275
FXUS63 KLSX 020357
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1057 PM CDT Wed May 1 2024

.KEY MESSAGES...

- An active pattern will promote multiple chances for showers and
  thunderstorms throughout the next week, though a majority of
  this time will be dry.

- The threat for strong to marginally-severe thunderstorms remains
  in place tomorrow, and is expanded a bit further east compared
  to earlier thinking. Confidence remains low (less than 30%) that
  any severe weather occurs, but large hail to 1 inch and damaging
  wind to 60mph would be the threats if storms can become strong
  enough.


&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 246 PM CDT Wed May 1 2024

A weak cold front associated with a shortwave aloft dropped south
this morning across the region, but with little in the way of
cold air aloft, a wind shift was the only sensible weather
associated with it. The region is more broadly situated under weak
ridging between two waves, one to our northwest and one to our
southeast. The front will begin to inch northward late this
afternoon as the shortwave exits and a surface low in the central
Plains slowly tracks northeast. While surface-based instability is
currently in the 1500-2000 J/kg range at least, convergence along
the front is meager at best. With little in the way of upper-
level lift or forcing to support deeper, more organized
convection, weak isolated to widely-scattered thunderstorms are
still forecast this afternoon and early evening.

The front advances north with greater speed overnight when the low-
level jet intensifies, but by that point all indications are that
the jet (and any low-level forcing for thunderstorms to persist)
shunts far north of the region into the Upper Mississippi Valley. As
such, while a rogue thunderstorm or two cannot be ruled out
overnight in northern Missouri, the region will stay almost
completely dry. 850mb/925mb warm air advection intensifies overnight
in northern Missouri, but we`re unable to realize much of that
warmth at the surface. By the time we can mix more deeply on Friday,
the warm advection becomes far more muted. While southerly surface
winds and some degree of insolation will help keep temperatures
above normal (mid-70s to mid-80s from northwest to southeast), more
extreme values will stay out of our reach.

Convection overnight in the central Plains will eject into northwest
Missouri, gradually tracking east and weakening by it arrives in our
northern forecast area around sunrise. How widespread any convective
debris will be may have some impact on our instability later in the
afternoon. Nevertheless, instability redevelops ahead of a slowly-
approaching, weak surface cold front from the west. Aloft, a modest
shortwave embedded in the upper-level ridge will also drift into the
Mid-Mississippi Valley during the late morning. With some degree of
mid-level ascent, albeit weak, in place, a more tangible threat for
strong to marginally-severe thunderstorms across most of the
forecast area during the afternoon and very early evening. Barring a
robust cold pool from the morning convection eliminating
instability, which is highly unlikely (less than 10% chance), MLCAPE
values are forecast to jump into the 750-1250 J/kg range across the
region. Particular focus for the most appreciable uncapped
instability is in central/northeast Missouri and west-central
Illinois, but areas as far east as the St. Louis metropolitan area
cannot be ruled out for a stronger thunderstorm. The one noteworthy
limit to the severe weather potential is a lack of deep-layer shear.
If convection manages to sustain itself, large hail to 1 inch and
damaging wind to 60mph would be the predominant threats. By sunset,
instability will wane and without a low-level jet to promote
continued forcing, thunderstorm strength will drop considerably.
High rain chances (upwards of 80%) continue overnight, but these may
be a bit too high given the very weak forcing available.

MRB

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 246 PM CDT Wed May 1 2024

The weak cold front barely makes it through the forecast area on
Friday, and the continued unimpressive forcing combined with gradual
height rises aloft will allow only weak showers and isolated
thunderstorms to develop early Friday. The more noteworthy impact
from the boundary will be the relatively-cooler (but still near-
normal) air it brings in its wake. Cool easterly winds usher in
enough dry air to keep most of the region rain-free from Friday
afternoon to Saturday morning before another wave glances the
region. This is the first of a series of weak waves that threaten
the region, with a cutoff low stalled over the northern High Plains
and ridging over the southeast CONUS promoting generally southwest
flow aloft. With subtle timing and amplitude differences leading to
large impacts on sensible weather, there`s not a lot of confidence
in the exact timing of any rain chances from Friday through the
weekend. However, the forcing looks consistently weak and transient,
and the region can expect plenty of dry time until early next week.
Any rain that does fall will not have immediate impacts on the
rivers at this stage, though heavier rain across the Missouri River
Basin may eventually lead to secondary rises.

By Sunday, global-scale ensembles converge on a trough deepening
once more in the Intermountain West, promoting broad, diffluent
southwest flow aloft and setting up another potentially-active
weather pattern in the central CONUS. At the same time, more
pronounced southerly flow in the boundary layer and resultant warm
air advection will bolster temperatures back into the low/mid-80s
yet again across most of the region through early next week. Exactly
how warm we get will come down to rain timing and cloud cover, which
isn`t all that certain at this point. While the synoptic-scale
pattern will be more favorable for stronger thunderstorms to develop
early next week, the devil is always in the details and there are
far too many differences in the ensemble guidance to be confident in
much beyond rain chances at that stage.

MRB

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1055 PM CDT Wed May 1 2024

An area of weakening showers continues to move east of the
Mississippi River this evening. Light and variable winds will
accompany clearing skies behind the departing precipitation with
VFR conditions continuing into late tonight.

Showers and thunderstorms over the central Plains will move into
western Missouri through early Thursday morning and eventually
impact KCOU/KJEF/KUIN by early afternoon. Thunderstorms are
expected to be weakening as they progress east with thickening
cloud cover remaining at VFR levels.

A weak cold front approaches the region later in the day,
bringing the potential for redevelopment in the mid to late
afternoon. The extent of redevelopment will be dependent on how
long morning convection hangs on. Nonetheless, guidance favors
MVFR ceilings running ahead of the front with IFR along and just
behind the front. Pockets of heavier rainfall may be accompanied
by gusty winds, frequent lightning and lower visibilities. IFR
 increase after 05z Thurs night/Friday morning. This potential
will have to be monitored in for later TAF package updates.

Maples

&&

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

$$

WFO LSX