Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 160406 AAA

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1006 PM MDT Wed Mar 15 2017

Issued at 1004 PM MDT Wed Mar 15 2017

Added 06z Aviation Discussion.


.SHORT TERM...Tonight through Saturday

Imagery continues to show deep large trof east of the MS river with
a robust occluded low over the New England area. West of the MS
river however, a large broad ridge is in control with upper level
flow over WY transitioning to flat or slightly swrly flow as an
embedded SW/jet approaches the FA. Orographic cirrus off the
Absaroka and Wind River Mountains is rather prolific today. The SFC
has general high P across the Great Basin and portions of the
intermountain west with a growing lee trof east of the Rockies from
central MT down along the CO Front Range. No precipitation is
currently falling across WY.

Today and tonight, the upper ridge is largely protecting the FA from
precipitation with only an outside chance of a few showers across
YNP. Expect a breezy warm day with afternoon moderately gusty west
to southwest wind across the Wind Corridor. Fire danger for portions
of Natrona and Johnson county will range from moderate to high this
afternoon. Snow melt and consequences thereof will continue to be
monitored...particularly over southwestern WY.

Late tonight and Thursday, the upstream SW/jet approaches and noses
itself into/through the ridge, flattening it and continuing to
induce lee side troffing across the Basins and High Plains east of
the Divide. This will lead to an increase in gusty west to southwest
winds across 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, 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 increase precip chances over
western and northwestern WY. A short period of moderate to localized
brief heavy snowfall will be possible Thursday morning...mainly
above 8500k ft as the embedded SW/jet and associated PVA moves
across the area. Expect areas of light rain-snow mixes or possibly
all rain, at times below 8000 ft. This system is fairly quick, only
putting down relatively light precip not upsetting the snowmelt
situation too much. Meanwhile, lee cyclogenesis over the CO High
Plains into the Central Plains and frontogenesis to the north of WY
in MT will eventually start to pull a front down through ern WY
Thursday afternoon with gusty north winds and light precip
(mainly rain showers) possible over northern Johnson county.

Thursday night through Saturday, the SW exits across the Plains and
ridging returns to the wrn CONUS and WY. Warming will return as well
while winds will be on the decrease as modest high P settles across
the FA in the wake of the exiting system. An increase in snow melt
is expected to return once again...particularly on Saturday due to
the warming conditions. Fire danger will also increase once again
with both Natrona and Johnson possibly reaching Red Flag levels for
low RH and gusty southwest winds a few hours Saturday afternoon.
Will continue to monitor both river levels and fire chances.


Agreement between medium-range forecast models is fairly good through
the bulk of the long term forecast period. Ridge moves off to our east
Saturday night and Sunday with strong trough moving across western
Canada. Tail end of this trough will bring moisture to northwest
Wyoming Saturday night and potentially across the far north on Sunday.
Have nudged precipitation chances upward Sunday over the far northern
Big Horn Basin and northern Johnson County. Accordingly, cooler air
will also push across northern and central sections of the forecast
area with highs Sunday about five degrees cooler than Saturday. Flow
off the Pacific remains open in west-southwest flow to begin the work
week. This will keep a chance of mountain snow and valley rain showers
in the forecast over the far west, especially the northwest, on Monday.
Temperatures Monday will be closer to seasonal normals. Flow begins to
buckle Monday night and Tuesday in response to deepening trough off the
West Coast. Therefore, transient ridge will build over the central and
northern Rockies early Tuesday, traversing the region later Tuesday and
Tuesday night. Warm advection and passing ridge axis will lead to a dry
and warmer day east of the Continental Divide. Southwest flow of the
Pacific will have moisture returning to the far west by late Tuesday.
Differences in the models begin to emerge Wednesday and Thursday. The
GFS is faster in bringing the West Coast trough onshore and into the
Great Basin by late Wednesday, whereas the ECMWF is about 6-12 hours
slower. Probably little tangible difference in the sensible weather as
it is appearing more likely that the energy with this trough may split.
With the uncertainty at this time range, strayed little from blended
guidance and remained consistent with surrounding WFOs. Temperatures
will remain above normal Wednesday with southwest flow aloft. Sunday
and Wednesday should be the windiest days, primarily on the east slopes
of the mountains and in the Wind Corridor from eastern Sweetwater into
Natrona County. Neither day appears to present anything out of the
ordinary for mid-March.




VFR conditions should prevail through most of the forecast period.
The ridge axis is finally progged to move east through the area
during the beginning of the forecast period. The models are
continuing to indicate that a low level flow will result in the
lower atmosphere gradually moistening. Will maintain the predominant
6sm -SN from 09z-15z at KJAC  and expect VCSH downstream at KBPI and
KPNA between 12Z and 18Z/Thu. Mountain top obscurations will be
widespread from 09Z-18Z/Thu along and west of the Continental
Divide. Substantial drying is expected in the wake of a shortwave
and cold frontal passage by late Thursday morning, so all terminals
will be VFR by 18Z/Thu. Surface winds will increase around 18Z-
19Z/Thu in the wake of this weather system. Speeds will generally be
15-25kts during the afternoon. Will gradually lessen wind speeds
across the TAF sites after the 02z Friday time frame toward the end
of the forecast period.


VFR conditions will continue through the forecast period. Will
continue with gusty west-southwest winds at 15-25kts at KCPR through
about 00Z/Thu. Still looks like late afternoon mixing will allow for
westerly wind of 10-18kts at KRIW and KLND. West-northwest down
valley flow will favor wind continuing through Thursday evening at
KRIW. KCOD will see off-again, on-again west wind of 10-20kts
throughout the period.

Shortwave and cold front will sweep east toward midday Thursday with
surface wind speeds increasing from the west toward 18Z/Thu.
Afternoon winds will continue to increase between 18Z-20Z/Thu with
widespread 15-25kts. Higher speeds are expected at KRIW where west-
northwest flow is favored and at KCPR where pressure gradient will
keep winds up. Will eventually lessen winds toward the 02z-03z time
frame as the pressure gradient begins to relax somewhat.



Fire Danger low west of the Divide...low to high, depending on
elevation, east of the Divide through Saturday. Today east of the
Divide, temperatures will warm into the 60s and lower 70s while
minimum RH values fall into the mid/upper teens and 20s for some
basins and lower elevations. This will increase fire danger into the
moderate to high category for portions of Natrona and Johnson
counties as near critical fuels, low RH and breezy conditions
combine...although it should only last a couple of hours this
afternoon. Thursday...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 also increase to the 20 to 30
percent range, keeping fire danger elevated...but not extreme.
Otherwise, light areas of precipitation will be possible west of the
Divide and across the north with the approach and passage of the
next weather disturbance and associated front. Post frontal on
Friday and Saturday, warming returns to the region with increasing
fire danger once again east of the Divide. 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.




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