691
FXUS63 KLSX 151957
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
257 PM CDT Mon Jul 15 2024

.KEY MESSAGES...

- Tuesday should begin the much anticipated break from the heat
  for most of the area. However, there is still potential for 105+
  heat index across the eastern Ozarks therefore the Heat
  Advisory remains in effect for that part of the area.

- A slow-moving cold front will trigger several waves of showers
  and thunderstorms from overnight tonight through Wednesday. Some
  of the storms late tonight could produce damaging winds and
  large hail. Flash flooding will also be possible, mainly from
  storms Tuesday night.

- Much cooler and drier air will move into the region behind the
  cold front which will bring us several days of below normal
  temperatures.

&&

.SHORT TERM...  (Through Late Tuesday Night)
Issued at 256 PM CDT Mon Jul 15 2024

Key messages:

The forecast through at least Tuesday night will hinge on where
thunderstorms develop and move tonight, and during the day on
Tuesday. Current thinking is that thunderstorms will develop over
Iowa or southern Minnesota this afternoon/evening and quickly grow
upscale into a linear MCS. Forward propagating Corfidi Vectors
indicate the MCS should move generally east-southeast tonight, and
lay down a strong outflow boundary that will also have some
southward push. MUCAPE ahead of the outflow boundary is forecast
to be in excess of 2000 J/Kg across northern Missouri into central
Illinois, and deterministic guidance shows a 40kt west-southwest
low level jet blowing across the boundary. The direction of the
jet is at about a 30 degree angle to the boundary...but it should
produce enough lift to generate additional convection on the
boundary itself. Timing the boundary into the northern portion of
our forecast area is difficult at best, but the current best
estimate is around midnight tonight. Determining how much
convection the outflow will bring with it into our area is also
very difficult, but isolated to scattered storms is plausible
given the jet`s less than favorable orientation to the boundary.
While a few strong to severe wind gusts are possible, the storms
should be elevated above a strong inversion that will be difficult
for downdrafts to punch through. There could also be a few large
hail stones with the strongest storms.

The strength and coverage of the storms that move into our area will
determine how far south the outflow boundary penetrates into
Missouri and southern Illinois, and that will determine where
convection persists or redevelops on Tuesday.  The CAMs vary pretty
widely on the position of the boundary as do the deterministic
models.  However, the general consensus is that it will make it down
to around the I-70 corridor.  Some models continue to produce
convection through the day, some have convection weaken and
dissipate, only to redevelop storms in the afternoon/evening along
the boundary.  The NAM even fires up what looks like a severe MCS
and drives it through central Missouri into the St. Louis Metro area
Tuesday afternoon...but it`s an outlier and I do not expect this
scenario to occur.  Going with the consensus again...a few showers
and storms will probably linger into mid-late morning over the area,
then dissipate and redevelop during the afternoon after we reach
peak heating.  Models are indicating MLCAPE values in excess of 3500
J/Kg, but the shear could be somewhat weak at only 20-30kts, so a
few storms could be severe, but widespread/well organized severe
thunderstorms do not appear likely at this time.

The afternoon/early evening storms will move the effective boundary
again, and the low level jet will once again increase to around 30-
40kts across the boundary.  Weak shear and lowered buoyancy during
the late evening and overnight will keep the resulting storms from
becoming too strong, but the warm cloud depth will be above 13kft,
and P-WAT values are now a little higher than previous forecasts at
2-2.5 inches.  Add to that slow storm motion and training and the
possibility for flash flooding becomes very real.  Where the
highest threat will be is still very uncertain, and will depend on
where the effective boundary settles Tuesday night. It is due to
this uncertainty that we have not issued a Flood Watch at this point.


Carney

&&

.LONG TERM...  (Wednesday through Next Monday)
Issued at 256 PM CDT Mon Jul 15 2024

The preponderance of guidance is now indicating that Tuesday night`s
convection will push the effective boundary into southern Missouri
or even farther south by Wednesday morning.  However a short wave
that looks suspiciously like an MCV moves slowly around the base of
the long wave trough Wednesday.  This produces more rain north of
the boundary across at least the eastern Ozarks and potentially as
far north as the I-70 corridor.  About 60% of the LREF members have
measurable precip across the eastern Ozarks Wednesday, so lingering
showers appear likely in this area, but the trends are for a dryer
Wednesday, so these PoPs may ultimately prove to be too high.

Cool high pressure builds in behind the front on Thursday, and the
forecast at least through Saturday is for dry weather and below
normal temperatures.  Temperatures warm a bit by early next week as
the airmass modifies and a slight chance for some showers also
returns due to increasing low level moisture and the presence of the
persistent long wave trough over the Central U.S.

Carney

&&

.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