Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 221840
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1240 PM MDT Wed Mar 22 2017

.SHORT TERM...(Today through Friday night) Issued at 1238 PM MDT
Wed Mar 22 2017

Imagery shows ridge axis moving into the cntrl CONUS with a large
upstream trof still offshore nrn California/OR with a modest jet
moving through the trof`s base across through central California
and into NV. At the nose of said jet, a small embedded disturbance
is moving over the nrn Rockies...approaching western WY. At the
SFC, post frontal high P continues across the northern High Plains
and Northern Plains with lower P centered over nrn UT along with
a nearly stationary front draped across southern, southwestern,
and west central WY then down along the Front Range of CO. Widely
scattered light rain or snow showers mainly west of the Divide
this morning with patchy areas of BR/FG over nrn WY.

Through tonight: Modest ridge axis continues to slowly translate ewd
and over the Plains` states with embedded disturbance and just
enough upper level flow/forcing across wrn WY to allow for isolated
to locally scattered rain and snow showers, including isolated
thunder, to occur. Tonight, as the large upstream trof deepens moves
over the western coastline, both a strong jet as well as increasing
sub-tropical moisture will start to make their way through the Great
Basin and into WY from the southwest. POP chances will increase as
well as the probable QPF values with the southwest quad of WY,
including the Wind River Mountains...receiving the better
moisture...both liquid and frozen. Stronger low topped thunderstorms
also possible over swrn WY in the afternoon and early evening as
CAPE may exceed 500j/kg with shear running 30+ kts. Additionally, up
to a couple of melting inches of snow may be possible by Thursday
morning...particularly over the region`s Mountains.

Thursday and Thursday night: The deepening and closing trof/low
swings into/through the Great Basin and portions of the Desert SW
with what looks to be setting up as a classic Four Corners Upper Low
and strong cyclogenesis proceeding across ern CO Thursday afternoon.
Strong low to mid-level frontogenesis will also take place to the
north of this area over central WY roughly from southwest to
northeast. This frontal zone will then push to the southeast through
Thursday afternoon and overnight. The entire FA will have decent
chance for precip...some convective with thunder Thursday afternoon
then more stratiform/upslope by evening and overnight. Best moisture
convergence through the cyclone`s maturation will come together
Thursday night across eastern/southeastern WY...possibly making it
back into portions of southeastern Natrona county. Snow levels will
be tricky and borderline at times below 7-8k ft and may have to wait
for the cold frontal genesis/passage later Thursday afternoon and
night before snow appears east of the Divide down to the Basin
floor. Locations like the Jackson Valley may remain protected from
the colder air all through this portion of the forecast, getting
little if any snowfall. Highlights will also be tricky and may best
fall into Advisory or SPS territory. SPSs now issued. Flood
potential will also rise appreciably across southwest WY where the
current Flood Watch resides with 48 hr QPF values ranging between
0.5 and 1 inch for much of the area...locally higher where
convective elements increase precipitation. Precipitation values
along these same lines will also occur in the Wind River Basin, the
southern Big Horn Basin as well as over the Bighorn Mountains...all
areas that may also experience some elevated flood
potential...although probably to a lesser extent than across SW WY.

Friday through Friday night: Post frontal and drying with upper
ridging moving back over the region and modest high P building
across the surface. Upstream, another trof will already be moving
through the wrn CONUS with current timing of the lead portion of the
wave arriving in wrn WY later Friday night with another round of
moderately unstable conditions producing periods of both rain and
snow. Snow levels across wrn WY will likely remain above 7500 feet
through Friday night.

.LONG TERM...(Saturday morning through Tuesday) Issued at 1238 PM
MDT Wed Mar 22 2017

Synopsis...A seasonably cool and unsettled pattern will prevail
through the period as a series of Pacific storm systems track across
the southern and central Rockies through the period.

Discussion...Synoptic pattern on Saturday morning features closed
upper low (#1) in extreme SE Kansas, split upper trough (#2) moving
across the 120W meridian along the west coast, with another trough
(#3) further upstream moving across the east central Pacific. 23/00z
GFS has trended toward the ECMWF`s faster solution moving southern
portion of split trough #2 across the Great Basin Saturday, across
the Four Corners area Saturday night.  However, GFS remains stronger
than ECMWF with northern stream portion of split across the northern
Rockies Saturday night.  Overall trend toward more progressive split
open trough will be less favorable for widespread significant
precipitation over the weekend.  Showers, with a few embedded
thunderstorms, will increase across the west during the day Saturday
with snow levels near 6kft.  Lee cyclogenesis in the southern High
Plains will pull a cold front south across central Wyoming Saturday
night/Sunday morning with chances of light snow mainly along the I-
25 corridor with any accumulations expected to be light.  Shortwave
ridging is expected to build into the area through the day Sunday as
trough #3 moves onto the West Coast.  GFS shows an upper low closing
and cutting off over Arizona/New Mexico Tuesday and Wednesday while
the latest ECMWF shows more progressive shortwave moving across the
southwest Monday before digging broader upper trough from the
Intermountain West the Central Plains by late Tuesday.  Confidence
in either of these scenarios is low, neither model has shown any
consistency with trough #3.  Therefore, forecast holds onto idea of
next round of rain and snow showers pushing into the west on Monday,
spreading into central Wyoming Monday night into Tuesday, drier
conditions Tuesday night and Wednesday.

&&

.AVIATION...(For the 18Z TAFS through 12Z
Thursday morning) Issued at 1238 PM MDT Wed Mar 22 2017

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Scattered showers will occur over the west through 12Z. Local MVFR
conditions may occur in NW WY. Otherwise, VFR conditions to prevail
today through this evening. Scattered showers will reform over
southwest WY this afternoon through this evening. An isolated
thunderstorm cannot be ruled out. Snow will fall in the higher
mountains with some mountain obscuration. Showers will increase
over southwest WY by 06Z Thurs with rain changing to snow in
parts of SW WY by 12Z Thursday. Areas of MVFR/IFR conditions will
occur in SW WY after 06Z. Please see terminal forecasts for more
detailed information.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

VFR conditions will prevail over the region through 06Z Thursday.
Scattered showers and isolated storms will develop over southern
and central WY this afternoon into the evening. Then showers will
become more numerous over this area by 06Z Thursday. After 06Z
Thursday, areas of MVFR ceilings will occur. Please see terminal
forecasts for more detailed information.

&&

.FIRE WEATHER...
Issued AT 143 AM MDT Wed Mar 22 2017

Fire danger will be low through the weekend. The exception to this
rule will occur today which shows signs of having elevated fire
danger across portions of central WY as gusty southwest winds
combine with lowering relative humidities and a few chances for
gusty isolated thunderstorms in the afternoon/evening. Thursday also
shows signs for embedded thunderstorms across southern Wyoming,
however the day on the whole will have increasing moisture spreading
across WY along with good chances for significant precipitation at
most locations...both liquid and frozen.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late tonight for WYZ013-023-025>027.

Winter Weather Advisory from midnight tonight to midnight MDT
Thursday night for WYZ008-009.

Winter Weather Advisory from 6 PM this evening to midnight MDT
Thursday night for WYZ014-015.

&&

$$

SHORT TERM...Braun
LONG TERM...Meunier
AVIATION...Skrbac
FIRE WEATHER...Braun



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