Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 192340

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
640 PM CDT Wed Jul 19 2017

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 259 PM CDT Wed Jul 19 2017

Oppressively hot and humid conditions will continue through
tomorrow given the broad ridge in place aloft and southerly to
southwesterly surface winds around the western periphery of a large
surface high pressure center. These features will maintain the
hot/humid air mass already in place across the area.

There is little opportunity for widespread precipitation tonight or
tomorrow. Although a shortwave is forecast to move from the northern
plains into the Great Lakes tonight and tomorrow, its path is too
far north to have much effect on conditions across the LSX CWA. This
disturbance will nudge a weak surface front southward into IA on
Thursday, but it is also expected to remain too far north to
influence SHRA/TSRA chances.


.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 259 PM CDT Wed Jul 19 2017

The synoptic pattern at the start of the long term period will
feature an amplified ridge across much of the central CONUS. This
ridge will remain in place through the first part of the weekend,
before it breaks down late this weekend into early next week due to
a fairly strong PV anomaly diving through the Great Lakes region.

The main story of the long term period will continue to be the
dangerous heat levels. Guidance remains consistent in showing the
850mb thermal ridge building into the area Friday and especially
Saturday. This should make for temperatures a bit warmer each day,
although Saturday`s temperatures could be affected a bit by clouds
streaming in from the north. Given the local effects of the urban
heat island in St. Louis, have nudged temperatures upward each of
these two days, now putting highs in the mid 100s Friday and
Saturday. If we continue to deeply mix each afternoon, then these
values could still be too cool.  Elsewhere, highs in the mid to
upper 90s seem reasonable with some locations across central MO
perhaps getting to near 100 degrees. These air temperatures coupled
with dewpoints in the upper 60s/70s will yield heat index values of
100 to 115 degrees each afternoon.

Guidance has come into slightly better agreement regarding
precipitation chances beginning Saturday afternoon and lasting into
Sunday. On Saturday afternoon, guidance suggests a wind shift will
move into portions of northeast MO and western IL.  The GEM/GFS/Euro
all now show isolated to widely scattered convection in this region
Saturday afternoon.  This could affect how warm we are able to get
in this area Saturday afternoon. However, given this is still
several days away and it looks as if convection shouldn`t be very
widespread, no changes to headlines are planned at this time.

The synoptic cold front will slide into the area Saturday night into
Sunday, once again focusing scattered convection. This front should
bring cooler temperatures at least to northeast MO/western IL by
Sunday afternoon. However, areas along/south of I-70 may once again
get rather warm ahead of the front depending on cloud
cover/lingering convection. Again, given this scenario, at this time
see no need to adjust current headlines for Sunday across southeast
MO and southwest IL.

Slightly below average temperatures will finally filter into the
region by early next week as the front drops south of the area.
However, the boundary will lift back north once again Tuesday into
Wednesday as upper-level ridging becomes established once again over
the Plains. This will allow for showers/storms to return to the
forecast for Tuesday/Wednesday of next week.



.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 631 PM CDT Wed Jul 19 2017

High pressure in the upper atmosphere will continue to bring dry
and VFR conditions to the region.


Dry and VFR conditions are expected through the period with light



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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