NWS St. Louis, MO wrote its last area forecast discussion before the first event at Apr 17, 11:49 AM CST, 3 hours 8 minutes earlier. Its key messages:
Area forecast discussion, Apr 17, 11:49 AM CST, 3 hours 8 minutes before
Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1249 PM CDT Fri Apr 17 2026
.KEY MESSAGES...
- Strong/severe thunderstorms are expected from late afternoon
into the evening, mainly in central/northeast Missouri and west-
central Illinois. Damaging wind gusts are the most likely
hazard, followed by brief tornadoes. Occasional large hail and
localized flash flooding are secondary threats.
- Weather hazards are minimal Saturday through mid week, but a
modest potential (30-50%) for patchy frost exists Sunday
morning.
&&
.SHORT TERM... (Through Late Saturday Afternoon)
Issued at 247 AM CDT Fri Apr 17 2026
With yesterday's showers and thunderstorms concluded, attention
turns squarely onto the potential for severe thunderstorms late this
afternoon through the early overnight hours, which remains on track
for a substantial portion of the area. Strong to severe
thunderstorms continue to be expected in portions of
northeast/central Missouri and west-central Illinois at a minimum,
with damaging straight line-winds the primary threat, followed by a
more limited potential for large hail and tornadoes.
As we approach sunrise, dry but relatively humid conditions have
developed throughout the area after a round of scattered
thunderstorms across the Ozarks yesterday. As the day progresses
though, strengthening southerly low level flow will transport
increasingly rich moisture (surface dewpoints in the mid-upper
60s) northward into much of Missouri ahead of an advancing cold
front, and underneath a plume of steepening mid-level lapse rates
(7-8C). By mid afternoon, the combination of these factors along
with afternoon heating is expected to generate a broad corridor of
substantial instability, with high confidence in 3000-4000 J/kg
of mixed-layer CAPE just ahead of the advancing front by mid-
afternoon. The strongest instability is expected to initially be
located just to our east across northwest and north-central
Missouri, but still- impressive values of 2000-3000 J/kg are also
projected to extend farther east into much of northeast/central MO
and west-central IL. Meanwhile, guidance continues to project
around 30-40kt of 0-6km bulk shear, with long straight-line
hodographs and perhaps some slight curvature in the low levels.
To put it more succinctly, this environment will have more than
enough of the baseline parameters to set the stage for severe
thunderstorms across a broad area.
Exactly what materializes in our portion of this event will depend
on a few more subtle factors. High resolution guidance continues to
suggest that rapid overturning and widespread convective
initiation will occur along and just ahead of the cold front
during the mid- late afternoon period, thanks to both the front
and cooling aloft/increasing divergence from an approaching
shortwave trough. However, exactly where this occurs will depend
largely on the timing, as earlier initiation would likely keep
the vast majority of initial storms to our west. Meanwhile, CAMS
continue to depict a few open warm-sector storms developing in our
area earlier in the day in our area, mainly central/NE MO, but
without an obvious forcing mechanism, along with recent failures
of such activity to materialize during other recent events,
confidence is low that this will occur. However, confidence
remains high that strong/severe storms will reach our area at some
point in the late afternoon/early evening given the ample
instability present, along with a steadily increasing low level
jet during this period.
As for storm mode, initial storms are likely to feature a mix of
modes, with supercells and multicells favored initially. However,
given the strong forcing along the front, numerous updrafts/cold
pools interacting with each other, and bulk shear vectors largely
parallel to the front, we expect that storms will develop upscale
into one or more QLCS' before they arrive or shortly afterward.
As such, damaging straight line wind continues to be the primary
hazard with this event, with perhaps some more significant
straight line winds approaching 70 mph (or more). The potential
for QLCS tornadoes will also exist, particularly along bowing
segments during the early evening that can become more favorably
oriented with 0-3km shear vectors. This will be most favored
during a window in the early evening when strong instability is
still present ahead of a strengthening low level jet and low level
shear/SRH, and LCLS can lower slightly, and while the advancing
line is still strong. Shear parameters are not particularly strong
(0-3km of 30-40kt), but with the right orientation there is
enough there to support a couple of brief tornadoes.
As the evening progresses into the overnight hours, model guidance
continued to suggest that instability will quickly wane, which
should limit the overall severe threat with southeastward extent.
Exactly when and where this occurs, though, is somewhat uncertain.
While we do expect at least some strong/severe storms in our area
during this event, the full geographic coverage is less certain this
far east than it is for our western and northern neighbors. Still,
there is a reasonable potential for severe storms as far southeast
as the I-44 corridor until roughly midnight or so, with quickly
decreasing confidence later and farther southeast. For the remainder
of the event, a weakening convective line with widespread stratiform
rain will be more likely, albeit with lingering lightning and a
limited potential for rogue strong/severe gusts throughout the
night.
Finally, while the advancing line of storms should have enough
forward motion to limit the potential for training and flooding in
most areas, the large parallel component of the wind shear with the
evolving line will maintain at least a limited potential for this,
particularly along the southwestern flank of the line in central
Missouri where forward motion may be a bit slower. LPMM output does
paint around 1.5-2.5 inches of rain in this area, and there are a
few locations around here and closer to I-44 that have already seen
1-3 inches of rain over the past couple of days. While there isn't a
high probability for widespread flash flooding, we can't rule out
some localized issues if storms can orient themselves favorably in
these areas late in the evening and overnight.
As lingering showers and thunderstorms move southeast late tonight
through early tomorrow morning, the cold front will filter in behind
with breezy northwest winds and quickly dropping temperatures, to
the tune of 30 to 45 degrees colder from this afternoon to tomorrow
morning. Persistent northwest winds will maintain these chilly
temperatures through the day, with highs in the mid 50s to mid 60s
Saturday afternoon (about 10 degrees below average). Most areas are
also likely to remain dry throughout the day, but a few showers may
linger across our far southeastern border into the afternoon.
BRC
&&
.LONG TERM... (Saturday Night through Thursday)
Issued at 247 AM CDT Fri Apr 17 2026
After today's thunderstorms and tonight's cold front, a relatively
quiet period of weather is expected through mid-week as northwest
flow aloft slowly transitions into a building ridge. This will lead
to a gradual warming trend through mid-week and mostly dry weather,
with perhaps a return to more active weather late in the period.
Before we get to our warming trend, though, we will need to monitor
the potential for patchy frost Sunday morning, as surface high
pressure finally allows winds to weaken. Current temperature
forecasts suggest morning lows may dip into the mid 30s in some
areas, and thus may not be fully capturing protected valleys where
cool air drainage could lead to some low 30s in a few spots. This
may also be true Monday morning as well, but more likely in our
eastern areas.
Otherwise, the main story through Wednesday will be steadily
increasing temperatures as southerly and southwesterly low level
flow resumes, and an upper ridge reasserts itself. Ensemble guidance
suggests that temperatures will likely climb back to near seasonal
averages by Monday afternoon and slightly above Tuesday and
Wednesday, with relatively narrow ensemble spreads. Meanwhile,
precipitation chances remain low throughout this period, with only a
few hints of isolated showers by mid-week. Until southerly flow can
become more firmly established, moisture return will be quite
limited, thus keeping precipitation chances relatively low through
this period.
However, this may change over the latter half of the week, as
southerly flow continues to bring increasing moisture northward, and
some variation of an upper trough approaches from the west. While
the upper flow pattern becomes more amplified in the extended as
this trough approaches, the timing and strength of this trough
remain highly variable among ensemble members and clusters. Still,
precipitation probabilities do increase substantially Thursday
onward, signaling perhaps a return to a more active period.
BRC
&&
.AVIATION... (For the 18z TAFs through 18z Saturday Afternoon)
Issued at 1235 PM CDT Fri Apr 17 2026
The focus for the TAF period is the potential for impacts from
severe thunderstorms late this afternoon into tonight. A line of
thunderstorms is expected to move into northeast and central
Missouri by 23z (St. Louis metro around ~03z), with gusts up to
70 mph and and a few weak tornadoes possible as it passes. Ahead
of this line, around ~20z through ~23z, a line of broken
thunderstorms may develop and move through central Missouri
northeastward through west-central Illinois. If this occurs, large
hail, damaging winds, and tornadoes will be possible.
The severe threat will end after the associated cold front passes.
Behind the cold front, expect gusty northwest winds up to 30 kts,
stratiform rain, and a few hours of MVFR ceilings. Widespread dry
weather and VFR conditions are expected by Saturday afternoon.
Jaja
&&
.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch from 4 PM CDT this afternoon through late tonight
for Boone MO-Callaway MO-Cole MO-Moniteau MO-Osage MO.
IL...None.
&&
WFO LSX