Area forecast discussion
National Weather Service Riverton Wyoming
541 PM MDT Wednesday Jun 19 2013
Short term...tonight through Thursday
A negatively tilted upper level trough has moved in from the
Pacific northwest over Idaho this afternoon. Cloud development
continues across western Wyoming...with showers and thunderstorms
possible late this afternoon and into the evening hours. Some
storms may be rather high based with the very warm and dry
boundary layer...so gusty outflow winds more than 40 or 50 miles per hour are
possible from the storms. Strong dry south to southwest flow has
developed ahead of the system as well across the entire forecast
area. These critical fire weather conditions will continue into
the evening hours before a cold front pushes into western Wyoming
early in the evening. The front will make its way eastward across
the forecast area into the early morning hours on
Thursday...bringing cooler temperatures...higher humidities...and a wind
shift to the west northwest. As the front pushes over the
absarokas...there is a possibility of enhanced downslope winds
from the Cody to Clark area from 9 PM to about 3 am. Will have to
keep an eye on observations in the area.
The center of the system remains to our northwest and moves
slightly to the NE into western Montana. Thus on Thursday
southwest flow will return to the area...although the wind will be
lighter with a weaker pressure gradient. Temperatures will be
cooler as well and Thursday morning will likely see some near
freezing temperatures in the western valleys. A Special Weather
Statement will be issued to address the cooler temperatures. Humidity
values will not be as dry as Wednesday too. The models are
suggesting very isolated thunderstorms Thursday late afternoon
around Casper Mountain and the eastern Big Horn Mountains.
Long term...Thursday night through Wednesday
There will be a couple of lingering showers and thunderstorms across
the northwest in the evening but these should end with the loss of
daytime heating. One thing to watch for later at night is the
possible development of nocturnal thunderstorms from the Big Horn
range and eastward as models show a 35 to 40 knot low level jet
setting up in that area along with somewhat deeper moisture than the
rest of the County Warning Area. This is far from certain
however...so for now we have kept only isolated probability of precipitation. So perhaps a
noisy way to welcome in Summer as the solstice occurs at 11:04 PM.
The models are split for the first full day of Summer on Friday with
the NAM showing energy and moisture moving a bit further to the
south with some convection across the northern portions of Wyoming
while the GFS keeps the precipitation further to the north and west
across Montana. For now we have compromised with having isolated
probability of precipitation across northern Wyoming and the rest of the area dry.
Temperatures will be range from somewhat above normal across areas
east of The Divide to even slightly below normal in the west. An
upper level low over Idaho is then expected to open up into the
trough and progress eastward for the start of the weekend. Saturday
has the same except the models have flipped with the GFS the wetter
models and the NAM keeping most precipitation in Montana. For
now...we made few changes to continuity given the lack of model run
to run consistency. However...at this time it does not look like an
all day washout for anywhere. Most models have the area dry on
Sunday with ridging building over the area so we have removed all
probability of precipitation from the area. Temperatures will also warm with flow become
southwest.
A more summertime pattern looks to return for early next week with
some ridging building across The Rockies. A couple of shortwaves
will ride over the ridge in the classic ring of fire pattern. This
would mean a chance of convection across northern Wyoming with warm
to hot temperatures across most of the central and southern portions
of the state. The models then split for Wednesday with the GFS
keeping the ridge a bit further to the south with maybe more showers
and thunderstorms in the northwest. Meanwhile...the 12z European
shows a possible Death Ridge in The Rockies by planting 598 upper
level high over western Colorado by Wednesday evening. This would
lead to...in the words of the classic kool and the gang
song..weather that is too hot...too hot...too hot baby for a lot of
people. Did not go that warm yet...but most basin locations should
be into the 90s nevertheless.
&&
Aviation.../00z issuance/
Strong wind will be found across much of the area through around
06z this evening. Some gusty wind will remain possible through
around 10z...though lighter wind will be found at the majority of
terminals. Other than wind...generally VFR conditions will be
found through the overnight hours. Wind is expected to increase
after 21z Thursday. Showers and thunderstorms across northwestern
Wyoming will weaken through the evening...though another
disturbance will move across the area Thursday afternoon. This
disturbance will again generate showers and thunderstorms...mainly
across the northwestern mountains. Most activity should remain
confined to the mountains...though some rogue showers moving
through the Jackson Valley. Should showers impact kjac...they
would result in brief MVFR conditions with stronger wind.
&&
Fire weather...
The upper level low continues to approach Wyoming late this
afternoon with clouds developing nicely in the western part of the
state. Strong dry very warm south to southwest flow has developed
ahead of the system across southern and central Wyoming. A red flag
warning is in effect until 10 PM this evening for four zones in
north central Wyoming where fuels are now critical. Although
similar or stronger conditions are present elsewhere across
western and central Wyoming...all other fire weather zones are not
critical at this time. Isolated to scattered showers and
thunderstorms are still expected late this afternoon and evening
across western Wyoming with very little precipitation expected
from the storms. The instability is likely to last until about
midnight in ynp. A dry cold front will push from west to east
through the area starting around 7pm along the western
border...and pushing into central Wyoming after midnight. Higher
elevation temperatures in western Wyoming will drop considerably
overnight. Thursday looks to be cooler and less windy than
Wednesday...with slightly higher humidities as well with very little
chance of any showers or storms.
&&
Riw watches/warnings/advisories...
red flag warning until 10 PM MDT this evening wyz280-281-285-300.
&&
$$
Short term...McDonald
long term...hattings
aviation...branham
fire weather...McDonald
