Area forecast discussion
National Weather Service Riverton Wyoming
1125 am MDT Sat may 25 2013
Short term...today through Monday night
Ridging remains across the plains as a trough along the Pacific northwest
ejects pieces of energy across the region through the short
term...eventually pushing the ridge further east and laying a broad
trough across much of the western US.
Warm...windy and dry southwest flow returns to much of the state
with minimum rhs expected to again drop below 10 percent across the
south. Some thunderstorm activity possible across the far northern
mountains and into Johnson County as some moisture tries to advect
in from the plains. All indications at this point are that the
Richer moisture and explosive instability will remain east of
Johnson County but one or two storms forming off of the bighorns
could intensify rapidly in eastern Johnson County if some of this
more moisture laden unstable air wafts further west than the models
would advertise.
The right entrance region of the jet will try and build in this
afternoon which could sustain an otherwise weak storm long enough
for it move into the lower elevations of the Bighorn Basin off of
the absarokas as some middle level moisture advects into the
west...expect a few storms going in Yellowstone and Teton Parks as
well as the absarokas and possibly into Cody and Jackson.
Sunday looks to be a fairly active convective day in northern
Wyoming...particularly in Johnson County where the best
cape...moisture and shear will be along a Theta-E ridge...flirting
with the northeastern reaches of the County Warning Area again. Expect possible
large hail and strong gusty winds with some of these storms.
Thunderstorms return on Monday with the strongest
instability...shear and a plume of higher dew points...possibly
reaching into the 50s working again into the Bighorn Basin and
Johnson County. Deep layer shear is expected to be strong enough to
encourage some more organized thunderstorms in the afternoon...with
large hail and strong winds the primary threats. Elsewhere...middle
level moisture and instability will be sufficient for thunderstorms
just about anywhere...they will likely be high based across the
basins and valleys with strong downburst winds and small hail.
Long term...Tuesday through Saturday
Strong jet stream across the central Pacific south of the Aleutian
chain early this morning will carve out its way southeast into the
Great Basin on Tuesday...followed by reinforcing Pacific energy with
500 mb low closing off over Wyoming on Thursday. This low expected to
progress slowly across the northern plains on Friday and Saturday.
The 25/00z GFS is faster with the overall progression of this
trough/500 mb low. The latest European model (ecmwf) is slower than the GFS but has
trended a bit faster from its previous runs...backing off its
earlier attempts to close off an upper low over the desert SW. The
main difference for Wyoming between these various solutions is mainly
a timing issue...especially with the exiting of the trough late
in the week...and forecast prefers the slower European model (ecmwf) Friday and
Saturday.
On Tuesday...plenty of showers are expected to develop through the
day in moist difluent southerly flow ahead of the trough. The best
instability to remain mostly over the High Plains with surface
trough across eastern Wyoming...though southeast flow may bring more of
this low level moisture and instability back into the the northern
Bighorn range...across the northern Big Horn basin especially per
the slower European model (ecmwf) scenario.
On Wednesday and Thursday...initial trough moves into the High
Plains on Wednesday quickly followed by reinforcing Pacific energy
diving southeast across the intermountain west into Wyoming. Jet becomes
more cyclonic through Wednesday night and Thursday across the Great
Basin/central rockies with Wyoming in the left front quadrant. We
could have a fairly widespread precipitation event across the
northern two-thirds of Wyoming under ecmwf's slower and further
south scenario Wednesday night into Thursday while the GFS would
place this precipitation shield further north into Montana and the
northern plains. Thursday will be the coolest day of this period
with highs in the 50s and 60s across the valleys and basins.
On Friday and Saturday...moist northwest flow will prevail across the north
and east on Friday as upper low slowly drifts across the northern
plains...with northwest flow and moisture gradually decreasing through the
day Saturday. Temperatures should begin to rebound on Saturday.
&&
Aviation.../18z issuance/
Large-scale upper low still spinning across the Pacific northwest
late this morning. This feature has various minor shortwaves
rotating counter-clockwise through the flow across the northwest
U.S. For the most part...these features will stay to our west
through the forecast period. 70kt+ jet streak working its way across
the northwest quarter of Wyoming late this morning. This should help
initiate convection across the absaroka and Teton ranges and
Yellowstone National Park this afternoon. Best instability for deep
convection will be in the northern Big Horn basin and northern
Johnson County late this afternoon. Gusty outflow wind to around
40kts would be the main hazard in association with the stronger
storms. Convection will wane in these areas between 03z-06z/sun.
Kcod and perhaps kjac would be the most likely candidates for thunderstorms in the vicinity
later this afternoon. Otherwise...VFR conditions will prevail
through the period at all terminals. Gusty south to southwest wind
generally 15-25kts anticipated to increase at most terminals between
19z-22z this afternoon. Sunday looks to be almost an exact Carbon
copy of today.
&&
Fire weather...
Very dry and breezy southwest flow takes over the southeastern
portions of the forecast area again with relative humidity below 10 percent
widespread across the Rawlins dispatch area. Wind gusts could
exceed 40 miles per hour at times in favored areas again today and Sunday.
Thunderstorms will again appear across the northern mountains with a
few expected in the Big Horn basin and Johnson County. These storms
will likely be high based and mostly dry...especially across the
lower elevations. Thunderstorms increase in intensity and coverage
across the north on Sunday...with a few severe storms possible.
Coverage increases even more on Monday as moire moisture arrives
with wet storms...some severe across the north and northeast...and a
mix of wet and dry storms across southern and central Wyoming.
&&
Riw watches/warnings/advisories...
none.
&&
$$
Short term...Allen
long term...aem
aviation...cnj
fire weather...Allen
