Special Weather Statement, NWS St. Louis, MO

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1223 AM CDT Thu Jul 27 2017

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 323 PM CDT Wed Jul 26 2017

Primary concern will transition this evening from heat to localized
heavy rainfall. Will be trimming far northwestern portion of heat
advisory off as thicker midlevel clouds have kept temperatures in
the low 90s preventing heat index values from approaching 105F.
Rest of advisory still looks good and will continue until 8PM.

Still lots of uncertainty with respect to convective trends for
overnight tonight. Retreating warm front is currently across the
Mississippi River in Illinois and continues to push northeastward.
Believe heaviest axis of rainfall may set up roughly along that
boundary which is forecast to also be roughly collocated with
strong upper-level forcing for ascent via vorticity maxima and
low-level moisture convergence. This axis however will likely stay
east/northeast of the CWA. Showers and thunderstorms will likely
fill in behind that area along trailing cold front. Due to copious
amounts of cloud cover and more stable air across the lower-mid
Missouri Valley, convection will likely not be as rigorous as
thought 24 hours ago. Not seeing too much of a severe threat as
freezing levels are very high with only modest midlevel lapse
rates. Cannot rule out a wet microburst however in a stronger
core.

Main concern with convection tonight will continue to be on
localized heavy rainfall. There are quite a few mitigating factors
for widespread flash flooding tonight including dry antecedent
conditions (and low soil moisture percentiles), more stable air
upstream than forecast 12-24 hours ago mentioned above, and very
high flash flood guidance (2-3" in one hour). That being said,
will be leaving the flash flood watch alone for now as still
expecting a round of showers and thunderstorms within the watch
area in a very supportive environment (deep warm cloud depths and
very high PWAT values) for efficient precipitation production.
Training would need to occur though to produce problems in local
areas.

Scattered-numerous storms will continue along and ahead of the cold
front through the overnight hours into early Thursday morning
bringing for most areas some very welcome rainfall. Believe there
will be a lull in activity during the late morning hours before
mostly scattered storms fire along the cold front in the early
afternoon. Highest PoPs for Thursday afternoon look to be across
portions of southeast Missouri and southwest Illinois where cold
front is expected to be located. Localized heavy rainfall again may
be possible if storms train across the same areas.

High temperatures tomorrow will be quite a bit cooler than that of
today due to an increase in cloud cover and convection. Highs in the
mid 80s are likely for most, which is near to slightly below normal
for late July.

Gosselin

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 323 PM CDT Wed Jul 26 2017

High pressure will build over the region by Friday night. Northwest
flow will dominate the pattern throughout the forecast period.
The area will see temperatures below climatological normals for
both afternoon highs and morning lows along with dry weather. High
pressure aloft will move west in response to a series of short
waves diving south out of the high plains starting Monday morning.
A weak boundary will setup in southern Iowa ahead of a cold front
that will impact the area later in the week.

Kelly

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1156 PM CDT Wed Jul 26 2017

Thunderstorm trends continue to be the biggest forecast challenge.
Expect numerous to widespread showers and thunderstorms to affect
the forecast area...mainly north of I-44 in Missouri and I-64 in
Illinois. Thunderstorms will be capable of producing MVFR and IFR
conditions in heavy downpours as well as isolated wind gusts to
25-35kts. Also, expect ceilings across northern Missouri and west
central Illinois to drop to MVFR over the next few hours as low
pressure moves along a cold front which will drift southeast
through the period. MVFR ceilings will likely get as far south as
central Missouri and may get as far east as the St. Louis Metro
area into southwest Illinois by Thursday morning. Shower and
thunderstorm activity should diminish after 14-17Z Thursday, but
may increase again south of I-44 Thursday afternoon due to daytime
heating.

SPECIFICS FOR KSTL:

Occasional showers and thunderstorms will affect the terminal
through the overnight hours. VFR conditions will prevail, but any
storm that moves over the terminal will be capable of producing
intermittent IFR conditions in moderate to heavy rain. Wind
direction will be very variable due to storms in the area...though
the general direction should be from the south most of the time.
Expect thunderstorm activity to diminish after 12Z...though timing
is uncertain. Could see additional thunderstorm activity Thursday
afternoon, but current indications are that afternoon activity
will be south of the terminal.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.
&&

WFO LSX

Hazardous Weather Outlook
National Weather Service Saint Louis MO
1115 PM CDT Wed Jul 26 2017

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-271130-
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-
1115 PM CDT Wed Jul 26 2017

This hazardous weather outlook is for portions of eastern and
central Missouri as well as west central and southwest Illinois.

.DAY ONE...Tonight

Strong thunderstorms are possible tonight across much of the
area. A few of the storms may be severe. Damaging winds are the
primary severe weather threat. Additionally, locally heavy rain is
possible with the stronger storms.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

A few strong to severe thunderstorms are possible on Thursday
across parts of east central and southeast Missouri and southwest
Illinois. Damaging winds are the primary severe weather threat.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed tonight, mainly in northeast
Missouri and west central Illinois.

Carney

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PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ST LOUIS MO
1033 PM CDT WED JUL 26 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0710 PM     TSTM WND DMG     FESTUS                  38.22N  90.41W
07/26/2017                   JEFFERSON          MO   TRAINED SPOTTER

            TREES DOWN IN SMALL RESIDENTIAL AREA AND THROUGHOUT
            SUNSET PARK. SOME SIDING ALSO REMOVED FROM A HOME.
            DAMAGE APPEARS TO BE FROM A MICROBURST.

&&

EVENT NUMBER LSX1700579

FGLASS

Special Weather Statement
National Weather Service St Louis MO
1256 AM CDT THU JUL 27 2017

MOZ051-052-060-061-270645-
Montgomery MO-St. Charles MO-Lincoln MO-Warren MO-
1256 AM CDT THU JUL 27 2017

...SIGNIFICANT WEATHER ADVISORY FOR WEST CENTRAL ST. CHARLES...
SOUTHWESTERN LINCOLN...MONTGOMERY AND CENTRAL WARREN COUNTIES UNTIL
145 AM CDT...

At 1256 AM CDT, Doppler radar was tracking a strong thunderstorm over
Bellflower, moving east at 35 mph.

Wind gusts up to 50 mph will be possible with this storm.

Locations impacted include...
Wentzville, Troy, Warrenton, Wright City, Montgomery City, Moscow
Mills, Davis, New Florence, Jonesburg, Truesdale, Hawk Point,
Innsbrook, Bellflower, Buell, Big Spring, Foristell, High Hill,
Middletown, Truxton and Cave.

This includes Interstate 70 in Missouri between exits 170 and 203.

This also includes  Cuivre River State Park.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 3915 9148 3911 9087 3871 9091 3880 9160
TIME...MOT...LOC 0556Z 271DEG 28KT 3897 9138

Glass