Area forecast discussion
National Weather Service Riverton Wyoming
1127 am MDT Monday may 20 2013
Short term...Monday through Wednesday night
The soggy pattern continues at this time with areas of rain moving
north to south across the state with an upper level low continuing
to spin over Nebraska and hints of an upper level low developing
over western South Dakota. As for rain today...satellite imagery
shows some cooling cloud tops over eastern Montana and Billings
radar showing some more rain moving southward into southern Montana.
As a result we expect another day of showers and isolated
thunderstorms. Models are showing less quantitative precipitation forecast than yesterday so
rainfall amounts for the most part should be less than yesterday.
Some creeks have risen quite a bit and have to be watched but at
this point since the rain has been more stratiform rather than
convective at the most I could see small stream flood advisories for
a few spots. There will also be some snow over the higher elevations
but similar to yesterday no winter weather highlights should be
needed since roads will be mainly wet to only slushy in places.
There is another dilemma with possible strong winds. Models are
showing strong 700 millibar winds with the NAM having 50 to 55 knots
aloft over northern Johnson County with favored northwest flow. The
GFS MOS is showing 40 knots sustained at Buffalo although this is
notorious for overdoing wind. However...the best conditions are
usually with a cold frontal passage and there is none. Also...the
pressure gradient is not that great without much cold advection. If
it occurred it would be with a heavier shower that could mix the
winds aloft down to the surface. At this time...we will go with an
Special Weather Statement but the day shift will have to monitor it closely.
Precipitation should taper on tonight as the low moves away to the
east. By Tuesday ridging will build over the area and bring a drier
and warmer day most of the area. There will still be a few showers
and thunderstorms around in the afternoon and evening...mainly in
the mountains with the high elevation heat source.
The weather may turn active once again Wednesday afternoon and
evening as the ridge moves a bit further to the east and a shortwave
approaches western Wyoming. With plenty of residual moisture around
with the still moist ground we have kept at least slight probability of precipitation for
the entire area with the highest chance across northwestern Wyoming
for that day.
Long term...Thursday through Monday
A large and complex upper low centered across the Pacific northwest
for much of the extended...will send multiple shortwave troughs
across the northern rockies in the southwest flow. Periodic cold
fronts with these systems will push into west/northwest Wyoming
before quickly retreating...making a tough call on temperatures especially
across the extreme west and northwest. Global models still show a
dry air mass across the Great Basin advecting northeast into the
area Thursday. This should reduce the coverage of convection
to...mainly across the north and extreme east. Instability does not
looks as good as yesterday...but could still see some vigorous
storms Thursday. The same scenario repeats itself through the rest
of the extended...with probably some intrusions of moisture
associated with individual shortwaves rotating around the large
complex upper low across the northwest. Hard to pinpoint best timing
for probability of precipitation this far out with these quick moving systems...and have
kept mainly slight probability of precipitation ongoing mainly in the afternoon and evening
for the northwest and north. Later shifts can adjust as needed as
timing of individual systems and extent of the dry air mass in the
southwest flow becomes clearer.
&&
Aviation.../18z issuance/
Isolated to scattered light rain showers continues today across much
of the region through the middle eveing perios...and perhaps a until
around 06z near krks. Shower conditions will keep MVFR conditions
around as well. Gusty wind will occur at many terminal
sites through this evening with the wind decreasing most terminals
after 03z this evening. Best area for wind today will occur in the
corridor between kriv and kwrl with gusts to 60 knots possible above
7000 feet mean sea level. VFR conditions will then prevail after 06z Tuesday although
isolated showers be a very slight possibility over the highest
elevations on Tuesday.
&&
Fire weather...
A cool and wet storm system will continue to impact the area through
today before improving conditions return on late tonight and
Tuesday. Most areas will see off and on rain with higher elevation
snow through Monday evening along with periods of gusty north to
northwest wind. Another storm system will bring a chance of
thunderstorms to the northwest on Wednesday with gusty south to
southeast winds across the area.
&&
Riw watches/warnings/advisories...none.
&&
$$
Short term...hattings
long term...wm
aviation...Braun
fire weather...hattings
