Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 302340

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
640 PM CDT Sun Apr 30 2017

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 318 PM CDT Sun Apr 30 2017

Will continue the flash flood watch until 06Z.  Band of widespread
showers that were over the area this morning and early this
afternoon has lifted north and east of the area.  Beginning to see
thunderstorms develop along the cold front over central Missouri
where the atmosphere has recovered somewhat.  Latest CAMS runs show
these storms continuing to develop and moving eastward with time
late this afternoon and early this evening.  There is still the
possibility that a few strong to severe storms to develop given the
limited instability and the more than ample shear available.  Any
additional heavy rainfall could easily produce flash flooding, so
will leave the flash flood watch in place through this evening. Then
the steady rain will gradually end from west to east as a dry slot
works into the area.  But then, will see a chance of rain moving
back into central and northeast Missouri as well as west central
Illinois as the upper low currently over the central Plains moves
northeast into Iowa.

Temperatures will drop behind the cold front tonight.  Tomorrow will
be colder with cold air advection and low clouds.


.LONG TERM...  (Monday Night through Next Sunday)
Issued at 318 PM CDT Sun Apr 30 2017

...Another moderate rainfall event looks to effect the region near
midweek which will impact river basins forecast to experience major
to record flooding...

The deep upper low will be over the Great Lakes region at the start
of the period will lift northeastward into southeast Canada on
Tuesday with lesser amplitude on the broad longwave trof in its
wake. This slight deamplification will be short-lived however as an
upstream ridge builds over the western U.S. Wednesday into Thursday,
resulting in a digging trof from the Plains into the MS Valley and
development of a new deep longwave trof by Thursday.

For sensible weather, decreasing cyclonic flow and diminishing winds
are expected Monday Night with weak high pressure sliding into the
area on Tuesday. This will bring the region slightly below average

Unfortunately the region seems to be in the cross eyes of another
round of rain beginning Tuesday Night. As previously mentioned, an
upper trof will dig into the area from the Plains and deepen
midweek, with an associated surface low tracking from TX across the
lower MS Valley and into the TN Valley by Thursday. Large scale
ascent/forcing associated with the amplifying upper flow, and the
associated mass/momentum adjustments as well as increasing low level
WAA/isentropic ascent will spread into the area late Tuesday night,
continuing Wednesday into Wednesday Night. The main focus will be
south of Interstate 70, with widespread showers/rain centered
through the river basins forecast to experience major to possibly
record river flooding this upcoming week. Present indications are
that any convection/thunderstorms will remain south of the area.
This looks to be a steady precipitation event with rainfall totals
of 1-2 inches, which will aggravate hydrologic conditions and
potentially impact the flood crests/longevity. How long the rains
lasts beyond Wednesday night is a bit more uncertain as the ECMWF is
more progressive with the digging and amplifying upper trof, and
thus would have the majority of additional precipitation shutting
down by early Thursday, while the GFS would keep the precipitation
continuing into Thursday evening. Once this rain event is over it
appears a period of dry weather lasting through next weekend with
gradually moderating temperatures.



.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 629 PM CDT Sun Apr 30 2017

Lines of showers and scattered thunderstorms will lift northeast
of the terminals early in the period. MVFR ceilings will be
mainly confined to areas around KUIN this evening. Then there will
be a period of dry and VFR conditions tonight before MVFR ceilings
with scattered showers move back into the area on Monday.

SPECIFICS FOR KSTL: The line of showers and scattered and
thunderstorms will move away from the area early in the period.
Then mainly dry and VFR conditions are expected through tonight
before MVFR ceilings move back into the terminal on Monday.



MO...Flash Flood Watch until 1 AM CDT Monday for Audrain MO-Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Lincoln MO-Madison MO-Moniteau MO-
     Monroe MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Reynolds
     MO-Saint Charles MO-Saint Francois MO-Saint Louis City MO-
     Saint Louis MO-Sainte Genevieve MO-Warren MO-Washington MO.

IL...Flash Flood Watch until 1 AM CDT Monday for Bond IL-Calhoun IL-
     Clinton IL-Fayette IL-Greene IL-Jersey IL-Macoupin IL-
     Madison IL-Marion IL-Monroe IL-Montgomery IL-Pike IL-
     Randolph IL-Saint Clair IL-Washington IL.



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