Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 252326

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
626 PM CDT Tue Apr 25 2017

.SHORT TERM...  (Through Late Wednesday Afternoon)
Issued at 323 PM CDT Tue Apr 25 2017

Storms are still expected to fire up along the frontal boundary over
western Missouri this evening and slide east across the region. This
first round of activity will reach central and northeast Missouri
around Midnight. Isolated strong to severe storms not out of the
question with this first round. Then will see showers and storms
weaken and diminish toward daybreak.

As the atmosphere recovers, timing/placement of front and surface
low will determine where the next round of storms fires up. For now,
models show redevelopment between 18z and 19z Wednesday extending
from west central Illinois southwestward towards Springfield MO,
then track east. Strong low level jet, decent forcing and strong
mean winds indicate we could expect fast moving bow echo
development, so main threat will be damaging winds, though large
hail and a few isolated tornadoes not out of the question either.

Mild temperatures to persist tonight with lows in the upper 50s to
mid 60s. Highs on Wednesday will depend on location of cold front.
For now have upper 50s to mid 70s, with steady or falling temps for
areas behind the boundary.


.LONG TERM...  (Wednesday Night through Next Tuesday)
Issued at 323 PM CDT Tue Apr 25 2017

Showers and storms will continue Wednesday night as the upper level
trough goes negatively tilted as it moves east-northeastward through
our area. The models depict strong upper level divergence ahead of
this trough.  The associated cold front may be passing just east of
the St Louis metro area by 00Z Thursday.  The severe weather threat
should continue into the evening hours, mainly south and east of STL
where the best instability will exist just ahead of the front. There
will also be favorable deep layer shear due to a strong mid level
wind max ahead of the upper level trough.  The rain should taper off
late Wednesday night and shift east of the forecast area by Thursday
morning as the upper level trough shifts northeast of our area with
the cold front moving east of the forecast area, allowing colder and
drier air to filter into our region.  Cooler temperatures can be
expected Wednesday night and Thursday.  Will only get a brief break
in the stormy pattern Thursday and Thursday evening, then another
round of showers and storms will move into parts of central and
eastern MO late Thursday night due to increasing low-mid level warm
air advection ahead of an approaching shortwave and weak surface
low.  A southwesterly low level jet extending into southern MO will
also bring low level moisture back into the area.  Much of this
precipitation may shift east of the forecast area by late Friday
afternoon, but then more significant convection will likely shift
slowly northward through much of our forecast area Friday night as a
surface low develops over the southern Plains well ahead of a
deepening upper level trough/low moving into the Plains.  A warm
front ahead of the surface low will lift northward into southern MO
Friday night.  Still model uncertainty as to the exact track of the
upper level low and surface low forecast to move through northern MO
later this weekend, but it appears that there will be the potential
for convection containing heavy rainfall this weekend, along with a
threat for a few severe storms across southeast MO and southwest IL.
The rain should taper off Sunday night as the dry slot moves into
our area south of the upper level low.  There may be lingering light
rain into Monday mainly across northeast MO and west central IL in
the wrap-around moisture/deformation zone. Cooler temperatures can
be expected Sunday night and Monday after the passage of the surface
low and trailing cold front.



.AVIATION...  (For the 00z TAFs through 00z Wednesday Evening)
Issued at 615 PM CDT Tue Apr 25 2017

SPECIFICS FOR KCOU, KUIN: VFR conditions are expected for the
first 0-3 hours and possibly a bit longer. SHRA/TSRA are expected
to develop near a cold front and affect KCOU/KUIN overnight. A
break is then expected before additional thunderstorms develop
along a cold front during the late morning or early afternoon.

SPECIFICS FOR KSTL, KSUS, KCPS: VFR conditions are expected for
the first 0-6 hours and possibly longer. Decaying SHRA/TSRA from
overnight convection should reach the St. Louis metro area
terminals towards daybreak and may last for a few hours. A break
of at least a few hours is then expected before a cold front
arrives during the afternoon. SHRA/TSRA are expected to develop
along the cold front and should affect the terminals after 18z.





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