359
FXUS63 KLSX 012358
AFDLSX

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

.UPDATE...
Issued at 642 PM CDT Wed May 1 2024

A small area of showers and thunderstorms have developed over
sections of southeast Missouri. Support of thunderstorms is
largely being driven by a mid-level disturbance that is running
east-northeast along an advancing warm front. The airmass ahead of
these thunderstorms is unstable with SPC mesoanalysis depicting
near 3000 J/kg of MUCAPE and mid-level lapse rates of 6-6.5C
outside rain-cooled locations. The limiting factor is shear with
20 knots of effective shear being plotted from approximately the
Mississippi River westward.

The 21z RAP in conjunction with the latest (22z) HRRR show this
area of thunderstorms weakening as it approaches the Mississippi
River and dissipating over Metro East communities in Illinois.
The question is just how long they survive, which could have
implications on rainfall in and around St. Louis Metro
communities. Thunderstorms are not expected to become severe with
the overall lack of support. However, there has been enough
instability to result in briefly stronger updrafts producing
small hail (pea size), gusty winds around 40 mph and locally heavy
rainfall. This potential will be greatest over the next couple of
hours before expected weakening after 01z or so.

Maples

&&

.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 00z TAFs through 00z Thursday Evening)
Issued at 626 PM CDT Wed May 1 2024

VFR conditions are expected through much of the period with few
exceptions. The initial concern is showers and thunderstorms that
have developed over southeast Missouri. The limited support they
have to work with begs to question how long they`ll survive.
Potential was held from the TAF package given the uncertainty, but
may need later amendments if they do hold together. Best estimate
of arrival, should they survive, is between 02z-03z.

Outside of this potential, winds become light and variable
overnight before increasing and turning out of the south
Thursday. Gust potential will be greatest near KCOU/KJEF with
increasing chances for showers and thunderstorms through that
afternoon. This potential will continue to gradually move east
into the remainder of the area near or just beyond the end of the
TAF package. Heavier rainfall within some thunderstorms could
result in visibility reductions at times with a mix of VFR/MVFR
and pockets of IFR ceilings spreading west to east Thursday
evening.


Maples

&&

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

$$

WFO LSX