749
FXUS63 KLSX 260339
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1039 PM CDT Fri Jul 25 2025

.KEY MESSAGES...

- Dangerous heat and humidity will spread gradually northward this weekend,
  with the hottest days expected to be Monday and Tuesday.

- Isolated to widely scattered thunderstorms are forecast through
  this evening, mainly across east-central Missouri and south-
  central Illinois. A few may be strong to severe through 9PM,
  with damaging wind gusts the main threat.

- A definitive end to the heat wave is in the offing next week as
  a cold front moves through the region. There is some uncertainty
  on the timing, but everyone by Thursday should experience
  temperatures back below normal for the first time in over 2
  weeks.


&&

.SHORT TERM...  (Through Late Saturday Afternoon)
Issued at 223 PM CDT Fri Jul 25 2025

Isolated to widely scattered showers along with a few
thunderstorms have developed over the past couple of hours, mainly
in southeast Missouri and southwest Illinois. This activity has
largely been along/southeast of what looks to be the remnant
composite outflow boundary from last night`s convection. So far
everything has been very weak, and latest GOES-16 satellite
imagery does not show much in the way of agitated cumulus.
However, the environment is at least marginally supportive for
semi-organized thunderstorm clusters given high MLCAPE (2000-3000
J/kg) and respectable effective shear (20-25 knots). Therefore,
would not be too surprising to see a few storms capable of
producing damaging winds (via microbursts) through about 8 or 9 PM
this evening. After that time frame, storms should gradually
weaken and eventually dissipate by midnight.

Overnight tonight, the chances of any showers and thunderstorms are
mostly confined to parts of northeast Missouri and west-central
Illinois. This is closer to the synoptic warm front, and where
modest low-level moisture convergence will reside. Better chances of
showers and thunderstorms however will remain to our northwest.
Model guidance is in pretty good agreement that an MCV will track
across the mid-Missouri Valley into southern Iowa. While the
majority of this activity will not impact us, parts of northeast
missouri may get clipped by the southeastern flank of the more
widespread area of showers and thunderstorms.

For Saturday, the expectation is for a composite outflow boundary
associated with early morning convection mainly in Iowa to drift
eastward. By early afternoon, scattered thunderstorms are expected
to develop along this southwest-northeast draped boundary. The best
chances (40-60%) are forecast from central Missouri through west-
central Illinois based on where the outflow boundary is most
likely to be located Saturday afternoon. Cannot rule out a couple
of stronger storms capable of producing gusty winds, but unlike
this afternoon there will be very little deep-layer shear. This is
because the mid/upper level winds really slacken off behind the
departing MCV as it heads toward the Great Lakes. Without much if
any deep-layer shear, thunderstorms should be very pulse like and
weaken before they get too strong.

Heat of course is the bigger concern on Saturday. For the most
part, conditions Saturday afternoon look pretty similar to this
afternoon with highs in the low to mid 90s and peak heat index
values in the 100-105+F. The highest values are expected to be
in/around the urban heat island of metropolitan St. Louis and
adjacent southwest Illinois. The one difference will be that the
dangerous heat should expand a bit northward. Exactly how far
north is a bit of a question mark (due to clouds and the
aforementioned outflow), but did elect to expand the heat advisory
another row or two of counties where confidence in seeing 100+
heat index values was highest.


Gosselin

&&

.LONG TERM...  (Saturday Night through Next Friday)
Issued at 223 PM CDT Fri Jul 25 2025

(Saturday Night - Tuesday Night)

Low chances of showers and thunderstorms exists Saturday night and
Sunday over portions of the area, but the focus should continue to
migrate northward as the mid/upper level anticyclone builds
poleward. Therefore, it is basically all about the intensifying heat
late this weekend into early next week. The peak of this heat event
is still expected to be on Monday and Tuesday afternoons as this is
when the mid-Mississippi Valley will be directly beneath the
anomalous mid/upper level anticyclone and when the 850-hPa thermal
ridge draped across the area. High temperatures should peak out in
the mid to upper 90s each day, with some locations (especially St.
Louis) at least flirting with the century mark. There is some
uncertainty with dewpoints, as conceptually speaking day after
day of dry weather and some deeper mixing suggests they may drop
more into the low 70s, or even upper 60s in parts of central,
southeast, and east-central Missouri. Regardless, it will be very
hot and humid with more of the area topping out in the 105-110F
range for peak heat index readings each of Monday and Tuesday
afternoons.


(Wednesday - Next Friday)

Confidence remains high that a major pattern change will take place
mid/late next week as the overall flow across the CONUS becomes very
amplified for mid summer. In fact, it looks a bit more like
something you typically see in late September/October. Anomalous
troughing (500-hPa heights 1-2 sigma below normal) is expected to
dig into southeastern Canada/New England, with a cold front moving
through the bi-state area sometime Wednesday/Wednesday night.
There remains some uncertainty on the exact timing, which will
have large ramifications on Wednesday`s high temperatures (and the
potential for one last day of this heat event). The latest spread
between the 10th/90th percentiles of the NBM is in the 10-15F
range, which is not surprising given the ongoing heat ahead of the
boundary and the strength of the air mass behind the front.
Speaking of which, there remains a very good signal for at least
slightly below normal temperatures to end the work week (and
likely into the following weekend). This is in large part due to
the strength of the incoming surface high due to the strong
mid/upper level confluent flow between the mid/upper level ridge
across the intermountain west and the aforementioned trough in
southeast Canada. In fact, the NAEFS has a 1026+ hPa surface high
sliding into the Upper Midwest Thursday. This would be a near-
record breaking surface high and a very strong signal from the
NAEFS mean some 144-168 hours out.

While there remains some uncertainty as to precisely "how cool"
it will be, the spread between the 25th/75th percentile for highs
Thursday/Friday of next week has lowered over the past 24-48 hours.
In general, most sites only have a spread of about 2-4F. Given
that the 75th percentiles are still some 5+ degrees below normal,
the overall confidence that not only below normal but WELL below
normal temperatures has increased. Highs in the mid 70s to low 80s
are forecast from north to south, with lows in the upper 50s to
mid 60s.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 1036 PM CDT Fri Jul 25 2025

While thunderstorms have developed over the last couple of hours
in the broader vicinity of KUIN, they are moving away from the
terminal with no development appearing imminent upstream. There
still remains a low chance for additional showers and
thunderstorms at the beginning of the period. Additionally,
scattered showers and thunderstorms are still expected to move
across the area during the late afternoon and evening Saturday.
Because of their scattered nature and uncertainty in timing, they
have been kept out of this round of TAFs, though will be possible
at any or all of the local terminals.

Elmore

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory until 7 PM CDT Tuesday for Boone MO-Callaway MO-
     Cole MO-Crawford MO-Gasconade MO-Iron MO-Lincoln MO-Madison
     MO-Moniteau MO-Montgomery MO-Osage MO-Reynolds MO-Saint
     Francois MO-Warren MO-Washington MO.

     Extreme Heat Warning until 7 PM CDT Tuesday for Franklin MO-
     Jefferson MO-Saint Charles MO-Saint Louis City MO-Saint
     Louis MO-Sainte Genevieve MO.

     Heat Advisory from noon Saturday to 7 PM CDT Tuesday for Audrain
     MO-Monroe MO-Pike MO-Ralls MO.

IL...Heat Advisory until 7 PM CDT Tuesday for Calhoun IL-Fayette IL-
     Greene IL-Jersey IL-Macoupin IL-Montgomery IL.

     Extreme Heat Warning until 7 PM CDT Tuesday for Bond IL-Clinton
     IL-Madison IL-Marion IL-Monroe IL-Randolph IL-Saint Clair
     IL-Washington IL.

     Heat Advisory from noon Saturday to 7 PM CDT Tuesday for Pike IL.

&&

$$

WFO LSX