Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 181959

Area Forecast Discussion
National Weather Service Riverton WY
159 PM MDT Wed Oct 18 2017

.SHORT TERM...(This evening through Saturday) Issued at 156 PM
MDT Wed Oct 18 2017

Imagery shows a broad ridge across the wrn 2/3rds of the CONUS and
the main storm track and nrn branch of the jet well to the north,
near or north of the border with Canada. A weaker srn branch of the
jet lays south near/south of the border with Mexico with WY under
relatively weak nearly zonal flow between these two areas. Much
further upstream near 50N/140W, is the approximate location of the
next trof (axis) that will affect the FA later Friday. The SFC has
low pressure across the Desert SW with high pressure entrenched over
the Great Basin and Central Rockies west of the Continental Divide
with a weak clipperish type front stretching all the way from a deep
low over cntrl Canada, down through the nrn Plains and across nern
WY and the border with MT. No precipitation is occuring within WY.

Rest of today and tonight, a relatively weak SFC P GRAD with weak
high pressure across most of the FA combined with nearly but not
quite unidirectional modest flow aloft will keep the region of the
central and northern mountains in addition to the Wind Corridor dry
and breezy today. this will in effect raise fire weather concerns,
especially across srn/ern Fremont and all of Natrona Counties this
afternoon...with borderline Red Flag conditions from time to time
over Natrona County. An SPS has been sent this morning for this
reason. Tonight, winds decrease and RH recovers. Winds later this
afternoon and tonight across the Absarokas and adjacent Cody
Foothills could occasionally gust 40 to 50 mph (a couple isolated to
60 mph not out of the question).

Thursday, a transition to swrly flow aloft occurs with the approach
of the previously mentioned upstream trof. Although winds through
the vertical do not really increase, the flow does become more
aligned and unidirectional. This along with slight better warming
near the SFC will cause low level lapse rates to rise a bit,
providing a better means to get slightly stronger winds aloft also
down to the SFC. This in turn will give a better chance for Red Flag
conditions to occur over Natrona and Johnson Counties in the
afternoon. As such, a Red Flag Watch has been issued covering
Thursday afternoon. Another brief period of high winds could also
occur later Thursday night/early Friday morning across the Cody

Friday, the FA remains under swrly and slightly divergent flow aloft
while the SFC and boundary layer P GRAD increases rapidly ahead of
an approaching and strengthening cold front from the west. Winds
will increase first across the higher elevations of the west Friday
morning before spreading east of the Divide int he afternoon and
continuing into the evening. Winds gusting out of the sw could
easily reach 40 to 55 mph (isolated higher at the usual spots) in
Sweetwater, Sublette, Fremont and Natrona Counties in addition to
the Cody Foothills. The Clark area could gust to high wind levels by
Friday afternoon especially if an elevated inversion under WAA
occurs. Minimum RH looks to "only" bottom out in the mid/upper teens
and 20s over most of the area east of the Divide. However, fire
weather conditions east of the Divide will still warrent "elevated"
status if nothing else because of the strong winds and Friday
evening wind shift. Otherwise, modest EPAC moisture flows into
wrn/nwrn WY from the west through the trof Friday afternoon. A rain
or rain snow mix across some valley locations can be expected Friday
aftenroon with all snow generally above 6.5k feet...lower all the
way to the valley floors by early Friday evening. 24 hr QPF totals
west of the Divide through Saturday afternoon will range from a few
hundredths across southwest WY to perhaps 0.3 to 0.4 inches across
northwest WY. Snow accumulation over the time frame will range from
a tenth to perhaps a couple of inches in the western valleys
(especially the northern Jackson Valley)...with anywhere from a half
inch across the southern mountains to as much as 6 inches over the
higher elevations (> 8500 feet) of the Teton and Gros Ventre Ranges.
The cold front will then move east of the Divide late Friday
afternoon through Friday night...with low chances for precip arrive
mainly over portions of the Cody Foothills and Big Horn Basin.
Saturday will be post frontal, quite cool, relatively dry and breezy
with lingering morning mountain showers giving way to partly to
mostly sunny skies by Saturday afternoon.

.LONG TERM...(Saturday night through Wednesday) Issued at 156 PM
MDT Wed Oct 18 2017

Our Saturday shortwave will be exiting the region Saturday night
with a few lingering flurries around the peaks. The one exception to
that will be the nw mtns where strong upglide signature will lead to
widespread mountain snow, especially from the Tetons northward. Snow
levels will rise on Sunday before ending but until then, even the
lower elevations of the nw, it will likely be snow. The other
significant weather to start this period will be the strong/high
wind potential along the lee of the Absarokas and Cody Foothills.
Strong nnw-sse jet will be replaced by a strong w-e jet for Sunday
night. Will hit the winds much harder than the blend is giving us.
Shortwave approaching from the nw will blast through later Sunday
night into Monday with a strong cold front moving south across the
area late Sunday night or early Monday through the day. Temps look
cold enough for potential rain changing to snow in the far nrn zones
Monday. Dry for most/all of the cntrl/swrn half. Surface high moves
overhead Tuesday for a quite cool (we`re likely too warm yet) day
for most areas but some recovery returning to the sw. Dramatic
warmup with building longwave ridge for the second half of the week.
H7 temperature reversal is quite amazing going from around -6C in
Casper Tuesday morning to nearly 12c (GFS) Wednesday afternoon.
Areas that mix will see a dramatic warmup from Tuesday to Wednesday
while calm basin/valleys likely struggle but still warm
significantly (if the GFS verifies). The new Euro though breaks down
the ridge some allowing a disturbance to move into the nrn plains
late Wednesday with some cooling spreading into the north half (it
still has very mild mid level temps over the area Tuesday night).
GEM keeps the ridge over the area even sharper than the GFS. GFS 5-
wave shows mean ridge building near 120W so there is some
possibility that a Euro type solution could verify so will certainly
not adjust any temps up yet this far out. Current forecast has low
to mid 60s east of the divide for now with 50s west.


.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 156 PM MDT Wed Oct 18 2017

VFR conditions will occur across the area through Thursday. A strong
and gusty west to southwest wind return to the Cody Foothills this
evening and continue overnight. Strong southwest winds will blow
from 50ne KRKS to KCPR on Thursday.


Issued AT 156 PM MDT Wed Oct 18 2017

Fuels remain at critical levels across over Johnson and Natrona
Counties. (Please see the Fire Weather Forecast product for more

...Elevated Fire Conditions Across Portions of Central Wyoming this

...Fire Weather Watch for Natrona and Johnson Counties Thursday

Today will continue with above seasonal temperatures and be quite
dry with minimum RH values ranging from 10 to 15 percent across the
lower elevations east of the Divide.Winds across central Wyoming
will gust out of the southwest 15 to 25 mph at times. No
precipitation expected today. Due to stated conditions, a brief
period of Red Flag conditions could present itself across Natrona
County this afternoon. Thursday, a slight improvement in relative
humidity is expected, although not enough with RH values still
ranging into the mid/lower teens (percent). Winds on the other hand
will increase ahead of the next approaching weather system and
become more unidirectional through the vertical making for easier
chances to see gusts in excess of 25 mph throughout the afternoon. A
Fire Weather Watch has been posted for these possible conditions
coming together Thursday afternoon. Otherwise, smoke dispersion will
be good to excellent both this afternoon and Thursday afternoon.


Fire Weather Watch from Thursday morning through Thursday
evening for WYZ280-281.



LONG TERM...Skrbac
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