Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 231951
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
151 PM MDT Mon Oct 23 2017

.SHORT TERM...Monday night through Thursday

We don`t have a lot of issues in the first 36 hours of the forecast
so I will no talk a lot about them. Expect near to slightly above
normal temperatures. Wind should decrease after sunset tonight with
light to moderate winds for all areas tomorrow. So all in all, a
fairly nice day.

Things turn more interesting on Wednesday. A strong cold front will
be moving toward the area from the north. The first area to see
effects will likely be the Absarokas and Cody Foothills where
increasing 700 millibar winds may mix to the surface and bring some
locally strong winds. It is borderline for high winds though, so we
will hold off on any highlights. The other concern is possible
critical fire conditions. The tightening pressure gradient ahead of
the front will increase southwest flow across the favored Rock
Springs through Natrona County corridor. Temperatures are also
expected to climb into the 70s and relative humidity falling toward
10 percent. There is a good chance a fire weather watch would be
needed across Natrona County. There is more uncertainty for Johnson
County though, since wind will be lighter and may not become strong
enough until later in the day when temperatures drop as the cold
front approaches from the north. It will be a warm day in many
areas, with many of the lower elevations seeing high temperatures
into the 70s.

The aforementioned cold front will begin to move into northern
Wyoming Wednesday night and across the area on Thursday and it will
have three main effects. The first effect will be some strong and
possibly high winds across areas favored by strong cold advection
with north to northwesterly flow, mainly the northern Big Horn Basin
and northern Johnson County. We bumped up winds significantly as a
result. The second will be a period of precipitation as the front
moves through. It would likely start as rain but then change to snow
as surface and 700 millibar temperatures plummet. The system has
little moisture to work with and is a quick hitter, so we do not
expect a lot of snow. However, the timing around the morning commute
could make things interesting.

The main effect will be the much colder temperatures, which will be
a slap in the face. All areas East of the Divide will see at least a
20 degree drop in high temperatures when compared to Wednesday. The
most dramatic drop will be in eastern portions of the County Warning
Area where temperatures could be as much as 35 degrees colder than
on Wednesday afternoon. With a brisk north to northwesterly breeze,
it will feel even colder.

.LONG TERM...Thursday through Monday

Mean longwave ridge holds between 130-135W through the period
keeping us in a nnw-nw flow much of the period. Our Thursday system
is passing off to the east and south to start this period. Cold
surface high will likely be overhead/slightly east of us around 00z
Friday. Coldest temperatures of the fall season are quite possible
Friday morning, especially (mainly) east of the divide. Any new
light snow cover (from our Thursday system) will just increase those
odds. Many low lying/snow covered areas could see teens. Chilly
airmass is short lived through with warming aloft moving in quickly
Friday through at least early Sunday. Another swd moving shortwave
may affect us Sunday night or Monday of next week. Some significant
differences in timing and location this far out but mean ridge
location supports another potential disturbance dropping down the
front side of the ridge (12z Euro is a day later but now at least
shows this system producing snow here but not until later Monday
night into Tuesday). This system that the GFS shows quite well is
currently way over in Asia and moves across the Pacific this week.
Another strong cold front may move down from the north next Sunday
night/early Monday with brief snow potentially, especially north
half/upslope and nrn mountain areas (not unlike our upcoming late
Wed ngt/Thur system). So cold start to the extended than warming up
over the weekend before dropping sharply, especially east of the
divide, Sunday night/Monday. So 40s and some 50s Friday with temps
warming into the 60s (50s for some basins) in many areas east of the
divide Saturday and Sunday (basin areas may begin to struggle though
with inversions starting to have an effect, especially Saturday)
with mainly 50s west. East slope foothill areas will be quite nice
this coming weekend with 40s and 50s (maybe even a few lower 60s
down low). Most of the middle to higher mountains (above 9000 feet
and up) will see 40s to lower 50s.

&&

.AVIATION...00Z Issuance

VFR conditions will prevail through Tuesday. Gusty west to northwest
wind areas will decrease after sunset.
&&

.FIRE WEATHER...

A ridge of high pressure will bring dry weather through Wednesday.
Gusty breezes this afternoon with decrease after sunset. Tomorrow
will be a dry and warmer day. Although relative humidity may fall
into the teens at times, wind should remain light to moderate for
most areas. Mixing and smoke dispersal will be generally fair to
good.

&&

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

$$

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


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