Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 252103
AFDLSX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE ST LOUIS MO
403 PM CDT Fri Jul 25 2014

.SHORT TERM:  (Through Late Tonight)
Issued at 403 PM CDT Fri Jul 25 2014

A thunderstorm complex, which began the day over far northeastern MO
and west-central IL, never fully died and has been working its way
inexorably southeast with several waves of precipitation.  The final
wave is progressing thru STL metro now and will deliver a quick shot
of rain.  It is weakening, despite being more successful at pushing
this far south than the other waves, and doubtful how much will
survive into southeast MO and far southern IL.

A warm front has been effectively held at bay over western and parts
of central MO today thanks to the rain-cooled air and thick cloud
cover downstream which has been steadily reinforced by the
persistent storms.  Temps vary from near 90 just south and west of
COU/JEF to the mid 60s in central IL.

TSRA complex should be out of the CWA by 00z early this evening, if
not dissipated, and at that point, warm front should begin its
advance.  If we clear out enough, could see a drop in the evening to
where overnight mins may occur around midnight before a steady or
slow rise occurs after midnight as warm front edges through.  Would
not be surprised to see another round of TSRA fire to the north
ahead of the warm front and where better upper support is, with what
does develop then trying to push southeast into our region very late
tonight again or Saturday morning.  The expected path of this pcpn
should be further east than what we saw with this event today due to
the anticipated position of the warm front.

Prefer something along the lines of the cooler MAV temps in the
east, and approaching the warmer MET MOS in central MO.

TES

.LONG TERM:  (Saturday through Next Friday)
Issued at 403 PM CDT Fri Jul 25 2014

Two items of concern for Saturday.  First:  the 850mb thermal ridge
is still forecast to to build almost directly across the I-70
corridor Saturday.  MOS temperatures in the mid to upper 90s still
look very reasonable under the thermal ridge with low to mid 90s on
either side.  With low to mid 70s dewpoints entrenched over the
area, would issue a wide ranging heat advisory along and either side
the I-70 corridor, however it looks like the atmosphere will mix
fairly deeply allowing drier air to mix to the surface, especially
east-northeast of the Ozark Plateau in central Missouri.  This
should keep heat index values below 105 degrees even though it will
be fairly hot.  However, St. Louis looks to be far enough to the
east northeast that dewpoint temperatures will remain a bit higher
and the heat island of St. Louis and surrounding urban areas will
likely keep ambient temperatures higher as well.  Heat index
values kiss 105 for a couple of hours Saturday afternoon, so will go
ahead and issue a Metro Area heat advisory.

The second concern is severe weather for Saturday afternoon and
night.  Models continue to show impressive instability between
4000-5000 J/Kg developing Saturday afternoon above a capping
inversion in the low levels.  Am leaning toward the GFS solution
which would initiate convection during the late morning or early
afternoon Saturday somewhere over western Iowa as a shortwave
ripples by and wipes out the cap.  Think storms should grow upscale
into a severe MCS and push east southeast with the primary threat
being damaging winds.  I think the greatest area of concern is
actually across southeast Iowa into central Illinois.  However,
given the degree of instability and uncertainty regarding the
strength of the cap on the northern edge of the low level baroclinic
zone, I cannot totally rule out severe weather over our area
Saturday afternoon and Saturday night.

Cold front will move thought the area Sunday morning and should be
down across the Ozarks by 18Z.  Expect there will be isolated to
scattered convection continuing along the front as it passes.
Should also see a decent warm up on Sunday as cool air lags behind
the actual wind shift by a bit.  However, it should get noticeably
drier fairly rapidly after FROPA so no heat advisory should be
needed for Sunday.

Medium range models remain locked on a highly amplified solution
next week with an abnormally deep longwave trof over the eastern 1/2
of North America and a very sharp ridge over the west.  This will
keep the Mississippi Valley locked in northwest flow aloft.  This
will allow cool and dry Canadian air to push across the eastern 1/2
of the CONUS.  The deep trof should allow steep lapserates to
develop over the area which will likely produce isolated showers and
possibly a few thunderstorms in the afternoon as the airmass begins
to modify later in the week.

Carney

&&

.AVIATION:  (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1255 PM CDT Fri Jul 25 2014

VFR conditions are expected to prevail outside of TSRA at the TAF
sites thru the valid period. TSRA expected to weaken as it moves
into STL metro with some activity expected to edge into the
northern vicinities of SUS and STL before final anticipated
dissolution. The TSRA is in the process of exiting UIN now. After
this, very iffy on TSRA chances heading into tonight and thru the
first part of Saturday...enough to preclude mention in TAFs.
Otherwise, look for veering wind directions from SE to SW.

Specifics for KSTL: VFR thru the period outside of TSRA with
veering SE winds to SW by Saturday. Current TSRA activity still
expected to fall apart before reaching the aerodrome, but believe
something will edge the northern vicinity before doing so.

TES

&&

.PRELIMINARY POINT TEMPS/POPS:
Saint Louis     72  97  77  91 /  20  30  40  20
Quincy          70  92  71  84 /  20  50  50  10
Columbia        73  96  75  89 /  20  20  20  10
Jefferson City  74  96  76  90 /  20  20  20  10
Salem           65  92  75  90 /  20  40  50  30
Farmington      67  95  73  91 /  10  10  20  20

&&

.LSX WATCHES/WARNINGS/ADVISORIES:
MO...HEAT ADVISORY from 1 PM to 7 PM CDT Saturday FOR St. Charles MO-
     St. Louis City MO-St. Louis MO.

IL...HEAT ADVISORY from 1 PM to 7 PM CDT Saturday FOR Madison IL-St.
     Clair IL.

&&

$$

WFO LSX






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