Area Forecast Discussion
Issued by NWS St. Louis, MO

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058
FXUS63 KLSX 290508
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1208 AM CDT Thu Jun 29 2017

.UPDATE...
Issued at 856 PM CDT Wed Jun 28 2017

I have updated the POPs across northeast MO and west central IL
several times this evening to account for the now in progress
elongated west-east MCS in northwest and north central MO. The
leading edge is now pushing into the CWA and will be encountering
diminishing instability as it moves east-southeast into northeast
MO with SBCAPE less than 500 j/kg east of the MS river. Thus any
strong-severe threat will be limited to a small portion of the CWA
in northeast MO over the next few hours. Backbuilding and
regerative convection continues on the western flank of this
complex as a stout southwesterly LLJ interacts with a well defined
convective outflow boundary and is feeding a reservoir of much
higher CAPE on the order of 2000-3000 j/kg into the system. This
pattern and overall motion favors a heavy rain threat along with
severe threat west of the CWA in northwest MO. Eventually it
seems a stout enough cold pool will result in either south or
southeast system motion and ultimately may need to increase pops
through central MO.

Glass

&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 323 PM CDT Wed Jun 28 2017

Mid-high level clouds clearing out of central MO this afternoon
after the passage of a weak shortwave.  Isolated convection moving
across parts of northeast MO where atmosphere has gotten quite
unstable and diurnal cumulus field has developed.  Organized severe
convection is expected to develop late this afternoon and this
evening across IA into extreme northern MO as a southwest low level
jet intensifies ahead of a shortwave moving through the northern
Plains.  Still model uncertainty as to how far south convection will
get into our forecast area tonight, but current thinking is that
convection, possibly severe will sag into northeast MO and west
central IL towards midnight as a cold pool develops and strengthens
within IA MCS. Convection should remain north of Interstate 70
tonight, and may weaken late tonight into Thursday morning as it
sags further south. Warmer lows can be expected tonight with
southerly surface winds along with higher surface dew points.
Additional convection should develop Thursday afternoon, mainly
across northeast MO and west central IL along any outflow boundaries
from nocturnal convection as the atmosphere destabilizes.  Slightly
warmer temperatures can be expected on Thursday across central and
southeast MO and southwest IL with more humid conditions as
southerly winds continue.

KD

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 323 PM CDT Wed Jun 28 2017

There will be a trend toward more seasonal temperatures through
the forecast period, but these could be impacted by residual cloud
cover from previous convection. A cold front with attendant low in
north central Kansas will very slowly approach the area from the
west. The front is projected to be just outside the forecast area in
northwest Missouri on Thursday afternoon. A series of shortwaves
will move across the Plains which could kick off an MCS. The
remnants of the MCS would move into central Missouri Thursday night.
The front will move just a little further south on Friday to just
north of I-70.  A number of factors will impact coverage of showers
and thunderstorms on Friday.  A lingering MCS from Thursday night or
cloud debris hanging around the area will not only impact
instability, but also high temperatures during the day. If enough
clearing is realized, the area will quickly destabilize during the
afternoon with forecast CAPE values in excess of 2000 J/kg and mid
level lapse rates over 7 C/KM and bulk shear up to 50 knots in
northeast Missouri and western Illinois. The primary threats will be
damaging winds and large hail with storms on both Thursday and
Friday.

An additional concern will be the threat for localized heavy
rainfall across parts of northeast Missouri and western Illinois.
The combination of a slow moving boundary, a series of shortwaves
pushing through the area Thursday night into early Saturday will
promote training of showers and storms over the same areas. A few
places in the aforementioned area could see localized rainfall
totals in excess of 3 inches with lesser rainfall totals further
south.

The front pushes south to the southern Missouri bootheel by
Saturday morning providing the area a temporary break from
rainfall until Sunday evening. Another storm system organizes in
the Plains and pushes east toward the area early next week. The
storm system will likely bring more chances of showers and
thunderstorms but there are timing and placement issues from model
to model.

Temperatures will be at or close to seasonal normals for afternoon
highs and morning lows during the time frame in areas not impacted
by showers and thunderstorms.

Kelly

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1149 PM CDT Wed Jun 28 2017

Main corridor of showers and thunderstorms overnight is expected
to be across northern MO. KUIN will remain on the northern edge
of the strongest storms and overall there should be a weakening
trend overnight across northeast MO and southwest IL owing to
limited instability. There is still some question overnight how
far south there might be a threat of showers or storms as an
outflow boundary pushes south. I certainly can`t rule out
something might make it into KCOU but my confidence is too low to
mention. I think by daybreak there will be little precipitation
left in the immediate area that would impact the terminals. VFR
flight conditions will then dominate much of the day. The next
threat of showers and thunderstorms will be focused again across
northern MO into west central IL during the later part of the
afternoon and especially the evening hours. Gusty southerly
surface winds will resume mid-late Thursday morning.

SPECIFICS FOR KSTL:

Still thinking the threat of showers and thunderstorms will
remain north of the terminal through the forecast period. Thus
expecting VFR flight conditions to dominate. Gusty southerly
surface winds will resume again mid-late Thursday morning. The
best chance of showers and thunderstorms will be after 06Z 06/30.

Glass

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

$$

WFO LSX



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