Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 220524

Area Forecast Discussion
National Weather Service Riverton WY
1124 PM MDT Mon Oct 21 2019

.SHORT TERM...This afternoon through Tuesday

Light mountain snow and snow flurries in the western valleys will
continue this afternoon, as northwest flow aloft continues. East
of the Divide could see isolated rain, snow, and/or graupel
showers this afternoon, as there remains some weak instability in
the flow and differential heating due to the highly variable cloud
cover. This snow will decrease in coverage and become confined to
the higher elevations in the mountains this evening, as the
shortwave pulls away, and western Wyoming transitions to the anti-
cyclonic side of an upper jet. Mainly dry conditions tonight
across the rest of the area. Snow chances increase after midnight
Tuesday over northwest portions, as warm air advection and
moisture move into the area. This area of snow will spread
southward through the morning Tuesday, with snow showers
continuing through the day. Snowfall amounts in the mountains
will generally range between 2 to 4 inches, with locally higher
amounts in northwest-favored areas. High temperatures on Tuesday
should be 5 to 10 degrees warmer across the entire area. Snow
levels over the west on Tuesday should increase to between 6500
and 7500 ft. The gradient will be tight enough Tuesday for gusty
west-southwest winds, 25 to 35 mph with gusts to 45 mph, to occur
over the Wind Corridor, from southern Lincoln County to Sweetwater
and Natrona counties.

.LONG TERM...Tuesday night through Monday

Northwest flow will prevail Tuesday night with the first of two
shortwaves pushing across the region in broad cyclonic flow over
the Northern Plains. This system will usher a cold front south
from Montana Tuesday night with gusty northwest wind in its wake.
Have adjusted surface wind speeds and gusts upward across
northern portions of the Bighorn Basin and Johnson County to
account for these gusty winds. Cyclonic flow remains across the
state Wednesday with another shortwave rotating south through the
forecast area later in the day. All told, Wednesday will see at
least scattered light rain/snow showers across the lower
elevations east of the Continental Divide. These showers will
linger into Wednesday evening before diminishing as the storm
heads south. North to northwest surface wind flow behind the front
will allow for upslope precipitation in Natrona County. However,
the Wind River Basin may be somewhat spared as the flow looks like
it may not turn enough to the northeast to provide favorable
upslope flow. Nonetheless, it will be wetter and cooler Wednesday
with high temperatures anywhere from around 10 degrees to 15
degrees below normal.

Conditions improve early Thursday morning as this storm dives into
the southern Plains and an eastern Pacific ridge pushes toward the
West Coast. This will leave Wyoming in cool, north-northwest flow
Thursday with temperatures remaining well below normal. It will be
dry Thursday with little notable wind to speak of. The ridge pushes
farther onshore Friday with warmer air filtering into the state from
the west. The upper-level flow will also back to the west Friday in
response to the next upstream trough heading toward the Pacific
Northwest. As a result, Friday looks to be dry and also the warmest
day of the medium range period.

Attention then shifts to the incoming Pacific Northwest trough which
will impact the weekend weather. West to southwest wind will pick-up
Saturday morning and afternoon ahead of this trough and associated
cold front. GFS is 20-30kts stronger than the ECMWF in depicting a
130kt+ west-southwest jet across northern Wyoming by late Saturday.
This orientation may slow the southward progression of the cold
front and delay it until late in the day Saturday or into Saturday
evening. Therefore, central and southern Wyoming may continue to see
seasonal temperatures Saturday along with gusty westerly wind. The
colder air will envelop the area Saturday night and Sunday along
with a gusty north to northeast wind. The pattern appears more
favorable for upslope flow in the central basins as the surface
and 700mb low set-up shop to our south. While this system does not
seem to tap a healthy dose of Pacific moisture, there does appear
sufficient moisture for at least a few inches, perhaps 2-4
inches, of accumulating snow Sunday and Sunday night on into
Monday. The position of the jet across central Wyoming combined
with the upslope flow would seem to favor Sunday as the best
period for widespread light snow. The pattern does not look very
favorable for heavy western mountain or valley snow. Temperatures
will again drop will below seasonal normals for Sunday and Monday
with most areas only reaching the upper 20s and 30s for highs.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1118 PM MDT Mon Oct 21 2019

Mainly dry conditions for all but the northwest corner of the
state overnight. Expect all terminals to be VFR through
12Z/Tuesday with KJAC the only terminal with the potential for
light snow showers between 09Z- 12Z/Tuesday. Light snow will
increase across northwest Wyoming through the day Tuesday with
IFR/MVFR likely at KJAC after 14Z/Tuesday. All other terminals to
remain VFR with the main aviation hazard being gusty westerly
surface wind. KRKS will see a slight decrease in wind speed
overnight, but overall 20-30kt wind speeds will prevail. Tuesday
afternoon will see 15-25kt surface winds blow at KCPR, KRIW, KBPI,
and KPNA. Mountain top obscurations will continue west of the
Divide through the forecast period, becoming more widespread and
lowering during the day Tuesday.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.



Northwest flow aloft will continue with light snow being favored
over the northwest mountains, particularly zones 140, 415, and
western portions of 286. After a brief lull Monday evening, the
light snow will slowly increase early Tuesday through Tuesday
afternoon. The snow will spread farther south and into the western
valleys Tuesday afternoon and evening. This will be the leading edge
of the next weather system which will drop south across the state
Tuesday night and Wednesday. A cold front is poised to push south
through areas east of the Continental Divide Tuesday night with
gusty northwest wind in its wake. This will be most pronounced
across zones 275 and 281. Therefore, after a dry and seasonal day
east of the Divide Tuesday, expect light rain/snow showers to
spread across northern and central Wyoming Tuesday night and
Wednesday. The surface flow pattern is not ideal for upslope snow,
but scattered snow showers will persist and it will be noticeably
colder. Highs Wednesday will be 10 to 15 degrees below normal for
many areas of the High Plains and Wind/Bighorn Basin districts.
Thursday will remain unseasonably cool before a rebound in
temperatures for Friday. Both days will be mainly dry regionwide.
The next storm system looks to arrive late Saturday and it will
impact the region into Monday. Lower elevation accumulating snow
looks quite possible by Sunday.





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