Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 151135

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
635 AM CDT Sat Jul 15 2017

.SHORT TERM...  (Through Late Tonight)
Issued at 313 AM CDT Sat Jul 15 2017

Area of high pressure at the surface is currently near the
Mississippi River upstream in the vicinity of the Illinois/Iowa
border. This area of high pressure is expected to control the
sensible weather across the bi-state region, and will slowly slide
south/southeast through the period, approaching the lower Ohio River
Valley by dawn on Sunday. Expect a clear to mostly clear sky through
tonight along with seasonably cool readings for mid July. Forecast
high temperatures should be in the mid 80s for the most part this
afternoon, with lows tonight in the 60s. MOS max/min temperatures
looked reasonable for the most part, but did lean toward cooler MET
guidance for tonight across the eastern Ozarks where a clear sky and
calm winds should yield favorable conditions for radiational


.LONG TERM...  (Sunday through Friday)
Issued at 313 AM CDT Sat Jul 15 2017

An upper level trof with several accompanying vorticity maxima will
dig southeastward through the Great lakes Sunday and Sunday Night.
This will bring a backdoor cold front into the northeast portion of
the CWA beginning on Sunday afternoon with the front dropping south-
southwest through Sunday night. Temperatures should warm nicely
ahead of the front thanks to a west-northwesterly wind component and
highs will once again rise to above normal levels. A ribbon of
low level moisture initially originating in the Plains will be
attendant with the backdoor cold front, and this moisture and
diurnal heating will contribute to seasonably moderate instability.
Low level convergence along the south-southwestward moving front
should be sufficient for at least isolated thunderstorms in the
afternoon and evening hours, with perhaps a bit greater coverage
in IL where there is some slight contribution from large scale
forcing. The model consensus suggests the front will stall
somewhere in the vicinity of central MO into southern IL by Monday
morning. Thereafter the surface flow and convergence weakens, the
front becomes less defined, and there is less confidence that it
will be able to support convective development on Monday afternoon
or evening. Cooling behind this front is rather neglible and
temperatures in central MO will likely we warmer Monday, while in
northeast parts of the CWA a few degrees cooler.

The model variances exhibited the last few days persist with the
most recent models runs. Confidence continues to increase as all
indications are a major heat wave will unfold beginning Tuesday and
continuing through at least the end of the week as an expansive
upper high becomes established throughout the central U.S.. The
position of the upper high center and mid/upper flow are the key
differences between the GFS and ECMWF. The ECMWF has been quite
consistent with the upper high center in the mid MS Valley, while
the GFS maintains a position shifting from the central Rockies
into the central Plains. In the EC scenario, the aforementioned
front retreats back to the north-northeast late Monday night into
early Tuesday. Low level south-southwest flow then persists
through the week`s end with the active east-west oriented
baroclinic zone well to the north through the upper Midwest.
Alternatively the more westward position of the GFS, results in a
more southward position of the east-west baroclinic zone and
northwest flow aloft, and a persistent threat of showers and
thunderstorms parts of northeast MO into IL. Once again we are
siding with the ECMWF and have some low pops only periodically
skirting north fringes of the CWA.

Dangerous levels of heat will envelope the CWA beginning Tuesday
and persisting through Friday. (Note the ECMWF would keep the heat
wave going into next weekend.) High temperatures from the mid 90s
to low 100s appear likely with afternoon and early evening heat
index values in the 100-110 degree range. The worst conditions
will likely impact metro St. Louis into southwest IL, where an
excessive heat watch and ultimately excessive heat warning appear
to be on the horizon, with advisory level conditions elsewhere.



.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 635 AM CDT Sat Jul 15 2017

Dry/VFR conditions through the period as area of high pressure
settles into the region.





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