Area Forecast Discussion
Issued by NWS Riverton, WY

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
000
FXUS65 KRIW 230952
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
352 AM MDT Sat Mar 23 2024

.KEY MESSAGES...

- Snow will spread into the west today, with the heaviest and
  steadiest snow expected tonight.

- Snow will spread East of the Divide late tonight into Sunday,
  with the greatest impacts in northern Wyoming.

- Strong wind is likely across the northern Big Horn Basin and
  Johnson County late tonight and Sunday.

- Temperatures slowly warm into mid-week, with mountain snow
  showers continuing until the next more potent weather system
  moves in Thursday.

&&

.DISCUSSION...
Issued at 346 AM MDT Sat Mar 23 2024

We have a complex forecast this morning. And this remains me of an
80s song (you know you were getting a dated Gen X reference this
morning). It was one Madonna;s earliest hits, "Borderline." As in
borderline for certain highlight criteria.

Right now the radar is largely quiet for many areas. The exception
is the WInd River Range were some snow is occurring and a few
showers central Wyoming. The HRRR has a decent idea of what is
occurring, although a few details are lacking. For the most part,
precipitation today looks to be on the light side with a couple of
exceptions. One is where there is orographic enhancement, like the
mountains. The other is the possibility for thunderstorms. Guidance
is showing some CAPE and negative lifted indices in northwestern
Wyoming. The current forecast shows isolated thunder, and we will
keep it for now. As for the highlights, if they didn`t start already
I would have seriously considered pushing the start time back. But
it is out already so we will leave it alone.

The heaviest precipitation will push into the west tonight as the
trough axis approaches and increases lift. There will also be some
left front quadrant jet energy to increase upper level divergence
and mid level flow switching from southwest to west-northwest should
increase orographic lift. Some favored orographic flow will continue
Sunday, but this may be restricted to northwestern Wyoming. As for
the borderline comment here, with new amounts it is borderline if
the Salt and Wyoming Range receive advisory amounts of snow, only
about a 1 out of 2 chance. We will leave the advisory for now but
future shifts may need to cancel it. Other ranges, especially the
Tetons and Wind Rivers, have at least a 4 in 5 chance of exceedance
so we will leave them as is.

Now for East of the Divide. Not a ton of concerns through this
evening as there will be a few showers and continued mild
temperatures. The problem here comes in late tonight and tomorrow as
the trough pushes eastward and temperatures drop. It will be slap in
the face on Sunday as temperatures will be 20 to 30 degrees colder
then the past few days. But then the borderline comes in again, this
time in regards to snow amounts and potential highlights.

The area of most concern is northern Wyoming, especially north of a
Cody to Greybull line. With the front coming down, this is a
favorable set up for snow. There is not a lot of jet energy though.
In addition, snowfall amounts have dropped somewhat. This is mainly
because temperatures are, again, borderline. Precipitation will
start as rain but will change to snow. The question is when, and
there is still uncertainty. And, with recent warm temperatures, the
ground is fairly warm so it will take a while for snow to stick to
roads. And, complicating things even further, snow that falls during
the day Sunday will have trouble sticking with the high sun angle.
So for now, we will go with a Special Weather Statement. We have the
same concern for areas from Worland to Thermopolis as a period of
northerly upslope develops behind the front. The start should be
later here though, and a lot of the snow would fall during the day,
limiting impacts. This is the same situation for Lander, where there
will be a period of upslope but the high March sun and warm ground
should keep impacts more limited. The Bighorns could pick up several
inches, but advisory amounts would be limited to the highest peaks.
As for probabilistic guidance, the highest any spot has for advisory
amounts of snow is around 1 in 3, with the highest chance at Cody,
Thermopolis and the Bighorn Range.

And there is one more concern with the system, wind. This will
mainly be the case for the favored cold advection / northwest flow
areas, mainly the northern Big Horn Basin and northern Johnson
County. It is, again though, borderline for highlights. It does not
look like the 700 millibar flow is sufficient and there is not a ton
of jet forcing. There could be some gusts to 40 to 50 mph at times
late tonight and Sunday though.

Conditions should gradually improve as the system moves away late
tonight. For the most part, we will be in a zonal flow for the next
several days, fairly typical of most of the winter. It does look
active across the west with a few systems moving through. The next
decent one looks to be on Tuesday. Meanwhile, areas East of the
Divide should remain largely dry through Wednesday with below normal
temperatures early in the week gradually moderating to near normal
by Wednesday. Wednesday at this point looks like a quiet day with
transitory ridging. Another more potent system may move in for the
end of next week, but timing and details are highly uncertain at
this point.

&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1020 PM MDT Fri Mar 22 2024

SCT-BKN FL150-200 clouds continue to stream across the Cowboy
State. LIFR clouds have developed over Johnson County and north
of KCOD in the last hour. These clouds are expected to surge
southward to KCOD between 06Z and 08Z and stay in place through
16Z. An area of showers will move over southwestern and central
portions from 06Z to 13Z this morning, with VCSH occurring over
KBPI/KPNA and spreading east of the Divide to KRIW/KLND and
eventually to KCPR. South to southwesterly winds will begin to
increase across southern portions into Natrona County through
the morning, spreading across much of the CWA through Saturday
afternoon. Gusts of 25 to 35 kt will occur in these areas during
this time. Isolated to scattered showers and thunderstorms will
be possible during the afternoon across areas west of the
Divide and the northern half of the Bighorn Basin. The speed of
these storms will be rather quick, so have left any mention of
VCTS out of any terminal forecast for now. Showers will continue
across the forecast area through the night, with the focus
shifting to the northwestern portion of the forecast area.

A strong cold front will push southward across areas east of the
Divide after 00Z. North to northwesterly winds will increase
over the Bighorn Basin and northern Johnson County toward the
end of the TAF period, with these winds spreading southward
through the night.

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 Weather Advisory from 6 PM this evening to 6 PM MDT
Sunday for WYZ001-002.
Winter Weather Advisory until 6 PM MDT Sunday for WYZ012-014-
015-024.

&&

$$

DISCUSSION...Hattings
AVIATION...LaVoie


USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.