Area forecast discussion...updated
National Weather Service St Louis MO
1011 PM CDT Tuesday may 21 2013
Update...
issued at 943 PM CDT Tuesday may 21 2013
Most of the shower and thunderstorm activity has remained to our
S and east so far this evening. Still thinking that additional
convection will likely develop and move through portions of southeastern
MO and southwestern Illinois late tgt...mainly S and east of stl as the strong
shortwave moving east-northeastward through eastern OK moves through northern Arkansas and
into southeastern MO by 12z Wednesday. Models still depicting upper level
divergence tgt across mainly southeastern MO and southwestern Illinois ahead of the
approaching upper level trough. A S-southwesterly low level jet will bring
low level moisture convergence across southeastern MO and southwestern Illinois. The
best low level moisture and instability will also exist over this
area ahead of the weak cold front extending from eastern Iowa and northestern
MO SW through southwestern MO and southeastern Kansas. Low temperatures tgt ahead of
this weak cold front should continue above normal due to the cloud
cover and relatively high surface dew points.
Gks
&&
Short term...(through late tonight)
issued at 314 PM CDT Tuesday may 21 2013
The overall severe weather threat has diminished significantly
from earlier thinking owing to a series of mcs's tracking to our
south from Oklahoma to Arkansas into the Tennessee Valley. These systems
have produced a good deal of cloud cover which has limited heating
and that combined with much weaker lapse rates has limited the
instability. The highest values of SBCAPE presently are from
1000-2000 j/kg extending from metropolitan St Louis into east central
Illinois...this half or less the amount of the previous few days. All
this said the precipitation chances still look pretty high tonight
across eastern MO into southern Illinois. We might see some isolated storms
develop in this higher cape air through sunset and we will also see
the northern portion of the strati form rain region of the Arkansas mesoscale convective system
impact southeast MO into southern Illinois. The main threat of precipitation however
appears to be later this evening into the overnight hours. Water
vapor imagery and vwp data is showing a powerful short wave across
extreme western OK and this will eject east-northeastward tonight and be centered
through eastern MO at daybreak on Wednesday. Deep large scale ascent associated
with this short wave should lead to an upswing in showers and
thunderstorms later this evening. I cant rule out an isolated
severe threat but again the marginal instability would suggest the
chance is low. Rainfall amounts will be substantially less than
last night...less than an inch.
Glass
Long term...(wednesday through next tuesday)
issued at 314 PM CDT Tuesday may 21 2013
The majority of the precipitation should be east of the MS river
in Illinois or further east at daybreak and continue to move
eastward as the shortwave trough and attendant cold front move east.
Cooler and drier air will overspread the region in the wake of the
front and any additional precipitation threat after middle-morning
should be from isolated diurnal showers. Another secondary cold
front will push through the area Wednesday night ushering high pressure
and even cooler air into the area. This large high will dominate
the area Thursday into Friday.
Not much has changed in the extended ranges of the forecast. An
upper ridge will dominate the central U.S. With the ridge axis
remaining just west of Missouri Sat-Mon. This will place US just
downstream of the ridge crest and subject to weak short waves
topping the ridge. These disturbances combined with return flow
and low level warm air advection on the backside of the retreating low level high
pressure system will bring a prolonged threat of showers and
thunderstorms to primarily the northern half of the County Warning Area from Sat
through Monday night. Thereafter the upper ridge axis moves to our
east...precipitation chances diminish and temperatures warm.
Glass
&&
Aviation...(for the 00z tafs through 00z Wednesday evening)
issued at 640 PM CDT Tuesday may 21 2013
Showers and scattered thunderstorms currently over far southern
MO/southern Illinois are staying well south of the area...so have
removed current mention of rain out of the St. Louis metropolitan taf
sites. Best chance of rain now will wait until late this evening
and overnight when an upper level disturbance currently over
central Oklahoma moves northeast into the area. Expect showers and
scattered thunderstorms to move across the area during this period...
so have -shra/thunderstorms in the vicinity to reflect this. Generally expect VFR conditions
outside of the rain when ceilings will lower to MVFR during this
period.
Specifics for kstl...expect best chance for showers and scattered
thunderstorms will occur overnight tonight when an upper level
disturbance moves across the area. MVFR ceilings and visibilities
are possible during the precipitation.
Britt
&&
Lsx watches/warnings/advisories...
MO...none.
Illinois...none.
&&
$$
Weather forecast office lsx
