Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201729

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1229 PM CDT Tue Jun 20 2017

Issued at 904 AM CDT Tue Jun 20 2017

Updated PoPs slightly based on latest guidance that is rolling in.
Do not anticipate drastic changes in coverage, remaining in the
scattered category. However, latest guidance suggests that a few
of these storms may be capable of generating marginally severe
hail this afternoon. As heating continues thru the afternoon, the
well mixed deep sub-cloud layer will pose a wind threat with these
storms. With deep layer shear increasing into the evening hours,
some of these storms may become more linear if a strong cold pool
can develop.



.SHORT TERM...  (Through Late Tonight)
Issued at 359 AM CDT Tue Jun 20 2017

Early this morning, a longwave TROF dominated eastern North America
with a HIGH dominating the southwestern CONUS, and NW flow aloft
over our region.  A weak surface RIDGE of high pressure prevailed
across the mid-South, yielding a light westerly flow over our
region.  Satellite imagery showed clear skies for much of our area,
but some bands of clouds and isolated showers continue to develop
over southern IL into the lower Ohio River Valley from some frontal
lift occurring aloft.  Temps were in the 60s.

The NW flow aloft will become less amplified with time thru
late tonight.  This will initially allow a cold front currently
over the northern Plains to rapidly move southeastward to northern
MO and central IL by late this afternoon, but with the cold front
then slowing down a lot and becoming nearly stationary over our
region later tonight.  The models have been coming into better
consensus on bringing together nearly sufficient moisture, some
instability, and low level convergence to develop the threat for
another round of widely scattered showers and thunderstorms
beginning late this afternoon over northern MO and central IL and
continuing into much of the evening for points further south to the
I-70 corridor in MO and I-64 corridor in IL.  The pcpn that develops
should dissipate by midnight.

Went aggressive on max temps for today, with what should be a very
ideal wind direction (230-250) and decent sunshine for much of the
day with at or above the higher MET MOS numbers preferred, equating
to widespread upper 80s to near 90, with low 90s in STL metro.  Min
temps tonight should be higher than persistence with the front
stalling in the northern CWA, with readings expected to bottom out
in the 60s with low 70s in the STL metro heat island.


.LONG TERM...  (Wednesday through Monday)
Issued at 359 AM CDT Tue Jun 20 2017

A weak front across northeast MO and west central IL will lift back
northeastward as a warm front on Wednesday.  Could not rule out a
few showers/storms across this area Wednesday morning.  Highs will
be above normal on Wednesday due to rising upper level heights and a
south-southwest surface wind.  Showers and storms are likely late
Thursday night and Friday as an upper level trough moves eastward
through the northern Plains and into the Great Lakes region and
sends a cold front southward into our area, while Gulf moisture
spreads northward into our area ahead of a tropical low moving
northward from eastern TX into AR.  Most of the convection should
shift southeast of the forecast area by late Friday night as the
tropical low moves northeastward into the TN Valley region.  The
models bring another deepening upper level trough eastward through
the northern Plains and Great Lakes region this weekend.  The GFS
model keeps its QPF north of our forecast area, while the ECMWF
model brings another round of shower/storms to our area mainly
Saturday night as a cold front drops southeastward through the
region.  Cooler conditions can be expected for Sunday and Sunday
night as the upper level trough deepens over the area, while a
relatively large and strong surface ridge builds southeastward into



.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1218 PM CDT Tue Jun 20 2017

An approaching cdfnt will help initiate TSRA late this afternoon
and thru the evening hours. Expect most of the storms to remain
east of the terminals, but some may develop further west. Some of
the stronger storms may be capable of producing large hail and
damaging winds. Otherwise, winds will be light and somewhat vrb
tonight behind the weak fnt. Winds shud become swly again Wed thru
the morning as the weak fnt lifts back nwd as a wrmfnt.

SPECIFICS FOR KSTL: Added VCTS early this evening to account for
TSRA potential. Otherwise, predominately swly winds.





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