198
FXUS63 KLSX 171744
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1244 PM CDT Thu Jul 17 2025

.KEY MESSAGES...

- A stalled front brings periods of showers and storms today and
  tomorrow. Isolated damaging winds or flash flooding is possible.

- Heat builds this weekend and into next week. A long duration
  heat episode begins as early as Saturday.


&&

.SHORT TERM...  (Through Late Friday Afternoon)
Issued at 247 AM CDT Thu Jul 17 2025

A cold front is currently stretched roughly west to east across the
center of Missouri eastward through Illinois and slowly stalling.
Numerous thunderstorms have formed along and north of the boundary,
especially across Kansas and western Missouri where low level flow
has a greater component up and over the boundary. This activity is
expected to decrease in intensity and coverage through the coming
hours. It`s possible that a remnant MCV is left over from this
convection which plays a role in additional thunderstorms this
afternoon.

The air mass north of the front is quite refreshing. Nearly all of
North Dakota and most of Minnesota is in the 40s at this hour, with
50s as far south as Nebraska and Iowa. This is an impressively cool
and dry air mass for this time of year, but unfortunately the main
push with this air mass is off to the east through the Great Lakes.
The northern half of our forecast area does feel the effects of the
cooler, drier air today with some areas not getting out of the 70s.
These cooler temperatures will be further enabled by extensive cloud
cover leftover from this morning`s convection. South of the front,
though, the heat and humidity continue with highs in the 90s.

With the stalled front around and plenty of instability rebuilding
along and south of it this afternoon, we expect additional rounds of
thunderstorms to develop in the vicinity of the front. Shear remains
fairly weak, but with high instability in a moisture-rich
environment we are likely to see heavy downpours and perhaps a wet
microburst or two. When and where is fairly difficult to ascertain
at this stage as much of today`s activity will be in some ways
related to the ongoing convection and any boundaries or MCV that
emerge from it. The general focus, though, is along and south of
Interstate 70. While there doesn`t seem to be any obvious forcing to
favor training convection (storms which perpetually form and track
over the same locations), precipitable water values in excess of 2
inches indicate very heavy rainfall rates are likely where storms do
form and track. Mesoscale convective evolution may result in a few
areas of more persistent thunderstorms which could lead to flash
flooding. However, confidence in this occurrence, or where it would
occur, is too low for a Flash Flood Watch at this point.

Overall convective activity likely decreases during the evening.
However, if there remains an MCV from earlier storms, then
additional development could continue through the night tonight. The
front remains in the vicinity tomorrow, perhaps lifting a bit to the
north. Thus we expect more rounds of showers and storms on Friday
with a continued threat for primarily locally heavy downpours.

Kimble

&&

.LONG TERM...  (Friday Night through Wednesday)
Issued at 247 AM CDT Thu Jul 17 2025

The primary factor driving our weather this weekend into next week
is a retrograding ridge building westward from the western Atlantic
into the south central US. Heat and humidity builds within the ridge
just as the remnant front gradually lifts back to the northeast. So
we`ll find our area becoming more and more under the influence of
those hot and humid conditions. Significant portions of our area
begin to see afternoon heat index values near or over 100 degrees as
early as Saturday, with the area impacted by heat expanding over the
following days. Initially the northern and eastern portion of our
area will remain affected by daily thunderstorms in the vicinity of
the retreating front and passing shortwave troughs rounding the top
of the ridge. This likely delays the onset of the hot and humid
conditions by a day or two. But eventually by early next week the
ridge becomes dominant across our entire forecast area, with the
heat and humidity affecting the entire region. There`s fairly high
confidence that the heat and humidity lasts through the end of the
week, but it may extend even a bit further. Some guidance shows a
trough moving through the Great Lakes late in the week that brings
relief via a cold front, while others hold the ridge strong into
next weekend too.

As a result, we have increasing confidence in a long duration heat
wave that will bring at least 100+ heat index values to much of the
area for several days to a week or more. We will need Heat
Advisories at some point, but lower confidence at the start of the
heat episode may lead to somewhat of a piecemeal approach to a
gradually expanding advisory as confidence increases on when the
more dangerous heat begins in each location. It`s also possible we
see more extreme heat index values over 105 which may necessitate an
Extreme Heat Warning. However, our confidence in those more extreme
values is lower. If that does occur, it will be more toward the
beginning to middle of next week when the ridge is strongest and its
center closest to our area.

Kimble

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1243 PM CDT Thu Jul 17 2025

As areas of MVFR ceilings scatter this afternoon, redevelopment of
scattered to numerous showers and thunderstorms is also expected.
The greatest chance of thunderstorms is at St. Louis metro terminals
and locations to the south and southeast. Thunderstorms will be
capable of MVFR to IFR flight conditions and possibly gusty winds.
As a front becomes draped across the region tonight, the chance of
showers and thunderstorms will remain present but confidence is
higher in the development of lower ceilings overnight. At least MVFR
ceilings are forecast across portions of the area with more
uncertainty regarding whether IFR ceilings and/or fog also develops,
and their coverage. Ceilings will lift/scatter late Friday
morning/afternoon as the front lifts back north. Another round of
showers and thunderstorms is also anticipated, although confidence
is low in their coverage and location.

Pfahler

&&

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

$$

WFO LSX