Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 212000

Area Forecast Discussion
National Weather Service Saint Louis MO
300 PM CDT Thu Sep 21 2017

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 259 PM CDT Thu Sep 21 2017

Early this afternoon, a sharp upper level RIDGE axis extended from
near Detroit, Michigan to northern Arkansas.  This has effectively
trapped an area of thick convective debris clouds (from a MCS early
this morning in southeastern IA that backbuilt some into northeast
MO) between STL Metro and the Quad Cities to the north where there
is very weak steering flow.  Consequently, this area of thick
cloudiness has kept temps mainly in the 80s this afternoon, while
surrounding areas have mostly reached into the low to mid 90s.  A
surface outflow boundary from this morning`s convection extends from
near Kirksville, MO thru the far southwestern edges of STL metro and
into southern IL, with easterly winds behind it, and southerly winds
ahead of it.  While the cumulus that has developed along this has
been more vertically developed than the rest to the south, it still
has struggled (for now) to develop enough to warrant concern for

It may still be possible to pop an isolated shower or storm along
the outflow just W or S of STL metro thru sunset, but looks unlikely
at this time.

The area of thick convective debris clouds will be slow to exit and
thin tonight but should eventually as they edge slowly east into IL
toward midnight.  Otherwise and elsewhere, skies will clear around
sunset.  Some patchy fog is expected mainly S and E of STL metro
closer to the surface RIDGE, where lighter wind flow and higher
crossover temps are anticipated.  MOS min temps, which are similar
to persistence, look good and could spell record high mins at STL
and possibly UIN--see CLIMATE section below.

The upper RIDGE axis will move little by Friday afternoon,
keeping high thicknesses in place, with southerly flow expected
during the daytime Friday.  Lack of a MCS to the north to generate a
new round of convective debris clouds to interfere with the heat
should mean a significantly warmer day tomorrow in spots, with max
temps surging into the 90s.  Record max temps could be threatened,
particularly at STL.  See CLIMATE section below.  Heat index values
are also expected to top out around 100, especially in what are
typically warmer temps in STL metro.


.LONG TERM...  (Friday Night through Next Thursday)
Issued at 259 PM CDT Thu Sep 21 2017

The unseasonably hot and humid conditions will continue into the
upcoming weekend as the upper level ridge holds its position across
Missouri and Illinois. Afternoon high temperatures in the lower to
middle 90s will combine with dewpoints in the 60s/70s to produce
afternoon and early evening heat index values around 100 degrees.
May need to hoist a Heat Advisory for the St. Louis Metro for
longevity (4 days of heat index values above 100 degrees) if
Saturday forecast remains on track. Will let midnight shift get
another run of model guidance in before hoisting an advisory.

Ridge finally shows some signs of breaking down/shifting east by
early next week as a deep trough of low pressure approaches the
region from the west. Have a few periods of slight chance to chance
PoPs but threat of a widespread meaningful rain event look slim at
this point. Temperatures should cool some Monday and Tuesday with
highs in the middle to upper 80s, with the real cold front bringing
temperatures back to seasonal normals on Wednesday of next week.



.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1244 PM CDT Thu Sep 21 2017

VFR conditions, dry wx, and light southerly surface winds will
prevail at the TAF sites for much of the valid period. Exceptions
to the wind will be areas for much of the STL metro and UIN which
are behind a convective outflow boundary with easterly winds and
this will persist for a few hours more before finally reverting
back to more southerly. Another potential issue is VSBY reductions
to fog at SUS and CPS and have included a minor reduction for now
until a better handle can be obtained for the 00z set, such as
observed crossover temps.



Record Max Temperatures Through Saturday

              KSTL  KCOU  KUIN
Sept 21     97 (1881)  100 (1893)   95 (1940)
Sept 22     95 (1956)   97 (1937)   96 (1937)
Sept 23     94 (1891)   94 (2007)   95 (1937)

Record High Min Temperatures Through Saturday

              KSTL        KCOU        KUIN
Sept 21     75 (1980)   73 (1970)   72 (1980)
Sept 22     73 (2005)   71 (2005)   71 (1930)
Sept 23     73 (1884)   73 (1937)   73 (1937)




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