Damaging Wind in Illinois, July 19, 2025

Area Forecast Discussion
National Weather Service Saint Louis MO
358 AM CDT Sat Jul 19 2025

.KEY MESSAGES...

- Showers and thunderstorms may cause localized flooding and
  occasional damaging wind gusts from mid morning through late
  afternoon, mainly across northeast Missouri and Illinois.

- Additional similar rounds of showers and storms are possible in
  the same areas through the remainder of the weekend and into
  Monday afternoon.

- Dangerous heat begins today roughly along and south of I-70,
  and spreads into the rest of the area early next week. This
  heatwave will extend through at least Thursday and potentially
  through the end of the week, and will likely cause significant
  impacts due to both the duration and magnitude of the heat.

&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 351 AM CDT Sat Jul 19 2025

An active stretch of weather is in store for our area over the next
week, with the potential for multiple rounds of showers and
thunderstorms, and also the development of a long-duration heatwave.

The most immediate concern early this morning is the potential for
one or more rounds of showers and thunderstorms throughout the day
today. Early this morning most of the area remains dry aside from a
few weak and very sporadic showers, but very high humidity and deep
moisture is firmly in place with surface dewpoints well into the 70s
and precipitable water values of 1.9 to 2.1 inches. Meanwhile, a
very modest southwesterly low level jet is also in place, providing
some slight isentropic ascent and moisture convergence. Over the
course of the morning, these factors will likely lead to a steady
increase in locally developing showers and thunderstorms, which
may pose a limited rate-driven heavy rain threat thanks to the
favorable thermodynamics, but little else. HREF members and more
recent individual CAMs maintain a fair amount of spread regarding
the potential coverage of this early activity, but it may become
robust enough to produce a few pockets of 1 to 3 inches of rain
from early to late morning.

Meanwhile, a much more organized line of thunderstorms is ongoing
well upstream across northern Iowa, and some semblance of this is
expected to reach northern Missouri and western Illinois sometime
around mid-day. Confidence in the timing has improved a bit now that
the complex has established itself, although there remains some
variability in models regarding just how well it will hold together
as it approaches. While the very high moisture content has the
potential to support rather robust instability as afternoon heating
commences, this may be stunted in some areas by early morning
convection. Likewise, CAMS continue to struggle with its evolution
as it dives into northeast Missouri and Illinois, although these
areas to remain heavily favored over areas farther southwest. If
enough instability can develop, another round of robust
thunderstorms will be possible during the afternoon, with very heavy
instantaneous rain rates (as much as 2 to 4 inches per hour).
Considering these potential rates, it would not be difficult to
quickly accumulate significant rain totals in small pockets, even if
this line is relatively progressive. As such, localized flooding
will be possible, even if this remains a somewhat conditional threat.

We will also need to monitor the potential for this line to
backbuild, as this would likely lead to more prolonged rainfall
and a more significant flooding potential. This remains a low
probability scenario, as the displacement of the most significant
low level jet to the west is not particularly ideal for this, and
there is limited support in recent CAMs. However, there will be
persistent, if not strong, moisture-rich inflow, and this as at
least a limited potential to cause some back-building as well.
While an unlikely (25% or less) scenario, this could lead to
amounts greater than 4 inches over small areas, especially if it
occurs in areas that see convection earlier in the morning.

Finally, there remains a limited damaging wind threat this afternoon
as well, which would become more likely if early morning convection
is weak and afternoon instability is stronger. Very warm profiles
and high freezing levels, not to mention linear storm modes do not
favor large hail today, and low level wind shear is not particularly
favorable for tornadoes, either. However, there may be just enough 0-
3km bulk shear and sufficiently low LCLs that a weak tornado can't
be completely ruled out.

Finally, dangerous heat remains set to begin across areas roughly
along and south of I-70, although this boundary is very approximate.
Morning and afternoon thunderstorms are likely to create an outflow
boundary that will surge south, and areas that can remain south of
the boundary by mid afternoon will likely see heat index values
climbing to near 105 degrees, and locally higher. Today is just the
beginning of this heat, though, as the worst of it is yet to come.

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&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 351 AM CDT Sat Jul 19 2025

Showers and thunderstorms are likely to exit the area to the
southeast by this evening, but a persistent pattern may lead to
repeat thunderstorm events again overnight tonight and Sunday night.
Not only will this keep the window open for heavy rain and other
thunderstorm hazards, but it will also likely keep the most
dangerous heat locked across our southern half through at least
Sunday, and possibly Monday until definitively expanding Tuesday
onward.

As in previous forecasts, day to day details regarding additional
rounds of thunderstorms remain relatively sparse after today's
potential, but it does appear that the overall pattern is not likely
to change much. A modest southwesterly low level jet will remain in
place and continue to advect rich moisture into the area, and fuel
overnight thunderstorms to our north both tonight and again Sunday
night. These complexes are then likely to drift south into our area,
bringing varying degrees of hazards that will depend largely on the
timing of their arrival. Each night the favored areas for initiation
remain across Iowa, although there is some variability and a few
recent CAMs have exhibited some more southerly solutions. Wherever
storms do develop, though, the pattern will continue to feature a
persistent low level jet overrunning a stalled front / composite
outflow boundary, and fits a classic pattern for heavy rainfall due
to the combination of intense rates and persistence. While again the
most favored areas for this remain just to our north, this will need
to be monitored very closely for a southerly shift, as such a
pattern could lead to a more significant flooding threat if it
develops locally and repeats itself again Sunday night.

While thunderstorms may be ongoing in the morning and afternoon
Monday, attention begins to shift toward the expansion of
significant heat early next week. Latest model projections suggest
that areas currently under the Heat Watch may be a bit slower to
warm up thanks to the aforementioned showers/storms, but rest
assured that by Tuesday this no longer becomes a factor, and heat
will spread into the rest of the area. This is due to a building
ridge that will finally shunt the storm track to the north, allowing
the entire area to bake under the full weight of the upper high.
Meanwhile, very high dewpoints will remain in place, and as a result
we will likely observe heat index values between 105 and 110
degrees, and possibly higher, from Tuesday through at least
Thursday. In areas south of I-70, this would likely equate to at
least 6 consecutive days of such temperatures, with the potential to
stretch through the end of the week. As such, the duration of this
event is likely to be as, if not even more important, than the heat
magnitude on any individual day, and there is potential for this
heatwave to create rather significant impacts as a result.

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&&

.AVIATION...  (For the 06z TAFs through 06z Saturday Night)
Issued at 958 PM CDT Fri Jul 18 2025

VFR conditions will generally prevail through the TAF period. The
chance for isolated to scattered showers and thunderstorms
increases overnight for areas along and north of I-70, including
KUIN and the St. Louis metropolitan terminals. Late Saturday
morning into afternoon, a thunderstorm complex is forecast to
enter northeast Missouri/west- central Illinois and dive
southeast. The exact timing on this feature is still uncertain,
but I gave my best estimate. Whether the complex holds up long
enough or extends southwest enough to impact the St. Louis
terminals is still uncertain as well, but I kept the PROB30 group
for it in and pushed back the timing since confidence is too low
to remove it.

Jaja

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Extreme Heat Watch from Monday afternoon through Thursday
     evening for Knox MO-Lewis MO-Marion MO-Monroe MO-Pike MO-
     Ralls MO-Shelby MO.

     Extreme Heat Warning from noon today to 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.

IL...Extreme Heat Watch from Monday 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 from noon today to 7 PM CDT Thursday for
     Madison IL-Monroe IL-Randolph IL-Saint Clair IL.

&&

WFO LSX

Hazardous Weather Outlook
National Weather Service St Louis MO
406 AM CDT Sat Jul 19 2025

ILZ058>060-064-065-069-070-074-079-095>102-MOZ018-019-026-027-
034>036-041-042-047>052-059>065-072>075-084-085-099-200915-
Greene IL-Macoupin IL-Montgomery IL-Bond IL-Fayette IL-Clinton IL-
Marion IL-Washington IL-Randolph IL-Adams IL-Brown IL-Pike IL-
Calhoun IL-Jersey IL-Madison IL-Saint Clair IL-Monroe IL-Knox MO-
Lewis MO-Shelby MO-Marion MO-Monroe MO-Ralls MO-Pike MO-Boone MO-
Audrain MO-Moniteau MO-Cole MO-Osage MO-Callaway MO-Montgomery MO-
Lincoln MO-Gasconade MO-Warren MO-Saint Charles MO-Franklin MO-
Saint Louis MO-Saint Louis City MO-Jefferson MO-Crawford MO-
Washington MO-Saint Francois MO-Sainte Genevieve MO-Iron MO-
Madison MO-Reynolds MO-
406 AM CDT Sat Jul 19 2025

This Hazardous Weather Outlook is for portions of eastern and
central Missouri as well as west central and southwest Illinois.

.DAY ONE...Today and tonight.

There is a chance of thunderstorms from mid morning through late
afternoon today. There is a low threat of damaging winds and flash
flooding across northeastern Missouri and Illinois.

A long-duration heat wave is expected to begin today along and south
of Interstate 70 into southwestern Illinois.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

There is a chance of thunderstorms Sunday through Tuesday. Low
threats for damaging winds and flash flooding continue across
northeastern Missouri and west-central Illinois.

Dangerous heat will continue along and south of I-70 Sunday, and will
expand into the remainder of the region either Monday or Tuesday.
This will continue through at least Thursday, and possibly through
the end of the week.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

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