Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 231755

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1255 PM CDT MON MAY 23 2016

.SHORT TERM...  (Through Late This Afternoon)
Issued at 330 AM CDT Mon May 23 2016

High pressure and the associated dry air mass has dominated the
region the last few days resulting in some nice diurnal
temperature ranges and warm daytime highs. Progression in the
pattern will begin today albiet change will be gradual. A
positively tilted long wave trof will remain centered through the
western U.S. while the upper ridge axis currently extending
through the MS Valley will shift east as the mid Atlantic upper
low/trof moves to the northeast off the east coast. This will
allow for both southwest flow aloft to begin spreading into the
region, and will also allow the low level anticyclone/surface high
to shift eastward. Gradually increasing southerly flow on the
backside of the retreating high will result in the beginning of
northeastward moisture transport, especially into western and far
central MO. I am not real excited about the precipitation
prospects today, but also can completely rule out a few showers
or thunderstorms into parts of central MO later this afternoon.
Temperatures will once again be above average and will really be
the main story today, with highs well into the 80s.


.LONG TERM...  (Tonight through Sunday)
Issued at 302 AM CDT Mon May 23 2016

Changes really begin to proceed tonight. Continued downstream
progression will bring southwesterly flow aloft and steeper mid
level lapse rates further east, while a veering southwesterly LLJ
will result in increasing east-northeastward moisture transport,
greater WAA, and development of elevated instability. The convective
trends tonight are far from clear cut. The aforementioned changes
and lift/MCON at the terminus of the veering LLJ would support
both maintenance of any ongoing convection from further west and
also new development. The most likely region would be from
central/northeast MO into west central IL - a location the new 00Z
ECMWF appears to be especially fond of.

From Tuesday into Thursday, much of the forecast depends on
mesoscale details and what occurs in proceeding periods and its
subsequent impacts. By late Tuesday a moisture-rich air mass will
be in place in the lower trop topped by steep mid level lapse
rates. This will contribute to strong instability through the
period, peaking with diurnal heating in the late afternoon/early
evening. Mesoscale boundaries, weak disturbances traversing the MS
Valley within the southwest flow aloft and a modulating
southwesterly LLJ will provide forcing/lift during this multiday
period. Deep layer shear will be sufficient for organized
multicell systems including one or more severe MCSs. I envision
that there will be ongoing convection on Tuesday morning that will
impact portions of MO, with potential for a nocturnal MCS late
Tuesday night into Wednesday morning across parts of northern MO
sinking east-southeast during the day Wednesday, and then perhaps
another MCS in its wake Wednesday afternoon and/or night.

The upper low/trof will emerge from the 4 corners area and lift
northeastward through the Plains and into the upper MS Valley from
late Thursday into Saturday. Large scale ascent associated with it
and several disturbances ahead of the main trof, along with a
prominent southwesterly LLJ will keep the threat of showers and
thunderstorms ongoing.



.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1247 PM CDT Mon May 23 2016

VFR flight conditions are expected to prevail at least through the
remainder of the afternoon...and likely through the evening and
overnight hours as well. Expect scattered showers and
thunderstorms to try to move into central Missouri during the
early evening, but it looks like they will be dissipating as they
do so. Rain chances continue to overspread the area tonight with
isolated to scattered showers and thunderstorms possible across
most of the area by tomorrow morning. Again...VFR conditions are
expected to prevail...but any heavier shower or storm could easily
bring visibility down to 2-3 miles or less.

Specifics for KSTL:

VFR conditions are expected to prevail at Lambert through the
period. Scattered showers are likely to be in the vicinity of the
terminal Tuesday morning, though timing and intensity are
uncertain at this time. Could see some thunderstorms as well.
Think shower activity will diminish through early afternoon. If a
shower or storm hits the terminal directly it could briefly drop
visibility to 2-3 miles or less.





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