Area forecast discussion
National Weather Service Riverton Wyoming
343 am MDT Wednesday may 22 2013
Short term...Wednesday through Friday night
After a fairly quiet weather day on Tuesday it looks like the
weather will turn active once again. Satellite imagery shows a thin
band of clouds working across central Wyoming at this time with a
few sprinkles but this should be out of the County Warning Area by
daylight. After the passage of these clouds precipitable waters
should stay relatively low. However...700 millibar flow will slowly
turn from south to southeast and east through the day and increase
precipitable water values up to 0.6 to 0.7 inches across many areas
east of The Divide with the highest values in northern and eastern
portions of the area. Cape values will also be on the rise with
values over 300 j/kg from the absarokas through the Wind River basin
and in areas from the Big Horn range and east. In addition...lifted
indices will fall to minus 3 in many areas and there is some
directional shear with southwest flow at 500 millibars. With low
pressure providing some lift look for isolated to scattered
thunderstorms in the forementioned areas...some possibly strong...to
develop this afternoon and evening with gusty winds and hail the
main threats. The threat of these storms should end late tonight
with the loss of daytime heating.
Still some uncertainty tomorrow as a lot depends on if a dry slot
sets up across portions of the County Warning Area...with the best
chance of that across the southern portions of the County Warning
Area. With the uncertainty we have left continuity alone for the
most part with the chance of showers and thunderstorms across
northern and eastern areas where moisture will be a bit deeper. With
increased south to southwesterly flow temperatures will continue an
upward trend to above normal levels. It will become windy as well
across southern portions of the state with the increased south flow
and good mixing from more sunshine. As for Friday...models now
indicate that much drier air will move as the best moisture gets
shunted to the north and east of Wyoming. Still some uncertainty so
we mainly nudged down the probability of precipitation rather than remove them at this time.
Long term...Saturday through Wednesday
Upper low over the northwest U.S. To fill and lift into southern b.C. Over
the weekend. This will leave Wyoming in SW flow between persistent
West Coast trough and expanding ridge over the plains. Medium range
global models show a slow progression of this overall pattern
through the extended period...the GFS is on the fast end of this
progression and the European model (ecmwf) is on the slow end. Forecast is weighted
toward the slower progression.
As is typical in warm season SW flow...the main surface feature will
be a quasi-stationary surface trough across northern and eastern
Wyoming with an east-southeast flow bringing in more moisture and instability
to the north and east of this boundary...a drier SW flow to the
south. On Saturday...this boundary will be near a Thermopolis to
Midwest line...with slight chances of afternoon/evening thunderstorms to the
north. Some stronger storms are possible in NE Johnson County.
On Sunday and Memorial Day...more convection is expected from eastern
Idaho into far West Wyoming ahead of a slow-moving cold front. A
stronger SW flow ahead of the front may push the surface trough east
of The Divide further to the north and east...thunderstorm initiation
primarily from Meeteetse to Buffalo and north.
Upper trough is expected to slowly progress across the Great
Basin/rockies Tuesday and Wednesday which will bring an increasing
chance of showers and thunderstorms across most of the area along
with a cooling trend.
&&
Aviation.../12z issuance/
VFR conditions will prevail over the region through 12z Thursday.
Isolated showers and thunderstorms will occur east of the
Continental Divide from 20z until 03z with local MVFR conditions.
West of The Divide...showers and thunderstorms will be more numerous
from 20z through 06z with areas of MVFR conditions. A few showers
may linger in northwest Wyoming through 12z Thursday. Windy conditions will
occur across southern and southwest Wyoming this afternoon with gusts to
30 knots.
&&
Fire weather...
More active weather will return to the area today as another storm
system brings a better chance for thunderstorms in the northwest
fire zones on Wednesday. These storms will be capable of very gusty
and erratic winds. Winds will largely be from the south to southeast
across the area. The threat of thunderstorms will remain across
northern zones on Thursday with warmer...windy and mainly dry
weather across the southern half of the state.
&&
Riw watches/warnings/advisories...
none.
&&
$$
Short term...hattings
long term...aem
aviation...Arkansas
fire weather...hattings
