Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 250216

Area Forecast Discussion...Updated
National Weather Service Saint Louis MO
916 PM CDT WED AUG 24 2016

Issued at 916 PM CDT Wed Aug 24 2016

Main concern overnight is potential for flash flooding.

MCS has developed over northwestern MO and eastern KS and is
heading towards our region. Most 1-hr flash flood guidance values
are in the 2 to 2.5 inch range which may seem decent enough to
preclude most flood potential but atmosphere is very moist, with
PWs of 2 to 2.5" moving in, and with good model agreement on
strong moisture transport moving into the northern CWA the next
few hours, a deep warm cloud layer in place in excess of 4km, and
what should be shear vectors that will be lining up more in
parallel with the orientation of the MCS overnight, it continues
to look like a good setup for heightened flash flood potential in
parts of northeast MO and west-central IL.

Current Flash Flood Watch area and timing looks good. Although
individual storms are moving a bit more to the east, which is
worrisome for areas down I-70, flow is expected to steer these
storms more to the north with time and should keep the focus over
northeast MO and west-central IL, especially between midnight and
4am where the main brunt of moisture transport will occur. It is
during this time we could see rainfall rates in excess of 2 inches
per hour and will support the current FFA.

PoPs look pretty meager to the south of I-70, with a gradient near
I-70 and the main area that will be affected across northern MO
and central-northern IL.

The MCS should largely exit the CWA just before or around 12z.

In its wake, the big mystery will be how far actual cold front
makes it south and how well it will be able to act as a focus for
new convection during the daytime, with post-frontal development
also in play. For now, looks like a broad chance for rain on
Thursday, but hard to get to specific. Temps will likewise be a
challenge and may be too warm in the north with low clouds
expected at least during the morning hours.



.SHORT TERM...  (Through Late Tonight)
Issued at 159 PM CDT Wed Aug 24 2016

A s/w ejecting out ahead of the main trof over the srn Plains will
continue to lift newd this evening. This s/w is progd to take on
more of an MCV structure as it lifts into MO/IA region late
tonight. Sct TSRA remain possible across much of the region this
evening. Storms have not developed as prev anticipated. However,
storms may still develop this afternoon across central portions of
the CWA where sfc convergence is greater.

Focus shud quickly become area of moisture convergence
across wrn portions of the CWA ahead of the approaching wave. This
wud suggest a longer duration of TSRA across nrn portions of the
CWA this evening. There is some uncertainty regarding duration of
precip tonight across the nrn portions of the CWA. Still, there is
still enuf threat for heavy rainfall/flash flooding tonight across
the current watch area to keep the watch as-is.

With ample CAPE across the region and deep layer shear around 40
kts, any storms that do develop will be capable of becoming severe
with large hail and damaging winds as the primary threat. However,
can not rule out some organized storms with a tornado threat.


.LONG TERM...  (Thursday through Next Wednesday)
Issued at 310 PM CDT Wed Aug 24 2016

Cold front to continue it`s slow progress to the south on Thursday,
finally stalling out just south of forecast area Thursday night. In
the meantime, still decent instability and moisture, so will see
storms along and ahead of boundary through Thursday evening, then
wind down a bit after midnight. Depending on cloud cover and
precipitation, highs will range from the low 80s far north to the
low 90s south on Thursday.

By Friday, models now a bit faster lifting front back north through
forecast area, so will have increasing pops during the day with best
chances along and south of I-70. By 12z Saturday, front will be
north of forecast area once again.

For the last half of the weekend and into early next week, weak
surface ridging will keep boundary just to our north with warm and
humid weather persisting. So active pattern to persist with chances
of storms each day, especially during the heating of the
afternoon/evening hours through next Wednesday.

Temperatures from Friday on will continue to be seasonable,
primarily in the mid 60s to low 70s for lows and low to mid 80s for



.AVIATION...  (For the 00z TAFs through 00z Thursday Evening)
Issued at 712 PM CDT Wed Aug 24 2016

A weak boundary will try to trigger some convection early this
evening, but anything that does develop should dissipate by 02z,
with the region remaining dry until convection from out west can
move in later tonight. Currently, the best chances for rain remain
for UIN with all other sites likely to remain dry overnight. Some
LLWS potential for later tonight for STL metro sites but want to
take another look with some new model data before deciding on
whether or not to place in these TAFs. A cold front is then
expected to drop down into our region early Thursday, and should
allow for some lower CIGs to move into UIN around 12z for a few
hours, but VFR conditions should persist elsewhere. Some rain
potential with the front dropping southward, with these chances
looking a bit better heading into the afternoon, but enough
questions regarding where this front will be Thursday afternoon,
will leave rain mention out of TAFs for now, with only low chance
PoPs going anyway.



MO...Flash Flood Watch until 7 AM CDT Thursday for Knox MO-Lewis MO-
     Marion MO-Monroe MO-Ralls MO-Shelby MO.

IL...Flash Flood Watch until 7 AM CDT Thursday for Adams IL.



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