Area forecast discussion
National Weather Service Cheyenne Wyoming
414 am MDT Sat may 25 2013
Short term...(today through sunday)
issued at 352 am MDT Sat may 25 2013
The water vapor imagery/upper air analysis showed troughs anchored
near the west and east coasts and ridge over the nation/S middle
section. A rather potent shortwave was moving eastward across
central/eastern Nebraska...perpetuating an active convective complex
in this region. The regional surface analysis placed a weak Pacific
front from western Montana through central Idaho and northwest
Nevada. A surface dew point discontinuity extended along the Laramie
range into the northern Colorado Front Range. Dew points east of this
boundary were in the 40s and 50s... with teens and 20s west.
Prevailing winds were light and variable. Temperatures were in the
40s and 50s. Infrared satellite/GOES fog cloud product showed stratus
cloud deck expanding over southeast Wyoming plains into portions of
the Nebraska Panhandle.
The 00z short range models forecast the surface boundary/dryline to
align north-south over the High Plains by this afternoon. Airmass
along and east of the dryline will become conditionally unstable
with Li/S prognosticated as low as -7c and sbcapes of 1500-2500 j/kg over
much of western Nebraska into northeast Wyoming. Diabatic heating
will result in steep lapse rates with a Theta-E ridge from
western/central Nebraska into northeast Wyoming. Widely scattered
convection will develop early this afternoon east of a Lusk to
Kimball line. The Storm Prediction Center has a slight risk of
severe thunderstorms north and east of a Harrison to Alliance line for
this afternoon and early evening. The main threats will be
damaging winds to 60 miles per hour and large hail...with a small chance of
an isolated tornado or two over the northeast Panhandle with
prognosticated shear values of 30-40 knots. 700mb temperatures near 12c will
yield afternoon highs in the 80s east of the Laramie range. 60s
and 70s will prevail to the west. Gusty southwest winds will
develop west of the Laramie range this afternoon.
Convection will diminish by late this evening as it shifts east
into west central Nebraska. Both the sref and NAM develop low
ceilings and patchy fog late tonight and early Sunday morning
for much of the Panhandle...similar to this morning. It will
be mild with low temperatures in the 40s to middle 50s. The dryline
will retreat westward to near the Wyoming border late this evening.
The models depict an identical setup for diurnal convection across
the High Plains Sunday as the dryline drifts east. Instability
parameters are similar with sbcapes of 1500-2500 j/kg...Theta-E
ridge axis and modest shear values from southeast Montana through
the Black Hills into western Nebraska. The Storm Prediction Center
outlook for Sunday shows a slight risk of severe thunderstorms for the
northeast Panhandle. Have not included in the zones or grids for
Sunday...but later shifts may need to consider if model trends
continue. 700mb temperatures Sunday are similar to today with
highs in the 80s east of the Laramie range. It will become
breezy once again west of the Laramie range Sunday afternoon.
Long term...(sunday night through friday)
issued at 352 am MDT Sat may 25 2013
A fairly active pattern will continue through the period as an upper
level trough slowly moves eastward over the Continental U.S. Through the week.
Models in general remain in good agreement with the pattern although
continue to show some differences through the latter part of the
week. Initially...southwest flow will be overhead Sunday night with
a shortwave moving over Wyoming late in the night. Could see showers
linger through early Monday morning as the dryline pushes eastward
with the shortwave. For Memorial Day...yet another weak wave will
move over the forecast area while the dryline remains situated along
the far eastern Panhandle counties and weakens. Warm temperatures and
increasing middle level moisture with the wave will act to destabilize
the atmosphere...so could see some high based convection in the
afternoon over the higher terrain and eastern plains. Best low level
moisture will remain over the eastern plains though...so will keep
higher probability of precipitation here while the west should remain mostly dry but breezy.
West winds will be gusty across southeast Wyoming through the day
with minimum humidities dropping to the middle teens to 20s from west
to east.
A much stronger and negatively tilted trough will move over The
Rockies on Tuesday. Some differences between model solutions
continue with this feature and embedded shortwaves...although the
00z European model (ecmwf) no longer develops a cutoff low on Tuesday. Instead...all
models now trend toward a deepening open wave that will slowly move
eastward over The Rockies and into the Central Plains midweek. The
surface pattern looks to be more upslope in nature across the Wyoming/NE
plains on Tuesday with continued breezy SW winds out west...with
general westerly flow settling in place beginning Wednesday. Will
maintain increased chances for showers and thunderstorms on
Tuesday...with a broader coverage of activity expected for Wednesday
as a stronger wave approaches the forecast area. A much deeper
moisture column will overspread the area with this wave...so looking
at a better chance for more widespread measurable rainfall
Wednesday and Wednesday night. Again...some differences exist
between models regarding the embedded waves within the mean
trough...but the general trend will be for the main trough axis to
move overhead Thursday and Friday. The stronger wave will move
across the County Warning Area either Thursday or Friday depending on which model
verifies...but regardless of the solution...will see westerly winds
increase quite a bit in the wake of this shortwave and attendant cold
front. Looks like a bora type wind pattern...with the potential for
high winds in the wind corridor of southeast Wyoming. Wrap around
moisture will maintain chances for showers and thunderstorms through
the end of the week.
Temperatures continue to look cooler through the end of the week as
the cold front associated moves across the forecast area. Highs will
only be in the 60s and 70s....with lows in the 30s and 40s.
&&
Aviation...(for the 12z tafs through 12z Sunday morning)
issued at 352 am MDT Sat may 25 2013
MVFR to IFR ceilings/visibilities will impact much of the Nebraska Panhandle
into portions of the eastern Wyoming plains through middle morning...
then improve to VFR. Elsewhere...VFR conditions will prevail. Thunderstorms in the vicinity
for the Panhandle terminals after 20z with brief MVFR ceilings/visibilities
possible in and near thunderstorms...along with gusty winds to 40 knots.
Convection may linger until late evening.
&&
Fire weather...
issued at 245 am MDT Sat may 25 2013
A surface boundary separating moist unstable air to the east and
dry stable air to the west will extend from northeast Wyoming to
the Nebraska Panhandle. This boundary will be the focus for
thunderstorm development this afternoon and evening...and again
Sunday afternoon. A few strong to severe thunderstorms will be
possible along and east of the boundary this afternoon and early
evening. Gusty southwest winds and low relative humidities will
result in near critical fire weather conditions for lower elevations
of Carbon County this afternoon and Sunday afternoon. Temperatures
will average 10 to 15 degrees warmer than normal this weekend...then
gradually moderate closer to normal by the end of next week. Passing
upper level disturbances in the southwest flow aloft will bring
scattered showers and thunderstorms and higher humidities to the
district next week.
&&
Cys watches/warnings/advisories...
Wyoming...none.
NE...none.
&&
$$
Short term...jamski
long term...mazur
aviation...jamski
fire weather...jamski
