Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 180842

Area Forecast Discussion
National Weather Service Riverton WY
242 AM MDT Sun Mar 18 2018

.SHORT TERM...(Today through Tuesday Night)

The convection that occurred earlier this evening associated with
the northeastbound lead shortwave that was ejected from the main
trough over the intermountain west has since dissipated. As this
trough digs, the main focus of the energy will transfer to the base
of the trough over the Southern Rockies today. Mid level
cyclogenesis will take place over Southeast Colorado today. Then the
surface low will bomb out over the Kansas/Co border by this evening.
Steady light, sometimes moderate snowfall will occur this morning
over the southern half of Sweetwater County this morning as
convective snow morphs into stratiform snow before shifting to the
Green Mtns and Natrona County, including Casper, this afternoon and
early evening. Some QG forcing will enhance lift over our southern
CWA. Once the cyclogenesis takes place over Colorado, upslope flow
will become established over Casper and Casper Mountain under low
level northerly flow. Once the sfc low over the Central Plains bombs
out, our snowfall should diminish as the low becomes tightly wrapped
and contracts in size early this evening. Since this portion of our
CWA will be on the northwest fringe of this activity, only one to
two inches of snow is expected in the aforementioned areas along
with 3 to 5 inches in upslope enhanced areas including Casper
Mountain. The strongest northerly winds should takes place over
Johnson County today, closer to the tighter sfc, wrap around
circulation. Otherwise, light snowfall will occur in the west and
Big Horn Mtns today and tonight as the baggy upper trough axis moves

A flat high pressure ridge will begin to build into the area Monday
marking the beginning of a gradual warming trend. There will be
enough instability, given the negative lifted indices, for a good
chance of snow showers in the north and west Monday. Then periods of
light snow will occur Tuesday and Tuesday night in the northwest due
to weak isentropic lift as temps warm sfc and aloft.

.LONG TERM...(Monday through Saturday)

Overview...A warming trend will occur Wednesday and Thursday ahead
of the next Pacific cold front.  Warmer temperatures with increasing
chances of valley rain showers will elevate flooding concerns across
the west on Thursday.  Breezy to windy conditions will develop
across central and southern Wyoming ahead of the cold front on
Thursday.  The front will push west to east across the area on
Thursday night and Friday bringing breezy to windy and slightly
cooler conditions.  The best chances of rain and snow showers will
be across the west and south Thursday through Friday.  Cooler and
unsettled conditions are expected to linger through next weekend
behind this cold front.

Discussion...Upper air pattern on Wednesday is expected to consist
of broad ridge across the Rockies and High Plains flanked by troughs
across the eastern U.S. and eastern Pacific.  ECMWF and GFS continue
to show a great degree of spread between solutions and poor run to
run consistency with the timing and amplification of the eastern
Pacific trough as it pushes across the western U.S. late in the week
through next weekend.  Given this uncertainty, we continue to focus
on the bigger picture and favor the slower trends overall.  More
weight was given to ECMWF for its consistent slower progression with
shortwave ejecting out ahead of the trough and associated Pacific
cold front.  On Thursday, most of the area will be in warm sector
with intensifying lee side surface trough ahead of the cold front.
Strong unidirectional SW flow may result in near high wind speeds
developing through the Rock Springs to Casper wind corridor.  Slower
cold frontal progression per ECMWF would keep snow levels at 7-8KFT
across the west Thursday as heavier bands of precipitation begin to
push in, before snow levels lower to valley floors Friday morning.
Rain on existing snowpack could result in some minor flooding
issues, especially in the Star Valley.  Westerly downslope flow with
the frontal and post frontal passage Friday will limit precipitation
chances east of the Continental Divide.  Both GFS and ECMWF
operational runs carve out a trough over the northern Rockies next
weekend with several pieces of energy sliding through its base,
though plenty of spread remains in the details and ensemble means,
and GFS 5-wave show much less amplitude than the deterministic


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)

Through around 15/16Z, some fog/low clouds are expected across the
far western valleys, as well as northern Big Horn Basin, and N.
Johnson County where MVFR/IFR conditions could occur. Otherwise,
areas of snow across Sweetwater County this morning will
spread/develop northeast, and begin to impact Natrona County around
midday. The most impacted terminal will be CPR where prolonged IFR
conditions are expected this afternoon into early evening due to
snow and/or low ceilings. After 21Z, isolated to scattered snow
showers could develop over far western Wyoming, and continue through
Sunday night. A cold front will push south, east of the Divide, this
morning shifting winds northerly. Low ceilings between 040-070 kft
agl are expected to develop gradually in the wake of the cold front
today for areas east of the Divide. The exception will be across
Natrona County where IFR to MVFR ceilings are expected.



The trough and cold front will spread showers across the south first
then over much of the area east of the Continental Divide with rain
snow mix changing to snow this morning. The snow then shifts across
the east later this morning.  Low clouds and light showers will
linger over the mountains with partial clearing in the western
valleys today and Monday with light snow accumulations. Temperatures
will cool slightly. Monday will see clearing most areas of the east.
The pattern changes little on Tuesday with mountain showers across
the west with partial clearing east with slight warming as west
winds pick up. With colder cloudier and showery weather across the
west mixing heights will be low with poor to fair smoke dispersal.
The windy areas east will have good to very good smoke dispersal in
the afternoons.  The less windy areas of the east will have fair to
low end good smoke dispersal mainly lacking transport winds.




LONG TERM...Meunier
FIRE WEATHER...Lipson is the U.S. government's official web portal to all federal, state and local government web resources and services.