940
FXUS65 KRIW 261733
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
1133 AM MDT Sat Apr 26 2025
.KEY MESSAGES...
- A warmer day with breezy to gusty winds expected today with elevated
fire weather conditions in southern and eastern areas.
- A weather system brings precipitation chances (60-80%), 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.
&&
.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.
&&
$$
DISCUSSION...Hattings
AVIATION...CNJ
