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 30 31 32 33 34 35 36 37 38 39 40 41
000
FXUS65 KRIW 220421
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1021 PM MDT Thu Mar 21 2024

.KEY MESSAGES...

- A weather system moving through southern MT today is bringing
  isolated to scattered showers to northern WY this afternoon,
  with a small chance of thunderstorms as well. A couple inches
  of snow is likely in the northwest WY mountains by tomorrow
  morning.

- Gusty winds and low humidity across central WY has created
  elevated fire weather conditions this afternoon. A Rangeland
  Fire Danger statement is in effect through 7 PM.

- Some rain/snow will impact southwestern and western WY Friday
  night into Saturday morning, with heavier precipitation moving
  into western WY Saturday afternoon and evening. East of the
  Divide will be mild with breezy southwest wind.

- Much colder air with gusty north winds and widespread
  precipitation will occur on Sunday as a strong storm system
  moves through the area. Unsettled conditions will continue for
  the following 3-4 days at least.

&&

.DISCUSSION...
Issued at 101 PM MDT Thu Mar 21 2024

With the ridge flattening across the central and southern Rockies, a
shortwave trough is moving through central and southern MT
today, bringing moister and stronger zonal flow to western and
central WY. Light showers over northwest WY this morning should
increase this afternoon, especially over the northwest WY
mountains and the northern half of the Bighorns. There is a
lower chance of showers for the lower elevations, though high
resolution models are indicating a 15-25 percent chance in the
Bighorn Basin and Johnson County through sunset. A weak cold
front has pushed its way into northern WY this morning, and it
will move southward through the afternoon through the Bighorn
Basin and Johnson County. SPC has about half of the CWA in the
Day 1 convective outlook due to the instability from the wave.
So, there is about a 10-20 percent chance of thunderstorms
across northern into central WY this afternoon and early
evening. A weak area of showers over northern UT will also be
moving eastward this afternoon to bring a low chance of showers
to the southern part of Lincoln County into Sweetwater County.
Parts of the higher terrain of northwest WY are likely to see
1-3 inches of snow accumulate by Friday morning.

The shortwave trough across MT is also serving to tighten up the mid-
level and surface pressure gradient, which is expected to increase
generally west winds this afternoon across central and southern WY.
Most areas should see winds around 15 mph with gusts 20-25 mph, and
higher speeds in the typically favored locations.  Giving the
stronger winds and low humidities below 20 percent anticipated
across central WY this afternoon, elevated fire conditions are noted
through sunset. At this time, the cold front from southern MT
and northern WY will have pushed southward to bring higher
humidities, though gusty north winds and cooler air will
alleviate the dry afternoon conditions. Showers are expected to
linger from northwest WY into central WY through midnight
before diminishing.

A weak ridge then builds in over the Central Rockies Friday
morning, though low-level moisture and northeast winds will keep
a low chance of showers along the northeast part of the Bighorn
mountains. Models are showing a weak shortwave trough moving
from southwest to northeast from central UT into central WY
late Friday. Ahead of the trough, light showers are possible
over the northwest WY mountains; otherwise, Friday appears to be
fairly seasonable. Moisture and instability increases over
southwest WY Friday evening for a 40-60 percent chance of
showers. Isolated showers then continue over the western WY
mountains Saturday morning and afternoon. The deeper part of the
West Coast trough then moves eastward late Saturday into
Sunday, increasing moisture and bringing in colder air. The
western WY mountains should see quite a bit of precipitation
Saturday night as the trough axis moves eastward by Sunday
morning. A surface low is expected to develop over SE WY early
Sunday, which brings down north winds and colder air from
Montana, thus concentrating precipitation (snow) over the
northern half of the state during the morning. As the trough
moves through the state on Sunday, precipitation then spread to
much of the rest of the forecast area along and east of the
divide, significantly cooling temperatures that had previously
been seen. What had been 40s and 50s with even some low 60s
Saturday afternoon will fall to the 30s to mid 40s on Sunday.
This system appears to have a moderate impact on the forecast
area, so stay tuned for more details as we get closer.

A relatively active pattern is expected to continue from Monday
through Thursday next week. Monday will remain cool with
lingering showers. Tuesday again remains cool with a weak
shortwave trough moving through the state to bring widespread
light precipitation. Wednesday should see the flow shift to a
more southwest direction and precipitation increase over
western WY, resulting in stronger winds over southern and
central WY. Some warming should occur as weak ridging bringing
in warmer mid-level temperatures. A strong trough then moves
west to east across the forecast area from Wednesday night into
Thursday, potentially bringing quite a bit of rain/snow to the
entire state along with colder temperatures and gusty north
winds. The GFS is more progressive with this system as the ECMWF
is delaying the movement by 12-18 hours. Thus, the typical
Spring Break week next week looks to truly be a Wyoming Spring!

&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1020 PM MDT Thu Mar 21 2024

Showers will continue to end across central portions to start
the TAF period, while weak showers will continue to move over
Sheridan/Johnson counties. These showers will end by 16Z.
IFR/MVFR conditions will occur at KCOD after 08Z, as low clouds
settle in. There is a chance that these clouds drop to
VLIFR/LIFR conditions. VFR conditions return by 17Z. Winds will
be lighter for all terminals through the forecast period.

Shower chances return from the southwest after 00Z, moving
toward the northeast. VCSH will be possible for several
terminals through the rest of the TAF period.

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...
None.

&&

$$

DISCUSSION...McDonald
AVIATION...LaVoie


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.