Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 170436
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1136 PM CDT Sat Aug 16 2014

.UPDATE:
Issued at 942 PM CDT Sat Aug 16 2014

The latest water vapor and VWP data suggest the mid level
low/vort max is located near KUIN. The associated surface low is just
east of Mexico with a warm front trailing southeast through the
St. Louis metro area and cold front extending to just east of the
Lake of the Ozarks. The air mass along and ahead of these features
is very moist with PWs average around 1.80 inches and warm cloud
depths are running 3.5-4.0 km. While this speaks to the efficiency
of rainfall production, thus far there has been a lack of overall
mesoscale organization and convection has been moving, resulting
in spotty rainfall totals of just over 2 inches since this
afternoon.

Short term guidance from the RAP and HRRR move the vort max to
just southeast of St. Louis by 12z with a nearly vertically
stacked surface low. The southwesterly LLJ is forecast to increase
some to around 30 kts ahead of the system while the current CAPE
stream of 1000-1500 J/KG will gradually wane. Present indications
given these trends suggest 2 zones with the highest probability of
locally heavy rain - 1) south of St. Louis across the eastern
Ozarks into southern Illinois more coincident with the LLJ axis
and 2) north of St. Louis from roughly Pike County IL to Fayette
County IL more associated with mesoscale forcing with the MCV. The
current mention of locally heavy rain has this covered well and if
a greater threat materializes will update accordingly.

Glass

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 347 PM CDT Sat Aug 16 2014

We have entered a quiet period of weather early this afternoon as
the atmosphere re-organizes itself.  Just entering extreme northeast
MO was a powerful 50-something MCV, and with the very moist
atmospheric column in place where PWs were in excess of two inches,
it is having no problem in perpetually generating rain around it.
Extending to the southwest of this important upper level feature was
a surface cold front:  from just W of Moberly to just N of Sedalia.
This front was intersecting a very moist atmosphere as well, where
surface dewpoints were in the low 70s and PWs near two inches, and
an unstable atmosphere as well, with MLCAPES up to 3000 J/kg.  This
front was just beginning to ignite some TSRA along it.  Further
downstream into eastern MO and southern IL, temps were just
beginning to recover into the upper 70s and low 80s where clouds and
rain had previously dominated.

The atmosphere is essentially primed and merely needs a trigger to
get the next round of rain going.  Moisture convergence should be at
sufficient levels for TSRA to get going along the front, but have
also noted that convergence was also beginning to increase over STL
metro and southeast MO where a window of opportunity for additional
development will present itself between now and early this evening.

Otherwise, primary foci will be surface front and MCV as they both
very slowly progress southeastward thru our region tonight and
continue on Sunday morning...finally expected to leave and pull away
Sunday afternoon.  As stated earlier, PWs will be in excess of two
inches, which is greater than two standard deviations from the mean
for mid-August, and models are also forecasting rather deep warm
cloud layers over 4km.  Greatest threat for excessive rainfall will
be an area sandwiched between the MCV, the broadscale lift from the
mid level shortwave, and the trailing surface front.  This all seems
to point to an area from east-central MO and throughout southwest
IL, including STL metro.  The problem is that 6hr FFG values are
mainly above 3 inches, with a small area just north of STL metro
where it is as low as 2.5 inches.  Given this, will pass these
concerns on to evening shift and let them see how convection
develops before issuance of any Flood Watch.

Categorical PoPs were forecasted thru this system`s conclusion on
Sunday with a warm and muggy night ahead tonight and a cool day on
Sunday with substantial lo clouds in the system`s wake expected.

TES

.LONG TERM:  (Sunday through Next Saturday)
Issued at 347 PM CDT Sat Aug 16 2014

A somewhat drier atmosphere is in place for Monday and with a lack
of upper level support and a decent lo level cap in place, should
see one day of dry wx.

A weak cold front drops down on late Monday night and lingers thru
late Wednesday and will result in a threat for thunderstorms.
Depending on where the front settles, it will separate seasonable
warmth to the north from summertime heat in the south.  This
summertime heat will be coupled with humidity that may result in 100
heat index values for parts of the area Tuesday and Wednesday
afternoons.

By late next week, there trend will be for drier and hotter wx as an
upper ridge builds overhead and may result in the summer we nearly
missed.  Looks like a period of a few more days of max temps in the
90s with heat index values between 100-105.  May need a heat
headline at some point for quite a few areas for next week as a
result.

TES

&&

.AVIATION:  (For the 06z TAFs through 06z Sunday Night)
Issued at 1114 PM CDT Sat Aug 16 2014

A slow-moving low pressure system will continue to bring scattered
SH/TS to the terminals over the next few hours. Coverage will
probably be greater at St. Louis metro area TAF sites due to their
proximity to the low. Once the precipitation moves out, ceilings
should drop to IFR and remain there for the rest of the night and
into tomorrow. There will probably also be transient MVFR to IFR
visibilities in fog overnight because of the highly saturated air
mass, but models are forecasting winds to stay high enough
overnight that it looks more like a low stratus situation rather
than widespread dense fog. Ceilings should slowly improve during
the afternoon as the low pressure system moves away from the area.
Prevailing winds will eventually turn around to the north behind a
cold front and on the back side of the departing surface low.

Kanofsky

&&

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

$$

WFO LSX





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