Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 170526
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1126 PM MDT Thu Mar 16 2017

.UPDATE...

Issued at 1125 PM MDT Thu Mar 16 2017

Added 06z Aviation Discussion.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 154 PM MDT Thu Mar 16 2017

Imagery shows flat ridge across the wrn CONUS with linear jet/SW
from the EPAC...through the PAC NW states and into WY. The large
deep trof across the ern CONUS is finally exiting into the Atlantic.
Upstream disturbance well out into the Pacific and due to arrive
near the FA Sat night and Sunday is currently located near 150W 35N.
SFC has high P across the Great Basin with lowering pressure to the
east/northeast with a deepening trof over the high/central Plains.
Upper jet/SW is causing scattered showers and a few embedded
isolated thunderstorms mainly across northwest and northern WY.

Today, the upstream SW/jet noses itself into/through the ridge,
flattening it over WY inducing lee side troffing across the Basins
and High Plains east of the Divide in the process. This will lead to
an increase in gusty west to southwest winds across the FA...and
especially the Wind Corridor from Rock Springs to Casper as the SFC
P GRAD increases and stronger flow aloft is adiabatically brought
to/near the SFC as lower level temperatures and instability
increases. Although high winds are not expected except for a few
isolated cases, these conditions will keep fire danger on the
moderate to high side for a few locations east of the
Divide...particularly across Natrona county. However, low to mid
level moisture will also be on the increase over the FA and will
mitigate any widespread fire threat. Additionally, this transition
to relatively deep westerly flow will open the door to a modest EPAC
moisture intrusion. Both the terrain lift and increasing convective
instability will keep some modest precip chances over western,
northwestern and northern  WY through early this evening. Isolated
thunder will be possible with some of this activity, particularly
north of a Jackson to Cody to Buffalo line. light to occasionally
moderate snowfall will be possible this afternoon mainly above 8500k
with areas of light rain at times below 8500 ft. With this system
being fairly quick, it will only put down relatively light precip
while not upsetting the snowmelt/potential flooding situation too
much. Meanwhile, lee cyclogenesis over the High Plains into the
Central Plains and frontogenesis to the north of WY is now pulling a
front down through ern WY with gusty north winds and light precip
(mainly rain showers) continuing to be possible over the northern
Big Horn Basin and northern Johnson county.

Tonight through Saturday, the SW exits into/across the Plains and
ridging begins to return to the wrn CONUS and WY. Warming will
return while winds will be on the decrease as modest high P settles
across the FA in the wake of the exiting system. An slow increase in
snow melt is expected, particularly Saturday, due to the warm and
windy conditions expected. Fire danger will also increase on
Saturday with both Natrona and Johnson possibly reaching Red Flag
levels for low RH and gusty southwest winds a few hours in the
afternoon. Have issued a Fire Weather Watch for both Natrona and
Johnson counties for Saturday afternoon.

Sunday morning, the upstream SW trof mentioned above makes its way
into western WY, increasing precip chances and bringing areas of
moderate to occasionally heavy snow to the western mountains above
8.5k ft with rain showers or some rain mixed with snow below that
elevation. Sunday afternoon...there will also be isolated thunder
chances once again east of the Divide as CAPE values begin to creep
up under relatively warm and somewhat moist conditions through the
boundary layer and decent shear aloft. Additionally, the warm and
windy conditions on Saturday and increased rainfall chances Sunday
may warrant a Flood Advisory or Watch across southwestern WY
beginning Saturday. Will continue to monitor river stages, snow melt
and river ice for the possibility.

.LONG TERM...(Sunday night through Thursday)
Issued at 154 PM MDT Thu Mar 16 2017

A flat high pressure ridge aloft will extend across the western
U.S. with a series of weak short wave low pressure troughs in
westerly flow bringing some Pacific moisture into western Wyoming,
with Scattered to numerous rain and snow showers over the western
mountains with rain in the valleys.  A cold front will extend from
north to south along the continental divide with some moisture
spreading east of the continental divide for scattered rain and snow
showers in the northern and central mountains and isolated rain
showers in the lower elevations.  Precipitation will generally
decrease on Tuesday as the High pressure ridge aloft builds north
over the area.  Some of the pacific moisture will still spread into
the west.  The flow aloft will become more southwest with warming on
Wednesday.  More moisture will mover up east of the continental
divide with the frontal boundary remaining over central Wyoming.
Showers in and near the western Mountains will be rather widespread.
Temperatures will cool Wednesday night as a cold front surges south
as a cut off low deepens over eastern Colorado.  Precipitation will
become more widespread east of the continental divide especially
along and near the east and north facing slopes of the mountains.
Precipitation will gradually tapper off Thursday afternoon with
high pressure building in from the west.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday)
Issued AT 1125 PM MDT Thu Mar 16 2017

VFR conditions will prevail across the area through 12z Saturday
under a dry westerly flow, scattered mid to high clouds. A
stationary frontal boundary near a vicinity KCPR-KRIW line will
lift north and east Friday morning with prevailing surface west
winds 10- 20 knots vicinity of a KCPR-30SM south KLND line.
Surface winds will be primarily S-SE north of this line. Mid-level
southwest flow will increase Friday night with areas of LLWS
possible east of the Continental Divide, vicinity KCOD-KDUB-KLND.

&&

.FIRE WEATHER...
Issued AT 154 PM MDT Thu Mar 16 2017

Fire Danger low west of the Divide...moderate to high today and
Saturday, depending on elevation, east of the Divide. Today...winds
increase out of the west to southwest east of the Divide and across
southern WY...gusting 25 to 40 mph at times. However, as low to mid
level moisture and clouds feed into the area, minimum RH values will
largely remain in the 20 to 30 percent range, keeping fire danger
elevated...but not extreme. Otherwise, light areas of showers
continue possible over northwest and north WY with the passage of
rather vigorous upper level disturbance and associated front. A
couple of lightning strikes will also be possible this afternoon
across northern WY. Post frontal Friday and Saturday with slow
warming returning to the region and increasing fire danger once
again east of the Divide on Saturday. Saturday afternoon in
particular could see Red Flag conditions for a few hours over both
Natrona and Johnson counties as warm temperatures, low relative
humidities and gusty winds combine. Please see the current Fire
Weather Product for further details.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Fire Weather Watch from Saturday afternoon through Saturday
evening for WYZ280-281.

&&

$$

UPDATE...Meunier
SHORT TERM...Braun
LONG TERM...Baker
AVIATION...Meunier
FIRE WEATHER...Braun



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