Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 171755
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1055 AM MST Sun Feb 17 2019

.SHORT TERM...Sunday through Tuesday night

The Arctic air is now sliding into northern Wyoming and will spread
across much of the area through the day. However, the air is oozing
in like a slug rather than charging in like a moose. This should
keep the threat of any strong winds to a minimum, although some
gusty breezes will still be possible in northern Johnson County and
the northern Big Horn Basin. There will also be more snow showers
around today. The atmosphere remains rather unstable so there will
be more snow showers around through the day, although maybe not
quite as numerous as yesterday.

Things will likely quite down from Monday into Tuesday. There will
continue to be some snow showers across the western mountains buy
any accumulations should remain rather light due to the moisture
starved atmosphere. Even quiet conditions are expected on Tuesday
with only isolated snow showers in the west. There is one small wild
card in the deck though with a couple of models showing some weak
QPF trying to sneak into eastern Natrona County Tuesday afternoon in
response to some lee cyclogenesis across southern Colorado. All the
models do not show a lot of moisture present due to the dry Arctic
air in place and the best dynamics remain well south of the area.
For now, we have some slight chance POPs in that area and will
monitor future trends. The GFS also shows a weak system moving into
the west Tuesday night. However, given the fast bias of the model
this far out, we kept the forecast largely dry through this period.

The main story will be the cold temperatures Monday and Tuesday. The
good news is temperatures should not be as cold as if this occurred
in early January given the longer days, somewhat higher sun angle
and lack of substantial or fresh snow cover in some areas,
especially East of the Divide. This won`t be the case West of the
Divide though with the abundance of snow from the recent snow
storms. As a result, we kept temperatures below MOS guidance,
especially Monday night which at this point looks to be the coldest
night.

.LONG TERM...Wednesday through Sunday

Overall, a longwave trough will remain persistent across the
Rockies through at least Friday. Well below average temperatures
by 15 to 25 degrees are likely in this time frame.

Early Wednesday, models prog a strong trough over British
Columbia/Pacific Northwest in the overall longwave trough. This
trough is expected to split with the bulk of the energy surging
south into the southern Great Basin by 00Z Friday, while the
northern stream pulls east into the High Plains. The southern
upper low is progged to pull slowly east across the Southern
Rockies into the central/southern Plains Friday into Saturday,
while global models show some additional weak energy in the
northern stream impacting the northern Rockies. There is a good
chance of light snow over the far west Wednesday, with the
potential for some light snow east of the Divide Thursday into
Thursday night in wake of a cold front. The 00Z ECMWF suggests the
light snow could last over the east into the day Friday which it
was doing last night. Thus have extended the slight PoPs in Friday
across the eastern parts of the forecast area. Overall the
dynamics stay south and/or east of the forecast area, and there is
limited moisture for this system so snow looks to be on the light
side. Having said that, a couple of inches of light, fluffy
snowfall is not out of the question in the lower elevations. Also,
winds look to be mainly light to occasionally breezy.

Global models are hinting the longwave trough over the past 10
days could become less amplified by early next week. This would
lead to warmer (becoming more seasonal), but potentially better
chances of active weather (precip and wind). There is of course a
large spread in solutions for early next week, and keep PoPs on
the low side and confined to the climatology favored western
areas.

&&

.AVIATION...For the 18Z TAFS through 18Z Monday

West of the Continental Divide (KJAC, KRKS, KPNA, KBPI)

VFR conditions will prevail at most terminal sites through the
period, the exception being KJAC. KJAC will see periods of IFR with
snow showers early this afternoon, then mainly MVFR through the
evening before getting to VFR conditions from 03Z on. Snow showers
will occur over and near the mountains today and tonight. Areas of
MVFR conditions will occur over and near the mountains with mountain
obscuration at times.

East of the Continental Divide (KCPR, KCOD, KRIW, KWRL, KLND)

Low VFR and MVFR conditions will occur today and tonight due to low
cloud formation. Isolated snow showers will develop this afternoon
over and near the mountains. The snow showers will decrease by 06Z
Monday. VCSH will be in most of the terminal forecasts. KCOD and
KWRL will snow showers on station until around 20Z this afternoon.
Expect MVFR conditions over and near the mountains with mountain
obscuration at times.

&&

.FIRE WEATHER...

Some snow showers will continue across the area today as much colder
air moves from north to south across Wyoming today. Other than some
gusty winds in northern Wyoming, expect less wind today than recent
days. Mainly dry conditions are expected Monday into Tuesday with
the exception of some snow showers across the western mountains.
Mixing and smoke dispersal will be generally fair to poor.

&&

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

&&

$$

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



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