Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 190917

Area Forecast Discussion
National Weather Service Riverton WY
317 AM MDT Mon Mar 19 2018

.SHORT TERM...(Today through Wednesday night)

Except for one isolated snow shower in the Red Desert, most of
the snow has exited to the southeast of the CWA as the trough
exits east and the energy shifts to the Southern Plains.

Then the focus shifts to western and northern Wyoming this afternoon
out ahead of a northern stream southeast bound shortwave as lifted
indices drop to below zero in these areas. Expect convection along
with isolated thunder snow to occur west of the divide and at the
higher elevations east of the divide in northern and western wyoming
this afternoon and evening. Precip will be mostly rain this
afternoon at the lower elevations in northern Wyoming east of the

Tuesday will be mostly dry except for isolated rain and snow showers
in the northwest as a dirty flat ridge builds in from the west. Some
isentropic lift will result in areas of light rain and snowfall in
the northwest Tuesday night. On Wednesday and Wednesday night, rain
and snow will increase in intensity in the far west as the flow
backs, increases, and becomes increasingly difluent along with
increasing upslope ahead of the next approaching main trough. Expect
accumulating snowfall in the far west Wednesday and Wednesday night
in the far west, but as of now will keep snowfall amnts sub

Overall, expect a gradual warming trend through mid week.

.LONG TERM...(Thursday through Monday)

...Valley Rain Showers Likely in the West Thursday will Elevate
Flooding Potential Thursday and Friday...

Overview...A Pacific cold front will move across the area on
Friday, followed by a stronger cold front late Saturday. Valley
rain and mountain snow showers will increase from the south and
west on Thursday. Flooding concerns across the west will be
elevated Thursday and Friday with warm temperatures Thursday and
the potential for 0.25" to 0.50" of rainfall amounts on remaining
snowpack below 8000 feet. The cold frontal passage on Friday will
shift the heavier precipitation into the northern Plains, and snow
levels will lower to valley floors across the west. A stronger
cold front will push into the west Saturday morning and across the
area through the day. This front may bring a period of moderate
to heavy snow to the west, and very windy conditions east of the
Divide on Saturday. Cool and unsettled conditions will likely
linger behind this cold front on Sunday and Monday.

Discussion...Our main weather makers during the extended period will
be a shortwave moving into western Alaska early this morning (the
kicker) and a cut off low over the eastern sub-tropical Pacific
(the kickee). The Alaskan system is expected to dive south across
the Gulf of Alaska in the shortterm and bottom out off the
northwest U.S. coast on Thursday. This system will begin to
kickout the eastern Pacific low Tuesday and Wednesday, with the
kickee opening up and lifting across the Great Basin, central
Rockies on Friday. Both GFS and ECMWF show robust fetch of sub-
tropical moisture extending from south/central CA into southwest
Wyoming on Thursday ahead of this system with both models trending
warmer with vertical temperature profiles over western Wyoming.
Thus, confidence is higher this morning that precipitation will
likely fall mostly as rain below 8000 feet across the west on
Thursday. Forecast also continues to trend temperatures warmer
across the area both Thursday and Friday with sub-tropical Pacific
origins of this first system.

ECMWF and GFS in better agreement moving the upper low off the NW
coast through the Intermountain West/Rockies over the weekend,
both models now bringing the surface cold front associated with
this system across the area on Saturday. This upper trough will
still have a -35c to -37c H5 cold pool in its core, so it will
pack quite a punch in late March. Baroclinic band of heavier
snowfall should hit the west Saturday morning with a good 12-18
hour period of moderate to heavy snowfall (likely winter wx
advisories, possible WS Watches/Warnings). The main concern east
of the Divide will be the potential for high winds with large cold
pool spilling down the east slopes. Cool, unsettled weather will
follow on Sunday under ~ -35c H5 cold pool.


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

The main aviation concern will be across Johnson and Natrona
counties where IFR conditions in low clouds and possibly some fog
are expected through about 15Z Monday. Snow showers with localized
IFR/MVFR conditions south and east of a line from KRKS-KRIW-KCPR
will gradually come to an end by 12Z Monday. Otherwise a mainly
dry morning is expected, except perhaps the far west where some
snow shower activity cannot be ruled out. This afternoon isolated
to locally scattered mountain snow showers are expected to develop
over northern/and perhaps central mountains spreading east into
parts of the lower elevations through the evening hours. This snow
shower activity should rapidly dissipate with loss of daytime
heating. Breezy west to northwest wind are expected across the
south and southwest part of the area, while most of the other
areas will be relatively light.


Issued AT 226 AM MDT Mon Mar 19 2018

Isolated to scattered snow showers redevelop over the northwest
today along with isolated thunderstorms. precipitation will in the
form of rain or mixed snow and rain in northern Wyoming this
afternoon east of the divide. Elsewhere, conditions will mainly be
dry across the area, with seasonal temperatures and gusty winds
across the south. A warming trend will begin Tuesday and continue
through the week. Gusty winds will also return across wind prone
locations (mainly during the afternoon) from Tuesday through
Thursday. At the same time, mixing heights will also continue to
increase through the week. Smoke dispersal will also be fair to




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.