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FXUS63 KLSX 301416 CCA
AFDLSX

Area Forecast Discussion...CORRECTED
National Weather Service Saint Louis MO
916 AM CDT Sun Jun 30 2024

.KEY MESSAGES...

- Cooler temperatures (5-15 degrees below normal) will be
  accompanied by a noticeable drop in dewpoints to result in a
  couple of very pleasant summer days today and Monday.
  Temperatures could near record lows Monday morning in some
  locations.

- Warm, moist conditions return Tuesday into Wednesday. The
  pattern becomes active once again late Tuesday onward with a
  large share of dry time between rounds of showers and
  thunderstorms.


&&

.SHORT TERM...  (Through Late Monday Afternoon)
Issued at 344 AM CDT Sun Jun 30 2024

Regional analysis shows a cold front at the southern fringes of the
forecast area with northerly surface flow that encompasses much of
the CWA. Temperatures and dewpoints continue to drop behind the
front as skies clear from north to south. As of 08z, temperatures
were 70s with dewpoints in the upper 50s to low-60s.

The reprieve from uncomfortably warm, moist air is thanks in large
part to a strong surface high (1026 hPa) that will take control
through Monday as it builds in from the northern Plains. Today`s
northerly surface flow will turn east to southeast Monday, which
draws even drier air into the region to start the work week. Though
there are no impacts to mention, it remains notable that the surface
high is anomalously strong, peaking (1028 hPa) at or near NAEFS
maximum climatological percentiles (99.5+). Dewpoints that drop into
the 40s and 50s tonight into Monday morning, where co-located with
light winds, will allow temperatures to become quite cool. Forecast
lows (upper 50s) around the STL metro could come close to record
lows for July 1st (56 degrees in 1937). It will be a stretch with
the center of the surface high slide to our north. However, roughly
25% of ECMWF ensemble members are within 2 degrees of a record low.
The GFS ensemble is a bit more aggressive with roughly 40% of the
members at or below a record low. Records at Quincy provide a little
more breathing room (52 in 1988), while Columbia isn`t going to come
close at all (45 in 1892). Additionally, NBM probabilities show a
small percentage (5-10%) of members bottoming temperatures out just
below 50 degrees east of I-55 and north of I-70 in Illinois. In
short, Monday morning could be jacket weather for northeast sections
of the forecast area.

Southwesterly mid/upper flow aloft draw moisture northward into the
area Monday morning into early afternoon. Some guidance brings low
chances (around 10 percent) for showers as far east as central
Missouri. However, dewpoint depressions of up to 30 degrees reside
near or below 850mb. This favors dry conditions, so I held PoPs west
of the CWA through the period.

Maples
&&

.LONG TERM...  (Monday Night through Saturday)
Issued at 344 AM CDT Sun Jun 30 2024

There is expected to be an eastward shift in the overall ridging
pattern late Monday into early Tuesday. Meanwhile, a broad upper
trough progresses west to east along the Canadian/U.S. border. While
south to southwesterly flow advects warm, moist air northward out of
the western Gulf and into the Plains, the orientation of the stacked
eastern ridge will make this process more gradual Tuesday into
Wednesday. Mid-level 850mb temperatures of around 20C encroach on
the Mississippi River Tuesday morning as warmer air holds over the
Plains at least temporarily. Temperatures do begin to warm with 90s
favored along and west of the Mississippi River, combining with
dewpoints in the low-70s for heat index values to reach near 100
Tuesday afternoon. Though warmer (mid to upper 80s) than Monday,
dewpoints in the 50s over interior section of Illinois should make
it more comfortable.

Late Tuesday into early Wednesday begins to get a little interesting
as a cold front drops into Iowa and extends southwestward into
northwest Missouri. The strong pressure gradient that lines up
between the ridge to the east and the approaching trough results in
intensifying jet dynamics as a shortwave rotates around the southern
edge of the upper trough. Copious moisture continues to feed
northward around the western periphery of the ridge as a strong low
level jet of 40-50 knots develops Tuesday night, extending from the
south-central Plains into the Great Lakes. There are some
differences in the position of the front between individual
guidance. The general theme is for convective initiation to occur
over Iowa Tuesday evening into Tuesday night. The front then sinks
southward into the CWA sometime late Tuesday night into Wednesday
morning before stalling. The setup looks to support severe
thunderstorms to the north, but the initial placement and timing of
the surface convergence will be key in how far south the severe
threat reaches. This will continue to be monitored over the next
couple of days to fine tune the spatial distribution of the greatest
threats with the potential for all hazards to be in play.

This boundary then has a play on the area from mid to late week,
including the Fourth of July holiday. Global guidance shows the
boundary stalling, becoming west-east oriented as multiple
shortwaves rotate around the parent upper low over southern Canada.
This supports thunderstorm potential along the front to some
magnitude during the day Wednesday. The strongest of the shortwaves
moves overhead late Wednesday night into Thursday morning, providing
ample upper ascent over the surface boundary, all within a region of
deep layer moisture. PWAT values of 2+ inches look to be 2-3
standard deviations above normal, which points to heavy rainfall the
night of the 3rd and heading into the 4th.

Additional thunderstorms are possible late in the week. Fortunately,
global guidance shows an amplified trough pushing the front south of
the area heading into next week, but not before yet another round of
thunderstorms late Thursday into Friday as another cold front
approaches the region. Ahead of the front, scattered, diurnally
driven thunderstorms could impact the Fourth of July. Given the
distance in time, finer details will become more clear to nail down
timing as the holiday approaches.

Maples
&&

.AVIATION...  (For the 12z TAFs through 12z Monday Morning)
Issued at 610 AM CDT Sun Jun 30 2024

VFR conditions are anticipated throughout the TAF period.
Light northerly surface flow will gradually turn out of the east
tonight into early Monday.

Maples
&&

.CLIMATE...
Issued at 344 AM CDT Sun Jun 30 2024

Record Lows
                   7/1

St. Louis      56 in 1937

Columbia       45 in 1892

Quincy         52 in 1988


&&

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

$$
WFO LSX