O'Fallon, Missouri 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

982
FXUS63 KLSX 021152
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
652 AM CDT Wed Apr 2 2025

.KEY MESSAGES...

- Severe thunderstorms are likely today. All hazards are possible
  including damaging winds in excess of 70 mph, hail in excess of
  2 inches in diameter, and a few tornadoes, some possible strong
  (EF-2+).

- Periods of heavy rain are expected Thursday through Saturday.
  Significant river and flash flooding are possible across parts
  of southeast Missouri and southwest/south central Illinois where
  a Flash Flood Watch is in effect for Thursday through Saturday
  evening.

- Gusty southerly winds are forecast today, with peak gusts of
  40-45 mph expected. A Wind Advisory is in effect for eastern
  Missouri and southwest/south central Illinois.


&&

.SHORT TERM...  (Through Late This Afternoon)
Issued at 416 AM CDT Wed Apr 2 2025

Low pressure centered over eastern Kansas will lift northeast into
the Upper Midwest today.  The warm front associated with this low is
moving into central and southeast Missouri at this time, and should
lift through the remainder of the area by 14-16Z.  The strong
pressure gradient between storm system lifting northeast and
strong/deep ridging over the eastern U.S. and into the western
Atlantic will produce very strong wind fields through the depth of
the atmosphere today.  This sets the stage for 1) strong surface
winds today...sustained in the 20-30kt range, with gusts in excess
of 40kts. The Wind Advisory therefore looks in good shape, and no
changes are expected at the moment.

Attention turns to convection today.  All CAMs show the broken line
of thunderstorms over eastern Kansas/northwest Missouri moving into
our northeast Missouri counties around 11-12Z.  Forecast soundings
as well as plan view CINH plots have been very consistently showing
that these storms will be elevated, with the primary threat being
large hail.  However, the strong wind fields will make it impossible
to rule out an isolated damaging wind threat. CAMs show the
morning convection weakening, although not entirely dissipating.
This potential for clouds and showers limiting daytime heating
remains the one potential issue with storms restrengthening in the
afternoon. However, the models are pretty insistent that this
will NOT be a hindrance, and instability will increase this
afternoon ahead of the front. The boundary itself will not be
moving very quickly due to the parent low pressure`s northeastward
movement, and the strong south-southwest flow ahead of it. RAP
and GFS show destabilization ahead of the front between 16-19Z
west of the Mississippi River, and CINH decreasing to nearly
nothing in the same area due to a combination of heating and
subtle cooling aloft. The RAP is showing around 1700 J/Kg MLCAPE
and 60-70kts of 0-6km shear at that time. While the majority of
the shear is linear, hodographs do show a little curvature in the
lowest kilometer of the sounding, and 0-1km helicity 200-300
m2/s2. With these kinematics and thermodynamics in play, all
severe threats are on the table including damaging winds in
excess of 70 mph, hail in excess of 2 inches, and tornadoes, some
strong (EF2+). The linear 0-6km hodographs hint there will be
lots of storm splits and mergers so this still looks messy, with
few if any discrete storms. More likely are lines and clusters of
supercells that move east through the afternoon. While the
strongest storms will likely be out of our area by 00Z or shortly
there after, convection may linger across parts of south central
Illinois well into the evening until the slow- moving boundary
finally clears the area.

Carney

&&

.LONG TERM...  (Tonight through Tuesday)
Issued at 416 AM CDT Wed Apr 2 2025

The primary concern beyond today remains the threat for waves
of heavy rain and flooding Thursday through Saturday.  Little has
changed in the forecast for this time period.  The effective
boundary gets pushed into Arkansas/Tennessee tonight where it takes
on an east-west orientation and becomes quasi-stationary.  The deep
long wave trough over the Desert Southwest ejects several subtle
waves in mid-upper level southwest flow across the lower/mid
Mississippi Valley which ride over the boundary.  These short waves
combined with strong and persistent warm/moist low level flow from
the Gulf overrunning the boundary will produce waves of showers and
thunderstorms.  The first, and probably least impactful wave of rain
still looks to develop and move into the forecast area on Thursday,
and persist into Thursday night.  Latest QPF from WPC shows less
than an inch in our area for this first round which matches well with
guidance.  While we should be able to absorb this much rainfall
pretty easily, it will prime the pump so to speak for Friday and
Saturday.  The persistent southerly low level flow pushes the
boundary up into northern Arkansas/southern Missouri on Friday
morning, bringing another surge of moisture with it.  The true heavy
rainfall threat begins at this point, and it looks like nearly
continuous rain for most of the area along and south of I-70 until
Sunday morning.  Highest rainfall amounts are still focused
generally along and southeast of I-44 in Missouri and I-70 in
Illinois where WPC QPF is 5-9 inches.  This agrees well with 5+ inch
probabilities from ensemble guidance.  Locally higher amounts are
certainly possible and moderate to major river flooding in southeast
Missouri continues to be a threat along with significant flash
flooding.  The Flash Flood Watch still looks like it`s in good
shape, but it may need to be expanded northward if the axis of heavy
rain shifts.

Briefly, for Sunday through Tuesday...

The long wave trough over the southwest U.S. finally shifts east
across the country which finally brings the rain to an end on
Sunday.  THe trough in the latest guidance doesn`t look quite as
persistent over the eastern U.S. as in previous runs...so the warm
up next week could be faster than forecast.  However, freezing
temperatures for parts of the area remain in the forecast for Monday
and Tuesday morning.

Carney

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 641 AM CDT Wed Apr 2 2025

Challenging forecast today with a line of weakening thunderstorms
moving into northeast and central Missouri this morning. The line
is expected to continue moving east through the morning, and could
partially dissipate before restrengthening this afternoon.
Thunderstorms this morning will be capable of producing 30-40kt
wind gusts and IFR conditions in heavy rain. Afternoon storms are
expected to be stronger with 50+ kt wind gusts, large hail and a
few tornadoes. Outside of storms, strong south-southwest flow
between 20-30kts with gusts of 30-40kts are expected until late
afternoon early evening. Some patches of MVFR clouds are likely
today outside of thunderstorms, otherwise VFR conditions are
generally expected to prevail.

Carney

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Wind Advisory from noon today to 7 PM CDT this evening for
     Franklin MO-Iron MO-Jefferson MO-Knox MO-Lewis MO-Lincoln
     MO-Madison MO-Marion MO-Pike MO-Ralls MO-Reynolds MO-Saint
     Charles MO-Saint Francois MO-Saint Louis City MO-Saint
     Louis MO-Sainte Genevieve MO-Warren MO-Washington MO.

     Flood Watch from Thursday morning through late Saturday night
     for Crawford MO-Iron MO-Jefferson MO-Madison MO-Reynolds MO-
     Saint Francois MO-Sainte Genevieve MO-Washington MO.

IL...Wind Advisory from noon today to 7 PM CDT this evening for Adams
     IL-Bond IL-Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene
     IL-Jersey IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-
     Montgomery IL-Pike IL-Randolph IL-Saint Clair IL-Washington
     IL.

     Flood Watch from Thursday morning through late Saturday night
     for Bond IL-Clinton IL-Fayette IL-Marion IL-Monroe IL-
     Randolph IL-Saint Clair IL-Washington IL.

&&

$$

WFO LSX