Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 250456

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1156 PM CDT SUN JUL 24 2016

Issued at 919 PM CDT Sun Jul 24 2016

Convection so far this evening has by and large stayed north
and/or southwest of the CWA. Showers/thunderstorms are trying to
form along a weak wind shift with the dewpoint gradient lagging
several hours behind. Tweaked PoPs for tonight focusing on the
wind shift boundary for the next few hours with additional
showers/storms possible also along/ahead of secondary
boundary/cold front and corresponding dewpoint gradient. Also
lowered PoPs beginning tonight and heading into the day on Monday
behind this boundary.

Also extended the excessive heat warning earlier this evening as
heat index readings have been slow to fall back below 105
degrees...mainly due to dewpoints being around 80 degrees. Warning
will expire at 0300 UTC for the area with the exception of the St.
Louis metro area with an expiration time of 0400 UTC.



.SHORT TERM...  (Through Late Tonight)
Issued at 320 PM CDT Sun Jul 24 2016

Oppressive heat and humidity blanket the area at mid-afternoon and
once again we are seeing a few short-lived showers and thunderstorms
across parts of south central and southeast MO. Like previous
days, there will continue to be a threat of these through early
evening. The greatest area of attention however turns to northeast
MO into west central IL this evening along and ahead of the slowly
advancing cold front, and then overnight as the front sags southward
to the St. Louis vicinity by 12z Monday. There is some CIN across
northern MO at this time, however the CAP will eventually erode
and the development of scattered thunderstorms is anticipated as
some weak ascent and slight cooling aloft spread across the area
attendant with an upstream impulse now in eastern KS/NE. Several
waves of multicell clusters are expected along and ahead of the
slowly advancing cold front both this evening and overnight. Any
severe threat would probably be this evening owing to greater
instability and be in the forming of damaging winds.


.LONG TERM...  (Monday through Next Sunday)
Issued at 320 PM CDT Sun Jul 24 2016

The cold front will continue to move slowly southward on Monday as
weak surface high pressure settles into the Mid-Upper MS Valley,
with the front in the vicinity of the bootheel/extreme southern IL
by early evening. Scattered thunderstorms/thunderstorm clusters
are expected to be ongoing at sunrise and continue along/ahead of
the advancing cold front. Both the precipitation and clouds along
with the frontal passage are expected to put an end to the
dangerous heat/prolonged heat wave with today being the last
thrust of extreme heat marked by HI values of 105-115+. The
excessive heat warning will be allowed to expire on schedule at
800 pm this evening. I should note that if the clouds are not as
great on Monday as expected and we see a tad higher temps, then
some HI values in the 100-102 range might occur from St. Louis
southward, however this is highly conditional and still a good
deal below the prominent readings of the last week.

From Monday night through Tuesday night I believe that the threat
of showers and storms should generally be confined to the frontal
corridor including southern MO and southern IL. Some of the
deterministic model guidance wants to generate precipitation much
further north into the cool sector, however the lack of a
"overrunning" south-southwesterly LLJ to provide the lift or an
appreciable short wave makes this suspect. Weak high pressure to
the north of the front will dominate, and the weather will largely
feature lower RH and cooler/near average temperatures.

A somewhat unsettled pattern is forecast to dominate the period
from Wednesday into early next weekend. An upper high will be
anchored in the western U.S. with broad mean trof through the
upper MS Valley-Great Lakes-northeastern U.S.. The resultant
broadly cyclonic flow - west/northwest aloft - will feature a
somewhat steady stream of short waves or lower amplitude
disturbances. These features and a wavering frontal boundary will
result in a continuous chance of showers and thunderstorms.
Temperatures are expected to be near average during this period.



.AVIATION...  (For the 06z TAFs through 06z Monday Night)
Issued at 1149 PM CDT Sun Jul 24 2016

Focus will continue to be on convective trends through the rest of
the night and into early on Monday. Front is through KUIN with
just a few showers behind it so belive Quincy will be dry save a
few showers in the vicinity. KCOU should get a couple of hours of
showers and thunderstorms early in the period with gusty WSW winds
and heavy rainfall in a TEMPO group. Tough call for the metro TAFs
but SW-NE oriented boundary is oozing SE very slowly with KSTL
having the best chance of seeing thunderstorms early in the TAF
period compared to KCPS and KSUS...though they may get in on the
action a bit later tonight. For now...continued VCTS groups due to
undertainty later on tonight with convective trends. Winds behind
the front will swing to the northwest with diminishing chances for
showers and storms as we head from Monday morning into the
afternoon hours as the front continues to sag to the south of the

Specifics for KSTL:

Greatest concern in the next 6-9 hours will be on possibility of
thunderstorms affecting the terminal. Believe highest chance will
be within the first few hours and have added a TEMPO group for
MVFR visbys w/ the thunderstorms. IFR visbys along with wind gusts
are possible if a stronger storm affects the terminal.
Thunderstorms continue to be possible into Monday morning but
chances will diminish toward the afternoon hours. Winds have come
around to the northwest behind an outflow boundary and may now
stay that way the rest of tonight...though they could be a bit
variable direction wise over the next several hours.





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