Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 242359
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
659 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 00z TAFs through 00z Saturday Evening)
Issued at 655 PM CDT Fri Apr 24 2015

Tough aviation forecast this evening, but in general we should
see a deterioration of ceilings with the persistent WAA
transporting deep low level moisture into the region. Timing and
ultimately how low ceilings will go is still a bit of a question
mark, but heading towards IFR conditions seems appropriate given
the moisture progged into the area. Bigger question mark is
when/were convection will form over the region. There seems to be
some consensus in the explicit model solutions that there will be
a split in the precip early in the period, with activity over AR
primarily moving east while increasing convection over KS
eventually develops into NW MO. However, during 06-10z time frame
all of the solutions are suggesting fairly strong WAA/isentropic
ascent focused over the mid-Mississippi Valley that should mean
showers and a few thunderstorms for most of our TAF sites. For the
KCOU and STL metro TAFS, expect morning stratus to lift and break
up a bit...and take on a more warm sector/cumuliform look before
the next round of convection fires along the cold front/dry line
in the early afternoon. At this point, have simply used a VCTS to
highlight this threat, and will leave the details to later
forecasts. However, expect KUIN to remain in IFR cigs throughout
the day as model consensus suggests the surface low passes near
or just to the south of this TAF site.

Specifics for KSTL: Expect deteriorating ceilings throughout the
evening, with IFR ceilings developing after 06z as showers and
thunderstorms develop over the region. This precip is expected to
lift out early in the morning, with ST breaking up a bit and taking
on a more warm sector look of a deep CU field by late morning.
There should be another threat of convection around or shortly
after midday as cold front/dry line sweep across the area, and
have used VCTS to highlight this threat attm.

Truett

&&

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

$$

WFO LSX






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