Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 191716
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1016 AM MST Mon Nov 19 2018

.SHORT TERM...Monday through Wednesday night

Mainly dry weather looks to continue through the period. The few
(and I do mean FEW) flurries in far northeastern areas should end by
morning. A weak Alberta Clipper dropping into the plains is bringing
some cold advection as well as some mid level clouds that should
keep a lot of fog from forming this morning. There could be some
local ground fog near unfrozen water sources, but that should be the
extent of it. Otherwise, expect a dry and cool day today. There will
be less wind than yesterday as well. Inversions will likely still be
present across some of the basins. However, with more sunshine
temperatures should be warmer than yesterday.

There is concern that the fog monster may crawl out of his cave
again on Monday night though. Warm advection will return aloft with
light winds across many areas will lead to a better chance of late
night fog. For now, we have put it mainly in the most prone areas.
Future shifts may want to increase coverage though if trends hold.
Across the southwestern wind corridor, a southwesterly breeze should
develop, and this should be enough to put a kabash on any fog
development.

With the warm advection aloft, inversions will likely set in again
across the basins and valleys. As a result, we once again kept
temperatures below guidance. By Wednesday, a Pacific system will
moving toward Wyoming. This will tighten the pressure gradient
across the area. And in the areas prone to southwesterly flow, AKA
Rock Springs through Casper, Wednesday looks like a rather windy
day. This will also warm temperatures, with highs in the 50s a
definite possibility in these locations. There is still some
question as to whether or not the inversions will break in the
basins and valleys though. For now, we played with middle ground. As
for system moving in from west, as expected the models have
continued to slow the progress of the system. As a result, we have
removed the POPs from the forecast for later Wednesday night. So,
people trying to reach loved ones houses to enjoy turkey and pumpkin
pie through western and central Wyoming should have a decent travel
day and night.

.LONG TERM...Thursday through Monday

Thanksgiving Day will see snow in the western mountains. A mix of
rain or snow may occur in the far west valleys Thursday morning
but should turn to snow by late morning as the colder air arrives.
Dry weather will occur over the rest of the region. Breezy to
windy in the normal wind corridors east of the divide ahead of
the cold front that moves through Thursday evening. Snow will
continue in the west on Friday as moisture will increase ahead of
the next weather system. Dry weather forecasted east of the
divide on Friday.

The GFS and ECMWF models are differing on the Saturday weather
system. The ECMWF model is stronger with more snow over the area
than the GFS, which has gone weaker on the 00Z run this night.
Have not backed away from previous forecast and will keep a chance
of snow over the region and may need to increase the chance with
time if the stronger model solution occurs. This system could
hamper holiday travel across the state as well.

Models differ on Sunday as well as the GFS model has another
weather system diving into the state from the NW. The ECMWF model
is dry and not showing that system at all. Have decided to keep
low snow chances in the western mountains and valleys with dry
weather elsewhere. Will see how things look tonight. Next Monday
has model differences too as the ECMWF is dry and the GFS has
lingering snow showers mainly in the high country and in the
eastern zones. Have gone with the isolated snow showers solution
and will see what the models do in the next runs and adjust
forecast as needed.

&&

.AVIATION...18Z Issuance

Mid level clouds continue over the northwest mountains as well as
from the Bighorn Basin to Natrona County. Clearing skies continue
over southern MT, leading to mainly SKC conditions this
afternoon.

Clear skies and warmer air at mountain top level will setup
conditions favorable for fog development Monday night vicinity KBPI,
KJAC, KPNA, KLND, KRIW and KWRL.


&&

.FIRE WEATHER...

Dry weather is expected for the next few days. Expect less wind than
yesterday. Inversions will be a bit weaker than yesterday. Any
patchy fog should burn off fairly early. Mixing and smoke dispersal
will be generally fair to poor. Relative humidity will remain above
critical levels. Inversions may strengthen tomorrow in the basins
and valleys. The next chance of snow will likely move into the west
on Thursday.

&&

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

$$

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


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