Area Forecast Discussion
Issued by NWS St. Louis, MO

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000
FXUS63 KLSX 010209
AFDLSX

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE ST LOUIS MO
909 PM CDT Sun Aug 31 2014

.UPDATE:
Issued at 905 PM CDT Sun Aug 31 2014

Progressive UA trof and associated cold front, as well as outflow
boundaries produced by intense convection so far this evening,
should cause convection stretching from western Iowa to central KS to
push across the NW half of our FA overnight. Rough timing based
on extrapolation of upstream radar data suggests leading edge of
precip moving into the Edina area by 05z/midnight.  While we are
missing much of today`s explicit guidance, from the data that is
available believe that the latest HRRR output has a best handle on
overnight trends in two aspects...being faster with the eastward
progression and suggesting that some precip may be knocking on the
western door of the STL metro by daybreak, and secondly more of a
focus on late night/predawn convection over eastern KS/W MO where
there should be good cold pool/low level jet interaction which
should focus the most robust convection in this area.

Forecast inherited from dayshift already trended specifics towards
the trends mentioned above, so going forecast looks quite good and
only minor adjustments needed to speed up onset of higher PoPs by
1-2 hours. If HRRR scenario does pan out I`m not too certain
about how much of a severe weather threat will reach our area, but
as mentioned by day shift there will likely be at least some
impact from upstream outflows that propagate east and produce
gusty winds.

Specific trends for Labor Day and Monday night...including the
potential for severe weather and heavy rain...will largely hinge on
how convection evolves across the region during the predawn hours
and into Monday morning;  specifically how long the early morning
convection lingers and where the associated outflow boundary will be
located during the afternoon.  While there is still quite a bit of
uncertainty on how the convection will evolve, it certainly appears
that outdoor plans may be altered in many areas due to the
weather.

Update will be issued as soon as 02x surface data can be ingested
and assimilated into database.

Truett

&&

.SHORT TERM:  (Through Late Tonight)
Issued at 350 PM CDT Sun Aug 31 2014

Isolated to widely scattered thunderstorms over southeast portions
of the CWFA should dissipate fairly quickly this evening.  Until
then, locally heavy downpours will continue over parts of the
eastern Ozarks.  Attention turns to tonight as a strong shortwave
will move from eastern Wyoming into the eastern Nebraska/northwest
Missouri.  An MCS is expected to develop ahead of it on the nose of
a 40kt low level jet over the eastern Plains.  The model consensus
keeps the vast majority of the precipitation over western/north
central Missouri tonight.  However, it does look like the leading
edge will nose into central and northeast Missouri between 08Z and
12Z.  Not sure how much of the strength of the storms by that
point.  Current thinking is that the cold pool may outrun the best
forcing causing the MCS to weaken as it moves east.  This scenario
would have the rain moving into western portions of the CWFA between
08Z and 10Z.  There may be some gusty winds as the outflow moves
through, but the most significant impact would likely be locally
heavy rain; and that looks more likely over western Missouri than in
central Missouri.

Carney

.LONG TERM:  (Monday through Next Sunday)
Issued at 350 PM CDT Sun Aug 31 2014

We will see a break in what has been a hot pattern for us this past
week, as the upper level ridge breaks down, thanks to a pair of
decently strong shortwave TROFs.  Much of the energy from these
shortwaves will pass us to the north, but nevertheless, we are
expecting high probabilities of rain from both of these systems
because of either a favorable position of the low level jet or
synoptic front and a very moist, deep warm-cloud atmosphere.

The first system is expected to be ongoing early Monday morning for
areas north and west of STL metro while in a slowly weakening
state.  This rain is expected to reach the STL metro area before
dissipating or scattering out, with an anticipated boundary setup
near or just south of the I-70 corridor for new development in the
afternoon.  There is also some potential for scattered development
Monday afternoon further north and west near the synoptic front in
northern MO and central IL.

The second system on Monday night should make more of a pass to the
south and interact with the synoptic front now deeper into our
region and favorable axis of the low level jet.  In addition, there
is potential for localized heavy rainfall with PWs of around 2
inches and warm-cloud depths in excess of 4km and training storms.

The front is then expected to stall over southern MO Tuesday before
pushing north as a warm front on Wednesday with persistent, but
lesser, chances for storms.

While the humidity air never really goes away thru late week, it
should be just cool enough to prevent any dangerous heat and
humidity conditions until perhaps when the warm front pushes thru
Wednesday and into Friday.

A period of dry weather is anticipated Thursday through early Friday
with a warm-sector setup, with the next, and what looks to be much
stronger, cold front pushing thru late Friday thru early Saturday.
Increased chances for rain will return with the front with all early
indications showing a cool and dry period next weekend for most
areas.

TES

&&

.AVIATION:  (For the 00z TAFs through 00z Monday Evening)
Issued at 533 PM CDT Sun Aug 31 2014

Could not rule out an isolated shower/storm in the St Louis metro
area until 02Z this evening, but it appears that the probability
is too low to include in the tafs. Convection developing across
eastern Nebraska and north central and northeastern KS along and 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 Monday morning as it weakens. New convectve development is
expected late Monday afternoon 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. Sely
surface winds this evening will gradually veer around to a sly
direction late tonight and a swly direction on Monday.

Specifics for KSTL: Will leave the STL taf dry this evening as
at most only isolated showers are expected until sunset. Weakening
showers and storms are expected to drop southeastward into STL
area Monday morning. There will be a break in the convection with
new development of storms expected late Monday afternoon and
evening. S-sely surface winds this evening will become sly late
tonight, then increase to around 15 kts by late Monday morning
from a swly direction.

GKS

&&

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

$$

WFO LSX






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