Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 270500
AFDLSX

AREA FORECAST DISCUSSION...UPDATED AVIATION
NATIONAL WEATHER SERVICE ST LOUIS MO
1200 AM CDT Wed May 27 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 CWA between 06-12z. Increasing H7-H5 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 CWA 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 LLJ pointed into southwestern MO by 06z.
The models which show a stronger LLJ 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 CWA.

Kanofsky

.LONG TERM:  (Wednesday through Next Tuesday)
Issued at 400 PM CDT Tue May 26 2015

In general, short range models are suggesting AMS over the central
CONUS 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 CWA. For the afternoon
hours AMS is forecast to destabilize, but aforementioned shortwave
energy will have exited the FA 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 CWA 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
UA 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 06z TAFs through 06z Wednesday Night)
Issued at 1140 PM CDT Tue May 26 2015

Isld/scttrd SHRAs will continue to dot the area thru the prd. Weak
forcing along a diffuse bndry will keep sporadic precip percolating.
Current activity across cntrl/NE MO and w cntrl IL should slowly
dsspt/move NE after 6Z. Guidance hints at another batch of convection
dvlpng across SW MO tonight. This precip, if it dvlps, will move
NE but appears to remain south of the STL metro terminals. Limited
coverage precludes mention attm. SHRA coverage may increase Wed aftn
with the highest coverage expected to be east of the STL metro. Winds
will become wrly Wed mrng.

Specifics for KSTL:

VFR fcst thru the prd though there will be isld/scttrd SHRAs. A weak
bndry will push thru the terminal drng the late mrng allwoing winds
to become more wrly and taking the better chance of rain with it.
Limited coverage precludes mention in the fcst attm.

2%

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     66  84  66  86 /  40  40  20  20
Quincy          62  80  62  83 /  30  10  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 /  30  40  20  20
Farmington      63  82  62  83 /  30  40  30  30

&&

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

$$

WFO LSX





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