818
FXUS65 KRIW 040015
AFDRIW
Area Forecast Discussion
National Weather Service Riverton WY
615 PM MDT Sat May 3 2025
.KEY MESSAGES...
- Above normal temperatures will continue Sunday, with
widespread readings in the 70s and 80s.
- A system will begin to push into the area Sunday. Isolated
showers and thunderstorms are expected across the area,
especially across western and southern Wyoming.
- Widespread rain and mountain snow is expected Monday through
Tuesday, especially across central Wyoming.
- Precipitation may lead to elevated river levels Tuesday into
Wednesday, mainly across central Wyoming.
&&
.UPDATE...
Issued at 550 PM MDT Sat May 3 2025
Clear skies and dry conditions will continue through the
overnight hours tonight into Sunday morning. Most showers and
thunderstorms Sunday afternoon look to remain over western and
southern Wyoming, (no changes there) though some chances also
occur for portions of central Wyoming. Overall, have chances
between 15% and 40%; showers/storms will be around, they will
just be isolated. Notably, the NAM has more widespread
convection than the HRRR, so there still is some uncertainty
with those mentioned chances.
For the precipitation Monday and Tuesday, amounts have come down a
bit, but the overall message remains: Monday and Tuesday
(especially Monday night into Tuesday) are wet and cool, with
the highest amounts over central Wyoming. Have moderate
confidence in the amounts, and would expect a few more
adjustments over the next 24 hours as more high-resolution
models come in. Would mainly expect rain shadow effects to show
more.
&&
.DISCUSSION...
Issued at 502 AM MDT Sat May 3 2025
High pressure will dominate today as the ridge reaches its maximum
extent over Wyoming. Accordingly, mostly clear skies are expected,
with little chance of precipitation across the area. High today will
peak around 15 degrees above normal as 700mb temperatures reach
nearly 10C. Humidity values will drop into the low teens today
across lower elevations, though relatively light wind will help to
mitigate fire weather concerns.
The ridge begins to break down on Sunday as a longwave trough
approaches from the West Coast. At the same time, an associated
500mb low will deepen across southern California. This will bring
the leading impacts from this system to Wyoming as moisture surges
northward. Showers and thunderstorms should kick off by early
afternoon, moving from southeast to northwest across the area given
the location of the low to our southwest. This activity will be
fairly widespread through the afternoon, though with the best
chances across western and southern areas. In reality, much of the
area will remain warm and dry again, with temperatures possibly even
peaking a little higher than Saturday for locations east of the
Divide. While brief heavy rainfall is possible with any storms, the
primary concern will be strong outflow winds given the still drier
boundary layer. Fire weather concerns may creep up a bit given the
outflow wind potential, though widespread strong winds are not
expected.
Guidance is coming into better agreement with the progression of the
low by Monday. The most likely scenario is for the low to track from
southern Arizona to the Four Corners region Monday afternoon, and
then move more easterly through Tuesday. This will further increase
moisture over Wyoming, leading to a respectable precipitation event
over the area. This will be most notable across central Wyoming
where a prolonged period of northeasterly upsloping flow will be
realized. Forecast precipitable water values are over 0.60", which
would put it near the climatological maximum for this time of year
at KRIW (SPC Sounding Climatology). Precipitation amounts across
central Wyoming are generally in the 0.50" to 1" range, with the
higher values toward the northeasterly facing mountain slopes. This
includes the Lander and Casper areas, which currently have about a
70% chance of seeing at least 1" of precipitation during the period
from Monday through Tuesday. Elsewhere, accumulations are generally
expected to be under half an inch. 700mb temperatures approaching
negative 6C Tuesday morning certainly mean mountain snow, and likely
even a mix or a change to all snow for a time in the lower
elevations as well. The primary concern with this event, while not
particularly high, is the potential for river and small stream
flooding. Elevated river levels can be expected, aided in part by
the warm weekend preceding the rain.
Precipitation will come to an end Tuesday evening. Further ahead,
ensemble cluster guidance continues to indicate a quick return to
rising 500mb heights by Wednesday. This favors warmer and drier
conditions through the end of next week.
&&
.AVIATION /00Z TAFS THROUGH 00Z MONDAY/...
Issued at 455 PM MDT Sat May 3 2025
VFR conditions continue through Sunday morning, with nearly clear
skies and light winds. The next weather system starts to impact the
region Sunday afternoon, bringing increasing cloud cover from the
southwest. General cloud cover should remain VFR through 00Z/05 with
thickening high clouds, but some convective showers and
thunderstorms may develop during the afternoon across most of
western and central WY. Best chances currently would be at KJAC,
KBPI, KPNA, and KRKS, so have added PROB30 groups to indicate a
thunder threat at those terminals.
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.
&&
$$
UPDATE...LaVoie
DISCUSSION...Myers
AVIATION...Straub/Rowe
