Area Forecast Discussion
Issued by NWS St. Louis, MO

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448
FXUS63 KLSX 151632
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
1132 AM CDT Fri May 15 2015

.UPDATE:
Issued at 1131 AM CDT Fri May 15 2015

Scattered thunderstorms have developed over parts of central and
southeast Missouri and will likely continue to develop into the
afternoon as shortwave trough currently over Kansas and Oklahoma
moves northeast. Expect current storms to spread northeast and
have continued likely pops into early-mid afternoon as this
shortwave moves over the CWA in an environment where SPC objective
analysis shows MUCAPES already in the 1000-3000 J/kg range. Upped
temps by a few degrees as we are already exceeding going highs in
a few locations because of sunshine this morning.

Britt

&&

.SHORT TERM:  (Through Tonight)
Issued at 346 PM CDT Thu May 14 2015

Ongoing SHRAs that have percolated all night should slowly
diminish this mrng with the weakening of the LLJ after 12Z. More
SHRAs are expected to dvlp later today. The region is under the
influence of SW flow aloft emanating from a closed low that is
just coming into CA. This system will set the stage for an active
prd thru the wknd. The key to the PoP fcst will be timing of
subtle short waves. The first one should pass thru this aftn. The
atmosphere will be largely uncapped so scttrd SHRAs/TSTMs are
expected to dvlp across the Ozarks drng the late mrng/early aftn
and then move NE in response to the approaching short wave.
Guidance is indicating that there will be a bit of a bndry/slight
wind shift across the nthrn CWA this aftn. Dps are also fcst to
pool some along this subtle feature enhancing the CAPE with values
in excess of 1500 J/kg by aftn. The SVR potential seems to be
limited tho due to marginal shear of around 20 kts. That being
said...can`t rule out strong to isld SVR storms for a few hrs drng
the aftn with hail being the biggest threat. Precip chances
diminish with the heating of the day as instability wanes and
forcing lifts NE of the FA. I keep slight chance PoPs going
overnight due to the favorable WAA pattern and a 30-40kt LLJ.
Temps should be above normal with highs in the low 80s.

2%

.LONG TERM: (Saturday through Thursday)
Issued at 337 AM CDT Fri May 15 2015

(Saturday thru Sunday night)

Saturday appears to be a repeat performance of Friday with another
low amplitude short wave fcst to affect the FA. This time the focus
appears to be a cross SE MO and sthrn IL tho. Atmosphere becomes
uncapped once again drng the aftn so SHRAs/TSTMs should dvlp thru
the aftn. Convective parameters vary by model with the GFS having
more CAPE than the NAM but less shear. I think the SVR potential is
fairly limited due to shear of less than 30 kts. This round of storms
should diminish with loss of day-time heat as the responsible short
wave lifts NE. The next round of precip is expected to approach the
CWA overnight as the main upper lvl system ejects NE. There should
be a decent shot of rain across the CWA as this feature passes.
Sunday represents the best chance for SVR storms with this episode.
Warm temps will combine with DPs in the 60s to near 70 to produce an
unstable atmosphere by aftn. Shear is higher...on the order of 40-50
kts due to the better upper lvl support. The low is fcst to move
from the Rockies on Sat mrng to the ern Dakotas by Sun evng. The
main thrust of precip should exit the CWA by evng with just some
lingering chances across the SErn portion of the FA Sunday night.
Temps will remain above normal thru the wknd with highs in the
low/mid 80s.

(Monday thru Thursday)

The upper lvl pattern becomes zonal by Mon evng with ridging
expected by the middle of the week. At the SFC...the cold front
assoc with the wknd system pushes thru Mon mrng ending any threat of
precip across the S/E portion of the CWA. Strong 1035mb SFC ridge
builds into the Plains Mon night thru Tue night. The next short wave
approaches the region on Wed bringing the next chance for rain for
Wed night and Thu. The SFC ridge builds back into the region for the
end of the week. This looks like an extended prd of aob normal temps
with a cool Canadian SFC ridge anchored to the north of the FA
producing persistent N/NE/E SFC flow. The week starts with 850 temps
a few degrees either side of 10 but cools to the mid/upper single
digits by Thursday.

2%

&&

.AVIATION:  (For the 12z TAFs through 12z Saturday Morning)
Issued at 710 AM CDT Fri May 15 2015

Extensive IFR area currently extends from north central MO into
eastern IA...in area that was dominated by rain-cooled air on
Thursday. On the fringes of this area at KCOU and KUIN there has
been spotty MVFR/IFR conditions over the past few hours, but
expect any residual lower conditions at these locations to improve
dramatically between 13-15z.

Otherwise, models are in good agreement that AMS across the FA
will become increasing unstable, with CIN totally eroding between
17-19z. These trends, combined with weak disturbances trickling
into the area from the southern Plains should lead to widespread
coverage of the convection by early-mid afternoon. While the
convection will likely be somewhat random in nature, some of the
hi-res guidance is suggesting a boundary over northern sections
of the CWA, which may tend to focus the convection a bit more over
the KUIN area.  Forecasts still attempt to convey a high
probability that most TAF sites will experience convection, but
obviously fine-tuning of forecast specifics will have to wait
until trends can be monitored by radar. Believe thunderstorm
threat should diminish at all locations by early this evening with
loss of heating and indications that shortwave ridge will
temporarily build into the area.

Specifics for KSTL: Sensible weather trends for KSTL should take
on a warm sector look...with rapid CU development and cloud bases
aoa 3kft by late morning followed by the development of convection
throughout the region by early afternoon. Believe coverage will be
great enough that TAF vicinity will also certainly be impacted by
some of the thunderstorm activity this afternoon, although as
mentioned in the main aviation AFD section radar trends will need
to be monitored for fine-tuning thunderstorm trends.

Truett

&&

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

$$

WFO LSX






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