Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 161754

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1254 PM CDT Sun Jul 16 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 333 AM CDT Sun Jul 16 2017

Backdoor cold front currently across the Upper Midwest is expected
to move slowly to the south/southwest through this evening. Moderate
surface convergence along this boundary in conjunction with
vorticity maxima skirting the the western Great Lakes should produce
isolated-scattered showers and thunderstorms. Timing wise, the mid
afternoon through the early evening is the most favored for storms.
Higher cores would pose a wind threat with very dry midlevels of the
atmosphere yielding high downdraft CAPE.

This afternoon will be fairly unpleasant temperature and humidity
wise. Highs are forecast to be in the low 90s for many areas.
These temperatures combined with dewpoints pooling along and just
ahead of the backdoor cold front will produce peak heat index
values of around 100 degrees in spots.

Cold front will continue to slowly push south/southwestward through
the night tonight. Convergence along the front weakens however
tonight This factor combined with lessening instability should lead
to a weakening, followed by a diminishment, of showers and
thunderstorm activity gradually with time. Due to the placement of
the front, there will be a temperature gradient from southwest to
northeast overnight tonight. Coolest conditions with lows in the mid
60s are expected across portions of west-central Illinois behind the
front with low 70s for portions of central and southeastern Missouri.


.LONG TERM...  (Monday through Saturday)
Issued at 333 AM CDT Sun Jul 16 2017

The latest data continues to support the development of a dangerous
heat wave this coming week. There are some geographical areas and
time frames with much greater confidence than others, owing to model
variances and predictability.

A backdoor front will extend across southern IL across southeast MO
arcing into central MO and into far northeast KS at daybreak Monday.
The front will either remain quasi-stationary or may even sag a bit
further south and southwest during the day as surface high pressure
settles into the Great Lakes region. Diurnal instability may be
enough to fuel an isolated storm or two in southeast MO, otherwise
the convergence with the front is quite weak. Temperatures should be
a bit cooler than today in eastern parts of the CWA and comparable
to today in central MO.

On Tuesday it appears the heat wave should begin to unfold although
the intensity of the heat on Tuesday may be a bit lower than we
expected several days ago. The upper high will begin to strengthen
and expand into the MS Valley. Present indications are that the
north and northeast retreat of the backdoor front as a warm front
will be a bit slower across eastern MO and IL, and the surface/lower
tropospheric winds are delayed in shifting to the south-southwest.
That said, Tuesday will still be quite warmer with areas along
and west of the MS River getting well into the 90s and afternoon
heat index values in the upper 90s to lower 100s.

The big thrust of the heat wave and dangerous heat will be
centered in the Wednesday through Friday time frame. A strong and
expansive upper high will dominate the Nation`s midsection, with
the high center slowly shifting east from KS into MO. More
uncertainty begins to creep into the forecast on Friday and into
the weekend. The ECMWF is a little faster sliding the upper high
center to the southeast and into the TN Valley and thus allows an
east-west front to penetrate well into the forecast area on
Friday, eventually retreating in back to the north on Saturday.
The GFS also slides the upper high center into the TN Valley
region, but not until Saturday. A prominent heat wave breaking
cold frontal passage could occur as early as late Saturday per the
GFS, or early the following week per the EC.

The greatest forecast confidence resides in the St. Louis metro area
and the previously issued excessive heat watch for the Tuesday
through Friday still looks on target. The hottest conditions remain
centered on Wednesday through Friday. High temperatures in the metro
area should be around 100+ with afternoon and early evening heat
index values in the 105 to 110 range. At some point in time over the
next few days a combination of excessive heat warnings and
advisories will encompass the forecast area for the Tuesday-Friday
time frame, potentially extended into the weekend depending on
forecast confidence.



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1236 PM CDT Sun Jul 16 2017

A cold front over southern Iowa will drift south across the area
over the next 24 hours. Scattered showers and thunderstorms are
expected along and ahead of the front. A small cluster of storms
has already developed along the front in southeast Iowa...and I
expect this cluster to grow in areal coverage and strength through
the afternoon. The storms will likely be capable of producing
occasional gusty winds to 30-45kts. Storms will weaken and likely
dissipate after 02-04Z...though a few showers may persist through
much of the night. Guidance is also indicating that a band of MVFR
ceilings will develop behind the front much the same as happened
this morning over northern Illinois. Think MVFR ceilings will lift
and scatter during the late morning or early afternoon Monday.


A cold front over southern Iowa will move into the vicinity of
Lambert this evening and stall southwest of the St. Louis Metro
area Monday morning. Scattered showers and thunderstorms will be
possible along and ahead of the front this afternoon and evening.
Most likely time for storms in the vicinity of the terminal will
be after 22Z...though the atmosphere is already unstable and some
isolated showers or storms area possible at almost any time.
Expect storms to dissipate by 02-03Z. MVFR ceilings are expected
to move into east central Missouri behind the front around
sunrise. Think MVFR ceilings will lift and scatter during the late
morning or early afternoon on Monday.



MO...Excessive Heat Watch from Tuesday afternoon through Friday
     evening for Jefferson MO-Saint Charles MO-Saint Louis City
     MO-Saint Louis MO.

IL...Excessive Heat Watch from Tuesday afternoon through Friday
     evening for Madison IL-Monroe IL-Saint Clair IL.



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