Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 200918
Area Forecast Discussion
National Weather Service Riverton WY
318 AM MDT THU OCT 20 2016
.SHORT TERM...Today through Saturday night
Imagery shows broad trof across the ern 2/3rds of the CONUS with
ridging over the west. The axis of the main trof has cleared WY and
is now located through the central Plains. SFC has general high
pressure from the Great Basin and into the central/northern Rockies.
Cold front well to the east over the Plains with a minor surface
trof through central/southeast WY and a few associated light snow
showers in tow.
Today through Saturday night: With modest ridging aloft building
across the FA, drying and warming will work across WY. The only real
weather of any significance will be with some breezy gusty winds
across the wind corridor each afternoon through
Saturday...increasing fire concerns somewhat...especially across
Natrona county. However, fire weather conditions look to remain
below Red Flag criteria for the most part...except for perhaps and
hour here or there. Saturday would be the day most likely to see
actual, but marginal, Red Flag conditions in the afternoon. Worth
watching. Otherwise, the late overnight through morning period in
the Cody foothills each night will see some increase in gusty
winds...with Saturday morning having the best potential for possible
high wind gusts near Clark and Hwy 120. Again, worth watching.
.LONG TERM...Sunday through Thursday
A strong upper low is progged to be off the Pacific Northwest
Coast Sunday. This feature is expected to remain nearly stationary
through Wednesday with the forecast area in a modest to strong
west to southwest flow aloft. Disturbances in the southwest flow
as well as some jet dynamics at times will give the
west/southwest areas periodic chances of precip Monday through
Wednesday. Areas east of the Divide are expected to remain mainly
dry. Above average temperatures are expected across the entire
area through Wednesday.
On Sunday, the upper level ridge axis will shift east across the
area with dry & mild conditions across the entire area.
By Monday, some moisture begins to shift north/northeast into the
area. This combined with a weak disturbance in the southwest flow
could result in some valley rain/mountain snow showers over the
west and southwest.
Monday night and Tuesday could have better jet support with
increasing chances of precip out west. However models have not
been consistent in jet position/timing so will keep PoPs
By Wednesday, models show the area on the anti-cyclonic side of
the upper jet with much of the precipitation possibly shifting
northwest/north of the forecast area. However, will keep some
chances of precip ongoing out west due to uncertainty on how
things will pan out.
00Z global models then show the main upper low off the Pacific
Northwest opening up, and shifting east across the Northern
Rockies as a strong shortwave trough with an associated cold front
Thursday and Friday.
WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES
Patchy fog and LIFR cigs may partially obscure terrain across the
western valleys through 18z. The snow-covered Upper Green River
Basin will be most prone to fog formation this morning. Otherwise,
VFR conditions will prevail through the period with generally light
surface winds - 10 knots or less. Increasing mountain top winds
will result in increasing turbulence Thursday night; however, these
stronger winds are expected to remain around FL 10kft and higher.
EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES
VFR conditions prevail through 12Z Friday. A strengthening lee side
surface trough will bring prevailing southwest surface winds 20-
30kts vicinity 50NE KRKS-KCPR late Thursday morning and afternoon.
Some gusty downslope winds will also develop vicinity KDUB-KCOD
early Thursday morning through Thursday night. Strengthening
mountain top winds may result in some mountain wave activity
and LLWS vcnty KCOD to the Montana border 00z-12z Friday.
Fire danger low most locations...moderate over portions of central
WY today. Minimum RH values remain at or above 25 percent for most
lower elevations on either side of the Divide. However, some
locations in both Natrona and Johnson counties will drop into the
upper teens (percent). These conditions combined with lingering
critical fuel values and winds occasionally gusting 25 to 30 mph
will allow for moderate fire danger over Natrona and Johnson county
this afternoon. Fire danger will remain much the same both Friday
and Saturday over these same areas with Saturday the most likely to
see at least brief high to extreme fire danger in the afternoon.