Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 040818
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
218 AM MDT Thu Jun 4 2020

.SHORT TERM...Today through Saturday night

We still have a couple of showers across northern Wyoming early this
morning but they are light and should be gone by morning. All in all
today looks fairly similar to today with a warm and breezy day with
a mixture of clouds and sunshine. There will also be the usual
mainly afternoon showers and thunderstorms around that we have seen
like the past couple of days. The best jet forcing will be over
Montana, but there may be just enough for a couple of storms to
become strong in Johnson County where the best instability is. Of
course, there could be a strong wind gust with a thunderstorm, but
that pretty much describes every shower or thunderstorm in Wyoming
this time of year. We will have another day of breezy to windy
conditions as well and with low humidity we will hoist a fire
weather statement as a result. Any showers or storms should end
around or after midnight, with mainly dry conditions into Friday
morning.

Most of Friday looks to be the minimum in activity as mid level
temperatures rise and help to put a cap on the atmosphere. This will
also likely be the hottest day of the week and possibly the hottest
day for several days. With a gusty wind continuing, fire behavior
will likely be elevated as well and this will be the main concern.

We will then begin a pattern change as an upper level low moves
onshore in southern California and gets kicked northward toward the
area. There are still some differences in timing though. As usual,
the GFS is the fastest but still has that bias of moving Pacific
systems too fast. The NAM and European are in better agreement
though and we will go with those. The result will be the chance of
showers and embedded thunder, roughly across the southwestern half
of the state. And then we come to Saturday...

Something interesting could be brewing on this day. Still, a lot of
ingredients have to come together to make it happen. The Storm
Prediction Center already has an enhanced risk of severe weather
across western South Dakota on this day with slight risk extending
very close to Johnson County. A slight risk in Wyoming on Day 3 does
not happen very often. We will have a strong, negatively tilted
trough swinging through the day along with an approaching cold
front. Shear parameters are decent but not great. However, there is
a limiting factor. And it is a big limiting factor. The question is
how much instability there will be. With the moisture moving in from
the southwest it could end up being rather cloudy which could limit
how unstable things get. The GFS in showing only weak instability
with lifted indices only around minus 2 and CAPE values topping out
at 400 J/KG. The NAM is a bit more bullish but nothing outstanding.
The really good numbers are east of the area but this will have to
be watched, especially if the system slows and more instability can
develop. The main threat for strong or severe storms would be over
the usual hotspots, mainly Johnson County, northern Natrona County
and possibly the Big Horn Basin. Still a lot of uncertainty but
certainly something to watch.

And finally to top it off. The "S" word will be returning to the
forecast. The "S" word of course is snow, as much cooler moves in
behind the cold front. During this period it would only be over the
higher elevations of the western mountains, above around 9500 feet.
However, it will lower in the Sunday. My partner will have more on
that in the long term.

.LONG TERM...Sunday through Thursday

On Sunday morning, models show a stout trough over the western U.S.
with the upper low centered over Oregon. The upper low is expected
to shift into central Idaho by 00Z Monday. Frontogenesis is expected
Sunday over the forecast area as the a strong west to east
baroclinic zone will develop across western/central Wyoming Sunday.
This strong Pacific cold front is then expected to surge east across
the forecast area Sunday night. The main mid-upper trough axis looks
to pull across the area on Monday/Monday night. There is a very good
chance of significant precipitation over the west as a strong jet
streak pulls across the area with decent QG forcing. Snow levels
over the far west could be down to around 8000 feet Sunday. Snow
levels will drop to valley floors west of the Divide Sunday night,
but the most of the precipitation should have shifted away, but
would not be surprised to see some snow in the western valleys and
basins Sunday night. The 00Z deterministic GFS and ECMWF are
spitting out between 0.50 and 1.00 inch of precipitation Saturday
night into Sunday night. Could see a few inches of snow in the
mountains while any higher amounts in the lower elevation might run
risk of some flooding. Will continue to monitor.

The bulk of chances of precipitaion with this system will continue
until the trough axis pulls across the area Monday/Monday night. The
precipitation Monday is expected to be much lighter than what occurs
Sunday. A trailing shortwave could give a slight chance of shower
activity especially over the northwest and north on Tuesday.

Ridging develops Wednesday and Thursday with mainly dry conditions
and a warming trend.

&&

.AVIATION...

TAF sites should remain VFR through the day Thursday. Isolated
showers and thunderstorms are possible across the north but should
not impact TAF sites. Westerly winds increase at most TAF sites
through the afternoon, but decrease around sunset and return to
typical nocturnal drainage flows.

&&

.FIRE WEATHER...

A weak weather system will move across the state and bring isolated
to widely scattered showers and thunderstorms, most numerous in
northern Wyoming. A gusty wind will develop in the afternoon with
some elevated fire behavior possible in portions of central and
southern Wyoming with relative humidity falling to around 15
percent. Mixing and smoke dispersal will range from good in portions
of the north to excellent in the west and south.
&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...Murrell
FIRE WEATHER...Hattings



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