River Flood Warning 118, NWS St. Louis, MO

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
626 PM CDT Sat May 16 2020

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 314 PM CDT Sat May 16 2020

Unsettled weather will continue through the weekend as an upper
level trough passes through the central CONUS. Two features of note
were analyzed within the broad upper trough this afternoon; a weak
negatively tilted shortwave extending from the central Plains into
Louisiana and a closed low moving ESE through the northern
Plains. A surface low was analyzed in the vicinity of the
Arklatex, with a warm front extending north and east into
southern SE MO and southern IL.

Several areas of showers and storms have developed in the mid-
Mississippi Valley along the elevated portion of the front. The most
vigorous convection is along and east of the Mississippi River,
where limited clouds have allowed for MUCAPE to build to around
2,000 J/kg. Effective bulk shear in this area is rather limited,
so individual updrafts have been fairly short-lived. Nonetheless,
at least one storm has pushed up to near severe levels and I
couldn't rule out another storm or two following suit.

These storms have also been very efficient rainfall producers.
With PWs in the upper quartile of seasonal climatology and a
tendency for new updrafts to develop in the immediate wake of
dying convection, we've had reports of 1-1.5" per hour beneath the
heaviest cores, resulting in flash flooding in a few areas.
Fortunately there does not appear to be any sort of anchoring
feature to focus these storms over a particular area, so I think
additional flash flooding will be rather isolated.

As we move into the evening hours, storm coverage will wane with
the instability. Meanwhile, the previously mentioned negatively-
tilted wave will continue to the ENE, drawing the surface low and
associated warm front north into the Mid-Mississippi Valley around
midnight. The surface low will then deepen and push to our NE
tomorrow morning as it interacts with the northern upper low,
before eventually dragging a cold front through the area tomorrow
afternoon.

As larger scale forcing intensifies in the early morning hours, CAMs
are in good agreement increasing precip coverage after roughly
midnight. Scattered to widespread storms are then expected to
continue until the cold front sweeps through the area during the day
tomorrow. Instability is expected to build to around 1,000 J/kg
ahead of the ahead of the front tomorrow, supporting yet another
round of thunderstorms. Much like today, effective bulk shear
looks fairly limited, with SREF guidance showing a less than 50
percent chance of us topping even 30 kts. Nonetheless this
CAPE/shear parameter space coupled with the frontal forcing would
support a limited severe threat in southern Illinois tomorrow
afternoon.

BSH

.LONG TERM...  (Monday through Next Saturday)
Issued at 427 PM CDT Sat May 16 2020

Large scale omega block still on track to take hold at the
beginning of the week while a closed low sits and spins over the
eastern third of the CONUS. Surface ridging will develop beneath
the block, kicking off an extended period of quiet weather in the
mid-Mississippi Valley. Our proximity to the eastern CONUS low
will keep things on the cloudy and cool side for the early portion
of the week. The deep moisture associated with that low will wane
as we move through the week, resulting in less overcast to broken
stratus and more broken to scattered cu. This will allow
temperatures to gradually build through the week. Some guidance is
showing a chance for diurnal showers toward the end of the week,
through the signal for this is quite weak, so have omitted it from
the forecast at this point.

BSH

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 611 PM CDT Sat May 16 2020

Showers and storms across the region will continue to decrease in
intensity and coverage this evening before picking back up around
06z as low pressure moves through the region. I think we'll only
see isolated to perhaps scattered thunder in the overnight hours,
so have left thunder out of the TAFs until storms become more
likely after sunrise. Scattered to widespread showers and storms
will then persist until a cold front moves through the region in
the afternoon, ushering the precip out of the area. Given
questions around exact timing and coverage of tomorrow's storms,
have just gone with VCTS and will update as details become more
clear.

Guidance in good agreement showing cigs quickly falling in the
early morning hours as ample low level moisture moves into the
region with the previously mentioned low. I'm not very confident
on exactly how low cigs will drop, but at this point much of the
guidance has all terminals dropping below 1,000 ft, so ended up
going IFR at all terminals.

BSH

&&

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

WFO LSX

Hazardous Weather Outlook
National Weather Service Saint Louis MO
356 PM CDT Sat May 16 2020

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-172100-
Adams IL-Audrain MO-Bond IL-Boone MO-Brown IL-Calhoun IL-Callaway MO-
Clinton IL-Cole MO-Crawford MO-Fayette IL-Franklin MO-Gasconade MO-
Greene IL-Iron MO-Jefferson MO-Jersey IL-Knox MO-Lewis MO-Lincoln MO-
Macoupin IL-Madison IL-Madison MO-Marion IL-Marion MO-Moniteau MO-
Monroe IL-Monroe MO-Montgomery IL-Montgomery MO-Osage MO-Pike IL-
Pike MO-Ralls MO-Randolph IL-Reynolds MO-Saint Charles MO-
Saint Clair IL-Saint Francois MO-Saint Louis City MO-Saint Louis MO-
Sainte Genevieve MO-Shelby MO-Warren MO-Washington IL-Washington MO-
356 PM CDT Sat May 16 2020

This Hazardous Weather Outlook is for portions of eastern and
central Missouri as well as west central and southwest Illinois.

.DAY ONE...Tonight.

Thunderstorms will continue through early this evening along and east
of the Mississippi River, where there is a marginal risk for large hail
and damaging winds. Localized flash flooding is possible in areas
that have already received heavy rainfall today.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

The chance for thunderstorms will continue on Sunday. Isolated
strong to severe storms are possible for areas near and south and
east of Saint Louis during Sunday afternoon.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed tonight.

BSH

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE ST LOUIS MO
442 PM CDT SAT MAY 16 2020

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

0425 PM     FLOOD            WILSONVILLE             39.07N 89.86W
05/16/2020                   MACOUPIN           IL   TRAINED SPOTTER

            WATER REPORTED COMING OUT OF FIELDS AND
            FLOWING OVER ROUTE 138 BETWEEN ROUTE 4 AND
            159.

&&

BRITT