Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 131705

Area Forecast Discussion
National Weather Service Riverton WY
1105 AM MDT Tue Mar 13 2018

.SHORT TERM...(Today through Thursday night)
Issued at 315 AM MDT Tue Mar 13 2018

Imagery shows shows amplified ridge/trough (west/east) pattern
across the CONUS with another large upstream trough/closed
low/occluding surface low lying offshore the PAC NW. The ridge axis
over the western CONUS currently cuts south from southwest Canada
through ID/NV and down along the CA/AZ border. The surface has high
pressure across the Northern/Central/High Plains with low
pressure/front just now coming into the PAC NW. Not much in the way
of any current precipitation is going on across the western CONUS
until you get to California.

Quiet ridging continues through tonight with nothing Wxwise going
on. Then the offshore trough tilts eastward and begins to move
inland. From late afternoon Wednesday through the end of the
forecast period Wyoming will be affected by this trough one way or
another. It`s tough to make too much yet with regard to the
placement, extent and strength of the isentropic lift, frontogenesis
(eastern/northeastern WY), and cyclogenesis (eastern CO). Lots of
question marks continue as the models (knee)jerk into to some
semblance of modest order with a few models returning huge amounts
(almost absurd) of water around the developing cyclone over eastern
CO where there just recently was hardly any. Not quite ready to bank
on that kind of heavy-hand at the moment. Betting that most of the
return upslope moisture will not make it back into this portion of
WY (central) with the rapid cyclogenesis that should be expected
from the High Plains of CO into the Central Plains from Wednesday
night through Thursday...placing some of the heavier QPF well east
of this FA. Rain, snow, showers and thunder will all be possible
Wednesday and Wednesday evening with snow levels rising to above 8k
ft Wednesday afternoon through part of the evening before falling to
valley floor levels between midnight and sunrise Thursday. At the
moment, new snow amounts west of the Divide look to remain at or
below Advisory levels for all locations (localized Advisory amounts
in the Tetons). Thunderstorms mainly west of the Divide on Wednesday
(where heating and lapse rates will have some positive affect) will
have some chance for graupel, but nothing large (just corn snow).
Ended these low topped thunder chances by early evening so not much
chance for most the low elevation areas east of the Divide...just
light showers. Light snow showers east of the Divide later Thursday
night...not amounting to much in accumulations. Again, keeping the
heavier QPF east of the forecast area.

.LONG TERM...Friday through Tuesday
Issued at 315 AM MDT Tue Mar 13 2018

Happy to report that the three long range models are in better
agreement with how this trough will evolve through Sunday night.
A piece of energy departs Northeast Colorado Friday and heads into
the plains states. Some morning rain and snow in the eastern
zones from Buffalo to Casper Friday will decrease during the
afternoon. Out west, scattered snow showers will occur Friday.
Otherwise isolated rain or snow showers for the rest of the
region. Saturday looks dry east of the continental divide while a
chance of rain and snow showers continues in west and southwest
WY. The rain and snow chances increase Saturday night and Sunday
across the area as the bulk of the trough moves into the Rocky
Mountain region. Rain and snow decreases late Sunday night as the
system moves into eastern WY or the western parts of South Dakota
and Nebraska. Hard to pinpoint areas of moderate to heavy precip
at this time as the track of the upper level system is likely to
change by the weekend. Monday will see some lingering snow showers
in the mountains with mostly dry weather at the lower elevations.
Next Tuesday looks dry with isolated snow showers in the western
mountains. High temperatures will be in the 40s to lower 50s Fri
and Saturday with 30s in the far west valleys. Cooler highs Sunday
and Monday with 30s and 40s, then a little warmer next Tuesday.


.AVIATION...18Z Issuance

Mainly SKC with light winds today. Tonight mainly after 06Z
Wednesday, mid-level cloudiness will overspread the west possibly
lowering to around 5 kft agl. Also, winds especially just above the
surface will begin to increase which could result in some marginal
LLWS developing over the west/southwest. A few light showers may
develop in the mountains late Wednesday morning but any steadier
precipitation should hold off until 18Z Wednesday.


Issued AT 315 AM MDT Tue Mar 13 2018

Seasonal to above seasonal temperatures expected across the the
region today for most areas. It will be dry all locations today
through most of Wednesday morning. Then the effects of the next
weather system begin to be felt west of the Continental Divide with
a little everything - from rain and snow to Thunderstorms with small
hail possible. Up to 6 inches of new wet snowfall will be possible
above 8000 feet...with rain/snow mixes and little accumulation below
that level. Cool and showery most locations, including areas east of
the Divide, Thursday and Thursday night. Gustiest winds most
probable across the Wind Corridor from eastern Sweetwater through
Natrona Counties both Wednesday and Thursday afternoons.





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