379
FXUS63 KLSX 151749
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1249 PM CDT Mon Jul 15 2024

.KEY MESSAGES...

- One more day of widespread dangerous heat and humidity is
  forecast today, with a Heat Advisory in effect for heat index
  values up to 110 degrees. Tuesday`s potential remains somewhat
  uncertain, but potential across the Ozarks and far southern
  Illinois was high enough to warrant extending the Advisory
  there until 8 PM Tuesday.

- A slow cold front will end the dangerous heat for some starting
  tomorrow. It will also trigger multiple waves of thunderstorms
  late tonight through Thursday that threaten strong to locally-
  damaging winds and at least localized flash flooding.

- Behind the cold front, a rare multi-day break from mid-July heat
  and humidity graces the late-week/weekend forecast.


&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 301 AM CDT Mon Jul 15 2024

The Mid-Mississippi Valley remains on the eastern periphery of a
strong, expansive ridge aloft, just missing the path of several
MCSs that have impacted the general region over the last 48 hours.
Further north across the Great Lakes and southeast Canada, a
longwave trough is slowly deepening and amplifying the flow
pattern while sending shortwaves into the region. Despite the
ridge being held to our west, a strong 850mb thermal ridge (23-25
degrees Celsius) will persist overhead today and expand over most
of the region. With little change in synoptic pattern compared to
24 hours ago except for consistently-hot 850mb temperatures and a
lack of cloud cover, dangerous heat and humidity will exist
areawide today. The Heat Advisory remains in effect through 8 PM
this evening for most (see below for details on the extension)
for heat index values as high as 110 degrees. These values
approach thresholds that are dangerous for anyone outside, not
just sensitive groups - exercise caution when outside today and
ensure those more sensitive to dangerous heat are cared for.

The hot and humid air at the surface will also act to build
instability across the region, with the RAP and similar short-range
guidance painting alarming MLCAPE values as high as 4000 J/kg this
afternoon into the evening. The shortwaves mentioned above will
continue to force a cold front south and are likely to initiate
severe convection at some point this afternoon to our north. Exactly
where and when this occurs is highly uncertain, which affects when
we`d expect any activity to arrive, but there is some concern that
the resultant MCS will dive south and threaten northern Missouri and
west-central Illinois with strong to severe winds (and perhaps a
weak tornado). While instability is helices, forcing aloft is
meager and strong MLCIN exists thanks to the thermal ridge aloft.
Both of these aspects to the forecast will limit the southward
extent of the severe weather threat later today, but the greatest
threat exists where the SPC Day 1 Convective Outlook`s Slight Risk
is drawn.

Because of the uncertain convective evolution overnight, Tuesday`s
forecast has a wide range of potentials. Sensible weather in the
region will hinge on where the cold front is, how far south it
reaches, and whether convection continues to percolate into the day.
Regardless, it will mark the beginning of the end for our dangerous
heat and humidity. Exactly how warm any one location gets is more
uncertain than normal for the reasons listed above, but with at
least more widespread cloud cover and a more westerly component to
the surface winds, we can reasonably expect to be a few degrees
cooler at least with similar humidity. If it`s actively raining
during the day, that will drop temperatures even further. That said,
sufficient confidence in at least 105 degree heat index values
exists ahead of the front (across the Ozarks and far southern
Illinois) to warrant extending the Heat Advisory there through
8 PM Tuesday evening. With the cold front forecast to be well-
within the forecast area during the afternoon on Tuesday (though
where exactly isn`t clear) there is some low risk for isolated
damaging wind during the afternoon if it can tap into low-level
instability.

MRB

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 301 AM CDT Mon Jul 15 2024

The front is still forecast to stall somewhere in the Mid-
Mississippi Valley later on Tuesday as a strengthening low-level jet
counters its southward progress. Confidence remains high that the
ambient environment will feature precipitable water values exceeding
2.00 inches, warm cloud depths of 13kft or more, and sufficient
instability. With this thermodynamic environment in the vicinity
of the front, LLJ, and modest jet-level ascent the threat for
heavy rain and flash flooding continues to exist overnight Tuesday
into Wednesday. A few details cast doubt on the forecast:

1) Deterministic soundings do have some degree of dry air in the
profile, decreasing rainfall efficiency.

2) The low-level jet is not very strong, nor is it oriented
favorably to the boundary to lend stronger low-level forcing.

Despite the rainfall not being at peak efficiency in the face of
weak dry air entrainment, Corfidi vectors are still parallel to the
front and continue to suggest training convection will be an issue.
All of this is to say, while the setup isn`t perfect, the heavy rain
will fall in a region that has seen 200-600% of normal precipitation
over the last two weeks. Flash flood guidance from the River
Forecast Centers remains relatively-low, which implies it will not
take very much rain to cause some degree of flooding issue. River
flooding is less certain due to the important dependence on where
exactly rain falls in a given basin, but that threat can`t be
discounted either.

A combination of remnant overnight convection and persistent digging
longwave trough will keep slowly pushing the surface cold front
south during the day on Wednesday. By this point, temperatures will
certainly fall below 90 degrees areawide and decidedly end any heat
threats. Depending on the front`s southward progress, we may deal
with another heavy rain threat Wednesday night in southern Missouri.
That said, the LLJ is markedly weaker and upper-level forcing is
further removed from the region that night which leads to a far more
marginal threat. By Thursday, an unusually-strong Canadian surface
high will become more established in the region. This airmass will
promote an unusual (but welcomed) period of dry, cool, comfortable
conditions through at least Sunday. While details become far less
certain by this stage, there is some low (15-20%) threat for rain
beginning early next week with some solutions establishing a
cutoff upper-level low over the central Plains. Along with this,
there`s roughly a 70-80% chance for sub-90 degree (thus below-
normal) high temperatures into at least early next week.

MRB

&&

.AVIATION...  (For the 18z TAFs through 18z Tuesday Afternoon)
Issued at 1238 PM CDT Mon Jul 15 2024

VFR flight conditions are expected to prevail across the area at
least through this evening...and possibly through the entire
period. There is a chance that thunderstorms over Iowa and
northern Illinois this evening will spread farther south to affect
parts of our area after 05-06Z. The most likely locations to see
storms will be along and east of the Mississippi River. However,
my confidence in storms affecting our forecast area is 30 percent
at best. Any storms that do move into our area will be capable of
heavy downpours which could produce IFR flight conditions and
gusty winds of 35-45kts.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT this evening for Audrain MO-Boone
     MO-Callaway MO-Cole MO-Franklin MO-Gasconade MO-Jefferson
     MO-Knox MO-Lewis MO-Lincoln MO-Marion MO-Moniteau MO-Monroe
     MO-Montgomery MO-Osage MO-Pike MO-Ralls MO-Saint Charles MO-
     Saint Louis City MO-Saint Louis MO-Shelby MO-Warren MO.

     Heat Advisory until 8 PM CDT Tuesday for Crawford MO-Iron MO-
     Madison MO-Reynolds MO-Saint Francois MO-Sainte Genevieve
     MO-Washington MO.

IL...Heat Advisory until 8 PM CDT this evening for Adams IL-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-Saint Clair IL-Washington IL.

     Heat Advisory until 8 PM CDT Tuesday for Randolph IL.

&&

$$

WFO LSX