Special Weather Statement, NWS St. Louis, MO

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
656 AM CDT Thu Jun 21 2018

.SHORT TERM...  (Through Late Tonight)
Issued at 425 AM CDT Thu Jun 21 2018

A slow-moving stacked low pressure system will bring periods of
SHRA/TSRA to the area today as it moves from IA into MO and IL. The
surface low associated with this system was located over
northwestern IA early this morning per METARs and satellite imagery.
This surface low will initially retrograde slightly towards the
IA/NEB border before moving south to southeastward into MO today.
Meanwhile, a new surface low will develop farther to the east.

The stacked low pressure system will push a cold front through the
region as it slowly moves southeastward today. The combination of an
upper low as well as surface boundaries will produce periods of
SHRA/TSRA across the area today. The greatest coverage is expected
after 18z when the surface cold front will be approaching the area.
Models depict 20-30 kts of 0-6km shear across the southern half of
the CWA, and when combined with the expected position of the cold
front, this suggests a few strong to severe storms are possible this
afternoon and evening across the southeastern one-third to one-
quarter of the CWA.

The concern with a slow-moving upper low overhead is that there will
be a large amount of ambient vorticity. This environment could
contribute to rotating updrafts in SHRA/TSRA, and even shallow
convection which otherwise appears unremarkable could quickly
acquire supercellular characteristics. One additional concern with an
upper low at this time of year is the potential for cold air
funnels. Cold air funnels are climatologically favored during the
late afternoon and early evening hours during the summer months when
a slow-moving upper low is overhead. Cold air funnels are attached
to the bases of shallow fair-weather cumulus clouds and rarely reach
the ground, but they are often photogenic and attract a lot of
attention.

Kanofsky

.LONG TERM...  (Friday through Wednesday)
Issued at 425 AM CDT Thu Jun 21 2018

The upper LO that was the primary driver of our weather for the
short-term period will begin Friday morning just east of STL and
will spend Friday slowly exiting our region to the east with its
influence waning as it does so.  By this point, the system is
expected to have become vertically stacked and will exhibit deep
cyclonic flow Friday for most of our region.  This will justify
likely or higher PoPs for most areas near and east of the
Mississippi River.

Synoptically, the position of the stacked LO pressure system will be
favorable for an extensive area of low cloudiness that will probably
persist for much of the day.  Being late June, it will be tough to
lock the sun out all day, but it does appear that the sun will have
precious few windows of time.  This should result in an unusually
cool day with max temps forecast in the mid to upper 70s for much of
the area.  If this forecast is realized, it will be the first time
max temps have failed to reach 80F since May 18th for STL, June 4th
for COU, and May 20th for UIN.  A reflection of the impressive
string of unseasonably warm temps that have prevailed the past two
months.

With the stacked LO pressure system passing to our east, the surface
cold front should finally make its push thru our region late Friday
and settle just to our south for the weekend.  A zonal flow is
expected to exist aloft and will continue to shuffle along upper
level disturbances to maintain a steady rain threat for the weekend,
but mainly in southern MO.  High pressure is anticipated to build
into the western Great Lakes by late in the weekend and should act
as a reinforcer of the seasonable temps (for a change) for another
day or so.

Already by early next week, the upper RIDGE will begin to re-assert
itself over the central CONUS, with another round of showers and
thunderstorms expected as a surface warm front moves thru, probably
around Tuesday.  This will end the string of near average to below
average temp days, with high levels of heat potentially making a
return by the middle of next week.

TES

&&

.AVIATION...  (For the 12z TAFs through 12z Friday Morning)
Issued at 637 AM CDT Thu Jun 21 2018

SPECIFICS FOR KUIN, KCOU: Satellite imagery and METARs showed an
area of MVFR clouds (with a few breaks) across the region at 12z,
and cigs should vary between VFR and MVFR during the first 0-6
hours of the valid TAF period until cloud bases rise. Periods of
SHRA/TSRA are expected between 15z and 22/03z. An extensive area
of MVFR clouds was located on the back side of a low pressure
system, and this area of clouds is expected to move over the
terminals after 21z. Cloud bases may lower to IFR after 22/00z,
but confidence was too low to include prevailing IFR in the KCOU
and KUIN TAFs at 12z.

SPECIFICS FOR KSTL, KSUS, KCPS: An area of MVFR clouds (with a few
breaks) was moving across the St. Louis metropolitan area at TAF
issuance. Ceilings should vary between VFR and MVFR during the
first 0-6 hours of the valid TAF period until cloud bases rise.
Periods of SHRA/TSRA are expected between 18z and 03z. An
extensive area of MVFR clouds was located on the back side of a
low pressure system, and this area of clouds is expected to move
over the terminals after 00z. It is possible that cloud bases may
even lower to IFR after 03z, but this appears more likely for
locations farther north of the St. Louis area.

Kanofsky

&&

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

WFO LSX

Hazardous Weather Outlook
National Weather Service Saint Louis MO
405 AM CDT Thu Jun 21 2018

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-221200-
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-
405 AM CDT Thu Jun 21 2018

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

Thunderstorms will be likely, mainly during the afternoon and
early evening hours. Isolated severe storms will be possible this
afternoon and early evening, with damaging winds the primary
threat.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

There is a chance for thunderstorms across at least a portion of
the area each day. Isolated severe storms will be possible
Saturday afternoon and evening, mainly south of Interstate 70.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon and early this
evening.

TES

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