Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 221736

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1236 PM CDT Sat Jul 22 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 350 AM CDT Sat Jul 22 2017

Warm front extending from surface low over south central Iowa east
southeastward through central Illinois will eventually lift
northeastward away from forecast area. In the meantime, could see
some isolated showers/storms over portions of west
central/southwestern IL through sunrise.

Beyond that, main cold front to begin sinking slowly southward into
northern Missouri today and tonight with isolated/scattered showers
and storms spreading south across region. As conditions become
unstable this afternoon through tonight, some of these storms could
be strong to severe. In the meantime, hot and humid conditions to
persist with highs in the mid 90s to low 100s. Heat indices will be
in the 100 to 115 range once again. Lows tonight will be in the 70s.


.LONG TERM...  (Sunday through Friday)
Issued at 350 AM CDT Sat Jul 22 2017

It continues to look more likely that most areas will see one
more day on Sunday of high heat and humidity before this event comes
to a close.

By Sunday, a transition to a NW flow aloft is expected in response
to a strong upper level storm system dropping southeastward into the
Great Lakes region from the Canadian Plains.  The NW flow aloft will
allow a pair of surface cold fronts to move thru our area.  By
Sunday morning, the first cold front is already expected to be in
southern MO and far southern IL on its way out of our region.
Impacts from this front will be mostly on rain chances, with the
best rain chances on Sunday being across far southern MO and IL and
predominantly during the morning hours.  The second cold front will
be just entering into northern MO and central IL by Sunday evening.
With the highest cloud amounts and PoPs expected during the morning
and not so much in the afternoon, and only slightly cooler air
accompanying the first cold front, the dangerous heat and humidity
for many areas on Sunday is really not going anywhere.  Currently
have forecast max air temps into the mid to upper 90s for most areas
with max heat index values of 100 or more for areas near and south
of I-70 with several areas topping 105.  Will expand coverage of
heat headlines on Sunday from what was done yesterday still further,
most notably now into central MO.  This headline may need to be
expanded to cover more areas further north yet until that second
cold front can get thru.

Speaking of which, the second cold front will drop down thru the
area on Sunday night.  It looks like surface convergence with this
front will not be as pronounced as the first front 12-24 hours
earlier and rain chances look small.  The main thing with this
second front will be the air that moves in behind it for Monday
and Tuesday will be noticeably cooler, to get us back to seasonable
temp values, and should finally bring an end to all heat headlines.

The building of the upper RIDGE still looks on target for late
Tuesday into Wednesday but the models are less certain to it lasting
much more than that, with decent model consensus on a regression to
the west with a NW flow aloft developing once again for late next
week. This flow should once again encourage a cold front to drop
down thru the area. Such as it is, we may need to re-issue heat
headlines for select areas for a day or two during the middle of
next week, perhaps longer.

At this time. rain chances look low for most days after Sunday, with
better chances returning late next week if the cold front does
indeed drop down thru the area.



.AVIATION...  (For the 18z TAFs through 18z Sunday Afternoon)
Issued at 1202 PM CDT Sat Jul 22 2017

SPECIFICS FOR KUIN: An outflow boundary was still affecting the
prevailing wind direction at KUIN at TAF issuance, but winds
should continue to veer this afternoon and become southwesterly to
west-southwesterly during the first 6-8 hours. Isolated to
scattered SHRA/TSRA are expected to develop late this afternoon
near a weak front, but KUIN may be too far north of the boundary
to experience much precipitation. Winds will become northwesterly
after fropa.

SPECIFICS FOR KCOU: Scattered SHRA/TSRA are expected to develop
late this afternoon and early this evening invof a weak front as
it sinks southward through the region. Winds will become
northwesterly after fropa.

SPECIFICS FOR KSTL, KSUS, KCPS: An outflow boundary moved through
the St. Louis metro area terminals a few hours ago, which shifted
the winds to 030-060. Winds have been slowly veering in the wake
of the boundary and were 130-160 at TAF issuance. Based on
upstream observations, winds will continue to slowly veer over the
next few hours before returning to the prevailing southwesterly
or west-southwesterly flow. Scattered SHRA/TSRA are then expected
to develop this evening invof a front as it slowly sinks
southward through the region. Winds will become northwesterly
after fropa.



MO...Excessive Heat Warning until 8 PM CDT Sunday for Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Lincoln MO-Madison MO-Moniteau MO-
     Montgomery MO-Osage MO-Reynolds MO-Saint Charles MO-Saint
     Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO-Warren MO-Washington MO.

     Excessive Heat Warning until 8 PM CDT this evening for Audrain
     MO-Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-Ralls MO-
     Shelby MO.

IL...Excessive Heat Warning until 8 PM CDT Sunday for Bond IL-Calhoun
     IL-Clinton IL-Jersey IL-Madison IL-Marion IL-Monroe IL-
     Randolph IL-Saint Clair IL-Washington IL.

     Excessive Heat Warning until 8 PM CDT this evening for Adams IL-
     Brown IL-Fayette IL-Greene IL-Macoupin IL-Montgomery IL-
     Pike IL.



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