Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 250448
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1148 PM CDT Fri Apr 24 2015

.SHORT TERM:  (Through Late Tonight)
Issued at 334 PM CDT Fri Apr 24 2015

Clouds and mild temperatures blanket the area late this afternoon
and we`ve seen very little precipitation thus far. The main forecast
issue tonight will be the precipitation chance/trends. Present
indications are two main areas to watch. The first is an area of
origination in south central and southeast MO this evening. The
guidance is in relatively good agreement showing a perturbation in
the southwest flow aloft will track northeast out of OK across the
aforementioned region. The latest water vapor imagery suggests the
presence of 2 upstream perturbations. This disturbance combined with
with strong moisture convergence/lift via an intensifying and
veering LLJ should result in an area of showers and thunderstorms
over the southeast third of the CWA. The second region of interest
is much further west. The short wave trof currently getting ready to
move into the central and southern High Plains combined with lift
via a secondary branch of the southerly LLJ should result in the
development of strong-severe thunderstorms along and east of the
dryline in KS over the next 2-4 hours. This initially scattered
convection should grow upscale into a convective system across
eastern KS this evening which will migrate eastward ahead of the
advancing surface low, and impact areas along/north of I-70
overnight into Saturday morning.

Glass

.LONG TERM:  (Saturday through Next Friday)
Issued at 334 PM CDT Fri Apr 24 2015

By 12z Saturday morning, strengthening surface low will be over
northwestern MO. Will see overnight activity sliding north and east
of forecast area as dry slot wraps around low with a brief break in
activity, mainly for St. Louis metro and areas to the south and east
through midday. Then for afternoon hours, possibility of severe
weather and location of it will depend on location of surface low,
frontal boundary and how much the atmosphere recovers from morning
activity. Reintroduced thunderstorms for all locations as there is
enough instability, at the surface and aloft and decent low level
jet for storms to develop even north of track of surface low. For
now best chances of severe weather will be over east central and
southeast Missouri as well as southern Illinois as surface low
slides east southeast Saturday afternoon. Models showing CAPE
between 1000-2500 J/kg, decent lapse rates and shear, with supercell
development possible and main severe threats being large hail,
damaging winds, and isolated tornadoes. SPC has placed this area in
a slight risk with an area of enhanced risk southeast of STL metro
area. Highs will vary from the low 60s far north to the mid 70s far
south.

System to exit region Saturday night with lingering showers tapering
off from north to south. Cooler air to slowly filter in with lows in
the low 40s far north to the low 50s far south.

Below normal temps and gradual clearing expected on Sunday as
surface ridge builds in. Beyond that, temperatures to slowly
moderate through the work week, with a shortwave sliding through
region on Wednesday with some scattered showers possible.

Byrd

&&

.AVIATION:  (For the 06z TAFs through 06z Saturday Night)
Issued at 1145 PM CDT Fri Apr 24 2015

MVFR cigs are in the process of overspreading the CWA attm...and
latest MET and MAV MOS still strongly suggest that this
deterioration will continue into IFR cigs during the predawn
hours as the persistent low level WAA increases the low level
moisture in the region. A few hit and miss showers have formed
over NE MO the last couple of hours, with the anticipation that
there will be a general increase in coverage of showers...as well
as thunderstorms...during the predawn hours as forcing via low
level jet and upper level low ramp up the vertical motion across
the Mississippi Valley. 00z guidance is a bit further south with
track of low, which would keep UIN in IFR conditions and precip throughout
the day, with only limited improvement at KCOU and the STL metro
area. Still appears that there will be a narrow window for
redevelopment of thunderstorms in the STL area during the early
afternoon, and have continued to highlight with VCTS attm.

Specifics for KSTL: Ceiings expected to drop below 1000 feet
during the predawn hours, with showers and a few thunderstorms
developing by around 09z. Anticipate that the early morning
convection will advect out of the region during the early morning,
with ceilings slowly improving to MVFR as surface low works into
the area from mid MO. Have maintained a mention of some
thunderstorm redevelopment during the early afternoon in the form
of VCTS...hoping later shifts will be able to refine the location
and intensity of this redevelopment.

Truett

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...NONE.
IL...NONE.
&&

$$

WFO LSX





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