349
FXUS65 KRIW 181836
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1236 PM MDT Sat May 18 2024

.KEY MESSAGES...

- Elevated fire weather conditions today and tomorrow, with
  gusty winds up to 40 mph and relative humidity values below
  20%

- Cool and wet Monday and Tuesday across the area. Snow in the mountains.

- Active weather pattern past Tuesday, keeping cool temperatures
  and precipitation chances through the workweek.

&&

.DISCUSSION...
Issued at 1235 PM MDT Sat May 18 2024

Fairly zonal flow today in the wake of the cold front that passed
through yesterday and last night. High temperatures will be a touch
cooler today as a result, but still in the low 70s for most. A gusty
afternoon again, though not as strong as yesterday. A southwest to
west wind gusting 25 to 35 mph develops for the afternoon across
most locations west of the Continental Divide, and across Fremont
and Natrona Counties, as well as the western Bighorn Basin. Today is
also dry, with minimum relative humidities (RH) under 20% for most
lower elevation locations. The HREF is highlighting Sublette,
Sweetwater, Fremont, and Natrona Counties for the lowest RHs, those
being around 10%. Elevated fire weather conditions for this
combination of wind and dry weather.

A Pacific system will be the main weather driver to end the weekend
and for the first half of the workweek. Starting with Sunday, flow
shifts southwesterly as the trough approaches, with a tightening
pressure gradient. Winds begin to increase in the morning, with
widespread gusts 30 to 40 mph (80% confidence for this range). RHs
again fall below 20% for much of the area, the HREF highlighting
Sublette and Fremont Counties with the best chance for lowest values
of 10% to 15%. Again, elevated fire weather conditions for Sunday. A
little moisture ahead of this system could bring some showers across
the area Saturday night through Sunday. Chances range from near 0%
to 60% depending on location.

The upper-level low tracks eastward Monday, centered on Idaho. This
is when the better moisture begins to make it into the Wyoming.
Precipitable water values increase to 100% to 150% of normal.
Widespread precipitation chances pick up Monday morning and into the
afternoon. Snow level around 7000 to 7500 feet will mean snow for
the mountains. The low tracks across the state Tuesday, shifting
upper level flow as it does. This is important, because by Tuesday
morning, favored flow will be for the eastern side of mountains.
Through Tuesday, 0.15 to 0.75 inches are possible across much of the
area. Of interest will be two locations, one being the mountains,
the other for the central area of roughly Fremont and Natrona
Counties.

Starting with the latter, models are indicated a roughly 30% to 50%
chance of Fremont to Natrona County getting more than 1 inch of rain
Monday through Tuesday. Being 3 to 4 days out yet, not all high-
resolution models are in range, so this can very well change.
Overall, this is something to watch in model trends.

For the mountains, the focus is on the eastern sides as the low
tracks eastward for Tuesday. Although snow begins Monday, rates and
amounts ramp up Monday night and through Tuesday. Totals through
Tuesday, as of now, are a little over a foot for the eastern Winds,
Absarokas, and Bighorns. Again, being a few days out, there can
still be some change in exact storm track on how much moisture comes
with this system. Will hold off on winter highlights for now, as
confidence is not yet high enough in any specific amounts at this
range out.

There is a little concern when it comes to flooding, namely over the
Bighorn Mountains. Some late season snow has kept liquid amounts
above normal for this time of year. We have already seen river
levels rise as warm temperature melt snow and recent storms bringing
precipitation. With the potential another large system (possible QPF
1 to 2 inches), could see more river rises.

Past Tuesday, it looks to remain active, with another system in the
Wednesday through Friday timeframe.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1133 AM MDT Sat May 18 2024

VFR conditions through the day today, with only high cirrus
across NW WY this afternoon. Wind gusts will again rise to 25 to
30kts this afternoon. Winds will decrease overnight, with most
locations remaining L/V. Some light shower activity will try to
spread into western and southern WY after 06Z tonight, but
will have minimal impacts on local airports due to the light and
isolated nature of the showers.

&&

.FIRE WEATHER...
Issued at 1235 PM MDT Sat May 18 2024

Elevated fire weather conditions for most lower elevation locations
today and tomorrow. Gusts today of 25 to 35 mph are common, with
gusts up to 40 mph tomorrow. RH drops below 20% for most locations,
with some areas nearing 10% minimum RHs today and tomorrow. Cooler
and wetter pattern starts Monday, with widespread precipitation
chances through Tuesday. An active weather pattern continues after
Tuesday.

&&

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

&&

$$

DISCUSSION...Wittmann
AVIATION...Straub
FIRE WEATHER...Wittmann