824
FXUS63 KLSX 051916
AFDLSX

Area Forecast Discussion
National Weather Service Saint Louis MO
216 PM CDT Thu Jun 5 2025

.KEY MESSAGES...

- An active weather pattern continues into this weekend with
  multiple chances of showers and thunderstorms. Some of these
  will have the potential to be severe this evening, Friday, and
  possibly Saturday.


&&

.SHORT TERM...  (Through Late Friday Night)
Issued at 216 PM CDT Thu Jun 5 2025

A boundary that has produced showers and thunderstorms in the bi-
state area for the past two days continues to hug our southern and
eastern borders. The now stationary front, according to recent
surface observations, extends from about Springfield, MO into
southern Illinois and north through Paris, IL. A passing shortwave
over the south central Plains and attendant surface cyclogenesis has
caused this boundary to begin to buckle northward over southeast
Missouri, placing portions of Reynolds, Iron, and Madison counties
back within the warm sector. According to the RAP, the environment
in these areas is characterized by 1500 J/kg of weakly capped or un-
capped SBCAPE with bulk shear values of 25 to 30 kts. These
parameters will be sufficient for strong to severe thunderstorms
later this afternoon with damaging winds being the primary threat.
Model soundings show a mixed boundary layer underneath a well
saturated cloud layer and pronounced dry layer in the mid-levels.
This supports accelerating downdrafts aided by rain-cooled air, and
thus damaging winds. Large hail also cannot be ruled out, but I
think damaging winds are the main threat for the day given the lack
of updraft strength from weak lapse rates. These thunderstorms will
diminish after sunset with the loss of daytime instability.

Focus shifts to Friday morning when we anticipate an MCS from the
Plains moving into southern Missouri early Friday morning. Models
are coming to more of a consensus with all CAMs showing the feature
moving through the southern CWA before spreading northeast during
the morning and early afternoon. This will be our best chance at
severe weather with damaging winds and a brief tornado or two being
the primary threats given the storm mode and 0-3 shear vector
orientation. Behind the MCS, isolated to scattered convection is
expected to develop along remnant outflow boundaries mainly during
the afternoon hours. These thunderstorms will have a difficult time
becoming strong in the worked-over air and low lapse rates. Most of
this convection will die after sunset, but an incoming mid-level
shortwave and strengthening low-level jet will keep the chance of
isolated showers and thunderstorms in the forecast overnight.

Otherwise, temperatures will remain on the cool side tonight through
Friday night with lows in the 60s and even touching the upper 50s in
northeast Missouri. Highs Friday will be in the 70s areawide.

Jaja

&&

.LONG TERM...  (Saturday through Next Thursday)
Issued at 216 PM CDT Thu Jun 5 2025

There is another chance for severe weather on Saturday, but it is
highly conditional. At this forecast issuance, two scenarios are
possible:

1) An MCS traverses the CWA in the morning and scours the
environment of instability. Severe weather is not favored.

2) An MCS misses the CWA or isn`t as impactful as in scenario 1.
Severe weather is favored. Ample SBCAPE on the scale of 1500 J/kg
with 40 - 50 kts of bulk shear will be more than enough to create a
favorable environment for strong to severe thunderstorms. Bulk shear
vectors and storm motion both coming out of the west also favors
discrete supercells. Potentially steep lapse rates >7 C/km with a
fat CAPE profile and mixed boundary layer suggest large hail and
damaging winds will both be hazards. Uni-directional shear favors
splitting supercells, but tornadoes can not be ruled out, as cell
mergers will likely be at play. Again, this is one of two scenarios
and is just as likely to occur as the first.

The stationary front responsible for this week`s ire will kick out
early next week, but not without a last round of showers and
thunderstorms. A cold front will drop from the northeast Sunday into
Monday. Moisture convergence will increase along the front as it
heads south, and most guidance has showers and thunderstorms to some
extent within the CWA as it passes. However, strength and coverage
of thunderstorms is still uncertain due to the impact of the next
few days on the environment. What is certain, is that once that cold
is through the area, the weather will quiet down for the area.

The weekend will start out cool with temperatures in the 70s on
Saturday. Sunday looks to be the warmest day until at least mid-week
with some locations south of I-70 hitting the mid-80s. The early
week cold front, due to a lack of strong and persistent CAA, will
not heavily impact the temperature forecast. 70s and low 80s on
Monday will turn to widespread low 80s by Wednesday.

Jaja

&&

.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)
Issued at 1230 PM CDT Thu Jun 5 2025

MVFR and IFR ceilings will keep gradually scattering this
afternoon. VFR conditions will last through the evening before fog
develops in most of the CWA save northeast Missouri. The next
feature of focus will be an MCS moving into the area from the
southwest. If this holds together, it has the best chance at
impacting the St. Louis metropolitan terminals, though I`ve added
PROB30s for mid-Missouri in case it builds or shifts track.
Otherwise, winds remain at or below 5 kts with variable direction.

Jaja

&&

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

$$

WFO LSX