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FXUS63 KLSX 201107
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
607 AM CDT Sun Jul 20 2025

.KEY MESSAGES...

- A few thunderstorms will continue to move through northeast
  Missouri and west-central Illinois this morning, posing a very
  limited threat for localized flooding.

- Another round of thunderstorms is likely to develop across
  northeast Missouri and western/southern Illinois late this
  afternoon and evening, and continue overnight through Monday
  morning. These storms will pose a threat for locally heavy rain
  and flash flooding, along with a more limited threat of
  damaging winds.

- While heat will be delayed until Tuesday in areas north of I-70,
  areas further south will see oppressive heat and humidity again
  today. Similar conditions are expected tomorrow, but dangerous heat
  will expand into all areas Tuesday through Thursday.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 432 AM CDT Sun Jul 20 2025

As of 4:30 AM, radar trends suggest that the complex that earlier
was heading into northeast Missouri has lost a significant amount
of structure and organization. It has essentially been absorbed
by a larger line of storms that extends northwest through a large
portion of Iowa, although remnants of it still extend into parts
of Knox county. While this likely means that the threat of
damaging wind gusts is lower than previously anticipated, even
this small portion of the line has exhibited relatively
substantial rainfall rate estimates, albeit from a very distant
radar range. As such, pockets of heavy rain will remain possible
across our far northern tier of counties for a few hours early
this morning, where a Flood Watch has been issued. This is most
likely across primarily Knox, Lewis, and Adams counties, although
we can`t rule out a few cells developing farther south along this
line, either. While the flooding threat is likely higher farther
to the north at least initially, we will need to continue to watch
for southward development.

As the day progresses, this activity is expected to drift off to the
east southeast, and a composite outflow boundary from both this
activity and another remnant complex to the west will settle into
the area. The impacts from this boundary will be twofold: First,
this will likely determine which areas cool off a bit, and which
areas see another day of oppressive heat. Second, this boundary is
likely to serve as a focus for renewed convection as it returns
northward and interacts with the low level jet later in the
afternoon and evening.

Regarding today`s heat prospects, we have opted to keep the Heat
Warning headlines in place as-is, but whether or not the entire area
sees extreme levels of heat is somewhat in doubt. In particular, the
northern tier of counties roughly along I-70 and the Missouri River
will likely see at least some initial relief from the previously
mentioned outflow boundary, and confidence in the afternoon
temperature forecast is very low as a result. This is less likely to
occur farther south across the Ozarks, which still managed to see
heat index values over 105 degrees yesterday. Considering that this
is a multi-day event, we opted not to trim any counties from the
headlines for now, but those in the northern 1 or 2 rows of counties
may see the most dangerous heat delayed one more day. In the areas
that are not impacted by this boundary or new thunderstorms, you can
expect heat index values to climb to between 105 and 110 degrees by
mid afternoon.

By late afternoon, return southerly flow is likely to allow the
outflow boundary to stall somewhere across our area, perhaps near I-
70 although this is not particularly certain. At this point, the low
level jet is expected to increase in speed and interact with this
boundary, perhaps even pushing it back to the north slightly. As
this occurs, another round of thunderstorms is likely to develop,
this time locally as opposed to farther north and then drifting
south into our area. As has been the case for the past two days,
very rich moisture will be in place, with HREF median precipitable
water values exceeding 2 inches, and reaching as high as 2.3 inches.
Not only this, but the position and strength of the low level jet
will be more favorable for training thunderstorms across our area,
with a westerly extension of 30 to 35 kt 850mb winds overrunning the
stalled boundary. Storm motions will favor thunderstorms moving
west to east along this boundary, which could lead to localized
training in combination with intense rain rates exceeding 2 inches
per hour in some cases. As such, while rainfall amounts will be
highly variable, concentrated corridors of 3 to 5 inches of rain
will not be difficult to achieve, with the potential to reach 7+
inches. This is also reflected in HREF LPMM guidance, adding
confidence to this potential. Rainfall totals such as that would
likely cause Flash Flooding wherever they occur. As such, the
ongoing Flood Watch expands further south and southwest into
additional portions of northeast Missouri and southwest Illinois
beginning at 5pm this afternoon and ending tomorrow at 1 PM.

In addition to the flooding threat, there remains a limited
potential for severe thunderstorms late this afternoon through
the overnight hours as well. Significant instability is expected
to build by mid afternoon along the stalled boundary, perhaps
aided by some moisture pooling along the boundary itself.
Meanwhile, modest 0- 6km bulk shear (20-30kt) will provide just
enough support for thunderstorm organization, with parallel
vectors to the initiating boundary supporting upscale growth. We
will also note some clockwise- curved hodographs that may
initially support transient supercell structures as well, although
eventually we expect cold pool processes to lead to linear /
cluster modes. While the overall weak shear limits the ceiling for
severe storms, isolated damaging wind gusts will be possible, and
we can`t rule out a few 1 inch hailstones or a brief tornado
along the boundary.


While the exact location of initiation is somewhat uncertain and
tied to the location of the outflow boundary, this is most likely
north of I-70. Storms are likely to develop late in the afternoon
and continue through the overnight hours, eventually drifting east-
southeast into southern Illinois before gradually losing strength
early Monday morning. Exactly when storms diminish is somewhat
uncertain, and it`s even possible that a few may linger through the
afternoon and evening across parts of Illinois as well. However, the
building upper ridge and increasing subsidence may act to suppress
new thunderstorm development Monday afternoon.

BRC

&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 432 AM CDT Sun Jul 20 2025

Given the potential for another round of nocturnal and morning
thunderstorms leading into Monday, yet another outflow boundary is
likely to be draped across the area for most of the day. North of
this boundary, temperatures are once again likely to be a bit cooler
than original projections, and this includes a majority of the
counties covered by the Heat Watch. As such, we have opted to delay
the start of the Watch by one more day, and it now starts Tuesday
afternoon as opposed to Monday. South of the boundary though, high
heat and humidity is expected to develop once again, with heat index
values climbing to between 105 and 110 degrees.

By Tuesday, though, the building upper level ridge remains very
likely to put an end to meaningful precipitation chances, and
allow oppressive heat and humidity to spread northeast and into
most of the area. This is then likely to continue unabated through
at least Thursday, with widespread heat index values of between
105 and 110 degrees, and morning lows generally in the mid 70s. As
such, areas in the Heat Watch area are very likely to see at
least 3 consecutive days of these conditions, while areas in the
Warning see roughly 4 to 6 days at minimum. While above average
temperatures are very likely to continue through the end of the
week, latest forecast temperatures have decreased very slightly
late in the week, and precipitation chances have ticked slightly
upward, likely in response to a gradually flattening upper ridge.
While this may just be a temporary blip, it has given us just
enough pause to refrain from extending the heat headlines any
further into the week.

BRC

&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 600 AM CDT Sun Jul 20 2025

The primary concern during the 12Z TAF period continued to be the
potential for thunderstorms at various times. Initially, a few
showers and thunderstorms have passed just to the north of UIN
this morning, plus a few weak showers near JEF/COU. While the
latter activity has moved east and dissipated, showers and a few
thunderstorms may continue to skirt UIN just to the north for a
few hours at the beginning of the TAF window. A few remnant
showers may also reach near JEF/COU again later in the morning,
but this activity is likely to dissipate before reaching these
terminals.

The main round of storms is likely to develop late in the
afternoon, likely somewhere between UIN and St. Louis area
terminals. While storms are likely to develop, the precise timing
and location of initial development is less certain. Still, UIN is
the most likely terminal to be impacted, with a relatively lower
chance at St. Louis terminals, and even lower chances at JEF/COU.
If a terminal is impacted, heavy rain and gusty winds are the most
likely hazards, with visibility and ceiling reductions as well.
This is most likely over the latter 6 to 12 hours of the TAF
period.

BRC

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch from 5 PM CDT this afternoon through Monday
     afternoon for Lincoln MO-Monroe MO-Pike MO-Ralls MO.

     Extreme Heat Watch from Tuesday afternoon through Thursday
     evening for Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-
     Ralls MO-Shelby MO.

     Extreme Heat Warning until 7 PM CDT Thursday for Audrain MO-
     Boone MO-Callaway MO-Cole MO-Crawford MO-Franklin MO-
     Gasconade MO-Iron MO-Jefferson MO-Lincoln MO-Madison MO-
     Moniteau MO-Montgomery MO-Osage MO-Reynolds MO-Saint
     Charles MO-Saint Francois MO-Saint Louis City MO-Saint
     Louis MO-Sainte Genevieve MO-Warren MO-Washington MO.

     Flood Watch through Monday afternoon for Knox MO-Lewis MO-Marion
     MO-Shelby MO.

IL...Flood Watch from 5 PM CDT this afternoon through Monday
     afternoon for Bond IL-Calhoun IL-Clinton IL-Fayette IL-
     Greene IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-
     Montgomery IL-Pike IL-Washington IL.

     Extreme Heat Watch from Tuesday afternoon through Thursday
     evening for Adams IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-
     Fayette IL-Greene IL-Jersey IL-Macoupin IL-Marion IL-
     Montgomery IL-Pike IL-Washington IL.

     Extreme Heat Warning until 7 PM CDT Thursday for Madison IL-
     Monroe IL-Randolph IL-Saint Clair IL.

     Flood Watch through Monday afternoon for Adams IL-Brown IL.

&&

$$

WFO LSX