Tornado and Flash Flooding in Missouri, May 11 to 12, 2016

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
714 AM CDT WED MAY 11 2016

.SHORT TERM...  (Through Late This Afternoon)
Issued at 340 AM CDT Wed May 11 2016

Main concerns today are dense fog early this morning and potential
severe weather.  Have issued a dense fog advisory through mid
morning.

Latest surface observations show that quite a few stations from the
eastern Ozarks into south central Illinois have had visibilities
fall to 1/4 mile the past few hours.  Visibilities in the St. Louis
metro area have also fallen recently to less than a mile at times.
With light winds and clear skies, think this trend will continue
through mid morning, so have gone with a dense fog advisory for the
eastern half of the CWA.  Conditions should begin to improve once
the sun comes up and mixing begins.

Then the focus turns to the potential for the thunderstorms the rest
of the day.  A weak warm front that thunderstorms are forming on
over southwest MO early this morning will lift northward this
morning. There is some potential that scattered thunderstorms will
develop along this front this morning into early this afternoon as
strong moisture convergence develops underneath increasing ascent
from approaching mid level trough.  Will show increasing chances of
showers and thunderstorms increasing from the west this afternoon.
With MLCAPES between 2000-3000 J/kg this afternoon and deep layer
shear between 20-35kts, a few severe thunderstorms will be possible.

Will likely see some sunshine which will allow temperatures climb
above normal today.  Highs should reach the lower to mid 80s.

Britt

.LONG TERM...  (Tonight through Tuesday)
Issued at 340 AM CDT Wed May 11 2016

Expect convection to be ongoing after 00Z Thursday ahead of the cold
front.  GFS and NAM continue to show 1200-2000 J/Kg of MUCAPE across
the CWFA during the early evening.  0-6km shear stays relatively
weak though...never really getting stronger than 20kts except over
northeast Missouri and west central Illinois which will be closer to
mid-level jet streak.  The mode convection takes will be very
dependent on what happens during the afternoon...but a forward
propagating MCS is still the most likely.  The system should weaken
through the night with waning instability as it moves southeast.
Convection allowing models aren't showing much behind the MCS ahead
of the front, but prefer to keep some low chance/slight chance PoPs
in until the front moves through and wipes out the instability.

The GFS has the front all the way southeast through the CWFA by 12-
15Z on Thursday.  Models are still printing out some very light QPF
behind the front, so will keep some low chance/slight chance PoPs
going in southeast sections of the CWFA Thursday morning.
Temperatures should be noticeably cooler with lower humidity behind
the front.  High pressure will move from the Great Plains down into
Arkansas by Friday morning.  Another shortwave will move southeast
across the area Friday afternoon and Friday night pushing another
cold front ahead of it.  Guidance continues to print out precip with
the FROPA Friday afternoon and Friday night.  Think the amount of
QPF might be overdone since the ridge over the southern CONUS will
be blocking moisture return, but all guidance highlights strong
moisture convergence ahead of the 850mb front.  Will keep PoPs going
but will stay in chance category.

Temperatures fall well below normal for the weekend as a sprawling
high pressure system builds over the area from the northern Plains
all the way down to the Gulf Coast.  Another longwave trof develops
over the western CONUS Sunday night into Monday.  850 mb flow turns
to the south/southwest bringing low level moisture back to the
region.  If medium range guidance works out, Monday and Tuesday look
fairly wet as this moisture is lifted over the cool air at the
surface.  Clouds and precipitation will likely contribute to
suppressing temperatures below normal for the end of the forecast
period.

Carney

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 653 AM CDT Wed May 11 2016

Current dense fog at the St. Louis area airports should begin to
lift in the next few hours, before VFR conditions develop by late
this morning. Showers and thunderstorms will be possible today
over the area, with the best chance late this afternoon and this
evening ahead of a cold front. Have timed this out with VCTS and
-SHRA groups in the TAFS. MVFR and possible IFR ceilings and
visibilities can be expected with the thunderstorms. The
strongest storms will be capable of producing strong wind gusts
and hail.

Specifics for KSTL: Visibilities will begin to improve in the
next hour and VFR conditions are expected by 15-16Z. Scattered
and thunderstorms are possible throughout the day, but current
thinking is that best timing at the terminal will be late this
afternoon and early this evening. Currently have VFR conditions,
but heavier thunderstorms could produce MVFR and possible IFR
ceilings and visibilities. MVFR ceilings will move into the area
later tonight.

Britt

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Dense Fog Advisory until 9 AM CDT this morning for Jefferson MO-
     Lincoln MO-Madison MO-Pike MO-Saint Charles MO-Saint
     Francois MO-Saint Louis City MO-Saint Louis MO-Sainte
     Genevieve MO.

IL...Dense Fog Advisory until 9 AM CDT this morning for Bond IL-Brown
     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.

&&

WFO LSX

HAZARDOUS WEATHER OUTLOOK
NATIONAL WEATHER SERVICE ST LOUIS MO
434 AM CDT WED MAY 11 2016

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-121200-
GREENE IL-MACOUPIN IL-MONTGOMERY IL-BOND IL-FAYETTE IL-CLINTON IL-
MARION IL-WASHINGTON IL-RANDOLPH IL-ADAMS IL-BROWN IL-PIKE IL-
CALHOUN IL-JERSEY IL-MADISON IL-SAINT CLAIR IL-MONROE IL-KNOX MO-
LEWIS MO-SHELBY MO-MARION MO-MONROE MO-RALLS MO-PIKE MO-BOONE MO-
AUDRAIN MO-MONITEAU MO-COLE MO-OSAGE MO-CALLAWAY MO-MONTGOMERY MO-
LINCOLN MO-GASCONADE MO-WARREN MO-SAINT CHARLES MO-FRANKLIN MO-
SAINT LOUIS MO-SAINT LOUIS CITY MO-JEFFERSON MO-CRAWFORD MO-
WASHINGTON MO-SAINT FRANCOIS MO-SAINTE GENEVIEVE MO-IRON MO-
MADISON MO-REYNOLDS MO-
434 AM CDT WED MAY 11 2016

THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF EASTERN AND
CENTRAL MISSOURI AS WELL AS WEST CENTRAL AND SOUTHWEST ILLINOIS.

.DAY ONE...TODAY AND TONIGHT

AREAS OF DENSE FOG WILL AFFECT PARTS OF EASTERN MISSOURI AS WELL
AS WESTERN AND CENTRAL ILLINOIS THROUGH MID MORNING. VISIBILITY OF
1/4 MILE OR LESS CAN BE EXPECTED.

SEVERE THUNDERSTORMS ARE POSSIBLE TODAY AND TONIGHT. THE MOST
LIKELY TIME FOR SEVERE STORMS WILL BE THIS AFTERNOON AND THIS
EVENING...THOUGH ANY STORMS THROUGH TONIGHT MAY BECOME SEVERE.
HAIL IN EXCESS OF 1 INCH IN DIAMETER AND WIND GUSTS IN EXCESS OF
60 MPH ARE THE PRIMARY THREATS. AN ISOLATED TORNADO IS ALSO
POSSIBLE.

.DAYS TWO THROUGH SEVEN...THURSDAY THROUGH TUESDAY

THERE IS A CHANCE OF SEVERE THUNDERSTORMS AGAIN ON FRIDAY
AFTERNOON AND EVENING ACROSS PARTS OF CENTRAL AND EASTERN MISSOURI
AND WEST CENTRAL ILLINOIS. LARGE HAIL AND DAMAGING WIND ARE THE
PRIMARY THREATS.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION MAY BE NEEDED TODAY AND TONIGHT.

CARNEY