290
FXUS63 KLSX 282331
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
631 PM CDT Fri Jun 28 2024

.KEY MESSAGES...

- Storms are likely this evening through the overnight hours. A
  few of these storms could be strong to severe with damaging
  winds and large hail the main threats, especially over central
  and northeast Missouri.

- Saturday afternoon into evening storms could be strong to
  severe, mainly along and south of I-70. The primary threats are
  damaging winds and large hail.

- With temperatures in the low to mid 90s on Saturday, expect heat
  index values near 105 over portions of southeast Missouri.
  Therefore, a heat advisory has been issued from Noon to 7 pm.


&&

.SHORT TERM...  (Through Late Saturday Night)
Issued at 257 PM CDT Fri Jun 28 2024

As the next system continues to slide east along the U.S./Canadian
border, the upper level ridge has begun to flatten overhead. A cold
front associated with the system will approach the region, moving
into northwestern Missouri this evening. On the western periphery of
the upper ridge, moisture continues to stream northward into Kansas
and western Missouri with the latest deterministic as well as
ensembles indicating an axis of high MU CAPES 3000-4000 J/kg. This
combined with 0-6km bulk shear of 25-35 kts, increasing LLJ (35-
45kts) and shortwave sliding along the cold front, will see storms
develop and eventually congeal into an MCS over western Missouri.
Then as the LLJ veers with time, the complex will slide east into
the forecast area late this evening and through the overnight hours.
Initially they are likely to be severe in this area, then as they
move into our area, CAPE as well as shear values quickly drop off,
so will see a weakening trend. However, could see a few strong to
severe storms with the main threats being damaging winds up to 60
mph and large hail up to an inch in diameter, especially over
portions of central and northeast Missouri.

The storms will diminish by mid morning on Saturday. Then focus
shifts back to the cold front. Still some uncertainty on exactly
where the boundary will be. The majority of the deterministic as
well as ensembles have the front just north of the I-70 corridor by
midday. Depending on how quickly areas along and south of the cold
front recover from the morning activity, will determine where and
when another round of storms develops. Guidance suggests surface
instability could recover rather quickly with MUCAPE of 3000-4000
J/kg, per the latest forecast soundings, so this combined with
better mid-level lapse rates near 8C, strong to severe storms are
possible Saturday afternoon into evening. The main threats would
be large hail and damaging winds.

As for precipitation amounts, with PWATs of around 2 inches, could
see fairly efficient rainfall, so locally heavy rainfall possible at
times.

One other concern on Saturday is the return of heat and humidity for
a brief period, south of the cold front. With highs in the low to
mid 90s and dewpoints in the mid to upper 70s, could see heat index
values near 105, especially for portions of southeast Missouri. So
issued a Heat advisory for this area from Noon to 7 pm on Saturday.

As the cold front exits the region Saturday night, the precipitation
will taper off from north to south.

Byrd
&&

.LONG TERM...  (Sunday through Next Friday)
Issued at 257 PM CDT Fri Jun 28 2024

By Sunday morning the front will be south of the forecast area.
Showers and a few storms will taper off by mid morning. Then as
surface high pressure builds into the region, much drier and cooler
air will filter in for Sunday and Monday.

The cool down will be fairly brief as the surface ridge slides off
to the east early next week and upper ridging over the western CONUS
moves in. Surface flow will veer to the south once again with well
above normal temperatures returning by Tuesday.

The latest deterministic and ensembles have some differences in
timing, location and strength with a system that slides east out of
the Pacific Northwest. This system will usher in another cold front
by midweek, introducing additional chances for showers and
thunderstorms, some of which could affect the July 4th holiday.
Confidence is low on timing, location, etc. thus stuck with the
latest NBM solution with chance POPs through the period.

Byrd

&&

.AVIATION...  (For the 00z TAFs through 00z Saturday Evening)
Issued at 618 PM CDT Fri Jun 28 2024

Southerly winds will slacken off this evening and attention will
turn to the northwest. Thunderstorms are expected to expand in
coverage into parts of northwest Missouri over the next few hours.
These storms will then move generally eastward, first reaching
KUIN around 0400 UTC and then central Missouri terminals closer to
0600-0700 UTC. For metro St. Louis, arrival time is likely to be
closer to 0900-1000 UTC. Storms should be a bit stronger further
west, so have a TEMPO for IFR visibilities in heavier downpours at
KCOU/KJEF. Further east/southeast, there is more uncertainty in
strength and coverage of storms. The threat for showers and
thunderstorms should depart from west to east late tonight into
early Saturday morning, with a lull into mid-late afternoon. That
is when additional thunderstorms are expected to form along the
cold front. The central Missouri and KUIN terminals should stay
dry as the front will move through earlier in the day (KUIN near
1500 UTC and KCOU/KJEF just after midday). However, the metro
terminals will likely be near the northern edge of this
development so have a 3-hour VCTS group from late afternoon into
the early evening hours. Winds behind the front are expected to
veer to the northwest.

Outside of any thunderstorms, VFR conditions are expected. Any
downpour could reduce visibilities to IFR and cannot rule out some
gusty winds at KCOU/KJEF. There is also a chance of MVFR ceilings
(bases around 2500 feet AGL) behind the front in northeast
Missouri and west-central Illinois mid-late Saturday morning.
Better chances are west of KUIN so held off for now, but did add a
SCT025 mention from 1500-1800 UTC.

Gosselin

&&

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Heat Advisory from noon to 7 PM CDT Saturday for Crawford MO-
     Iron MO-Madison MO-Reynolds MO-Saint Francois MO-Sainte
     Genevieve MO-Washington MO.

IL...None.
&&

$$

WFO LSX