Area Forecast Discussion
Issued by NWS Riverton, WY
FXUS65 KRIW 211700
Area Forecast Discussion
National Weather Service Riverton WY
1100 AM MDT SUN AUG 21 2016
.SHORT TERM...Sunday through Tuesday night
The main concerns for the next few days are the flaming variety,
other wise known as fire. Things are quiet right now. Flow will turn
more west to southwesterly today and raise temperatures 10 to 15
degrees warmer than on Saturday. In addition, a weak shortwave
moving across the area will mix the atmosphere a bit and bring
breezy conditions. This, combined with low humidity will bring some
critical fire conditions to northwestern Wyoming. Elsewhere,
although humidity will be low, wind should not be strong enough for
critical fire weather. As for the chance of precipitation, a couple
of models have high level heat source but with little moisture to
work with we left the forecast dry.
Fire danger will increase Monday for most areas as breezy conditions
develop across much of the area. Temperatures will continue to be
rather hot with highs in the 90s across many of the lower
elevations. The culprit for the breezy and warmth will be a
tightening pressure gradient ahead of an advancing cold front. We
anticipate fire weather highlights spreading across more of the map.
As for the chance of convection, it still looks to be mainly in the
west where a bit more moisture may try to creep in. But even here,
it will be limited so coverage will be sparse and most places should
stay rain free.
The front will move through on Tuesday. Moisture still appears to be
limited however so any convection would again be sparse with most
areas seeing none. The NAM and GFS have both backed off on the
chance and suppress the moisture mainly across southern Wyoming and
further south. The models also delay the arrival of the cooler air a
bit so we raised high temperatures a couple of degrees. Fire weather
concerns should ease a bit with the cooler temperatures but could
still approach critical in some locations.
.LONG TERM...Wednesday through Sunday
A 1025 MB surface high will slide southeast along the front range
of the Northern Rockies Wednesday and Thursday. This will keep a
cool northerly or easterly breeze in many locations. A couple of
disturbances pushing across the area, and a mid-level baroclinic
zone set up along the Continental Divide should give much of the
area isolated to locally scattered convection. Models show weak
instability mainly along and west of the Divide while areas east
of the Divide should be relatively stable. Will still have
thunderstorms in the forecast east, but have limited to isolated.
Based on 700 MB temperatures, snow levels Wednesday and Thursday
east of the Divide should range between 8500 and 9500 feet. Snow
levels west of the Divide will be much higher. Right now looks
possible for some minor accumulations over the northern mountains.
On Friday, a warming trend will commence, but will still remain
cool for late August especially for areas east of the Divide.
Another weak disturbance pushing across the area will continue to
give the area a shot of isolated convection mainly east of the
Divide and over the far south.
On Saturday, both the GFS and ECMWF show a mainly dry westerly
flow with the forecast area on the northern periphery of the
monsoonal moisture. At the same time, both models show yet another
upper low diving into southern British Columbia.
Models show the upper low pushing east into southern Alberta
Sunday with a strengthening west/southwest flow developing. This
will need to be watched for critical fire weather conditions.
However right now, models show slightly higher humidity compared
to what is expected in the short term.
Temperatures well below average east of the Divide Wednesday and
Thursday, with seasonal to slightly below average west/southwest.
Temps returning to seasonal to perhaps slightly above average next
VFR conditions will occur through Monday. It will be breezy across
much of the area this afternoon into the early evening. Wind speeds
of 12 to 18 knots will be common at most terminal sites with gusts
up to 22 knots. Monday will see even more wind...especially along
and west of the divide. Areas of smoke from wildfires in NW WY may
impact visibility downwind of the fires.
...RED FLAG WARNING FOR PORTIONS OF NORTHWESTERN WYOMING...
A combination of warm temperatures, low relative humidity and a
gusty west to southwest wind will bring critical fire conditions to
portions of northwestern Wyoming. Elsewhere, although relative
humidity will fall into the teens, winds will not be strong enough
for a long enough period for red flag conditions. Elevated fire
danger for a brief period will be possible in the afternoon however
in the normally windier locations. Mixing and smoke dispersal will
range from fair in the Basins east of the Divide to good to very
good elsewhere. Fire danger will become elevated in more areas
Monday with hot temperatures and gusty winds for many areas.
Red Flag Warning from 1 PM this afternoon to 9 PM MDT this
evening for WYZ140-286-415.