Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 182323

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
623 PM CDT Tue Jul 18 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 255 PM CDT Tue Jul 18 2017

The synoptic pattern early this afternoon features a strong upper-
level ridge anchored across the central Plains. This ridge will
slowly meander eastward through the period, enhancing its influence
over the Midwest which will help bring even warmer temperatures to
the region on Wednesday.

Main forecast concern will be the dangerous heat which has started
across the area today and will only worsen on Wednesday.  Latest air
temperatures this hour sit in the low to mid 90s (STL at 97) which
coupled with dewpoints in the upper 60s/70s have yielded heat index
values from 95-105.  As we head into Wednesday, the upper-level
ridge building overhead should help push a weak boundary off to the
northeast of the region. The further northward displacement of the
front should enable dewpoints to mix out more efficiently than they
have today.  That being said, increasing 850mb temps and 1000-850mb
thicknesses suggest temperatures will likely be warmer than today,
with values in the mid 90s and into the 100s in the STL metro. This
will translate to heat index values topping out near 105 in many
areas (especially along/north of I-70). Given the heat is only
expected to worsen Thursday into the upcoming weekend (see long term
section below), have decided to upgrade portions of the Excessive
Heat Watch to an Excessive Heat Warning in all areas but southeast
MO and extreme southwest IL. In these areas, slightly lower
dewpoints should limit heat index values to the 100-104 range.

The other more minor forecast concern will be on convective chances
this afternoon into this evening. Latest radar imagery depicts some
very isolated showers along a weak front over southeast MO and along
a differential heating zone over central MO. Given a fairly uncapped
atmosphere, we will likely continue to see isolated shower/storm
development in these regions through early this evening.  Will have
to keep an eye on an MCV/compact shortwave trough diving through the
weak northwest flow aloft this evening. This mid-level forcing has
been enough to force some showers and isolated storms over northwest
MO, which could move into northeast MO and west-central IL through 2-
3Z (perhaps even lingering a bit later). Guidance has not handled
this system well today so confidence in convective coverage
associated with this feature is fairly low.


.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 255 PM CDT Tue Jul 18 2017

A widespread and significant heat wave will be ongoing at the start
of the period with a strong upper level RIDGE dominating our region.
 Although the strength of this RIDGE will weaken some heading into
Friday and Saturday, it is enough to maintain the going pattern and
effects thru this time period, as it will have the effect of hanging
a surface cold front to our north with southwest surface flow over
our region and minimal chances for rain.  Look for max air temps
from the the mid 90s to the low 100s with heat index values from 105
to 114 each afternoon and early evening.  The remnant Heat Watch
will be converted to a Warning for all areas outside of southeast MO
and far southern IL, with those areas being converted to an Advisory
where the cooler effects of the Ozarks will place peak heat indices
a bit more in check.

All of the models develop, at some point, increasing NW flow aloft
in response to a TROF settling into the Great Lakes region with this
to continue into early next week.  The point of contention remains
just exactly when do they do it, which will directly impact on when
this current heat wave ends.  The GFS continues to suggest the need
for an extension of heat headlines for many areas into Sunday, while
the CMC and to a slightly lesser extent, the EC are showing much
less of a need, especially in the northern CWA.  We currently have a
Heat Watch for our southeast MO and far southern IL counties thru
early Sunday evening and this remains a plausible and valid time for
that area given a decent consensus that the front will remain
upstream.  All other areas further north, the uncertainty increases
and decided to keep the end of the heat headlines at early Saturday
evening for now until we get a better handle on timing of the late
weekend cold front.

Models then come into better alignment by Monday, with NW flow aloft
and a cold front thru and bring to an end this latest heat event
with more seasonable temps, if not a little below average.  What
looks to be more dubious is the pcpn chances, and prefer to keep
that at or below CLIMO values for now.



.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 619 PM CDT Tue Jul 18 2017

Isolated thunderstorm near Quincy is expected to move east of the
terminal by 00Z. Otherwise, mainly dry and VFR conditions are
expected through the period has a surface high currently over
Illinois moves off to the east. Winds will remain light.


Dry and VFR conditions are expected through the period with light



MO...Heat Advisory from noon Wednesday to 8 PM CDT Sunday for
     Crawford MO-Iron MO-Madison MO-Reynolds MO-Saint Francois
     MO-Sainte Genevieve MO-Washington MO.

     Excessive Heat Warning until 8 PM CDT Saturday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Franklin MO-Gasconade MO-
     Jefferson MO-Lincoln MO-Marion MO-Moniteau MO-Monroe MO-
     Montgomery MO-Osage MO-Pike MO-Ralls MO-Saint Charles MO-
     Saint Louis City MO-Saint Louis MO-Shelby MO-Warren MO.

     Excessive Heat Warning from noon Wednesday to 8 PM CDT Saturday
     for Knox MO-Lewis MO.

IL...Heat Advisory from noon Wednesday to 8 PM CDT Sunday for
     Randolph IL.

     Excessive Heat Warning until 8 PM CDT Saturday for Madison IL-
     Monroe IL-Saint Clair IL.

     Excessive Heat Warning from noon Wednesday to 8 PM CDT Saturday
     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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