Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 240518
Area Forecast Discussion
National Weather Service Riverton WY
1118 PM MDT TUE AUG 23 2016
.SHORT TERM...Tonight through Friday (Issued at 148 PM MDT)
Imagery shows large broad trof across the western CONUS with modest
ridging over the eastern CONUS. Embedded shortwaves in the trof are
currently moving across northern WY...with another following and
located from western MT into northern ID and WA/OR. SFC has
front/trof through eastern WY and into the Plains with higher
pressure off to the west. Very dry airmass continues over most of
the FA today.
Weak cold front swept through the northern/eastern last
night...mainly affecting areas east of the Divide...bring smoke from
the northern fires into portions of central WY. Today will see
similar conditions to that of yesterday, but with slightly cooler
temps and increased RH values across the northern forecast zones.
The gustier portion of the wind field will shift further south
across far western WY and be slightly less vigorous too. However,
winds and low RH values will be at or near critical south of this
frontal passage mainly across southern and southwest WY and also
portions of the central/northern mountains which once again will see
extreme fire weather conditions...and where the above mentioned RFW
resides. Only very low chance for a couple of isolated
showers/storms across the far south once again.
Tonight another front moves through the northern FA, then east of
the Divide and into the southern zones by Wednesday morning,
bringing another a change in the weather...at least east of the
Divide and over portion of northwest WY. As the area goes post
frontal with the passage of the PAC NW trof there will be some
increase in moisture with yet another stronger front moving into the
northern zones by Wednesday afternoon. There will be increased
chances for precipitation across the north and the east of the
Divide. But with only modest increase in moisture, most of the
showers/storms will initially provide little rainfall...at least
until late Wednesday night/Thursday morning...but gusty erratic
winds to 45 mph will be possible. Cooler everywhere Wednesday with
higher RH values. However, southwest WY will again remain very dry.
Winds, however, look to behave under the trof`s influence and
frontal passages...so no fire weather headlines expected...although
zone 277 will be close to Red Flag conditions and warrants
Wednesday night through Thursday night, with much cooler
temperatures, increased moisture (finally) liquid showers and some
light snow expected across the higher terrain of central and
northern WY overnight through Thursday morning...especially in the
Beartooth and Bighorn mountains with perhaps and inch or two on the
ground above 9500 feet by noon Thursday. Otherwise, most
precipitation for the day Thursday is expected to fall across
northern WY and to a lesser extend over portions of central WY. A
few storms may become briefly strong over northwest/northern WY with
brief heavy rain, small hail and gusty outflow winds. Light snow may
return to the northern mountains for a brief period Thursday night.
Friday will see the final portion of the upper trof pass through.
With areas of sun and modest warming at the surface under cold temp
aloft, the FA will remain somewhat unstable so that by afternoon
showers and storms will likely break out over the mountains at
least...trying to move east to southeast into the adjacent
foothills and/or basins.
.LONG TERM...FRIDAY NIGHT THROUGH TUESDAY
Models have been struggling as to when our developing troughiness
will swing east of us. Today the majority of the guidance has it
swinging east of us to start this period while yesterday it was
Saturday. Mean ridge is forecast to be out near 160W/40-50N to start
this period while positively tilted trough is forecast to extend
from Nrn Canada sswd to the Great Basin which would still favor some
energy over us into Saturday. Blend is fairly dry and for now will
go with a warming trend for Saturday through Monday before another
system passes to the north on Tuesday with at least a little
cooldown in the north. Otherwise, it just looks increasingly warm
and dry with increasing fire weather concerns once again. H7 temps
warm about 1C each day from Saturday into Monday starting out in the
10-11C range Saturday. Tuesday may see a little cooling behind the
front in the far north/nern zones. Otherwise, 80s east of the divide
with some 90s possible by Sunday and Monday in the lowest
elevations. The west will be in the 70s with a few spots touching
80. Little pcpn in this pattern expect potentially right at the
beginning Friday evening with a decent shortwave expected to be
passing se across the area.
VFR conditions will occur through 06z Wednesday. Isolated showers and
thunderstorms will develop over northwest WY after 18z and continue
through 06z Thursday. KJAC will be impacted by this activity at times.
Smoke from nearby wildfires may reduce visibility in the Jackson area
at times. Southwest WY will be mostly dry through the period.
VFR conditions will occur east of the divide through 00z Wednesday.
Isolated showers and storms will develop after 19z...initially near
the mountains...then spreading over the lower elevations during
the afternoon and evening hours. Low clouds will develop over
northern WY after 00z Wed with MVFR conditions prevailing by 06z
over the north.
Fire weather conditions elevated to critical across the forecast
area today...then across the far southwest corner of WY Wednesday.
With a warm and very dry period continuing until the next fall-like
system arrives later tonight, RH values will drop quickly again
today, ranging from the single digits to lower 20s (percent) for
most locations across the forecast area. Winds gusting upwards of 25
mph across the critical zones will combine with these very dry
conditions and critically dry fuels to give extreme fire weather
conditions across these zones. A reprieve from these fire weather
conditions looks to arrive Wednesday for most of the forecast area
as cooler weather with some modest chances for precipitation invades
the region...especially across northern and central Wyoming. The far
southwest corner of Wyoming may be the only area that does not see
significant improvement and could get close to critical fire danger
yet another day...at least for a couple of hours. Will wait and
monitor for another forecast cycle or two before making a decision
at this time.