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Area forecast discussion...updated aviation 
National Weather Service St Louis MO
731 PM CDT Tue may 26 2015

Short term: (through late tonight)
issued at 400 PM CDT Tue may 26 2015

The primary forecast issue for tonight is convective trends. A
nearly stacked low pressure system will continue progressing
northeastward towards the Great Lakes tonight, trailing a weak cold
front in its wake. This front will edge into northern MO overnight
before stalling across the region tomorrow. Most models depict a
couple of vorticity maxima rotating into the back side of the
lifting shortwave trough tonight, and the strongest of these
features will provide large-scale ascent across the northern part of
the County Warning Area between 06-12z. Increasing 700 mb-500 mb lapse rates spreading
across the region will contribute to instability. There are also
some hints of a h25 jet streak developing across parts of MO which
places the northern County Warning Area beneath the favorable ler. None of these
features by itself looks particularly strong, but taken together
they support a chance of sh/ts accompanying the cold front as it
sinks slowly southward overnight.

One area of uncertainty for tonight is that some models also develop
a southern branch of the low level jet pointed into southwestern MO by 06z.
The models which show a stronger low level jet appear to be developing it as a
response to a vort Max lifting out of the Southern Plains, but it's
not clear whether that vort Max exists in the real atmosphere. On
the other hand, a small thunderstorm complex recently developed over
south-central OK, which is approximately where that feature ought to
be if it does exist. Whether it's an actual shortwave or just a
remnant MCV, it could provide support for sh/ts given the greater
MUCAPE and better lapse rates which are forecast to be in place over
the southern part of the County Warning Area.

Kanofsky

Long term: (wednesday through next tuesday)
issued at 400 PM CDT Tue may 26 2015

In general, short range models are suggesting air mass over the central
Continental U.S. Will remain rather unsettled into the end of this week. It
appears that vorticity maxima/shortwaves rotating around the upper
low overnight tonight will exit our area on Wednesday, with
shortwave ridging in the wake of these features advecting into the
mid-Mississippi Valley by Wednesday night. At this point, not
exactly sure how to handle convective trends on wednesday;
certainly, primary threat during the morning will be tied to
movement of predawn convection across the County Warning Area. For the afternoon
hours air mass is forecast to destabilize, but aforementioned shortwave
energy will have exited the forecast area by that time which may tend to
inhibit coverage of any redevelopment, and convergence along the
weak cold front will be rather feeble. I've continued some mention
of convection over most of the County Warning Area during the afternoon, with this
threat diminishing and sinking south Wednesday night with the
front, and as upper level ridge builds into the area.

From Thursday into the weekend, southwest flow will dominate the
upper air pattern over the central conus, with a series of shortwaves
forecast to eject into our area from the longwave trof over the
western U.S. Initially on Thursday, threat will remain in the
chance category as large scale lift is weak and moisture return is
still somewhat limited. However, likely pops should overspread
the region Friday-Saturday as there is model consensus that a
fairly robust shortwave and associated cold front will be
impacting the region at this time.

Lesser shower and thunderstorm chances will linger into the start
of the new week as weak ripples continue to propagate through the
area.

Temperatures will remain a bit above average through Friday, then
cool off over the weekend as the colder air works its way into the
mid-Mississippi Valley. For now i've pretty much stayed with
guidance that has highs in the 60s to lower 70s on Sunday, but
wonder given the progged upper air patter if this may be a bit too
cool. Will have to monitor upcoming medium range solutions on this
cool down over the next few days and nights.

Truett

&&

Aviation: (for the 00z tafs through 00z Wednesday evening)
issued at 642 PM CDT Tue may 26 2015

Stacked area of low pressure across sthrn Minnesota will lift north
overnight. Isld shras rotating around it across nthrn MO should
diminish after sunset. A weak bndry is progged to slowly sink
south thru the area thru the prd. Expect isld shras in assoc with
this feature overnight. The limited coverage keeps mention of
precip out of the fcst...for now. Coverage may increase some wedn
aftn but the bndry should be just east of the stl Metro sites by
then. Otherwise expect VFR cu and a SW breeze bcmng wrly tomorrow
mrng as the bndry passes.

Specifics for kstl:

VFR fcst thru the prd. Isld shras are possible tonight in assoc
with a weak bndry slowly sagging south. Due to the expected
limited coverage of the shras, have not included mention of precip
is the taf attm. Coverage of precip should increase some drng the
aftn but the bndry should be beyond the terminal by that time. The
bndry passes tomorrow mrng swinging the SW wind to the west.

2%

&&

Preliminary point temperatures/pops:
Saint Louis 66 84 66 86 / 40 40 20 20
Quincy 62 80 62 83 / 30 20 10 20
Columbia 62 81 63 84 / 30 30 20 40
Jefferson City 63 81 63 84 / 40 30 20 40
Salem 65 82 65 86 / 20 40 20 20
Farmington 64 82 62 83 / 20 40 30 30

&&

Lsx watches/warnings/advisories:
MO...none.
Illinois...none.
&&

$$

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