491
FXUS65 KRIW 070340
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
940 PM MDT Mon May 6 2024

.KEY MESSAGES...

- Deepening low pressure in northeast Wyoming will continue to
  generate very gusty west to northwest wind through Tuesday.

- Western mountain snow has been limited Monday. However,
  another round of precipitation is expected late tonight and
  Tuesday.

- The area of low pressure shifts back south Tuesday night and
  Wednesday, setting the stage for more widespread rain during
  the daylight hours. There is the potential for lower elevation
  light snow in upslope-favored locations late Wednesday night.

&&

.DISCUSSION...
Issued at 255 PM MDT Mon May 6 2024

Satellite imagery is showing an impressive upper-level low reforming
over northeast Wyoming early Monday afternoon. In fact, WPC
mentioned that this system may threaten monthly low pressure
records for the region. Instability has remained to our east,
while 500mb southeasterly flow fed moisture into portions of
central and northern Wyoming through midday before the upper low
wrapped up and switched the flow to the northwest. Precipitation
was heaviest in Johnson and Natrona Counties where 0.75 to 1.25
inches fell over a 6-hour period. Elsewhere, the corresponding
pressure gradient has generated widespread 40 to 50 mph gusts,
particularly in those locations favored by northwest flow and
along the I-80 corridor. The heavier snow anticipated over the
far west mountains has not materialized as the best moisture was
well to the east. As a result, much of the western snow has
been confined to mountain areas along and just east of the
Continental Divide.

The tight pressure gradient favors continued gusty west to northwest
wind through Monday evening. Gusts of 40 to 55 mph persist,
especially in the central Wind River Basin, the I-80 corridor, and
northern Johnson County. Wind speeds increase in northern Johnson
County Monday evening as the low strengthens on the Plains.
Favorable 700mb wind of 50+ knots and the anomalous nature of this
low lend credence to the potential of high wind around Buffalo
Monday evening into early Tuesday morning. Most high-resolution
guidance keeps gusts just below warning thresholds, but Buffalo
is susceptible to northwest wind. The gradient decreases enough
that speeds should drop below high wind criteria by sunrise
Tuesday. Elsewhere, speeds take a downturn beginning late Monday
evening. However, this decrease will be short-lived as mid-
level winds mix to the surface by midday Tuesday. Westerly wind
speeds will be less than those seen today, so no highlights
anticipated.

As for precipitation, the overall trend will be to shift showers to
the east tonight. Precipitation continues to pinwheel southward
across areas east of the Continental Divide as it wraps around the
developing low pressure system. The southward extent of the
precipitation shield has generated advisory snow amounts over
Casper Mountain today along with the Bighorn Range. The
precipitation shield shifts slowly north and east by mid-evening
Monday as the low tracks slowly into the western Dakotas. This
should confine rain and snow showers to mainly the Bighorns and
Johnson County by midnight Tuesday with decreasing chances
through early Tuesday morning. Current highlights for the
aforementioned mountain locales are on track and look good.
Wrap-around precipitation and a more favorable mid-level
orographic flow aids mountain snowfall in the far west mountains
late tonight and Tuesday. QPF amounts were too high Monday and
there is concern that terrain effects are again inflating QPF
totals tonight and Tuesday in the western mountains. This is
most pronounced over the Teton Range. For now, have continued
the winter storm warning for the Tetons, but storm totals have
been tempered accordingly. The best chance for any accumulating
light snow in the far west valleys would be late tonight into
Tuesday morning. As for Tuesday, the best chance for
precipitation will remain across the far west mountains and
valleys in favored west to northwest flow aloft. Instability is
sufficient to produce isolated thunderstorms during the
afternoon in the west. While thunder is less likely elsewhere,
there is still enough instability and moisture to generate
scattered showers during the afternoon.

After a short respite Tuesday evening, the pesky upper low begins to
pinwheel and push into the northern forecast area early Wednesday
morning. Shortwave energy and a favorable northerly flow provide
cooler air and increasing upslope flow across central Wyoming by
late afternoon, overnight, and even into Thursday morning. A new
round of winter highlights may be needed for the Absarokas,
Bighorns, and east slopes of the Wind River Range to account for
this potential. Additionally, a difference of a few degrees or a few
hundred feet of elevation, along with precipitation intensity could
lead to lower elevation snowfall Wednesday night. These light
accumulations would be most possible in Natrona and Fremont
Counties. In general, conditions dry from northwest to southeast
through the day Thursday into Thursday evening as the low sags
south and east. Temperatures remain unseasonably cool both
Wednesday and Thursday. The overall trend Friday through Sunday
will be toward warmer and drier conditions.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 940 PM MDT Mon May 6 2024

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals

A deepening low pressure system will continue to spin over the
Northern Plains through the TAF period. On the backside of this low
strong northwest flow will persist. Tonight, wind will be relatively
light (sustained 10-15 knots gusting 20-25 knots) compared to Monday
afternoon. Wind will increase by mid-morning across all terminals.
The strongest wind will occur at KRKS where gusts as high as 45
knots will occur Tuesday afternoon.

Tonight should remain dry as the low shifts to the north and east
through the overnight hours. Then, this low will retrograde back
toward WY Tuesday afternoon. Some breaks in cloud cover is expected
Tuesday morning/afternoon, which will give way to convective
showers. The best chance of snow/rain will be at KJAC with
prevailing MVFR conditions by 12Z. Conditions may drop to IFR
briefly if a stronger shower directly impacts the terminal. For now
have PROB30 for snow showers at KPNA/KBPI. Mountain obscuration
continues for the next 24 hours.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

A deepening low pressure system will continue to spin over the
Northern Plains through the TAF period. On the backside of this low
strong northwest flow will persist. Tonight wind will be relatively
light for KCOD/KWRL/KLND terminals. At KCPR/KRIW wind will remain
strong through the overnight hours into tomorrow. Wind will increase
by mid-morning across all terminals. The strongest wind will occur
at KRIW where gusts as high as 45 knots will occur Tuesday afternoon.

Tonight should remain dry as the low shifts to the north and east
through the overnight hours. Then, this low will retrograde back
toward WY Tuesday afternoon. Some breaks in cloud cover is expected
Tuesday morning/afternoon, which will give way to convective
showers. The best chance of rain/snow will be at KCPR Tuesday
evening. Conditions may drop to MVFR if a shower directly impacts a
terminal, but the chance is too low (20%) at this time for
prevailing rain. Mountain obscuration continues for the next 24
hours.

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

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT Tuesday for WYZ001.

Winter Weather Advisory until 11 PM MDT this evening for
WYZ008-009-022.

High Wind Warning from 11 PM Tuesday to 6 AM MDT Wednesday for
WYZ010.

Winter Storm Warning until 11 PM MDT Tuesday for WYZ012.

&&

$$

DISCUSSION...Jones
AVIATION...Rowe