431
FXUS63 KLSX 031130
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
630 AM CDT Mon Jun 3 2024

.KEY MESSAGES...

- An active weather pattern, marked by numerous rounds of
  thunderstorms in various parts of the region and above-normal
  temperatures, will exist through Tuesday night. A few of these
  thunderstorms may be marginally-severe this afternoon and
  Tuesday afternoon.

- Heavy rain from thunderstorms today and Tuesday will threaten
  isolated flash flooding, but exactly where remains uncertain.

- Starting mid-week, a pattern change marked by somewhat-cooler
  temperatures and lower (but non-zero) precipitation chances,
  will take hold through the weekend.

&&

.SHORT TERM...  (Through Late Tuesday Afternoon)
Issued at 302 AM CDT Mon Jun 3 2024

A mix of mid-level clouds and cirrus from various areas of
convection exist overhead this morning, with an MCV from an
overnight complex tracking into Iowa and stimulating weak
convection in northern Missouri. A stationary front is draped
from north to south in Missouri from a weak surface low in the
northern Plains, forced by a weak shortwave amidst quasi-zonal
flow aloft. On the warm side of the boundary, MLCAPE values
approaching 1500-2000 J/kg and higher precipitable water values
(near the 90th climatological percentile) are analyzed on the SPC
Hourly Mesoscale Analysis. Today`s convective evolution is still
quite a bit uncertain, with run-to-run inconsistencies abounding
among convective-allowing models. That said, the stationary front
is forecast to track east this morning, enveloping the region in
warm (a few degrees above normal for early June), moist, and
unstable air.

This environment will be ripe for at least isolated deep
convection, with SBCAPE values forecast to be at least 1500-2000
J/kg in the more conservative estimates, but where CI occurs will
depend on mesoscale boundaries other areas of low-level lift. With
nothing synoptic in scale to focus convection, thunderstorm
location and timing may not be known until within a few hours of
CI. That said, there is general consensus that the best chances
will exist along a remnant outflow boundary in the warm sector
that tracks from west to east this afternoon. With unimpressive
flow aloft, resultant weak deep-layer shear will preclude much
threat for more than isolated strong to severe winds and
marginally-large hail in the strongest convection this afternoon.

Anomalously-high PWATs will maintain some threat for locally-heavy
rain and isolated instances of flash flooding, but the greatest
threat is shifting south compared to the previous forecast. Model
soundings now highlight layers of drier air aloft that would
entrain into thunderstorms and lower overall efficiency. That
said, HREF LPMMs (showing a worst-case magnitude scenario of
sorts) continue to highlight isolated, yet alarming values of
3-5"+. There`s also greater uncertainty of nocturnal convection in
general, further throwing this aspect of the forecast into doubt.
Because of the uncertain precipitation placement and increasing
uncertainty for impacts, no Flood Watch was issued. However,
isolated flash flooding is still a real threat overnight tonight
into Tuesday along and west of the Mississippi River if convection
can persist.

A similar situation develops on Tuesday: anomalously-warm, moist,
and unstable air remains across the region with a low threat for
strong to marginally-severe thunderstorms during the late morning
and afternoon. The SPC Day 2 Convective Outlook highlights a
majority of our counties in Missouri with a Marginal Risk, but the
threat may extend further east into Illinois as well. That said,
there are some subtle differences in the shear parameter space
(stronger further west) that may lead to some spatial stratification
in the threat.

MRB

&&

.LONG TERM...  (Tuesday Night through Sunday)
Issued at 302 AM CDT Mon Jun 3 2024

Starting Tuesday morning, a well-defined shortwave will traverse the
U.S.-Canada border and intensify as it approaches the Upper
Mississippi Valley through the Great Lakes late Tuesday. At the
surface, a low will take a similar track and draw a cold front down
into the bi-state region, sparking more widespread showers and a few
thunderstorms overnight Tuesday. While severe weather is still
not forecast with this feature, it will mark the end of our more
active weather pattern and usher in drier air at the surface and
aloft for Wednesday. Current deterministic and ensemble guidance
agree that this first cold front will actually not feature a
drastic temperature change, with colder 850mb temperatures being
held back across the Great Lakes and Upper Mississippi Valley.
However, the drier air will be an unusual respite from the typical
humidity of early June. A secondary, reinforcing cold front looks
to cross through the region late Wednesday into early Thursday
amidst the cyclonic flow aloft. While there is a low threat for
rain with this feature, the low- level convergence is weak and
moisture availability will not be robust. More notably, this
front will further scour the atmosphere of moisture, potentially
sending dewpoints to around the 25th climatological percentile by
the end of the work week.

Differences in the exact evolution, amplitude, and position of the
ridge-trough pattern still exist in the global-scale ensemble suite,
which will impact temperatures and precipitation chances into the
weekend. The low-end potential for high temperatures Friday through
Sunday are a few degrees warmer than previous runs, suggesting the
coldest air stays to our north and guidance consensus migrating
to a warmer (near-normal) solution. That said, there are still
noteworthy differences in the flow pattern that preclude much
certainty. The pesky, low-end rain chances (15-35%) still exist
from early Saturday through Monday owing to a few scenarios that
lead to precipitation. If we stay amplified aloft, as some members
suggest, we may see lobes of vorticity in the cyclonic flow
threaten rain from the north and west. Others favor a more west-
northwest, less amplified pattern that opens us up to shortwaves
and rain chances from the west. There are a few that keep us
completely dry, but there is increasing doubt that we are rain-
free through the weekend into early next week.

MRB

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 621 AM CDT Mon Jun 3 2024

VFR conditions currently prevail at the terminals, with the main
aviation concerns for the TAF period surrounding the threat for
thunderstorms this afternoon and early evening. While the
thunderstorm timing at any one location is relatively uncertain
still, the best chances for thunderstorms will be this afternoon
and evening from west to east. While earlier forecasts kept VCTS
well into the overnight hours, there`s little confidence that this
is realistic anymore and thus was removed. That said, I cannot
rule out a few thunderstorms overnight in the region overall.

MVFR CIGs will enter the region from the south tonight, impacting
all TAF sites except KUIN. Given how much low-level moisture will
exist, fuel-alternate conditions or even IFR may occur, but those
conditions are rare this time of year and were not included in the
forecast. Another round of thunderstorms is possible after the
end of most of the TAFs, and near the end of KSTL`s TAF, but
timing confidence for this round is even lower than for today.

MRB

&&

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

$$

WFO LSX