Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 300433

1133 PM CDT Fri May 29 2015

Issued at 930 PM CDT Fri May 29 2015

Have issued a Flash Flood Watch for much of central, east-central,
southeast MO and the entire STL metro area for increased potential
for heavy rainfall and flooding.

Potentially dangerous situation developing with good synoptic
support for heavy rainfall with deep warm cloud layer of 4km
present, high PWs of near 2 inches. In addition, a MCV is present
just west of AIZ and is moving E-NE, and a low level jet is in the
process of pushing north into eastern MO to further focus the
developing rainfall. That it is occurring at night is also a
concern where this type of wx tends to focus better anyway.
Finally, a deep TROF axis aloft that will take much of tomorrow
to finally get thru the region with most models are taking the
primary disturbance at the base of this upper TROF and bringing it
N-NE into the STL area on Saturday.

Therefore, have issued a FFA thru noon Saturday, with the primary
concern over the southwestern portions of the Watch, where the
heaviest rainfall is expected.



.SHORT TERM:  (Through Late Tonight)
Issued at 340 PM CDT Fri May 29 2015

Complex of showers and thunderstorms ahead of an MCV over E OK will
slowly translate north and eastward overnight. Main threat with this
feature looks to be the potential for localized heavy rainfall due to
long/narrow CAPE profiles with high PWAT values and slow
translational speed of convection. Best bet right now of where
heaviest axis of rainfall would be across south-central Missouri
extending toward the Missouri River between KCOU and KSUS where
1.00-1.50"+ of rain is likely. Threat of widespread severe weather
continues to look low as low-level atmosphere nocturnally stabilizes
through the evening hours along with weak deep-layer shear.
However...cannot rule out an isolated strong wind gust or two in any
heavy precipitation core.

Another seasonably mild night should be in store ahead of synoptic cold
front which will move into northwestern sections of the are very late
tonight. Lows are expected to be in the 60s.


.LONG TERM:  (Saturday through Next Friday)
Issued at 340 PM CDT Fri May 29 2015

(Saturday-Saturday Night)

Front will slowly progress south and east on Saturday clearing the
CWFA by late afternoon per latest model guidance. Showers and
thunderstorms along and ahead of the front along with post-frontal
showers as UL jet streak backbuilds. A few storms across southeast
Missouri and southwest Illinois may be strong to severe with CAPE
values of 1000-1500 J/kg and 0-6km shear on the order of 20 knots or
so. Isolated severe wind gusts along with small hail look to be the
primary threats. A cooler day compared to this afternoon is also
expected due to copious amounts of cloud cover...precipiation...and
cold advection commencing to the northwest of the low-level
baroclinic zone. Coolest conditions will be across portions of
northeast Missouri with highs only in the mid 60s with warmest highs
near 80 degrees toward KFAM and KSLO.

Cold/dry advection begins to really amplify and take hold late
Saturday night as low-level anticyclone slides southeast toward the
Great Lakes. Chances of showers should wane overnight from north to
south as the drier/more stable air filters in from the north and
east. A cool night for very late May should be in store just from
the strength of the cold advection as clouds look to linger across
the area. Look for lows to range from the upper 40s to mid 50s from
north to south across the bi-state area.


The Sunday through Friday timeframe will be marked by mostly dry
weather along with moderating temperatures. Cool day still appears
to be in store on Sunday with highs likely failing to hit 70 degrees
across the area. Upper-level low as previous forecaster mentioned
looks to get stuck beneath/to the south of building ridge axis
across lower Mississippi Valley. This should yield fairly
tranquil/dry weather through much of next week with highs warming
back above normal by Wednesday. Next chance of showers/storms still
appears to be on Thursday though large model disagreements do exist
between the GFS/ECMWF with the European solution continuing the dry
trend while the GFS brings the area some showers/storms.



.AVIATION:  (For the 06z TAFs through 06z Saturday Night)
Issued at 1057 PM CDT Fri May 29 2015

Showers and storms will continue late tonight as the large area
of showers and thunderstorms across south central MO ahead or
northeast of an upper level disturbance and associated surface low
shifts northeastward through the taf sites late tonight. A strong
southerly low level jet will also lead to redevelopment of
convection across our area late tonight. The MOS cloud guidance is
consistent in dropping the cloud ceilings late tonight and
Saturday morning down to around 1000-1500 feet. Prevailing surface
winds will be mainly sly for most of late tonight, then shift to
a nwly direction Saturday morning as the surface low moves
northeastward through southeastern MO and as a cold front now
entering northwest MO drops southeastward through our area. The
showers and thunderstorms will taper off from northwest to
southeast on Saturday.

Specifics for KSTL: Showers and storms will continue late tonight
as a large area of showers and storms across south central MO
moves northeastward into STL late tonight lingering into at least
Saturday morning. The cloud ceiling will drop late tonight and
Saturday morning down to around 1000-1500 feet. Prevailing surface
winds will be s-sely late tonight, although there will be some
variability near thunderstorms. The surface wind will become nwly
Saturday morning. The showers and storms should taper off Saturday
afternoon and evening after passage of the surface low and cold
front, but cloud ceilings will remain low.



MO...FLASH FLOOD WATCH through Saturday morning 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-
     Montgomery MO-Osage MO-Pike MO-Reynolds MO-St. Charles MO-
     St. Francois MO-St. Louis City MO-St. Louis MO-Ste.
     Genevieve MO-Warren MO-Washington MO.

IL...FLASH FLOOD WATCH through Saturday morning FOR Calhoun IL-Jersey
     IL-Madison IL-Monroe IL-St. Clair IL.



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