Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181754

Area Forecast Discussion
National Weather Service Riverton WY
1154 AM MDT Mon Sep 18 2017

.SHORT TERM...(Today through Wednesday night)
Issued at 230 AM MDT Mon Sep 18 2017

Imagery shows a broad longwave trof across most of the CONUS with
wrn Atlantic high pressure only affecting the eastern Sea Board
(with the exception of the embedded Hurricane JOSE. Nearly zonal
flow is currently in place ovr WY with the axis of the upstream trof
extending from the BC coastline into WA and OR. SFC has weakening
high pressure over the cntrl Rockies and High Plains with low(ering)
pressure west and south of WY. A cold front is currently coming
into/through the PAC NW...headed this direction. No precipitation
currently falling anywhere in Wyoming this morning.

Today, a transition to southwest flow aloft is expected the next
rather strong upper level trof approaches from the west. With
warming at the surface and relatively cool temperatures aloft lapse
rates will will range from 8.8 - 9.0 deg C/km at 500 mb to around
10.5 deg C/km at the SFC...enough to allow strengthening winds aloft
to affect winds at the SFC. Today will see west to southwest winds
across much of the FA gusting 25 to 45 kts. Isolated showers
possible across the western/northwestern mountains late this
afternoon. Then the effects of the upstream PAC NW trof and
attending cold front will be felt tonight/overnight as the system
approaches the FA from the west...moving into/through
northwestern/western Wyoming beginning this evening. Precipitation
chances will increase rapidly with what now looks like another
period of significant EPAC precipitation. Snow levels will begin
rather high at well above 10.5k ft Monday evening...lowering to
around 6.5k ft around sun-up Tuesday. Measurable snowfall does not
really look to begin in earnest until sometime after sunrise
Tuesday. Decent amounts of precipitation look to be in place across
much of the western mountains/valleys tonight and Tuesday...however
the changeover to snow will be tricky in the nearly saturated
atmosphere and will be nearer the freezing mark as it drops from top
to bottom. With that in mind, have kept the total snowfall through
Tuesday night at or below Advisory amounts for all of the western
and central mountains except the Wind River range which looks to be
in the best position to take advantage of the left exit region of
the jet on Tuesday. Additionally winds will also gust around the FA
due to an increasing pressure gradient and frontal passage. By
Tuesday night, a trof axis moves through WY with some drying taking
place and precipitation amounts dropping off significantly until the
transition back to zonal flow allows a new batch of EPAC moisture
into WY Wednesday afternoon and evening...heralding the beginning
of a relatively long period of precipitation of one sort or another
across western WY.

.LONG TERM...(Thursday through Monday)
Issued at 230 AM MDT Mon Sep 18 2017

A longwave trough will slowly pull east across the Western U.S.
with inclement and cool conditions.

Strong frontogenesis should be ongoing Thursday morning across
northern/western Wyoming. The front is expected to push south and
east across the area Thursday night/Friday morning with the first
significant wave in the south southwest flow ahead of the mean
trough. The best chances of precipitation will be across northern
and western Wyoming northwest of the main baroclinic zone
spreading eastward with time. By Friday morning it appears the
best dynamics with the first wave begins to lift away. Models are
showing a band of significant precipitation across the northwest
Thursday/Thursday night. Snow levels begin high, but should lower
to near valley floors by 12Z Friday.

As the main trough/upper low begins to slowly pull across the
area Friday night into Saturday, models are showing some
isentropic lift as well as some upslope flow developing. This
could result in widespread precipitation forming east of the
Divide Friday night and possibly lasting off and on through
possibly Sunday. Significant mountains snowfall is possible with
snow levels dipping down to around 6500 feet Saturday, and even
possibly lower Saturday night/Sunday.


.AVIATION...For the 18Z TAFS through 18Z Tuesday afternoon
Issued at 1142 AM MDT Mon Sep 18 2017

VFR conditions will occur at most terminal sites through 12Z
Tuesday. Breezy to windy conditions will occur today at most
terminal sites from late morning through sunset this evening.
A cold front will move into NW WY around 06Z tonight and push
eastward to a KBYG-KRIW-KEVW line at 12Z Tuesday. Rain showers with
high mountain snow will begin in NW WY between 00Z and 03Z this
evening. This precipitation will spread across much of Western WY by
10Z Tue. MVFR conditions will develop over the northwest by 06Z
Tue and increase in areal coverage from 09Z to 12Z Tue in the
west. Mountains will become more obscured with time tonight in the
west. Please see terminal forecasts for more details.


Issued AT 230 AM MDT Mon Sep 18 2017

Fuels at critical levels for most of the low elevation forecast
zones in addition to many mountain zones below 8500 feet. (Please
see the Fire Weather Forecast product for more information).

Today will be somewhat warmer all locations and similar to slightly
drier than on Sunday. A few isolated showers will be possible across
the west this afternoon...increasing to numerous showers - both rain
and snow - tonight and Tuesday. Winds will increase and become quite
gusty out of the southwest today ahead of an approaching fall
storm...gusting 25 to 45 mph at times over much of the forecast
area. Minimum relative humidity today will generally range from the
upper teens across the low elevations...mid 20s to 40 percent in the
mountains. Smoke dispersion in the afternoons will tend from very
good to excellent today through Wednesday.




LONG TERM...Murrell
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