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FXUS63 KLSX 152255
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
555 PM CDT Fri May 15 2026

.KEY MESSAGES...

- A cluster or broken line of showers and thunderstorms is
  expected to move southeast into the area after midnight. The
  strongest storms could produce a few damaging wind gusts and 1
  inch hail.

- Another round of marginally severe storms is possible Saturday
  afternoon and early evening. Some storms may also produce
  locally heavy rainfall.

- More widespread severe thunderstorms will be possible again
  early next week, most likely Monday and Tuesday as a cold front
  moves slowly through the area.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 359 PM CDT Fri May 15 2026

The primary concern in the short term is thunderstorms tonight and
tomorrow and their potential for severe weather and possibly flash
flooding.  Guidance blows up thunderstorms across Iowa and eastern
Nebraska this evening on the nose of a strengthening low level jet.
Discrete storms grow upscale into an MCS which moves into northeast
Missouri and west central Illinois after midnight.  Instability and
shear aren`t exceptionally favorable for severe storms at 1000-1500
J/Kg MUCAPE and 25-30kts of 0-6km shear, so the MCS should be
weakening as it moves into our area.  Nevertheless, there is an
isolated damaging wind/large hail threat overnight, north of the I-
70 corridor, and mainly over northeast Missouri into west central
Illinois where storms will likely be strongest.  The GFS and RAP
build up the instability again Saturday afternoon, mainly across
northeast Missouri and west central Illinois again, with CAPE values
in excess of 2000 J/Kg.  Once again the deep layer shear is in the
25-30kt range in the afternoon.  It does increase across northern
Missouri early in the evening, but CAPE falls off quickly after
sunset.  The stronger instability in the afternoon and then the
brief combination of stronger shear but weaker instability in the
evening will be good enough for another limited large hail/damaging
wind threat.  This severe threat will also be contingent on enough
daytime heating to realize those CAPE values, and it`s possible that
scattered showers and thunderstorms with considerable cloud cover
could keep temperatures in the 70s.  This would likely severely
limit the amount of instability available for storms.

In addition to the severe weather threat, the RAP and GFS are
hinting at the possibility for training thunderstorms with locally
heavy rain across northern Missouri and west central Illinois which
could lead to flash flooding Saturday afternoon and evening.
Forecast soundings show freezing levels around 12,000 ft with
precipitable water values up to 1.75+ inches. Both models print out
a bullseye of around 2 inches of QPF either in our west central
Illinois counties, or just north of them Saturday afternoon/evening.
 Several CAMs also show training thunderstorms, although the
positions of the storms is not consistent with some over Iowa,
others in northern Missouri, and one as far south as central
Missouri.  The HREF 24hr LPMM QPF shows a stripe of 2-4+ inches
along and south of I-70 stretching from Warrensburg to Montgomery
City by 06Z Sunday, and another two bullseyes in north central
Missouri and west central Illinois.  The inconsistencies in these
forecasts do not lead to particularly high confidence in a heavy
rain forecast, however there is enough of a signal to be concerned
and a Flood Watch may be needed if guidance comes into better
agreement.

Carney

&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 359 PM CDT Fri May 15 2026

The upper pattern becomes more amplified on Sunday as a long wave
trough deepens over the western U.S. and a ridge builds over the
east.  A short wave ejects from the base of the trough forcing lee-
side cyclogenesis over the Plains  which enhances the south-
southwest low level flow resulting in strong warm advection.  850mb
temps rise to 16-18C by Sunday afternoon across the Mid Mississippi
Valley which mixes down to the mid to upper 80s.  A mostly clear sky
and strong May sunshine will also help heat things up so the going
forecast of upper 80s to around 90 still looks good.

Another short wave ejects northeast across the Plains on Monday
which develops another low that pushes a cold front into Eastern
Kansas/Nebraska and western Iowa.  Models continue to develop
thunderstorms along the front Monday afternoon that move east
through Monday night.  The deterministic GFS and ECMWF show
instability dropping off fairly quickly in the evening across our
area ahead of the storms.  This should lead to a weakening trend in
the storms as they move into central and northeast Missouri,
although there should still be sufficient instability ahead of the
convection that severe storms will remain possible.  This agrees
well with SPC`s Day 4 Severe Weather Outlook which drops from 30%
across northwest/north central Missouri down to 15% in northeast and
central Missouri as well as west central Illinois.

Timing the synoptic front remains uncertain as it will be moving
fairly slowly through Missouri and Illinois.  The deterministic GFS
and ECMWF are in very good agreement for the 12Z runs this morning
with the front stretching to the southwest from near Quincy to near
Columbia Missouri by 18Z Tuesday.  Unfortunately this would allow
the atmosphere to recharge after Monday night`s storms.  The EC
produces between 1500-2000 J/Kg of CAPE at 18Z ahead of the front,
while the GFS shows as much as 2500-3000 J/Kg.  However the deep
layer shear ahead of the front is only 25-30kts.  While there will
probably be some severe storms due to the moderate to strong
instability, the weaker shear should cut their intensity somewhat.
However, the LREF cluster analysis is showing variance in the speed
of the upper trough which will push the cold front through the area.
 Even though the deterministic guidance is in good agreement, this
variance lends a good deal of uncertainty to the timing of the
front.  A faster FROPA would lead to a lower severe weather threat
for most if not all of the area than discussed above.  A slower
front could allow for even warmer temperatures on Tuesday which would
lead to more instability and a higher threat.

The remainder of the week looks cooler and dry as high pressure
moves across the Mississippi Valley.  High temperatures drop back to
near normal in the 70s on Thursday with overnight lows in the 50s.
Temperatures begin to warm on Friday as southerly flow returns to the
area...although the extent of the warm up is uncertain.  LREF high
temperature IQRs double from around 5 degrees on Wednesday and
Thursday to 10 degrees on Friday due to what looks like some
variance in the position of an upper level ridge and the next up
stream short wave trough.

Carney

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 545 PM CDT Fri May 15 2026

Periods of active weather are expected during the 00Z TAF period,
most notably one or more rounds of showers and thunderstorms
between late tonight and tomorrow afternoon. While a few showers
are lingering in parts of the area at the start of the period,
most of this initial activity will remain south of local
terminals. Late tonight, more widespread and robust showers and at
least a few thunderstorms will move into the region, potentially
impacting all of our local TAF sites between late tonight and
early tomorrow afternoon. Visibility reductions due to heavy rain
and gusty winds will be the most likely hazards from these storms,
in addition to lightning.

This activity will likely diminish during the early-mid
afternoon, and areas are more likely than not to remain dry the
rest of the period. However, there is a low probability potential
for redeveloping showers and storms during the mid-late afternoon
along I-70, which is represented by a new PROB30 group. Confidence
is relatively low that this latter round of convection will
materialize.

&&

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

$$

WFO LSX