Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 271745 AAA
Area Forecast Discussion
National Weather Service Riverton WY
1044 AM MST Sun Nov 27 2016
.SHORT TERM...Sunday through Tuesday night
We have some things we know and some things we don`t know tonight.
What we do know is that we are entering into an unsettled weather
pattern over the next few days. However, what we do not know are a
lot of the details since the models are differing on things. And
with the differences in weather...we will split the discussion into
West and East of the Divide.
West of the Divide...
As of now there are some decent echoes across eastern Idaho
associated with some upper level divergence and the left front
quadrant of a jet passing to the south of Wyoming. These should move
into western Wyoming overnight and bring a few inches of snow. There
could be a brief period of heavier snow. However, the models are
consistent in keeping the steadier snow fairly brief with the best
upper level forcing moving away by morning.
After that and following a cold front passage is where the
uncertainty moves in. Flow will turn west to northwesterly and with
the atmosphere still unstable there will be some snow showers
around. The problem is these showers and bands are very difficult to
pinpoint. We will not issue any advisories now but some may be
needed if one place gets a band to set up for a few hours. The wind
will also increase behind the front, especially across the south
where things could get interesting across Interstate 80 at times
with the snow showers and possible blowing snow. These showers should
dissipate this evening and tonight.
Another wave will move in for Monday. There will be favorable flow
along with some instability with this system. However...there is not
a ton of moisture associated with the wave. As a result, we kept QPF
amounts on the light side with POPS fairly high. Again, pinpointing
the heaviest areas of snow is difficult. Conditions should settle
some for Monday night into Tuesday as heights rise and instability
lessens allowing a reduction in coverage of showers.
East of the Divide...Most areas should start the day dry. However,
as the cold front passes through there will be an increase in
showers and yes, some bands of snow and possibly rain. The
uncertainty here is where they will set up. Most models are zeroing
in on places like from eastern Fremont into Natrona County. However,
the exact position cannot be pinpointed. In addition, there are
differences in QPF, with the NAM being much wetter than the GFS.
The truth is probably somewhere in the middle though. In addition,
with temperatures expected to rise into the 40s at times, some
precipiation may fall as rain. For now we kept the trace to 2
inches in the lower elevations, but some areas could get more
where bands set up late today and especially this evening. This
should wane later tonight as the best forcing moves away.
Monday at this point looks to be a cooler but more settled day. It
will feel a bit cooler through with cloud cover and a brisk wind.
There could still be a few showers but coverage will be less with
the vast majority of the area dry most of the time.
Things may get more interesting again Monday night into Tuesday. The
progress of an upper level low that is passing to the south of
Wyoming today will be slowed and may even stall over the Dakotas or
Minnesota for a time with a highly amplified pattern and strong
ridging along the east coast. This will have two effects. One,
enough moisture may wrap around the low to bring some light
precipitation to portions of Johnson and Natrona Counties. The
biggest effect will be wind. 700 millibar winds will strengthen to
50 to as much as 60 knots across portions of northern Wyoming. In
addition, these areas, the northern Bighorn Basin and Johnson
County are favored for strong winds with north to northwest flow.
MOS is showing sustained winds of 40 knots at Buffalo. There is a
decent possibility that high wind highlights may be needed for
later Monday night or Tuesday. Elsewhere, temperatures will
continue a downward trend. There will still be a couple of showers
around but coverage should not be that much. Conditions should
improve Tuesday night as the low begins to fill and slowly move
.LONG TERM...Wednesday through Sunday
Synopsis...A mainly dry and seasonably cool pattern will prevail
over the area through next weekend.
Discussion...Synoptic pattern at the outset of this period consists
of longwave trough east of the Rockies with upper low over
Minnesota, upper ridge across the interior west into central Canada
and shortwave off the NW coast. Wraparound precipitation with upper
low over Minnesota will be east of the area by Wednesday morning
along with a relaxing pressure gradient and decreasing NW wind
across the Interstate 90 corridor.
ECMWF and GFS have more or less swapped solutions over the last 24
hours with ECMWF digging upper low south roughly along the 120W
Meridian, into NW Mexico on Friday, while GFS shows a track across
Idaho and western Utah. GFS amd EC ensemble means are somewhere
between these two extremes. This mean solution would not bring
precipitation as far east as GFS (across most of the area Thursday
and Thursday night) nor go completely dry per ECMWF. Forecast
leaves low PoPs across the west, with any snowfall amounts expected
to be on the light side Wednesday and Thursday. Either way upper
low should be far enough to our southwest for dry forecast on
Friday. WNW difluent flow may bring some light snow into the NW
over the weekend, otherwise gradual warming is expected with
westerly flow bringing in milder Pacific air.
WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
Made some slight adjustments to ceilings at BPI, JAC, PNA and RKS
involving ceilings and snow shower timing in the short term portion
of the forecast. Expect occasional -sn and MVFR/IFR conditions to
continue at JAC and RKS until between the 01z-02z time frame. VFR
conditions with higher based low clouds are expected and VCSH at BPI
and PNA during this same time period. Will continue with MVFR
visiblities due to -SN and generally IFR ceilings at JAC through the
rest of the forecast period. After 13z, MVFR ceilings at around 2KFT
are expected at BPI, PNA and RKS.
EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
Scattered snow is still expected to develop through the afternoon
with MVFR conditions expected until around 23z with local IFR
conditions in some locations. The snow shower bands will decrease by
04Z Monday. VFR conditions are expected at all sites after 04z with
the exception of CPR, with VFR ceilings expected after 07z. A gusty
west to northwest wind will be common through the afternoon with the
wind decreasing after 01Z Monday.
Please check the latest terminal forecasts for details.
An upper level disturbance will move across the area today and bring
some rain and snow showers with the greatest coverage across the
west. A gusty southwest wind will continue from the Red Desert
through Casper. Mixing and smoke dispersal will be generally poor in
the Basins, fair to good elsewhere.An upper level disturbance will
move across the area today and bring some rain and snow showers
with the greatest coverage across the west. A gusty southwest wind
will continue from the Red Desert through Casper. Mixing and smoke
dispersal will be generally poor in the Basins, fair to good