Area Forecast Discussion
Issued by NWS St. Louis, MO

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FXUS63 KLSX 092334

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
634 PM CDT Sun Aug 9 2020

.SHORT TERM...  (Through Late Monday Night)
Issued at 309 PM CDT Sun Aug 9 2020

A weakening line of showers remaining from earlier convection in
western MO will move through the area this afternoon and evening.
PWATs approaching 2 inches and SBCAPE ahead of the line in excess of
3500 J/kg will provide plenty of energy for thunderstorms to develop
if the remnant forcing holds together. CAMs have been teetering back
and forth on whether storms develop, with the latest iterations
weakening. The chance for a stray thunderstorm still exists, however
coverage nor intensity will be anything of note.

Tonight, showers and thunderstorms will redevelop with the passage
of a shortwave trough through southern Iowa into Illinois. PWATs
will remain near 2 inches, and will interact with additional forcing
provided by an enhanced low-level jet. Models continue to trend the
heavier precipitation eastward out of the CWA, but the chance of
thunderstorms still exists in northeastern MO and
central/southwestern IL given MUCAPE of at least 1000 J/kg in the
presence of this forcing. Steering flow is slightly stronger than
that of the storms last night, which should limit the threat for
flash flooding to occur.

Monday will feature a return to much warmer temperatures and higher
humidity, supported by stronger southwesterly flow in the low
levels. 850 mb temperatures will climb to 22-24C, making for surface
temperatures in the mid-90s across the southern CWA. Low 90s are
expected in northeast MO and west-central IL due to increased cloud
cover from morning convection. Dewpoints will be range from the low-
to-mid 70s across the region. Heat index values are expected to peak
near 105F in east-central MO and southwestern IL, which has prompted
a Heat Advisory for this area through tomorrow evening. The only
uncertainty for reaching heat indices of 105F is the presence of
precipitation in the morning. If showers/thunderstorms are more
widespread than currently predicted, residual cloud cover may be
enough to suppress temperatures to an extent. Despite this
uncertainty, enhanced warm air advection and increased low-level
humidity will keep heat index values elevated.

A more robust shortwave trough digs south from southern IA into the
CWA Monday afternoon and evening. Since the northern CWA will likely
see precipitation associated with this forcing first, they will
likely be spared from high heat indices. Showers and thunderstorms
should overspread the region along a low-level boundary associated
with the shortwave. CAPE values, both surface-based and most
unstable, are notably elevated, with SREF probabilities of
exceeding CAPE of 2000 J/kg around 60-70% in the late afternoon
through the evening. The SPC has issued a slight risk for severe
weather in central MO, and a marginal risk for the remainder of
the CWA. Large hail and damaging winds are the main threats Monday
evening in the northern CWA, with these threats moving into the
southern CWA after midnight. While diminishing to an extent,
showers and thunderstorms should persist through the night Monday.


.LONG TERM...  (Tuesday through Next Sunday)
Issued at 309 PM CDT Sun Aug 9 2020

The shortwave disturbance that enters the CWA Monday afternoon will
slowly move south and east Tuesday, exiting the region by the
Tuesday evening. Showers and scattered thunderstorms should
redevelop along the slow-moving front Tuesday afternoon in
southeastern MO and southwestern IL , especially as daytime
heating enhances instability ahead of it. The remainder of the
period is marked by cycles of shortwave passages embedded in the
periphery of an upper-level ridge. With each passing wave exists
the chance for thunderstorms since low-level moisture should
remain seasonably elevated.

Deterministic and ensemble solutions are beginning to converge on a
more amplified pattern starting late Wednesday. This leaves the Mid-
Mississippi Valley caught in northwest flow in the mid/upper levels
between a ridge to the west and broad troughing to the east. While
confidence is not very high at this time, this pattern is generally
conducive to unsettled weather as shortwaves will continue to pass
through the area. Temperatures should top out in the 80s through the
rest of the week as the hotter temperatures are confined further
south of Missouri.



.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 627 PM CDT Sun Aug 9 2020

Isolated to scattered storms are possible overnight tonight,
mainly along/east of the Mississippi River. Best chances appear to
be at KUIN where a VCTS group is in place form 0800 to 1200 UTC
Monday. Elsewhere, cannot rule out a stray storm, but
probabilities are too low to mention in the TAF. Much of Monday
should be dry, but chances of storms should increase as a cold
front approaches late in the period as it enters northern
Missouri. Added a VCTS group at KUIN based on this timing, and
held off for KCOU and the metro terminals as better chances should
hold off until Monday evening.


A stray storm cannot be ruled out tonight, but better chances are
north and east of the terminal. Dry weather is favored for much of
Monday before a cold front approaches during the evening. Added a
VCTS group in as at least scattered storms are forecast along this
frontal boundary. Light southerly winds tonight should veer to the
southwest ahead of the cold front on Monday.



MO...Heat Advisory from noon to 7 PM CDT Monday for Franklin MO-
     Jefferson MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO.

IL...Heat Advisory from noon to 7 PM CDT Monday for Clinton IL-
     Madison IL-Monroe IL-Randolph IL-Saint Clair IL-Washington



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