Area Forecast Discussion
Issued by NWS Riverton, WY

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
FXUS65 KRIW 192019

Area Forecast Discussion
National Weather Service Riverton WY
219 PM MDT MON SEP 19 2016

.SHORT TERM...Tonight and Tuesday

The main forecast challenge in the short term will be fire
weather concerns. Through early afternoon, red flag criteria is
beginning to occur across the south. Thus the red flag warning
already in place looks good with no changes needed. A drier air
mass will push across much of the forecast area late tonight and
Tuesday as the flow aloft backs from the west to southwest. At the
same time, winds aloft begin to weaken with models showing the
weakest surface winds over the far south with the strongest
surface winds over the northwest. Surface gradient remains weak,
but steep lapse rates from surface to 500MB will allow for mixing
of stronger winds aloft down by early afternoon. For the most part
winds look to be marginal /Red Flag speaking/ where RH is a slam
dunk, and where winds are the strongest, RH should not reach
criteria. Have elected to have a fire weather watch where winds
are marginal, but RH will be low to very Low with a Haines of 6 is
expected which includes fire weather zones 280 and 289. Other
areas to keep an eye will be over the southwest, but winds look to
marginal. Overnight shift can take another look before deciding.

Otherwise isolated showers will be possible across the northwest
and north this evening associated with a weak disturbance in the
westerly flow combining with just enough moisture and lift from
the southern periphery of a right entrance dynamics with a jet
steak across the MT/Canadian border. Instability and forcing is
lacking for nothing more than a strike or two of lightning in a
few areas. Dry and warmer conditions are expected Tuesday with the
only precip chances over northwest half of Yellowstone, and that
might be too optimistic for precip chances.

.LONG TERM...Tuesday night through Monday

WSW mid level flow Tuesday night ahead of large shortwave trough
over the Pacific Northwest will continue breezy warm conditions over
WY into Wednesday morning.  Tuesday night may see some light showers
in NW portion of YNP due to weak instability, but expecting bulk of
the showers to be in SW Montana.  As the large trough digs into
Oregon on Wednesday, flow over WY expected to be more SSW.  However,
with the surface low over the central Rockies into the eastern Great
basin, the surface pressure gradient will bring NNE surface flow
into northern WY, with SSW surface flow over southern WY, creating
a boundary across central WY. Cloudiness will increase considerably
Wednesday, helping to decrease the max temps as 700mb temps also
decrease by about 4C. Moisture drawn up from the Great Basin will
heavily impact western WY from Wednesday through Friday.  Showers
with isolated thunderstorms expected to develop over the western
mountains Wednesday afternoon and continue into the evening hours.
Moist NE flow in NE WY should also allow isolated showers over
Johnson County. Thursday morning should see the 500mb low center
move into northern Ca and NW NV.  A weak shortwave trough in the SW
flow over Utah is forecast to move into SW WY before 18Z Thursday,
enhancing precipitation over the lower elevations of SW WY. Slightly
drier SSW surface flow to also increase from south central into
central WY, keeping precipitation over western and northern WY.
Precipitation amounts across western Wyoming likely to range from
0.25" to over 1.0" in 36 hours.

As the 500mb low center moves to northern Utah Friday morning, warm
and moist mid level air from central Rockies begins to be drawn up
and around the low center.  The forecast position of the low center
is in good agreement by three global models - GFS, ECMWF, and CMC.
Precipitation should become much more widespread across the area
Thursday evening into Friday morning.  Surface flow across northern
WY should remain from the north, while winds over southern WY likely
to be from the S to SW, depending on the track of the low.  Coldest
air appears to move over the area Friday mid day into Saturday
morning, bringing the snow levels down to 8000 to 8500 feet in the
western mountains, including the Wind River mountains.  Scattered
precipitation looks to continue through Saturday mid day with the
low center continuing to move to the NE into the northern plains.

The medium range models differ a bit with the pattern from Sunday
into the first part of the week.  Current GFS solution appears to be
most plausible at this point, with the bulk of the energy moving NE,
yet developing a weak closed low over the desert SW.  Both the ECMWF
and CMC global appear to keep most of the energy in the desert SW
low.  At this point, the differences don`t affect the forecast over
the area much though.  Sunday and Monday should then see clearing
skies with warmer seasonal temperatures as weak high pressure builds
over the area from the northwest.



The main aviation concern will be very isolated -shra and even one
or two thunderstorms through early evening over the northwest and
north. KJAC and KCOD could see those showers, but will decide at
issuance if to include them in the 00Z TAF. Main hazard will be
gusty winds. Except for brief MVFR with showers, VFR conditions
will prevail through the TAF period. Breezy to windy west to
southwest wind in most areas decreasing around sunset. On Tuesday,
isolated shower activity will be confined to the extreme
northwest. Breezy south to southwest wind will be observed over
the northwest including KJAC with weaker winds over the far south.



Red flag warning continues through the evening for the southern
fire weather zones for low RH and windy southwest flow. No changes
to current red flag warning for today. On Tuesday elevated to
extreme fire weather conditions will occur across much of the
area. Winds will be weaker in most areas than what occurred Monday
afternoon, but RH will be lower. Have issued a fire weather watch
Tuesday afternoon and evening for zones 280 and 289 where
winds...RH and a Haines of 6 could combine for extreme fire
weather behavior. Could see red flag conditions across the Rawlins
Dispatch zones as well as the Wind River Basin Tuesday, but winds
and RH look even more marginal and not for long enough time to
warrant a fire weather watch.

Decreasing fire weather conditions Wednesday from a combination of moisture
being pulled northward, and a weak mainly dry cold front pushing across
the area. The best chance of seeing any precipitation Wednesday will
be across the west. A significant storm system will impact Wyoming
Thursday and Friday with a good chance of widespread precipitation
with low to very low fire weather concerns.


.RIW Watches/Warnings/Advisories...
Red Flag Warning until 8 PM MDT this evening for WYZ277-279-289.

Fire Weather Watch from Tuesday afternoon through Tuesday
evening for WYZ280-289.



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