Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 272045
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
345 PM CDT Wed Aug 27 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 344 PM CDT Wed Aug 27 2014

Area of showers and thunderstorms over northern Missouri has almost
totally dissipated as of 20z, and convection has refocused to to the
south and east along the outflow boundary produced this morning by
convection over northern Missouri, as well as the very ill-defined
remains of the surface frontal boundary.  Storm coverage should
remain scattered due to upper level ridging over the area, but
continue to monitor the stronger storms for an isolated/pulse-type
severe weather threat over the next hour or two.

With no upper support, expect this largely diurnally-driven
convection to dissipate this evening with loss of heating, with
thunderstorm threat once again shifting back to our northwest
overnight.  Shortwave topping ridge and the associated low level
forcing/warm advection should lead to a a fairly large convective
complex forming over the mid Missouri River valley (northwest of our
FA) during the predawn hours.  While the bulk of this activity
should remain northwest of our forecast area, there should be some
threat of convection in northern sections of our CWA, either from
the leading edge of this activity, or perhaps from additional
convection that will attempt to form in the very weak zone of low
level warm advection in advance of the complex.

Truett

.LONG TERM:  (Thursday through Next Wednesday)
Issued at 344 PM CDT Wed Aug 27 2014

Nocturnal convection should be winding down during the morning, but
not before producing some thunderstorms over parts of central and
northeast Missouri.  It looks like the main area of precip will be
further west over northwest Missouri again nearest to the low level
baroclinic zone.  With lingering cloud cover and the upper level
ridge slowly breaking down, expect that temperatures will be cooler
Thursday than in recent days.  With highs ranging from the mid 80s
to around 90 instead of mid 90s, it looks like heat index values
will miss the magic 100 degree mark by 2-3 degrees in most
locations.  While still hot and humid, it should feel...less bad.
Expect widely scattered afternoon thunderstorms to help keep a lid
on temperatures as well.  Will therefore let the heat products
expire on schedule this evening.

Thursday night and Friday will be a repeat of tonight and Thursday
as a weak low level jet provides just enough lift to potentially
force some isolated to widely scattered convection. With overnight
lows in the upper 60s to mid 70s and highs Friday in the mid 80s to
around 90, we should be able to stay heat-headline free.
Additionally, the trof of low pressure currently spinning over
southwest Wyoming will be moving into the eastern Plains
Friday/Friday night which will further weaken the upper level ridge
and allow more widespread convection late Friday afternoon and
Friday night primarily over central and northeast Missouri.  The
trof will be directly over the CWFA Saturday which should continue
to produce thunderstorms and keep temperatures suppressed in the 80s.

Another stronger trof will dig into the Great Plains on Sunday.
This will keep the upper ridge at bay and continue to keep
temperatures in the 80s.  Can`t rule out a few afternoon
thunderstorms Sunday as well since the upper ridge will be well to
our east.  The trof continues eastward Sunday night and Monday which
will bring us increasing chances for rain.  medium range models are
in decent agreement regarding the cold front associated with the
trof.  It should push into the CWFA late Monday afternoon/Monday
evening and bringing the threat of more thunderstorms to the area.
Models stall the front out over southern Missouri Tuesday and then
allow it to creep back to the north Wednesday.  This won`t be a big
blast of cold air behind the front...rather just enough cool air to
bring temperatures down to near or slightly below seasonal normals.

Carney

&&

.AVIATION:  (For the 18z TAFs through 18z Thursday Afternoon)
Issued at 1245 PM CDT Wed Aug 27 2014

Have gone with the idea that storms over north central MO will be
weakening this afternoon and will not impact UIN as low level jet
(finally) loses it`s punch. Meanwhile, scattered convection is
epected to develop along outflow boundary and/or the very ill-
defined remnants of synoptic frontal boundary, and the latest
surface data and hi-res output suggests this activity will be east
of COU and primarily impact STL metro sites. For now have stayed
with VCTS in the STL area TAFs, and will update if/when convective
trends become more clear. Much like yesterday, these storms will
be capable of very localized, microburst winds, and will be
monitoring radar trends for this threat as well. Have allowed the
afternoon thunderstorm threat to end by 01z, with generally
tranquil conditions during the late evening and overnight hours.
Heading towards daybreak have maintained some MVFR vsbys in fog
along the I-70 corridor in the warm humid AMS, and reintroduced
VCTS at UIN as leading edge of complex that forms to our NW
overnight should be threatening northern parts of the CWA by that
time.

Specifics for KSTL: Aviation concerns can expect scattered
TSRA to form over the Bi-State area this afternoon, but some doubt
about whether the storms will actually impact within 10nmi of the
STL aerodrome. Aviation interests should also be aware that these
storms will be capable of producing micro-burst wind, much like
affected parts of STL county yesterday afternoon. Have allowed
this threat to diminish early this evening, with generally
tranquil conditions overnight. Area has been experienced MVFR
vsbys in fog the past several mornings, and without a change in
airmass have included this in the 08-12z time frame. More ams-type
TSRA are possible again on Thursday, but have not included in TAF
attm due to the large uncertainty of where the axis of
development will be.

Truett

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     76  91  76  90 /  30  40  20  20
Quincy          69  87  71  88 /  20  50  30  30
Columbia        72  92  71  88 /  20  30  20  40
Jefferson City  72  92  72  89 /  20  20  20  40
Salem           71  89  71  89 /  30  40  30  20
Farmington      71  90  70  88 /  30  20  10  20

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...HEAT ADVISORY until 8 PM CDT this evening FOR Audrain MO-Boone
     MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Madison MO-Moniteau MO-Montgomery MO-Osage MO-
     Reynolds MO-St. Francois MO-Ste. Genevieve MO-Warren MO-
     Washington MO.

     EXCESSIVE HEAT WARNING until 8 PM CDT this evening FOR Jefferson
     MO-Lincoln MO-St. Charles MO-St. Louis City MO-St. Louis MO.

IL...EXCESSIVE HEAT WARNING until 8 PM CDT this evening FOR Bond IL-
     Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey IL-
     Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-
     Randolph IL-St. Clair IL-Washington IL.

&&

$$

WFO LSX





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