Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 172330

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
630 PM CDT Mon Jul 17 2017

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 340 PM CDT Mon Jul 17 2017

Primary concern for the short term is the onset of excessive heat
across the area.  The upper level ridge over the Great Plains will
continue to build east into the Mississippi Valley over the next 24
hours.  The surface ridge currently over the area will drift
eastward...and weak southerly flow will return to the region.
Current heat index values range from the low 90s to around 100
across the area...and expect that temperatures will be a bit warmer
tomorrow given the southerly flow.  The only reason not to upgrade
the heat watch to a warning is the possibility that mixing may cause
dew point temperatures to be a bit drier than previously expected
which may keep heat index values a little lower.  Heat island effect
of St. Louis will likely mitigate this drier air to some degree, so
went ahead and upgraded to a warning for tomorrow for the STL Metro.
Will stick with the watch elsewhere and let the evening/midnight
shift re-evaluate the threat.

Elsewhere and otherwise...the lingering boundary draped across
southeast and central Missouri continues to be a focus for isolated
thunderstorms.  Current convection should weaken and dissipate this
evening as we lose daytime heating. Introduced some slight chance
PoPs over the eastern Ozarks again on Tuesday since that boundary
just doesn`t want to go away.  While there`s little or no QPF being
forecast by the models, surface flow indicates convergence and most
guidance shows 850mb moisture convergence in the area Tuesday


.LONG TERM...  (Tuesday Night through Next Monday)
Issued at 340 PM CDT Mon Jul 17 2017

Upper level RIDGE will be in the process of building overhead
Tuesday night and early Wednesday.  This position will allow for NW
flow aloft for this period and with model guidance supporting MCS
development in IA Tuesday night, would leave the door open to some
intrusion from that heading into Wednesday morning, likely as a
period of debris cloudiness, possibly with some weakening rain in
the northern CWA.  This is not anticipated to have much impact by
afternoon with respect to heat headlines/impacts with the first of
several days of the widespread very hot/humid air to take hold of
our region.

From the start of the period thru at least Saturday evening, an
upper level RIDGE will dominate our region with SW flow at the
surface.  A surface cold front may eventually set up and stall near
the MO/IA border late Thursday thru Saturday, but with it just north
and the air very warm and capped to the south, little to no impact
is expected.  Look for max air temps from the mid 90s to the low
100s with heat index values from 105 to 113 each afternoon and early
evening.  Heat headlines now exist as a Watch for the entire CWA
with the highest confidence area of STL metro now converted to a
Warning.  All heat headlines stop on Saturday evening except for
southeast MO and far southern IL which go until Sunday evening.

The models all diverge beginning Sunday into Monday with some
maintaining the RIDGE for another day into Sunday (GFS) and others
breaking it down and swinging a TROF thru the Great Lakes region
giving us NW flow aloft and allowing a surface cold front to barrel
thru Sunday.  Even in the more aggressive front scenario, it won`t
push far enough south in time to affect the heat which is why
headlines go for another day there but uncertainty increases
substantially heading north into I-70 and points northward and thus
heat headlines were left as-is on a Saturday expiration at this

Models come into better alignment by next Monday, with NW flow aloft
and a cold front thru and one way or another shed a decent
likelihood on a conclusion for this latest heat event with what will
also be the best chance for rain the next week.



.AVIATION...  (For the 00z TAFs through 00z Tuesday Evening)
Issued at 618 PM CDT Mon Jul 17 2017

Isod TSRA possible at KCOU for the first few hours of TAF period.
Expect fog to develop late tonight across much of the region,
impacting KCOU/KSUS/KCPS. Otherwise, winds will be light and
generally nely thru tonight. Winds will gradually veer to
eventually become sely to sly on Tues.



MO...Excessive Heat Watch from Wednesday afternoon through Sunday
     evening for Crawford MO-Iron MO-Madison MO-Reynolds MO-
     Saint Francois MO-Sainte Genevieve MO-Washington MO.

     Excessive Heat Watch from Tuesday afternoon through Saturday
     evening for Audrain MO-Boone MO-Callaway MO-Cole MO-
     Franklin MO-Gasconade MO-Lincoln MO-Marion MO-Moniteau MO-
     Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Shelby MO-
     Warren MO.

     Excessive Heat Warning from noon Tuesday to 8 PM CDT Saturday
     for Jefferson MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO.

     Excessive Heat Watch from Wednesday afternoon through Saturday
     evening for Knox MO-Lewis MO.

IL...Excessive Heat Watch from Wednesday afternoon through Sunday
     evening for Randolph IL.

     Excessive Heat Warning from noon Tuesday to 8 PM CDT Saturday
     for Madison IL-Monroe IL-Saint Clair IL.

     Excessive Heat Watch from Wednesday afternoon through Saturday
     evening for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-
     Fayette IL-Greene IL-Jersey IL-Macoupin IL-Marion IL-
     Montgomery IL-Pike IL-Washington IL.



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