000
FXUS63 KLSX 151128
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
628 AM CDT Mon Apr 15 2024

.KEY MESSAGES...

- Anomalous warmth persists today, with high temperatures
  reaching the mid/upper 80s yet again.

- There is up to a 40% chance for scattered thunderstorms late
  this afternoon until about 10pm across most of the area. One or
  two of these may produce quarter-sized hail and gusty winds, but
  most will be sub-severe.

- Strong to severe thunderstorms are increasingly likely across
  central/northeast Missouri and west-central Illinois Tuesday
  afternoon into the evening, with greater uncertainty further
  south and east. All severe hazards (large hail, damaging wind,
  tornadoes) are possible, with the area mentioned above at risk
  for very large hail and a strong tornado.

&&

.SHORT TERM... (Through Tuesday Night)
Issued at 348 AM CDT Mon Apr 15 2024

Current regional surface analysis depicts a weak east- west
oriented frontal boundary slowly advancing south from northern
Missouri and west-central Illinois. The very weak surface
convergence, mid-level subsidence, and absence of appreciable
moisture is allowing for the front to pass with no sensible
weather besides lowering dewpoints and slightly-cooler air. Strong
subsidence aloft exists across the central CONUS, keeping skies
mostly clear in the Missouri Valley and Upper Mississippi Valley.
Further west, a very amplified cutoff trough across the
Intermountain West is resulting in strong diffluent flow across
the Front Range. Leeside cyclogenesis is underway near the
Colorado/Wyoming border, and this deepening low will be the focus
for our upcoming active weather through early Wednesday morning.

A combination of daytime heating and the previously-mentioned
surface low (to which this weak front is attached) ejecting into the
Great Plains today will allow for the boundary to advance north as a
warm front today. Temperatures will only be marginally impacted by
the presence of the front, with no cold air advection of which to
speak. As such, highs today will climb once more into the mid/upper
80s and prolong our highly-abnormal warmth. While the front will be
largely unimpactful for most of the day given the stronger upper-
level subsidence, the ridge aloft will begin to erode with most
guidance showing a subtle shortwave advancing into the bi-state area
this evening. That subtle cooling aloft will allow for isolated to
scattered convection to develop along and ahead of the front this
evening around sunset. Unless thunderstorms form very close to or
slightly behind the front, they will be elevated and pose mainly a
marginal large hail threat. Instability slowly wanes after sunset,
which will limit the threat for severe weather beyond 10pm.

By early Tuesday morning, the region will be well-within the open
warm sector of the deepening surface low. Aloft, the closed upper-
level low will eject into the central Plains and promote broad
ascent across the Mid-Mississippi Valley. There will some degree of
cloud cover during the morning, and perhaps some convection in
southeast Missouri and southwest Illinois, but nothing widespread or
intense enough to tamper instability for later in the day. There
remains a very tangible threat for severe thunderstorms across the
region, with particular focus still across central/northeast
Missouri and west-central Illinois. This concern manifests when a
dryline approaches the region from western Missouri. Convective
initiation is still forecast during the early afternoon along and
ahead of this feature as upper-level support improves further. Mid-
level lapse rates will increase atop a very warm boundary layer,
which will destabilize the atmosphere and intensify the discrete
convection amidst strong deep-layer shear. Discrete supercells will
race northeast from the boundary into central and northeast Missouri
(as fast as 50-60kts) during the early afternoon, reaching west-
central Illinois in the mid-afternoon.

Across central/northeast Missouri and west-central Illinois, model
hodographs lengthen and exhibit increased curvature into the
early/mid-afternoon in an area of lower LCLs. This environment will
be most favorable for a tornado threat compared to surrounding
areas. If a tornado develops, the stronger effective inflow shear
and storm-relative helicity suggest a potential for it to be on the
stronger side. The steep mid-level lapse rates and strong (but not
overly-robust) instability will also lead to efficient hail growth.
This same general area is also at risk for not only severe hail, but
hail to the size of hen eggs (2.00") at least. Uncertainty regarding
the southeastward extent of the strongest severe thunderstorms still
exists, but intensity decreases gradually further from the upper-
level shortwave. That said, large hail, damaging wind, and
tornadoes are all possible across Missouri and Illinois outside of
the previously-mentioned areas.

More severe thunderstorms may develop closer to the dryline itself
Tuesday evening, prolonging the threat for very large hail, damaging
wind, and tornadoes. Stark differences remain in the location and
coverage of any convection, but the best potential again is across
central/northeast Missouri where the strongest upper-level support
will exist through 12am. Finally, it`s worth noting that non-
convective winds and gusts will be on the stronger side during the
day on Tuesday as the mature cyclone approaches. Wind speeds and
gusts remain below threshold for a Wind Advisory at this point, but
will come very close across the region.

MRB

&&

.LONG TERM... (Wednesday through Sunday)
Issued at 348 AM CDT Mon Apr 15 2024

Wednesday will likely (80% chance) be largely uneventful, as long as
the dryline exits the region at the speed it`s forecast to. Some
guidance does slow it down, leading to redevelopment Wednesday
morning into the early afternoon in southeast Missouri and southwest
Illinois and a threat for more stronger thunderstorms. The best
threat of this is south and east of the forecast area, but it bears
monitoring. Otherwise, temperatures will be cooler areawide but will
vary as the cold front advances through from the west. Highs will
struggle to warm past the mid/upper 60s in northern/central Missouri
and west-central Illinois, while those south and east will see
marginally-warmer values.

Uncertainty still abounds into the latter half of the week with
strong longwave trough centered near the US/Canada border forcing
near-zonal flow across the central CONUS. This pattern will result
in a surface boundary meandering near the region from late Thursday
into the weekend. Shortwave impulses, nearly impossible to time with
any accuracy at this stage, have potential to bring several rounds
of rain to the region along the front. That said, where the front
and shortwaves interact is highly uncertain and key to where rain
falls. With each shortwave, it appears the boundary shunts further
and further south, promoting cooler air than we`ve seen recently
across the region going into the weekend. While some in southeast
Missouri and southwest Illinois may (30% chance) see rain on
Saturday, we dry out Sunday when more defined northwest flow drives
the front further south.

MRB

&&

.AVIATION...  (For the 12z TAFs through 12z Tuesday Morning)
Issued at 609 AM CDT Mon Apr 15 2024

Other than a small area of MVFR stratus across south-central MO,
clear skies exist across the region at this hour. A very weak
front is currently analyzed roughly along the Missouri River
extending into south-central Illinois. The front will advance back
north today, and winds will gradually return out of the south as
they do this afternoon and evening. Isolated showers and
thunderstorms look to develop along and just ahead of the front
this evening. While a few rumbles of thunder can`t be ruled out at
the St. Louis and central Missouri terminals, there`s little
confidence in direct terminal impact given the isolated nature of
the convection. KUIN may see some residual showers, but chances
for thunder are lower.

The warm front advances north of the region Tuesday, and a
tightening pressure gradient along with strong mixing will lead to
gusty south winds during the day on Tuesday. Thunderstorms are
forecast across the region, but beyond the current valid TAF
period.

MRB

&&

.CLIMATE...
Issued at 225 PM CDT Thu Apr 11 2024

DAILY HIGH TEMPERATURE RECORDS

          |=== 4/15 ==|
St. Louis | 89 (2002) |
Columbia  | 90 (1896) |
Quincy    | 88 (2002) |


&&

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

$$

WFO LSX