Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 301757

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
1257 PM CDT Sun Apr 30 2017

Issued at 1232 PM CDT Sun Apr 30 2017

Widespread showers and scattered thunderstorms today has limited
the amount of available instability over the area. Current band of
widespread rain should gradually exit to the northeast this
afternoon. There still may be scattered showers and thunderstorms
that develop along the cold front this afternoon. While the
atmosphere will lack instability, it still will have ample shear
to produce a few strong to severe thunderstorms, so will keep a
mention a few strong to severe storms going. Otherwise, clouds and
rain will keep temperatures down this afternoon.



.SHORT TERM...  (Through Late Tonight)
Issued at 439 AM CDT Sun Apr 30 2017

Periods of SHRA/TSRA with heavy rain will continue across the area
today, and severe thunderstorms are possible across the eastern half
of the CWA this afternoon.

The stalled boundary which has been responsible for the widespread
heavy rain across the region will finally move out of the area this
afternoon and tonight. The eastern portion of the boundary will lift
northward as a warm front and pivot about the surface low while the
western portion of the boundary moves eastward as a N-S oriented
cold front. A persistent southerly LLJ will continue to funnel
moisture over the warm front this morning, resulting in continued
SHRA/TSRA today which should gradually shift northward/northwestward
with the movement of the surface boundary. Precipitation should
finally begin to taper off tonight once the occluding system has
moved away from the region. Forecast PW values remain at +3SD for
April, therefore the threat for heavy rainfall will continue

Even with limited warm sector instability due to cloud cover and
precipitation, there are several factors which would still support
strong to severe thunderstorms this afternoon across the eastern
half of the CWA. These factors include steep H7-H5 lapse rates due
to cold air aloft, large-scale ascent ahead of the approaching upper
low, forcing associated with the cold front, timing of the cold
frontal passage during peak heating, and 0-6km shear values of 30-

Temperature trends today will follow complex rising/falling patterns
due to the lifting warm front across the northern CWA, the
approaching cold front across the western CWA, and influences from
ongoing precipitation and cloud cover. In general, highs today will
range from around 60 to the mid-70s, but the daily high temperature
will occur at different times for different locations. Temperatures
should fall into the 40s areawide tonight within the cooler post-
frontal air mass.


.LONG TERM...  (Monday through Saturday)
Issued at 439 AM CDT Sun Apr 30 2017

Lingering light rain can be expected on Monday, mainly across
northeast MO and west central IL, associated with the deformation
zone/wrap-around moisture on the backside of the upper level and
associated surface low moving northeastward into the Great Lakes
region.  It will also be windy on Monday due to the tight surface
pressure gradient.  High temperatures on Monday will be around 15
degrees below normal due to cold 850 mb temperatures below zero
degrees C during the morning hours, along with low level cloud cover
hindering solar insolation.  Any rain should shift northeast of the
forecast area by Monday evening as the upper level and associated
surface low move well northeast of the region. Warmer, more normal
high temperatures are expected on Tuesday with well deserved
sunshine and dry conditions.  The dry weather will be fairly short
lived though as showers will spread eastward into our forecast area
by late Tuesday night, particularly across central and southeast MO
and southwest IL as low-mid level warm air advection increases
across this area over an 850 mb front and north of a surface front
as well as ahead of an approaching upper level trough.  Upper level
divergence will also increase over this area ahead of the
approaching upper level trough.  Showers will continue on Wednesday
with the highest pops and QPF still across central and southeast MO
and southwest IL as the deepening upper level trough approaches with
at least a weak surface wave developing along the surface front
south of the forecast area.  The model solutions begin to diverge
after Wednesday with the ECMWF model being more progressive with the
upper level trough causing much of the precipitation to shift
southeast of our forecast area by late Wednesday night with only
some patchy light rain Thursday on the backside of the deep upper
level trough.  The GFS model is less progressive as it closes off an
upper level low in the base of the upper level trough by Thursday
and keeps precipitation going across southeast MO and southwest IL
as late as Friday evening.  With model uncertainty and differences
for now will gradually reduce pops beginning Wednesday night and
begin shifting them southeast with time down to only slight chance
pops across parts of southeast MO and southwest IL by Thursday
evening, then finally totally ending any pops across the entire
forecast area by Friday evening.  Should see warmer temperatures by
Saturday due to gradually rising upper level heights as the upper
level trough shifts east of the region coupled with more sunshine on
Saturday with surface ridging over the area.



.AVIATION...  (For the 18z TAFs through 18z Monday Afternoon)
Issued at 1232 PM CDT Sun Apr 30 2017

Expect mainly MVFR/some IFR conditions through the afternoon
hours with showers and scattered thunderstorms. The rain and
MVFR/IFR conditions will move out of the area tonight before more
MVFR ceilings move into the area after 12Z tomorrow.

SPECIFICS FOR KSTL: Expect mainly MVFR conditions with showers
this afternoon. Showers will move out of the terminal with VFR
ceilings expect through tonight. MVFR ceilings are expected to
move back into the terminal around 15Z tomorrow.



MO...Flash Flood Watch through late tonight 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 through late tonight 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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