Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 010854
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
354 AM CDT Mon Sep 1 2014

.SHORT TERM: (Today through Tonight)
Issued at 344 AM CDT Mon Sep 1 2014

There are three interconnected forecast issues covering today
through tomorrow: convective timing, the potential for severe
weather, and the potential for heavy rainfall. The main factors
involved with the forecast are the ongoing early morning
convection with its outflow boundaries, an upstream synoptic cold
front, and a couple of shortwaves moving through a broad longwave
trough.

If the early morning convection dissipates across our area, then the
air mass is likely to destabilize ahead of the approaching synoptic
cold front. Models are forecasting up to 2000-3000 J/kg of CAPE
and between 30-60 kts of 0-6 km bulk shear across most of the CWA
during the afternoon/evening hours ahead of the front. These
parameters are more than sufficient to support organized severe
thunderstorms. With at least a few models depicting a small but
nonzero perpendicular component to the shear vectors with respect
to the H85 boundary orientation, it would not be surprising to see
a few supercells somewhere across the region before convection
starts to congeal and become more linear with time. If the models
are correct with the orientation of the shear vectors with respect
to the cold front, then aftn/eve convection should become linear
very quickly.

As of 0830z, early morning convection appears to be diminishing
over northern/northeastern MO, lending credence to the above
scenario. However, if the early morning convection does not
dissipate and if the outflow boundary is pushed farther to the
south, then the overall coverage of severe thunderstorms would
probably be much lower and would tend to be limited to the
southern CWA.

Moving into tonight, the cold front becomes oriented nearly parallel
to the 0-6 km shear vectors, and the primary forecast issue then
shifts from severe weather to heavy rainfall. Overall, this looks
like a decent setup for locally heavy rainfall. PW values
increase to over 2" tonight, which is not only at +2SD and the 99th
percentile for August, but also exceeds both of those thresholds for
September. Models also depict broad lift from favorable jet coupling
(between the RER of a jet at H25 and the LER of a jet at H85) which
develops by 06z in response to a shortwave moving through NEB/IA.
In addition, MUCAPE remains quite high overnight and the mid-level
flow also appears to be nearly parallel to the cold front. Because
of these factors, I`m reasonably sure that there is going to be an
elongated band of convection overnight, but I`m not sure whether
it will be with the true synoptic cold front (resulting in a band
of rainfall near the I-70 corridor) or with the outflow/effective
boundary (resulting in a band of rainfall located much farther
south, possibly only affecting the southern CWA).

Flash flood guidance numbers are around 2-3" for 1 hour, 2.5-3.5"
for 3 hours, and 3-5" for 6 hours. There hasn`t been much rainfall
over the last few days, therefore the ground is not saturated and
can probably absorb a fair amount of water before widespread
flooding becomes a concern. There is also a question about where
the axis of heaviest rainfall will occur. For these reasons, we
will hold off on any flood/flash flood watches for now.

Kanofsky

.LONG TERM:  (Tuesday through Sunday)
Issued at 344 AM CDT Mon Sep 1 2014

The front and/or effective boundary should linger across the
region on Tue/Tue night before lifting northeastward as a warm
front on Wed/Wed night. The precipitation threat will also lift
northeastward with the warm front during the middle of the week.
Model solutions then depict a cold front moving into the area
towards the end of the week and into next weekend, bringing a
chance of rain and a cooler air mass into the region.

Kanofsky

&&

.AVIATION:  (For the 06z TAFs through 06z Monday Night)
Issued at 1103 PM CDT Sun Aug 31 2014

Convection extending from IA southwest through northwestern MO and
KS just ahead of a cold front will drop southeastward into the
UIN and COU areas late tonight, and then eventually into the St
Louis metro area towards morning as it weakens. New convective
development is expected late Monday afternoon and early evening
mainly in COU and the St Louis metro area between the approaching
cold front and an outflow boundary left from the late
night/morning storm complex. S-sely surface winds late tonight
will gradually veer around to a swly direction Monday morning.

Specifics for KSTL: Weakening showers and storms are expected to
drop southeastward into STL area by early Monday morning. There
will be a break in the convection with redevelopment of storms
expected late Monday afternoon and evening. S-sely surface winds
late tonight wlll increase to around 14 kts by late Monday
morning from a swly direction.

GKS

&&

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

$$

WFO LSX





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