044
FXUS65 KRIW 040940
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
340 AM MDT Sat May 4 2024

.KEY MESSAGES...

- A nice start to the weekend today with mild temperatures and
  light to moderate wind.

- Rain and snow returns to the west Sunday, spreading east at
  night. Strong to high wind is likely East of the Divide Sunday
  afternoon.

- Much of next week looks cool, damp and blustery with rounds of
  showers and mountain snow. Details on the placement and timing
  of heaviest precipitation remain rather uncertain this
  morning.

&&

.DISCUSSION...
Issued at 332 AM MDT Sat May 4 2024

This morning I have some good news and bad news. The good news at
least one half of the weekend, mainly today, looks like a nice
Spring day with mild temperatures and light to moderate wind. With
the rest of the period rather active, this is all I will say about
this. Because, the rest of the forecast, if you like warm and dry
weather, will not be to your liking. And with the contrast in
conditions, I will split the discussion into West and East of the
Divide starting on Sunday.

West of the Divide...The main concern here will be snow, mainly in
the higher elevations. The approaching Pacific system continues to
slow it`s progress somewhat though. So, we have removed the POPs for
this evening and some models even have late night dry. The
precipitation will get underway on Sunday. During the day though,
snow levels will remain rather high, remaining above 8500 feet for
the most part. Levels will then fall to the valley floors during the
evening after the Pacific cold front passes through. Most models are
keying in on Sunday night with the heaviest snow as this will
associated with the trough axis passage. An assist from the left
front quadrant of an 100 knot jet streak should enhance
precipitation rates during this time, with over an inch an hour
snowfall rates possible. Most models are keying in on the Tetons and
southern Yellowstone with the heaviest snow, and as a result we have
issued Winter Storm Watches starting at noon Sunday, although the
main impacts would not be until at night. Decent snowfall rates will
continue through Monday as mid level flow turns to a favorable
westerly direction, bring orographic enhancement to the snow. The
one question this morning was how long to run it, as guidance is
split on decent snow extending into Tuesday. For now, we decided to
run it through Tuesday night, since it is only a watch. Future
shifts can decide on duration when they are upgraded. These areas
were selected since there is greater than a 1 in 2 chance of
exceeding 12 inches of snow. As for the other areas, the chance of
warning criteria is less than 2 in 5 right now. As a result,
advisories would be more appropriate in places like Jackson, the
Star Valley and the Salt and Wyoming Range. In addition, with
borderline temperatures and the strong May sun, accumulation
would be limited on roads during the daylight hours.

More uncertainty creeps in starting Wednesday. It will still be
unsettled and cool with a longwave trough hanging around, but
details are very hard to figure out with slow moving upper level
lows. For now, we made few changes to continuity. To sum up the
extended, we have high confidence (greater than a 3 in 4 chance) of
cool and damp weather continuing, but confidence remains very low in
specific timing and precipitation amounts.

East of the Divide and Sweetwater County...After a nice day today,
things will turn more active Sunday as the aforementioned Pacific
low approaches the area. Most of Sunday still looks to be rain free
and warm. We have left some small chances of convection late as a
hedge, but the chance of an location seeing a shower or thunderstorm
before 6 pm is 1 in 4 or less. The main concern here continues to
be wind. There will be a good set up for potential high wind with a
tightening pressure gradient, southwest flow, 700 millibar wind
approaching 55 knots at times and enhanced downward momentum from
the right front quadrant of an 100 knot jet. These areas have at
least a 3 in 5 chance of wind gusts past 55 mph. As a result,
we have issued a high wind watch for the favored areas from
Sweetwater County through Natrona County starting Sunday. A
tight pressure gradient may keep high wind going into Monday as
well. So, we ran the watch through Monday with a mention that
wind will decrease somewhat Sunday night as the wind decouples
after sunset. Strong wind may continue into Tuesday as well,
especially across Sweetwater County where the best jet energy
will be. We do not have enough confidence to extend the watch
this far though. Temperatures will be quite warm on Sunday as
well with the southwest flow, rising well into the 70s in some
locations.

Now for precipitation, showers will overspread the area Sunday
night as the cold front moves through. Exact coverage is still
in question though as the position of the low will be key. Some
models take the low further north which could dry slot the area
somewhat, keeping coverage down somewhat. So, QPF amounts were
kept on the low side through Sunday night with POPs remaining
higher.

There continues to be uncertainty for coverage of showers for much of
the week as well. It, again, depends on what happens with the low.
With blocking ridging to the east and the low getting cut off from
steering flow; it will only move slowly over much of next week.
The big question is how much moisture can wrap around the low.
Most models favor northern Wyoming, especially the Bighorns and
Johnson County through Wednesday night, which makes sense given
the low will be somewhere east or northeast of the area. There
are still large differences in how far south the moisture can
get though and a lot depends of the exact position of the low.
There is the potential for a decent amount of snow in the
Bighorns with this though, given the favorable upslope flow. It
is too early for highlights, since the decent snow would not
begin until Monday at the earliest. The greatest chance may be
Wednesday or Wednesday night when the low, emphasis on may, get
pushed further south. But consistency with the guidance has been
very low. With 700 millibar temperatures remaining at minus 5
or warmer, snow levels should remain at 6000 feet or higher. The
result will be the lower elevations having mainly rain. This
unsettled weather will continue through the workweek. So to sum
things up, we have high confidence (greater than 9 in 10) of a
cool, blustery and unsettled pattern through much of next week.
Confidence in details of timing and placement of the heavier
precipitation remains very low, especially starting on Monday.


&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 949 PM MDT Fri May 3 2024

Showers have ended this evening and skies have steadily been
clearing. The trend of clearing skies will continue tonight, with
mostly clear skies expected through Saturday morning. Patchy fog is
the only hazard tonight/Saturday morning due to the recent moisture
and clearing skies. Terminals with a chance of patchy fog are
KCPR/KRIW. The better chance (50%) is at KCPR, so have MVFR VIS and
FEW003 from 10Z-15Z/Sat. At KRIW kept VCFG as it is less likely
(30%) to occur/impact the terminal.

The prevailing wind will be light and variable at most terminals
tonight before becoming southerly late Saturday morning. High clouds
(SCT200) will pass overhead Saturday afternoon, with increasing
cloud cover heading into Saturday evening. All terminals will have
prevailing VFR conditions through the TAF period, with the exception
of KCPR due to fog.

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...
Winter Storm Watch from Sunday evening through Monday evening
for WYZ001-012.

High Wind Watch from Sunday morning through Sunday evening for
WYZ018.

High Wind Watch from Sunday morning through Monday evening for
WYZ019-020-028>030.

&&

$$

DISCUSSION...Hattings
AVIATION...Rowe