604
FXUS65 KRIW 261939
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
139 PM MDT Sat Apr 26 2025

.KEY MESSAGES...

- Above normal temperatures this afternoon and Sunday with
  breezy to gusty winds expected with elevated fire weather
  conditions in southern and eastern areas.

- Isolated showers with a few embedded rumbles of thunder
  cannot be ruled out (up to 30% chance) across northwestern
  Wyoming this afternoon.

- A weather system brings precipitation chances (60-90%), rain
  showers and higher elevation snow, to western and northern
  Wyoming Sunday and continues into Monday.

- Precipitation chances (50-80%) again across much of the area
  midweek.

&&

.UPDATE...
Issued at 138 PM MDT Sat Apr 26 2025

Water vapor satellite imagery shows the upper-level low that will
influence our sensible weather over the next 48 hours, over the
central California coast. This afternoon, the upper-low`s associated
trough is keeping strong south to southwest unsaturated flow into
the Intermountain West. With mostly sunny skies, the atmosphere will
continue to decouple into the afternoon bringing warm temperatures
and breezy gusty winds. 700mb temperatures this afternoon are
forecast to be between 5-6C (80-95% chance) for Fremont, Natrona,
and Sweetwater Counties. These 700mb temperatures translate to
afternoon highs around 70 degrees. These above normal temperatures
combined with drier air and gusty winds between 20-30mph will
continue to create elevated fire weather conditions across southern
and eastern portions of the CWA.

A weak shortwave embedded within the southerly flow will traverse
western Wyoming this afternoon and provide support, in combination
with topographic lift, for isolated showers (up to 30% chance). An
embedded rumble of thunder cannot be ruled out with steep mid-level
lapse rates.

The forecast remains on track with the upper-level low trekking into
the Intermountain West on Sunday. The eastern and southern half of
the CWA is expected to be under an associated south-north oriented
jet streak Sunday afternoon with strong low-level flow. This will
support gusty winds and low relative humidity values Sunday
afternoon and create another day of elevated fire weather conditions
in the aforementioned locations. Western Wyoming should begin seeing
showers, high elevation show and low elevation rain, Sunday
afternoon. As the low pushes across northern Wyoming Sunday night
through Monday, coverage of showers expands across northern Wyoming.
Timing of the bulk of the precipitation across the north has shifted
a bit later to begin around around sunrise Monday morning. The
current deterministic forecast has 6-10 inches of snow Sunday night
through Monday night across the Absarokas and northern Bighorns with
very locally higher amounts in the highest elevations. However,
because of the timing of the anticipated snow, no highlights have
been issued at this time.

&&

.DISCUSSION...
Issued at 223 AM MDT Sat Apr 26 2025

We are now in the last five days of April, and we continue to ride
the rollercoaster of the transition time between the departing
winter and coming summer, a.k.a. Spring. And we will have a rather
large variety of weather through the next several days, some you
would see in summer and some you would see in winter. Flow will turn
to the southwest today as shortwave ridging transits across the
state. And this, combined with sunshine returning for most and a
gusty southwest breeze developing for many after the trough axis
passes, will bring a much warmer day, especially east of the
Divide. Above normal temperatures will return to much of the
area, with many locations being 20 to 25 degrees warmer than
yesterday. Most people will like this part. However, there is a
down side. This will be accompanied by low humidity and gusty
winds for many locations. The result will be elevated fire
weather in the windier areas, mainly from Rock Springs to
Casper. Fire weather highlights are not needed though since
fuels are in green up right now. And finally, there will be a
bit of a weak surface southeasterly upslope flow along the
Continental Divide and this may be enough for isolated afternoon
showers or thunderstorms, but the chance is at most 1 in 6 at
any location. Any shower could have a strong wind gust though,
given the large dew point depressions.

Sunday will be a transition day across the area as an upper level
low approaches from the west, most likely passing across southern
Wyoming Sunday night into Monday. This day should have a dry morning
but showers and isolated thunderstorms will increase through
the afternoon, with chances mainly across western and far
northern Wyoming. As for precipitation type, 700 millibar
temperatures should remain fairly warm through the day, only
dropping to around minus 2C by the end of the day, so the lower
elevations should stay all rain. Areas in the south and east
will have another warm and dry day. And with humidity remaining
low, elevated fire weather will still be a possibility.

As the low passes through the state, flow will turn back to the
north later Sunday night and into Monday and moisture wraps around
the backside of the low, bringing much cooler air back into the
area. Any precipitation will transition to more stratiform. The
question is, what type of precipitation? Through most of the
night east of the Divide, 700 millibar temperatures remain
above 0C, which would keep snow levels above 8500 feet. Areas
west of the Divide will see 700 millibar temperatures fall to
minus 5C though, which would drop snow levels even down to the
valley floors later Sunday night. Accumulations would remain on
the light side though, with a greater than 9 out of 10 chance of
2 inches or less through Monday and much of this would only be
on non paved surfaces.

The mountains may be interesting though. Models continue to
show the heaviest precipitation across far northern Wyoming. We
will likely need advisories for some of the northern mountains,
mainly the Absarokas and Bighorns with a greater than 4 out of 5
chance of 6 inches or more through Monday afternoon. As for any
potential watches, we will hold off for now. There is only a
very small area of the Absarokas with a greater than 1 out of 2
chance of 12 inches of more. There is a better chance in the
Bighorns. However, even here, the chance is generally less than
1 out of 2 except for the higher northern peaks, where impacts
are still minimal this time of year. In addition, the heaviest
precipitation would fall during the day, when the high late
April sun angle would mitigate impacts on travel. Still time to
change our mind here though.

As for the lower elevations, it still looks to be mainly rain.
There could be a period of snow at times, especially if
precipitation rates are heavy but accumulations would be small
and largely on grassy surfaces. The lowest 700 millibar
temperatures drop to is minus 5C to minus 6C, which would keep
snow levels generally above 5500 feet. The most likely locations
for accumulation east of the Divide would be a place like
Meeteetse, Cody or Dubois with somewhat higher elevation.
Buffalo could see some snow, but this would be late after the
deeper moisture moves away. This definitely looks like a decent
rain, with a greater than 1 in 2 chance of a quarter inch of
rain across the northern half of the state, with a greater than
2 in 3 chance of a half an inch around Buffalo. Most guidance
shows the precipitation ending by Monday evening as the low
moves away.

Transitory ridging then brings a dry and milder Tuesday.
Another system then moves into the area for Tuesday night and
Wednesday with the next chance of precipitation. Model guidance
has more uncertainty with this system and given the concern with
the Sunday / Monday system, we didn`t have time to take a good
look at it. Ridging should then build over the area Thursday and
Friday, bringing a return to drier and warmer conditions. So, a
perfect example of the rollercoaster that is spring weather in
Wyoming.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1133 AM MDT Sat Apr 26 2025

All terminals to be VFR through the forecast period. Cumulus are
already developing over the western mountains as of late Saturday
morning. These clouds will lead to isolated showers that push
northeast onto the lower elevations after 20Z/Saturday. KCOD
would have the best chance at a stray shower until 00Z/Sunday,
but confidence is not high. Otherwise, mid and high cloud cover
increases over the west Sunday morning as a closed low
approaches from the west. Southwest surface wind 10-20kts across
the southwest terminals and KCPR will diminish around sunset
Saturday evening. The pressure gradient tightens Sunday morning
ahead of the upper low. This allows for southerly surface wind
to increase to 12-25kts during the 15Z-18Z/Sunday timeframe at
these same terminals.

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

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

UPDATE...Gerhardt
DISCUSSION...Hattings
AVIATION...CNJ