Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 020209
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
909 PM CDT Wed Oct 1 2014

.UPDATE:
Issued at 853 PM CDT Wed Oct 1 2014

Forecast looks to be generally on track and I don`t foresee any
major changes for the rest of the evening. Instability is waning,
but obviously there`s enough to keep some rather strong storms
going. Will most likely let WW511 expire naturally at 1000 PM. The
challenge after 1000 PM will be trying to figure out exactly where
the heaviest rain will fall for the rest of the night. It still
looks like the best threat for heavy rain and training will be
over parts of our central Missouri counties. RAP is persistently
showing moderate to strong moisture convergence at 850mb. Storms
this evening have been efficient rain producers, and I expect this
will not change. No change to flash flood watch at this time, and
will probably only tweak PoPs to match current radar trends.

Carney

&&

.SHORT TERM: (Through Thursday Night)
Issued at 340 PM CDT Wed Oct 1 2014

Lots to keep track of in the next 24 to 36 hours. First: severe
thunderstorms have developed along the outflow boundary in central
Missouri left over from storms earlier today. Modest instability
between 1000-1500 J/Kg combined with around 40 kts bulk shear have
produced a supercell which is riding the boundary. A few more
cells are developing ahead of the big storm and may continue to
strengthen. Expect the severe threat to continue into the early to
mid evening hours...generally along and south of I-70. Should see
storms weaken within a few hours of sunset, but then the low level
jet strengthens in southwest flow ahead of the broad trof over the
Rockies and developing low level circulation over the Plains. This
will bring the second area of concern into play: heavy rain.
Thunderstorms this morning and now this current round of severe
convection will saturate the ground and increase the potential for
flash flooding. Aforementioned 30-40kt low level jet is bringing
plenty of Gulf moisture up into the area. Forecast soundings show
a warm cloud depth of nearly 12,000 FT and BUFKIT is indicating
that individual convective elements may move very slowly and train
over the same locations. Current QPF from WPC shows rainfall in
excess of 3 inches over the next 24 hours with locally higher
amounts in addition to what`s already fallen. Therefore have gone
ahead and pushed a flash flood watch for parts of central,
northeast, and east central Missouri. Watch may need to be
extended further east or northeast tonight as the situation
develops.

Attention turns to tomorrow when the upper level trof moves into the
Great Plains and the associated surface system moves from northern
Oklahoma into eastern Iowa through the day.  Our CWFA will be
squarely in the warm sector through the day, and even if there are
no breaks in the clouds temperatures should be at least between 80
and 85...if not a tad bit warmer.  With warm, humid airmass in
place, NAM is forecasting SBCAPE values in excess of 2500 J/Kg in
some parts of the CWFA. With 30-40kts of deep layer shear, and the
approaching shortwave becoming negatively tilted , this looks like a
good setup for severe weather during the afternoon and evening
hours.  The shear vector looks like it will be parallel to the front
so the most likely convective mode will be a squall line with bows
and LEWPS possible.  The front will sweep through the area overnight
Thursday night and we should be into cold advection by 12Z Friday
morning.

Carney

.LONG TERM:  (Thursday through Next Wednesday)
Issued at 403 PM CDT Wed Oct 1 2014

Fall returns with a vengeance Friday and Saturday.  Temperatures
will drop 25 to 30 degrees behind the front Friday and Saturday.  It
will also be pretty breezy behind the front on Friday.  Forecast
soundings show some pretty good mixing which is likely to produce
wind gusts in excess of 30 mph.  Guidance is hinting at lows in the
30s across most of the area on Saturday morning, and this looks
reasonable given the strength of the cold advection.  This could
easily be the coolest night of the season so far.

Temperatures slowly moderate Sunday through next Wednesday. The
highly amplified upper level pattern featuring a strong vortex just
north of the Great Lakes region and a longwave trof over the eastern
1/2 of the CONUS will slowly become more zonal through midweek.  By
Wednesday the flow aloft should be nearly zonal and low level warm
advection will bring temperatures back to near or above normal.

Carney

&&

.AVIATION:  (For the 00z TAFs through 00z Thursday Evening)
Issued at 650 PM CDT Wed Oct 1 2014

Primary concern for tonight is thunderstorms. Best chances for
storms look like it will be from central Missouri, north of I-44
into the STL Metro area and then along and north of I-70 tonight.
IFR visibilities in heavy rain will accompany these storms at
times. Think flight conditions should be mainly VFR outside of
thunderstorms. Numerous thunderstorms are expected across the area
on Thursday as well. Expect VFR conditions and southwest flow to
prevail outside of storms.

Specifics for KSTL:

Occasional thunderstorms will affect the terminal tonight. IFR
visibilities in heavy rain will accompany these storms. Expect VFR
flight conditions to prevail outside of storms. Numerous
thunderstorms are expected on Thursday. Timing is uncertain,
however the best chance for storms will likely be during the
afternoon and evening hours.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...FLASH FLOOD WATCH through Thursday evening FOR Audrain MO-Boone
     MO-Callaway MO-Cole MO-Gasconade MO-Moniteau MO-Monroe MO-
     Montgomery MO-Osage MO.

IL...NONE.
&&

$$

WFO LSX





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