms in the afternoon.

For the weekend, most areas will be dry with near normal
temperatures, there will be slight chances of showers and
thunderstorms across the far south and northwest.  On Tuesday and
Wednesday, there will be slight chances of showers and thunderstorms
across the south and west with temperatures remaining a few degrees
from seasonal normals.

Discussion...Upper air pattern on Saturday morning features
shortwave trough moving across central Canada with a dry westerly
flow to the south across the northern Rockies.  The shortwave trough
will push a weak Canadian cold front south into north and central
Wyoming during the morning.  Monsoonal flow will be suppressed
mainly south of the area, with far southern Wyoming expected to be
on the northern fringe of the plume.  On Sunday and Monday, both the
GFS and ECMWF build up a ridge across the NW U.S. and western Canada
as a low drops south along the west coast underneath the ridge.  A
clipper system is shown diving SE into the northern High Plains
Monday morning with its attendant cold front dropping south across
Wyoming, east of the Continental Divide.  GFS and ECMWF both show
an area >70 percent RH (potential for high clouds) in the upper
layers of the atmosphere along and ahead of the cold front, but
there remains little agreement on the orientation, or run-to-run
consistency for that matter. The best guess at this point for
eclipse time, scattered to broken high clouds possible along the
south of the path of totality, slight instability along and west
of the Divide will lead to some cumulus buildups over the
mountains by 11 am. The good news is that widespread
precipitation or opaque cloud cover from Jackson to Casper looks
highly unlikely at this point.

After the big event Monday, the ridge is expected to build over the
Rockies Tuesday and Wednesday with possibly some monsoonal moisture
advecting northward into the area from the Great Basin and/or an
upper low along the southern CA coast.


.AVIATION...12Z Issuance

Conditions to be VFR through Wednesday night. Overall trend will be
for less convection and drier conditions Wednesday. Some isolated
convection between 21Z/Wed and 02Z/Thu, mainly over the higher
terrain of northwest Wyoming. Moisture drifting east from Idaho
combined with moderate instability will assist convection, while weak
shear will likely limit intensity and ability to sustain over the
basins. Have left VCTS in the KCOD terminal given its proximity
to the Absaroka Range. Convection will wane with the setting sun
and diminish by 03Z/Thu. Afternoon and early evening west-
northwest breezes in favored basin locations, but all terminals
should generally be less than 10kts through the forecast period.


Fire conditions should remain below critical today. High pressure
will build over the area today and bring a warmer and mainly dry
day. Any thunderstorms development would likely be restricted to the
northern mountains and adjacent foothills and would be very isolated
in nature. Relative humidity will fall to around 20 percent in some
areas. However, wind should remain light to moderate except in the
vicinity of any shower or thunderstorm. Mixing and smoke dispersal
will range from fair in some of the basins to very good in the
mountains. Somewhat breezier conditions may return to the west and
south on Thursday.



SHORT TERM...Hattings
LONG TERM...Meunier
FIRE WEATHER...Hattings is the U.S. government's official web portal to all federal, state and local government web resources and services.