Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201721

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1221 PM CDT Thu Jul 20 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 311 AM CDT Thu Jul 20 2017

Dangerous levels of heat will really crank up today. Make no
mistake, it has already been hot the last few days, but today marks
the beginning of a 3 day stretch of the hottest conditions with this
heat wave. The large upper high/ridge currently centered in eastern
KS will remain the dominant weather feature as the high center
shifts eastward into the vicinity of SW MO by this evening, and to a
position in the western TN Valley by 12Z Fri. West-southwesterly
lower trop flow will become more pronounced and the guidance shows a
decided eastward progression of the hottest temps aloft. This should
translate into hotter temperatures today. The last several days
clouds impacted temps in central and north central MO. There is some
blow-off high cloudiness at this time across northern MO into
central IL from thunderstorms much further north but this should
thin this morning and become a non-factor. Afternoon heat indices
within the excessive heat warning will generally be in the 105-110
range through early evening with a few spots possibly topping 110
depending on localized dew point trends. Metro St. Louis will
remain the hottest area due to the urban heat island effect, and
also resulting from the lack of rainfall and thus ease of more
pronounced superadiabatic lapse rates. Afternoon mixing however in
the heart of the metro will also result in a noted lowering of
dew points and help keep the dangerous heat in check to a degree
(ie. HI closer to 110 vs. 115+). The east-west baroclinic zone is
expected to remain anchored well to the north of the forecast area
keeping the threat of showers and thunderstorms at bay. Mild to
warm conditions are expected again tonight.


.LONG TERM...  (Friday through Wednesday)
Issued at 311 AM CDT Thu Jul 20 2017

(Friday - Sunday Night)

Worst of the ongoing heat wave still is expected to continue on
Friday and Saturday. Midlevel anticyclone moves across the mid-
Mississippi Valley with the west-east oriented 850-hPa thermal
ridge also across the bi-state area. High temperatures across most
areas on Friday as well as Saturday should be about 1-2 degrees
warmer than those expected of today with similar dewpoints.
Saturday may end up being the hottest day of this heat wave with
very deep mixing and a favorable southwesterly surface wind
direction. A couple of caveats or negating factors however for
Saturday include a dampening of the upper-level ridge in addition
to a prefrontal trough entering the northern CWA Saturday
afternoon. This trough axis is expected to help focus isolated
thunderstorms, and at the very least, an increase in cloud cover.
Because of these issues raised above, have highs on Saturday very
similar to Friday for most areas and just a bit cooler across the
northern CWA. Peak heat index values each day are expected to be
on the order of 105-110F each afternoon, with some areas slightly
above 110F including portions of the St. Louis metropolitan area.

The prefrontal trough looks to interact with a strengthening low-
level jet Saturday night so expect thunderstorm coverage to
increase. This trough axis quickly exits the CWA Sunday morning so
expect lots of dry time on Sunday. Actual synoptic cold front looks
to move through the area about 12 hours after the prefrontal trough
and bring another chance of showers and thunderstorms to at least
central and southern portions the area.

Still quite a bit of uncertainty to high temperatures on Sunday due
to a variety of factors including amount of cloud cover, possibility
of convection, and slight timing differences in both the prefrontal
trough and synoptic cold front. Due to the lack of confidence in
high temperatures, will hold off on any headline changes for now,
but it still does appear very possible that heat headlines north
of the current advisory for Sunday may become necessary down the

Cold front will push move through the southeastern portion of the
CWA by late Sunday night with a cooler and drier airmass in its
wake. This frontal passage will thankfully put an end to this
heat event across the area.

(Monday - Wednesday)

While early next week will be quite a bit cooler and less humid,
temperatures will only return to near normal. Mid/upper level ridge
begins to amplify once again to our west by midweek. Position and
strength of this ridge will be critical in determining temperatures
as well as thunderstorm chances. GFS/ECMWF differ on the placement
heading into midweek next week. The GFS keeps the ridge across the
Great Plains while the ECMWF builds in the ridge bringing a return
to much above normal temperatures.



.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1204 PM CDT Thu Jul 20 2017

VFR through the period with diurnal cumulus and occasional cirrus.
Southwest winds will diminish overnight and become more southerly,
then increase and become southwesterly again after 21/12z.



MO...Heat Advisory until 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-Knox MO-Lewis 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.

IL...Heat Advisory until 8 PM CDT Sunday for Randolph IL.

     Excessive Heat Warning until 8 PM CDT Saturday for Adams IL-Bond
     IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-
     Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Pike IL-Saint Clair IL-Washington IL.



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