060
FXUS63 KLSX 132335
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
635 PM CDT Sat Jul 13 2024

.KEY MESSAGES...

- Dangerous heat and humidity will affect the region for Sunday
  and Monday. Heat index values of 105-109 are expected each
  afternoon. A Heat Advisory is now in effect for the entire area.

- The heat may linger into Tuesday for at least part of the area,
  though there is some uncertainty due to a cold front which is
  expected to move into the area by Tuesday afternoon.

- The front will bring a chance for rain for Tuesday through early
  Thursday morning as it sags slowly southward. Temperatures are
  expected to drop to 5-10 degrees below normal for Wednesday
  through Saturday.


&&

.SHORT TERM... (Through Monday)
Issued at 326 PM CDT Sat Jul 13 2024

The primary concern in the short range, at least through Monday,
will be dangerous heat. A sprawling upper level ridge centered
over the Four Corners region will spread east across the Lower
Missouri and Mid-Mississippi Valleys Sunday into Monday.
Meanwhile, persistent southerly flow will continue to draw warm
moist air into Missouri and Illinois. While the low level thermal
ridge axis looks like it will be just north of our area across
Iowa, 850mb temperature are still forecast to be 22-24C on Sunday
and 24-26C on Monday. This should easily be good enough for mid to
upper 90s both days, and with dew point temperatures in the low
to mid 70s, heat index values will rise to around 105 Sunday and
105+ Monday for the vast majority of the area. Have therefore
issued a Heat Advisory starting Sunday afternoon and continuing
until Monday evening. Aside from being hot and humid, we`ll
probably see an isolated shower or thunderstorm this
afternoon...most likely along and east of the Mississippi River in
southeast and south central Illinois where the outflow from this
morning`s convection has settled and . the energy from a weak
short wave can weaken the low level cap. A few showers are also
possible late tonight into early Sunday morning across northeast
Missouri and west central Illinois as an outflow boundary from
nocturnal convection farther north makes its way into the area.
Other than those slight chances, dry weather is expected through
Monday night.

Carney

&&

.LONG TERM...  (Monday Night through Next Saturday)
Issued at 326 PM CDT Sat Jul 13 2024

The long wave pattern will begin changing Monday night as a trough
digs into the northern Plains and Upper Midwest.  Guidance appears
to be in good agreement in handling this trough as it continues
southeast through the Mississippi Valley into the Tennessee and Ohio
Valleys by Thursday.  It does take all of Tuesday and much of
Wednesday for the trough to overpower the ridge which will be pushed
back over the Rocky Mountains.  This will mean the surface front
associated with the trough will also take time to push through
Missouri and Illinois.  Current guidance shows the front entering
northern Missouri and west central Illinois Tuesday morning and
taking until at least Wednesday evening and possibly early Thursday
morning until it gets into northern Arkansas.  The wild card of
course will be convection associated with the front which could push
the effective boundary farther south than the current forecast. This
may be true for any period from Tuesday morning through Thursday
morning.  As such, my confidence level in the temperature forecast
is especially low for Tuesday as outflow and or convective debris
from storms could significantly lower temperatures.  Indeed, the
difference between 25th and 75th percentile temperatures at most
locations is 6+ degrees Tuesday afternoon.  There was some chat
among neighboring NWS offices about keeping the Heat Advisory going
through Tuesday afternoon, however we agreed that the uncertainty in
the temperature forecast was high enough that we`d wait until the
forecast became more clear.

As the front sags south the plentiful moisture over the region will
provide juice for several waves of showers and thunderstorms.  The
heaviest and most widespread of these waves will likely be Tuesday
night ahead of the mid/upper level trough axis.  Latest QPF from WPC
is showing 1 inch plus in spots across the eastern Ozarks Tuesday
night.  Of course, the position of the low level boundary will
dictate where the heaviest rain will fall, and the operational GFS
and ECMWF show the highest QPF closer to the I-70 corridor.  The
rain finally ends on Thursday as the front finally pushes into
Arkansas.  A cool dry high pressure system builds into the Midwest
behind it with temperatures between 5 and 10 degrees below normal.

Carney

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 633 PM CDT Sat Jul 13 2024

Confidence is high in dry and VFR flight conditions at all local
terminals through the forecast period. There is a very low chance
for isolated showers and thunderstorms in the vicinity of KUIN
tomorrow morning, but confidence is low enough in them directly
impacting the terminal that their mention has been left out of the
TAF for now.

Elmore

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from 1 PM Sunday to 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 from 1 PM Sunday to 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