Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 201146

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
646 AM CDT Wed Jun 20 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 340 AM CDT Wed Jun 20 2018

A cluster of showers and a few thunderstorms that have developed
near the I-70 corridor last evening has maintained itself the past
few hours with the most extensive development entering the western
half of the STL metro area.  This development can be attributed to
what looks like a MCV in the vicinity with an old outflow boundary
further helping things along for areas further east into southern
IL.  Skies were mostly cloudy across our region early this morning
with temperatures in the upper 60s and lower 70s.

The MCV should slowly slither its way east northeastward with the
mid level flow and the area of showers and a few storms being driven
by it will take several hours to exit to the east.  Localized heavy
rainfall will still be possible until this exits being a very slow
moving focus that promotes training of echoes.  A lull is then
expected for most areas during mid and late morning.  Heading into
peak heating during the afternoon, a similar setup is then expected
from most of the recent past days with weak shear (less than 25kts 0-
6km), moderate instability (1000-2000 J/kg MLCAPE), very weak CINH,
with no obvious mechanisms to focus convection in any one place over
another, with scattered thunderstorms expected.

A feature that looks like it will act as a focus is a lobe of
vorticity, an arm of lift associated with an approaching upper LO.
This upper LO is currently over the interface of Wyoming/Montana and
is expected to track southeastward towards our region by late this
afternoon. It will this lobe of vorticity from this approaching
upper LO that should be just west of our forecast area by mid to
late afternoon and should enhance and increase convection for
northeast and central MO with what does result here then tracking
northeastward heading into the evening hours.  By overnight, the
upper LO itself will be in close proximity enough to maintain
persistent chance PoPs.

Max temps today will range from 85 to 90 most areas, a welcome break
from the persistent and dangerously high levels of heat and humidity
since late last week.  Min temps tonight will range from 65 to 70
most areas, with low 70s in the STL metro area.


.LONG TERM...  (Thursday through Tuesday)
Issued at 340 AM CDT Wed Jun 20 2018

The upper LO will continue tracking slowly southeastward, into
northern MO by late Thursday afternoon, thru the STL metro area
Friday morning and into southeastern IL by late Friday afternoon.
This will be a primary driver of our weather for this time period,
with generally high PoPs (peaking during the afternoon and early
evening) and an even cooler stretch of daytime temperatures.

While severe thunderstorm potential will be low Thursday and Friday
with low CAPE (less 1000 J/kg MLCAPE) for this time of year, shear
will be bit higher thanks to mechanical mixing around the upper LO
and adding in enhanced cyclonic flow, potential for cold air funnels
does appear to be higher for those days and could be the main severe
threat, such as it is.

In the wake of the upper LO passing to our east, the surface cold
front should finally make its push thru our region and settle just
to our south for the weekend.  A zonal flow is expected to exist
aloft and will continue to shuffle along upper level disturbances to
maintain a steady rain threat for the weekend, but mainly in
southern MO.

Already by early next week, the upper RIDGE will begin to re-assert
itself into the central CONUS to remind us that we are now
officially in astronomical summer, with another round of showers and
thunderstorms expected as a surface warm front moves thru at this

Thanks to an upper LO sliding thru Thursday and Friday and being on
the cool side of a front for much of the upcoming weekend, look for
daytime max temps to be at or...wait for it...BELOW AVERAGE...thru
the weekend with a return to above average temps for early next



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 637 AM CDT Wed Jun 20 2018

SPECIFICS FOR KCOU, KUIN: Scattered showers and thunderstorms will
continue for the next few hours, mainly east of KCOU. There will
probably be a break in the rain at KUIN for a few hours before
additional SHRA/TSRA develop after 18z and affect both KCOU and
KUIN. MVFR ceilings are possible after 21/00z.

SPECIFICS FOR KSTL, KSUS, KCPS: Scattered showers and
thunderstorms will continue moving across the St. Louis
metropolitan area for the next few hours. There will probably be a
break in the rain before additional SHRA/TSRA develop after 18z
and reach the terminals after 20z. MVFR cigs are possible after
21/06z, but confidence was too low to include in the 12z TAFs.





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