577
FXUS63 KLSX 252346
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
646 PM CDT Sat May 25 2024

.KEY MESSAGES...

-Severe thunderstorms are expected late tonight through Sunday
 morning, primary threat is damaging winds.

-Severe thunderstorms are expected again during Sunday afternoon
 and evening, with damaging winds, tornadoes, and large hail
 possible.

-Locally heavy rainfall, up to 2-5 inches, is possible with both
 rounds of thunderstorms on Sunday.

-Next week will usher in a pattern change toward drier, more
 seasonal weather.


&&

.SHORT TERM...  (Through Late Sunday Night)
Issued at 334 PM CDT Sat May 25 2024

Surface ridge will continue to slide off to the east this
evening. Otherwise, the upper level trough over the Rockies has
begun to lift out towards the Great Lakes region. The majority of
the latest deterministic as well as ensemble models have good
agreement with the timing, strength and placement of the system as
it moves through the area late tonight through Sunday. As a
surface low deepens in the lee of the Rockies, the accompanying
warm front will lift northward across the forecast area late
tonight. Still expect convection will initiate on the nose of a
strong southerly low level jet (40-60kts) over central Kansas,
advecting in plenty of low level moisture and instability as it
congeals into an MCS. As the low level jet veers, the MCS will
slide east across the region. Still some uncertainty on the exact
track of the complex, with some guidance solutions pushing it
along the I-70 corridor, while others push it across southern
Missouri and Illinois.

As for the threat for severe storms late tonight through midday
Sunday, strong deep layer shear (0 to 6km bulk shear of 40-50kts) as
well as 3000+ J/kg MU CAPE combined with cold pool support from the
MCS will allow portions of the MCS to become strong to severe. The
main threat will be damaging winds and a low threat of tornadoes.

The first round of storms will weaken and exit the region by midday
Sunday. Then there is the matter of a second round which will be
conditional depending on where the atmosphere can recover from the
morning activity. So where ever the outflow from the morning
activity ends up, thunderstorms are expected to redevelop during the
afternoon and into the evening as the mid level trough moves through
and erodes inhibition. Latest RAP soundings have MU CAPEs back
around 3000+ J/kg with 45 to 50 knots of effective shear, which will
support supercells with primary threats of very large hail (2"+)
and damaging winds (up to 80 mph). Even though confidence is
highest that the best location for severe storms will be in the
warm sector south of the outflow boundary, could see enough
recovery north of the outflow where the environment would be quite
favorable for tornadoes, so a few significant tornadoes (EF2 or
stronger) are possible.

Another issue to deal with will be locally heavy rainfall as each
round of storms move through. Yesterday portions of west central and
southwest Illinois received up wards of 3 to 5 inches of rain, with
few locations in excess of 6 inches. If thunderstorms train over
this area, the threat for flash flooding will be greater. If the
storms either don`t train or they move over areas that have not
gotten as much rain, the threat for flash flooding will be lower.
With the possibility of 2 to 5 inches of rain, have issued a flood
watch for areas along and south of a Pittsfield, IL to Monroe City,
MO line from 06z Sunday until 06z Monday.

Byrd
&&

.LONG TERM...  (Monday through Next Saturday)
Issued at 334 PM CDT Sat May 25 2024

Even though dry and mild weather is expected through most of the
work week, another shortwave will rotate around the surface low over
the Great Lakes region. This could bring another round of showers
Monday afternoon, but for now confidence is low. Beyond that,
pattern begins to change late in the work week with next chances for
rain by next weekend.

Byrd

&&

.AVIATION...  (For the 00z TAFs through 00z Sunday Evening)
Issued at 646 PM CDT Sat May 25 2024

The focus for the TAF period will be two rounds of severe
thunderstorms passing through the forecast area. The first will
arrive late tonight/early tomorrow morning. The time of arrival of
this round is trending earlier, so made that adjustment in this TAF
issuance. Although we have prevailing MVFR conditions, a direct
impact from a robust thunderstorm will create brief heavily reduced
flight conditions due to heavy rain. We expect prevailing wind gusts
of 20-25 kts during this round, though stronger thunderstorms may be
capable of producing wind gusts of up to 60 kts.

We expect a break late Sunday morning into the early afternoon
before convection develops ahead of a cold front. This round is also
expected to produce severe thunderstorms with all hazards possible
including up to 70 kt gusts, 2+ inch hail, flash flooding, and
possibly a significant tornado in the strongest thunderstorms.
Again, heavily restricted flight conditions are expected in case of
a direct impact to a terminal. This round will last into Sunday
evening.

The wind field will remain generally southerly for most of the area
through Sunday evening when winds veer to northwesterly behind the
cold front. They`ll also continue to increase tonight and tomorrow
ahead of the front. Wind speed and direction will be impacted by
convection during most of the period, and may not always reflect
the overall pattern.

Jaja

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Flood Watch from 1 AM CDT Sunday through late Sunday night for
     Audrain MO-Boone MO-Callaway MO-Cole MO-Crawford MO-
     Franklin MO-Gasconade MO-Iron MO-Jefferson MO-Lincoln MO-
     Madison MO-Moniteau MO-Monroe MO-Montgomery MO-Osage 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.

IL...Flood Watch from 1 AM CDT Sunday through late Sunday night for
     Bond 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.

&&

$$

WFO LSX