Area Forecast Discussion
Issued by NWS Riverton, WY

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
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

000
FXUS65 KRIW 270808
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
208 AM MDT Thu Apr 27 2017

.SHORT TERM...Thursday through Saturday night

The forecast again today is causing many headaches in the humble
Riverton abode. And again, I an going to remake this version of
reasoning that I have remade more than the Beatles classic song
"Yesterday." The song is 1 or 2 degrees. This will make all the
difference in the forecast today. However, there are a couple of
other considerations, including 200 or 300 feet, time of day and 5
or 10 miles. That has a lot of considerations and I will explain
below.

As for 1 or 2 degrees, this is a consideration for today and
tonight. Conditions aloft will be borderline for a lot of locations
today and even into this evening. And as we all know, this can make
a big difference in snowfall amounts. There is less uncertainty on
this however tonight and even Friday as conditions will be cooler
aloft and more favorable for snow. It is the same for elevation
today. However, the higher elevations East of the Divide still look
good for warning amounts of snow so we upgraded most of those. The
exception is the western Bighorns where downsloping may limit
accumulations so we went with an advisory there.

As for time of day, this has to do with the sun angle. Here in late
April, the sun is as strong as it is in mid August. And what
surfaces absorb more sunlight...dark ones, especially roads. This
looks to have an impact in some areas. The most recent runs of the
models show the heaviest QPF across the Natrona and Johnson Counties
falling at night. Therefore, a better chance for snow to stick. As a
result, we went with winter storm warnings in that vicinity.
However, in places like Fremont County, the heaviest QPF is during
the daylight hours. This would limit accumulation significantly,
especially on roads. This is why we went with an advisory for these
areas since impacts would be less. Some areas could see over 6
inches of snow on the grass but only some slush on the roads.

The last is the one weather event in Wyoming that I hate the most,
snow bands. This system is a bit less disorganized than the system
earlier this week so snow will be more diffuse. This will make the
highest amounts almost impossible to pinpoint. A few spots will
likely see heavy amounts of snow but you go 5 to 10 miles away and
there will be very little. This is the wild card in places like the
Bighorn Basin and Wind River Basin. This is something that you react
to.

So to sum it up, we went with warnings from Johnson County to
Natrona County. However, this looks to be a reverse of what usually
happens in Natrona County. With flow more easterly than
northeasterly, the highest amounts should fall in the northern
part of the county with lesser amounts toward Casper. Further
west, we went with advisories. Meanwhile, West of the Divide the
heaviest snow will fall this morning. Snow showers will continue
through the day but nothing really heavy.

Low pressure should move away Friday night with most steady snow
ending by midnight. With some weak cyclonic curvature hanging over
the area a few light snow showers will linger into Saturday, but
nothing of consequence. With a lot of clouds hanging around,
temperatures will continue to average below normal.Dry weather
should return to the state Saturday night.

.LONG TERM...(Sunday through Thursday)
Issued at 115 AM MDT Thu Apr 27 2017

A couple of quick moving systems in a strong northwest flow, and
their associated weak cold fronts will impact the area Sunday
through Wednesday. Looks like the first wave and front will push
across the area Sunday night/Monday, with another shortwave
impacting the area Tuesday. Currently the wave on Tuesday looks to
be the stronger of the two, so will hit it a bitter harder with
PoPs. Ridging begins to build into area Wednesday, but still
could see some showers...mainly over the east. Thursday looks to
be dry with strong ridging over the Rockies. Precipitation Sunday
through Wednesday looks to be mountain snow, and mainly valley
rain. The exception could be Monday night/Tuesday where enough
cold air could works into the area with rain changing to snow in
the valleys. The best chances of precipitation with each event
will be across the northwest/northern mountains.

Temperatures will be below average through Tuesday, with a warming
trend to seasonal Wednesday, and even above average Thursday.

&&

.AVIATION...12Z ISSUANCE

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

An upper level low pressure system and associated cold front will
move across western Wyoming through 12z bringing numerous snow
showers with widespread mountain obscurations and MVFR, occasional
IFR conditions early this morning. Gusty west to northwest winds and
drier air, snow showers becoming more scattered, will occur across
the southwest behind the cold front 12z-15z Thursday, while areas
vicinity and north of a KAFO-KPNA line will continue to see numerous
snow showers, widespread MVFR, occasional IFR conditions into Friday
evening. Some gradual improvement is expected from the northwest
after 03z Friday.


EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

An upper level low pressure system and associated cold front will
move across the Continental Divide into central Wyoming 12z-15z
Thursday.  Scattered rain and snow showers will occur ahead of the
cold front. A northeast upslope flow will keep KCOD under MVFR
ceilings and occasional showers of rain and snow through 12z.  The
low pressure system will stall out over east central Wyoming
Thursday afternoon with widespread rain and snow along and north of a
vicinity KTHP-50SM north KCPR line 15-18z pushing south into KRIW-
KLND-KCPR in the afternoon.  Rain will change to snow in most lower
basins Thursday evening, widespread MVFR/IFR conditions expected to
develop across the area Thursday evening and continue overnight.

&&

.FIRE WEATHER...

A cool and wet pattern looks to persist through Friday. Low pressure
will move across the area over the next couple of days and bring a
mix of rain and snow today with mainly snow from tonight into
Friday. The precipitation will be banded and difficult to pinpoint
the heaviest amounts. Mixing and smoke dispersal will be generally
fair to good.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 6 PM this evening to midnight MDT
Friday night for WYZ017>019-022.

Winter Weather Advisory until noon MDT today for WYZ012-013-023-
024.

Winter Storm Warning from 6 PM this evening to midnight MDT
Friday night for WYZ010-011-015-020.

Winter Weather Advisory from 6 AM this morning to noon MDT
Friday for WYZ003-004.

Winter Weather Advisory until 6 AM MDT Friday for WYZ001-002.

Winter Weather Advisory from 6 AM this morning to midnight MDT
Friday night for WYZ005>008.

Winter Storm Warning from 6 AM this morning to midnight MDT
Friday night for WYZ009.

&&

$$

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



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