Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 251158

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
658 AM CDT WED MAY 25 2016

.SHORT TERM...  (Through Late This Afternoon)
Issued at 219 AM CDT Wed May 25 2016

Wish I could say with confidence how today will play out in terms of
convective coverage and timing, but yet again, it is muddled with
ongoing thunderstorms to our west across the Central Plains and
little help from the plethora of model guidance available to us.
That being said, I do believe we have a better chance of seeing
measurable precipitation across a larger portion of the CWA today
given better synoptic forcing, so have maintained likely PoPs.

The best I can tell is that the convection stretching from central
Nebraska to southeast Oklahoma will slowly edge east this morning in
a weakening state and either completely fall apart across central
Missouri or be reinvigorated by the approaching shortwave and
diurnal instability that develops ahead of it.  There are also signs
in the convective allowing models that some elevated thunderstorms
may develop in an arcing fashion from southeast Missouri to north
central Missouri during the predawn hours as the LLJ veers.  I told
you it was muddled, and therefore I just broadbrushed some likely
PoPs from west to east across the CWA today. As far as the threat
for severe thunderstorms, I would have to say it is conditional on
some surface instability being realized either ahead of the decaying
MCS or behind it, especially across north central Missouri late this
afternoon.  Needless to say, todays forecast will need to be fine
tuned or nowcast as trends become evident.


.LONG TERM...  (Tonight through Tuesday)
Issued at 219 AM CDT Wed May 25 2016

Interesting signals in some of the model guidance tonight across
northern Missouri and west central Illinois. Moisture convergence is
maximized between 06Z and 12Z and there is some indication that
convection may train across this region with a threat of heavy
rainfall and flash flooding. Instead of jumping with both feet into
this solution, have decided to increase PoPs to likely across the
northern CWA and mention heavy rainfall. This area may require a
flash flood watch if model guidance continues to show this setup in
the 12Z runs.

This thunderstorm activity should slowly sink/spread south through
18Z on Thursday on the collective outflow and veering low level jet.
Redevelopment will certainly be possilbe Thursday afternoon given
the degree of instabilty forecast, so maintained high chance Pops.

A more widespread shower and thunderstorm setup is still on tap for
Thursday night and Friday as the main upper level trof finally
approaches and lifts through the Midwest.  Temperatures are forecast
to be a bit cooler on Friday given clouds and precipitation.

Threat of thunderstorms does not end with the passage of the trof.
In fact, chances of thunderstorms litter the extended as the upper
level pattern keeps southwest flow alive and well. Weak shortwaves
and a nocturnal low level jet will invigorate convection daily as
the area remains warm and unstable.



.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 643 AM CDT Wed May 25 2016

Another difficult aviation forecast with lots of uncertainty in
coverage of showers and thunderstorms, timing, and location.
Scattered showers and thunderstorms have been developing from KCOU
eastward since around 10z and the threat for SHRA/TSRA either at
the terminal or in the vicinity will continue this morning. The
general thought is that most of the activity should shift to the
east of KCOU by mid-morning and east of the St. Louis area
terminals by midday. Another round of thunderstorms is anticipated
this afternoon originating over eastern MO/west-central IL and
hence the TEMPO group. A lull in precipitation is then expected
until later tonight when more showers and thunderstorms are
expected across northern MO into western IL impacting KUIN.

Specifics for KSTL:

Scattered showers and thunderstorms have been developing between
KCOU and KSTL since around 10z and the threat for SHRA/TSRA
either at the terminal or in the vicinity will continue until late
morning. Another round of thunderstorms is anticipated later this
afternoon originating over eastern MO, and hence the TEMPO group
from 21-00z. Most of the shower and thunderstorm activity tonight
is expected across northern MO into western IL, possibly drifting
southward to KSTL by early Thursday morning. Confidence is not
high enough to add any overnight mention at this time.





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