Area forecast discussion
National Weather Service Cheyenne Wyoming
614 am MDT Friday may 24 2013
Update...
issued at 604 am MDT Friday may 24 2013
Winds across the northern Panhandle are expected to remain below
advisory criteria through the remainder of the morning...except
for along the Pine Ridge where gusts up to 50 miles per hour are still
possible as the low level inversion begins to lift. So went ahead and
extended the Wind Advisory for northern Sioux and Dawes County
until 9 am to account for these winds.
&&
Short term...(today through saturday)
issued at 528 am MDT Friday may 24 2013
Low clouds have overspread the eastern plains once again
tonight...with a few areas of fog and drizzle occurring mainly from
the southern Laramie range eastward to Sidney. Upper level vorticity maximum
is moving across northern Utah and eastern Idaho at this hour...with
the associated surface low over central Wyoming moving steadily to
the northeast. 3hrly pressure falls of 1 to 2 mb are analyzed by
the laps to the Lee of The Rockies extending eastward to the
eastern plains ahead of this wave. 3 to 5 mb pressure rises are
developing across western Wyoming behind the cold front moving
into western Carbon County. The dryline boundary looks to be
situated roughly along the Carbon/Albany County line. As the
vorticity maximum continues to move northeast through the morning...the
surface low and attendant surface boundaries will follow a similar
trajectory. By 18z today...models show the dryline situated from
Chadron to Pine Bluffs with south-southeasterly flow out ahead of
the boundary across the southeastern Panhandle. The slightly more
westward placement of the southern extent to the boundary looks to
be influenced by stronger southerly winds developing ahead of a
second shortwave moving northeast across Colorado in the late
morning and early afternoon. In general...the passage of the
dryline will allow for much more heating across the forecast area
with widespread high temperatures into the 60s and 70s west of the
Laramie range...and 70s and 80s across the eastern plains. In
addition...dry and breezy conditions will prevail west of the
dryline...with humidities looking to drop to the low to middle teens
and southwest winds gusting 20 to 30 miles per hour. The initial vorticity maximum
will also push the Stout midlevel thermal ridge to the east...with
700 mb temperatures looking to hover around the 8c line today.
As the second shortwave approaches the forecast area...winds in
the southern Nebraska Panhandle will begin to back to a more
southeasterly component through the late afternoon. The 00z and
06z GFS along with the 00z European model (ecmwf) and Gem favor a slightly
stronger shortwave than the NAM...which seems realistic based off
currently water vapor imagery. This will favor an even more
easterly component to the surface winds across the southern Panhandle
which is depicted by a more bullish backing of the winds in the
GFS and European model (ecmwf) through the afternoon. This will result in a push
further west of the dryline through the late afternoon and evening
back up along the Cheyenne Ridge toward the Laramie range.
Weakly diffluent flow aloft will combine with the approaching
shortwave to enhance broadscale lift across the eastern plains.
Strong convergence along the initially stalled boundary across the
Panhandle looks to initiate isolated to scattered thunderstorms in
the afternoon. With cooler temperatures aloft and more heating
expected at the surface...forecast soundings show much less of a cap
today. Models forecast a range of 1000 to 2500 j/kg of cape on the
eastern side of the dryline by 21z with a steadily eroding cap
through this time. 0 to 6 km bulk shear will increase to 40 to 50
kts through the afternoon as surface winds back and strengthen
ahead of the shortwave. Therefore...the environment looks primed
for storms that fire off the boundary to move east-northeast into
a more favorable environment for organization of storms into a
supercell type Mode. One concern though will be decreasing
midlevel winds through the day which could prevent the storms from
developing strong enough midlevel rotation conducive to
maintaining a discrete supercell Mode for long. Needless to
say...the storms look to develop in a favorable environment to
become severe in nature...mainly east of a line from Alliance to
Pine Bluffs in the late afternoon and early evening. The main
threat will be large hail and damaging downburst winds...but any
storm that can form or attach themselves to terrain induced
boundaries off the Cheyenne Ridge certainly have the potential to
produce a tornado.
As the dryline retreats westward in the late afternoon and
evening...think the chance for thunderstorms will increase across
the plains of southeast Wyoming. Not really expecting anything
severe as the cap quickly strengthens when the boundary retreats
westward...but small hail and gusty winds are not out of the
question with quickly developing storms.
The next upper level shortwave will move across Wyoming on Saturday
and will strengthen the leeside pressure trough and push the dryline
eastward through the day. Models show some differences with how
far eastward this boundary will progress with the NAM pushing the
boundary well east of the County Warning Area while the GFS is much more slow to
move this boundary. Will maintain a chance for thunderstorms
across northern and eastern portions of the County Warning Area to account for the
differences and would not be surprised if a few develop into
severe thunderstorms with cape values of 1000 to 2000 j/kg ahead of the
boundary in the Panhandle under weak inhibition.
Elsewhere...conditions will remain very dry and breezy with
southwest winds gusty 30 to 40 miles per hour once again along and west of
the Laramie range.
Long term...(saturday night through thursday)
issued at 528 am MDT Friday may 24 2013
There is good agreement with the 00z GFS/European model (ecmwf) and ensembles
depicting a mean ridge over the central Continental U.S. And mean trough
over the west. Embedded middle level shortwaves in southwesterly
flow aloft will migrate across the region late in the weekend
through the middle of next week. The initial wave will interact
with a stalled surface boundary and unstable airmass to produce
scattered convection Sunday from northeast Wyoming into the
Nebraska Panhandle. The Storm Prediction Center day3 outlook shows a slight risk
of severe thunderstorms extending into the northeast Panhandle Sunday
coincident with steep lapse rates...sbcapes of 1500-2000 j/kg and
modest deep layer shear in the vicinity of the dryline. This
situation will continue to be monitored over the next few days.
Dryline shifts east into central Nebraska Sunday night...but
retreats west toward the Wyoming border Monday. Another shortwave
will once again aid in convective development Monday from the
east central Wyoming into the Nebraska Panhandle...however severe
tstorm threat appears low at this time.
The European model (ecmwf) continues its trend of developing a closed upper low
over the Desert Southwest Tuesday and slowly tracking east across
The Four Corners through Wednesday night before opening up. The
GFS maintains a broad negatively tilted trough from the Great
Basin to the southern rockies...lifting northeast into the Great
Plains by Thursday. Both models develop surface low pressure to
the Lee of the Front Range Tuesday and track it northeast into the
northern and Central High plains through Wednesday. The dryline
becomes more active both Tuesday and Wednesday with passing
shortwave energy aloft. Convection will be more widespread these
two days. The GFS advects drier air into the County warning forecast area Thursday with
the surface boundary advancing as far east as central South
Dakota and Nebraska. The European model (ecmwf) lags the boundary over the eastern
Wyoming and Colorado plains with more widespread quantitative precipitation forecast. For now...
trended toward the drier GFS solution with more isolated
convection across northern and western portions of the County warning forecast area.
Temperatures during the long term will average above seasonal
normals. Warmest day will be Sunday...followed by a gradual
moderation closer to seasonal normals by the end of the week.
&&
Aviation...(for the 12z tafs through 12z Saturday morning)
issued at 528 am MDT Friday may 24 2013
MVFR to IFR conditions will continue along and east of Interstate
the Laramie range through 15z. VFR conditions and gusty
southwest winds of 20 to 30 kts will then occur across southeast
Wyoming through this evening. Isolated to scattered thunderstorms
will develop across the southern Nebraska Panhandle after 21z and
will produce gusty and erratic winds...brief MVFR ceilings and
visibilities...and localized severe turbulence. MVFR ceilings may return
across the eastern plains tonight...but likely will stay a scattered
deck.
&&
Fire weather...
issued at 528 am MDT Friday may 24 2013
Much warmer and drier conditions will occur across
the district through this weekend...with humidities dropping to
the middle teens to middle 20s across southeast Wyoming...and 20 to 30
percent readings across the Nebraska Panhandle. In
addition...locations in far western Carbon County could see
humidities drop as low as 10 percent both days. Southwest winds
will become breezy across southeast Wyoming as well this
weekend...with gusts generally 20 to 30 miles per hour. Further east in the
Panhandle winds will remain at a more south to southeasterly
component at 10 to 20 miles per hour. Isolated to scattered thunderstorms are
possible both days mainly across the far eastern Panhandle...with
a few isolated storms possible over the plains of southeast
Wyoming this evening.
&&
Cys watches/warnings/advisories...
Wyoming...none.
NE...Wind Advisory until 9 am MDT this morning for nez002-095.
&&
$$
Update...rjm
short term...rjm
long term...jamski
aviation...rjm
fire weather...rjm
