259
FXUS63 KLSX 160652
AFDLSX

Area Forecast Discussion
National Weather Service St Louis MO
152 AM CDT Thu Jul 16 2026

.KEY MESSAGES...

- Typical summer heat and high humidity will continue through
  early next week, with the warmest days Saturday and Monday
  when heat index values have the best chance to reach between
  100 and 105 degrees.

- Scattered pop-up showers and thunderstorms are expected across
  more of the area today and tomorrow, with best chances
  along/south of I-70 today, and along and east of the
  Mississippi River tomorrow. Brief bursts of heavy rain and
  lightning are the main hazards expected.

&&

.SHORT TERM /THROUGH FRIDAY/...
Issued at 146 AM CDT Thu Jul 16 2026

As expected parts of the Ozarks saw a round of scattered afternoon
showers and thunderstorms yesterday, with the greatest coverage
across Reynolds, Iron, southern Crawford counties. In these areas,
pockets of radar-estimated 1 to 2 inches of rain were observed, with
isolated totals of 2 to 3 inches just to the south of our forecast
area. These showers formed in a weakly-sheared but moisture-rich
environment, with PWAT values between 1.8 and 2 inches, and likely
got a slight boost in forcing from a slowly approaching mid-level
vorticity maximum. These conditions are expected to largely continue
today, but with a few slight modifications. First, the
aforementioned vort max and rich moisture will drift further north,
and the already modest easterly flow will weaken and become more
southerly. This will spread shower chances slightly further north,
although we still expect the greatest coverage south of I-70. Storm
motions are also expected to be slightly slower, and still feature
substantial rain rates (1-2 in/hr), which means we may see a few
more pockets of 2-3 inches of rain where the stronger cells persist
a bit longer. Still, given how weakly sheared these storms will be,
individual cells may have trouble lasting for much longer than an 1-
2 hours. As such, while we can`t rule out some nuisance flooding
issues, this is not expected to be widespread.

For tomorrow, the aforementioned vort-max appears to weaken and
drift northeast along with the deepest moisture, carrying the best
chances for more afternoon thunderstorms into Illinois and perhaps
right along the Mississippi River. Similar hazards are expected with
Friday`s showers as well.

As for heat and humidity, temperatures will generally remain near
seasonal averages and similar to what was observed today (upper 80s
to near 90). However, a slight increase in surface humidity
(dewpoints in the low to mid 70s, and locally higher) may nudge heat
index values to near or even slightly above 100 degrees,
particularly in southwest Illinois where the highest dewpoints are
forecast. We will continue to watch the potential for a duration-
driven Heat Advisory (4+ days of 100 degree heat index), but with a
backdoor cold front likely over the weekend, confidence is too low
that we will reach the duration criteria for such a headline.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
Issued at 146 AM CDT Thu Jul 16 2026

From Saturday onward, the synoptic scale upper flow pattern is
expected to be one of persistence, with a building longwave ridge
across the intermountain west, a longwave trough across the Great
Lakes and eastern CONUS, and northwest flow aloft locally as we will
be in between the two. Across the Great Lakes, a series of
reinforcing shortwaves will dig southeast and send weak backdoor
cold fronts into the Mississippi Valley, and the timing/strength of
these features will be the primary focus of sensible weather changes
in our area this weekend through the middle of next week. The first
of these fronts is expected to approach the area sometime over the
weekend, and while there is some timing uncertainty even with this
first front, the most favored timing for this appears to be sometime
Saturday night or early Sunday. In any case, this front does not
appear to be particularly strong and may not even push through our
entire forecast area before it washes out. As such, temperatures
Saturday may even get a slight boost before it arrives, perhaps into
the low-mid 90s, with only a modest "cooldown" Sunday. Meanwhile, we
are also maintaining some low chances (20-40%) for
showers/thunderstorms Saturday afternoon/evening and again Sunday
afternoon/evening, to account for the presence of the boundary and
lingering humidity.

By Monday, ensemble guidance continues to suggest that our area will
be in-between passing shortwaves, with warming southwesterly low-
level flow developing as well. As a result, Monday is slightly
favored to be the warmest day of the next 7, with actual
temperatures perhaps nudging back into the low to mid 90s across
most of the area. As mentioned previously, this development keeps a
few areas in play for 100+ degree heat index values for 4 or more
consecutive days, although Sunday`s modest cooldown may prevent a
lot of areas from technically meeting this criteria. Given that this
is a rather marginal heat event for this time of year in our area,
and that we have already seen significant heat this summer, we will
continue to refrain from issuing headlines until we gain more
confidence in this weekend`s temperatures. Sometime mid-week, it
does appear that a stronger Great Lakes shortwave will send a more
notable cold front southward, but this has consistently been delayed
further and further into the week in ensemble guidance. As
such, while long-range ensemble means do hint at an eventual
cool-down, this may not occur until potentially as late as
Wednesday, and even then confidence is not particularly high
given recent trends. This front will also reintroduce additional
chances for showers and storms next week, but considering the
timing discrepancies, day-to- day precipitation probabilities
remain low overall.

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 944 PM CDT Wed Jul 15 2026

Confidence in fog development has increased enough that impacts
have been mentioned at KCPS and KSUS. The threat at KCOU, KJEF,
KUIN, and KSTL remains low, and if impacts were to occur, would
most likely be less so than at KCPS and KSUS.

Widely scattered showers and thunderstorms are expected during
the afternoon into the early evening. Any of the TAF sites have
the potential for impacts, but given the scattered nature of
this convection, the chance for direct impacts at this lead time
is very low. If impacts are to occur, reduced visibilities are
the main concern.

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...None.
IL...None.

&&

$$

SHORT TERM...BRC
LONG TERM...BRC
AVIATION...Elmore