038
FXUS63 KLSX 142311
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
611 PM CDT Sun Jul 14 2024

.KEY MESSAGES...

- Dangerous heat and humidity will will continue at least through
  Monday. Heat index values Monday afternoon are expected to
  reach 105 to 110 degrees..

- The temperature forecast for Tuesday remains uncertain due to
  the possibility of scattered showers and cloud cover as a cold
  front moves into the area.

- Multiple rounds of thunderstorms are forecast along the slow-
  moving front late Tuesday through Thursday which could lead to
  flooding. Much cooler weather is expected for Wednesday through
  Sunday.


&&

.SHORT TERM...  (Through Late Monday Night)
Issued at 242 PM CDT Sun Jul 14 2024

The upper level ridge over the Four Corners region continues to
dominate the weather pattern. Many locations have already reached
100-105 heat index despite mid level clouds and a few sprinkles
earlier today. The heat and humidity will only intensify over the
next 24 hours as southwest flow will continue to prevail ahead of
the next system. All signs point to Monday being dangerously hot
and humid as 850mb temperatures approach 25C which mixes down to
right around 100F at the surface. Dew point temperatures this
afternoon are mostly in the 73-76F range and are expected to
remain in that range if not increase slightly tomorrow. This will
produce dangerous heat index values Monday afternoon ranging from
105 to 110. There is a very small chance that the tail end of an
MCS over Illinois may graze parts of west central and
southwest/south central Illinois early Monday morning. If this
does occur, the outflow could blunt some of the worst of the heat
in those areas. However, current thinking is that the MCS will
stay well north of the area and the outflow will wash out due to
the prevailing southwest flow.

Carney

&&

.LONG TERM...  (Tuesday through Next Sunday)
Issued at 242 PM CDT Sun Jul 14 2024

Models are still advertising a dramatic pattern shift beginning on
Tuesday.  An unusually strong mid-upper level trough will dig
through the Midwest starting Tuesday, and continue southeast all
the way through the Mississippi Valley to the Gulf Coast by
Thursday.  This trough will drag a slowly moving cold front into
Missouri and Illinois Tuesday.  It`s still uncertain where the
effective boundary will be on Tuesday due to potential outflows from
convection Monday night.  Indeed, several CAMs show the tail end of
an overnight MCS making it well into the northern 1/2 of the CWA by
12Z, and the NAM pushes it south of the I-70 corridor in central and
eastern Missouri into southwest Illinois by mid-Tuesday morning.
The uncertainty with the position of the boundary continues to make
the temperature/dew point forecast difficult at best.  Therefore we
will continue to hold on extending the Heat Advisory. That being
said, there is potential for 105+ heat index Tuesday afternoon south
of the cold front particularly if higher dew points pool ahead of
it.

Thunderstorm chances during the day on Tuesday along and ahead of
the front remain in doubt as there is a significant capping inversion
in place.  There is also a possibility for a few severe storms
capable of producing damaging winds during the afternoon and evening
(if they can break the cap) as guidance suggests it will be quite
unstable with more than 3500 J/Kg SBCAPE ahead of the front.
Thunderstorms look more likely on Tuesday night, mainly after
midnight, as the southwest low level jet strengthens to around
30-35kts, overrunning the now stalled front and producing moderate
to strong moisture convergence over Missouri and Illinois.  The low
level jet is forecast to veer a bit with time, which is a good set
up for multiple rounds of training storms.  P-WAT values should be
around 2 inches, and forecast soundings show the warm cloud depth to
be in excess of 13Kft. All this adds up to a good potential for
heavy rain and possibly flash flooding.  Where the heaviest rain
falls will depend on the position of the front, but current thinking
is somewhere near or south of the I-70 corridor.

The front is expected to begin slowly moving again on Wednesday and
Wednesday night as the trough aloft continues digging into the
Mississippi Valley.  There may be another round of thunderstorms
capable of producing heavy rain Wednesday afternoon as what looks
like an MCV generated from convection over the Plains Tuesday night
phases with the trough right along the front.  It`s unclear at this
point how likely this is to happen as the GFS seems to be
contaminated with grid-scale convective feedback, but if it does
occur it would certainly exacerbate any flooding from the previous
night.

Cool Canadian high pressure will build into the region behind the
front on Thursday.  Ensemble cluster analysis shows very good
agreement with the pattern shift and position of the trough over the
eastern 1/2 of the CONUS for Thursday through next Sunday...although
there are some differences in the strength of the trough by the end
of the period.  This lends good confidence to the forecast of below
normal temperatures for Thursday through Sunday.

Carney

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 611 PM CDT Sun Jul 14 2024

Dry and VFR conditions will persist through the TAF period. There is
an off chance of thunderstorms skirting through west-central and
south-central Illinois late tonight, but confidence in any activity
brushing KUIN is low and will need to be monitored this evening.
Otherwise winds will fall below 10 kts for most locations tonight
before becoming gusty from the southwest again tomorrow afternoon.

Jaja

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 8 PM CDT Monday for Audrain MO-Boone MO-
     Callaway MO-Cole MO-Crawford MO-Franklin MO-Gasconade MO-
     Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln MO-Madison MO-
     Marion MO-Moniteau MO-Monroe MO-Montgomery MO-Osage MO-Pike
     MO-Ralls MO-Reynolds MO-Saint Charles MO-Saint Francois MO-
     Saint Louis City MO-Saint Louis MO-Sainte Genevieve MO-
     Shelby MO-Warren MO-Washington MO.

IL...Heat Advisory until 8 PM CDT Monday 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-Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX