850
FXUS63 KLSX 150331
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1031 PM CDT Wed May 14 2025

.KEY MESSAGES...

- The warmest day so far this calendar year is forecast on
  Thursday as highs reach into the mid 80s to low 90s. Daily
  records may be threatened at all 3 of our official climate
  stations (KSTL, KCOU, and KUIN).

- There is a highly conditional threat of a few severe
  thunderstorms late Thursday afternoon/early evening, mainly
  across south-central Illinois.

- More widespread severe thunderstorms are possible on Friday
  afternoon/early evening, with the potential for all severe
  hazards.


&&

.SHORT TERM...  (Through Late Thursday Night)
Issued at 205 PM CDT Wed May 14 2025

A diffuse synoptic warm front is currently across northern Arkansas
and is forecast to move northward through the area tonight. Along
and north of this feature, broad/weak low-level moisture convergence
may help initiate some isolated showers and thunderstorms through
late this evening. The remainder of the night is expected to be dry
as the axis of low-level moisture convergence centered near 850 hPa
lifts quickly off to the northeast. Mild conditions are expected
overnight as winds veer more to the south and begin to increase in
speed heading toward dawn. Lows in the mid to upper 60s are
forecast, or about 10-15 degrees above normal for the date.

Anomalous warmth still looks like the main story for Thursday as an
axis of +20 to +24C air at 850 hPa advects across the CWA. Those 850
hPa temperatures are very rare this early in the year, and are
generally about +3 sigma above normal and near daily records for the
date. Plenty of sunshine is also expected during the morning/early
afternoon hours, with surface winds veering a bit more to the
southwest ahead of a Pacific cold front. This veering of the surface
winds is more favorable for downslope winds off of the Ozark
Plateau, though would prefer another 20 degrees or so (more
west/southwest) for maximum warming. Nevertheless, the conditions
tomorrow are certainly ripe for a very warm day as highs are
expected to top out in the mid 80s to low 90s. Daily records may
be threatened at all 3 of our official climate stations (KSTL,
KCOU, and KUIN), but the current forecast falls about 2-4F short.
Please see the CLIMO section below for more details.

Another concern for Thursday afternoon/early evening is on the
potential for convective initiation along the weak Pacific cold
front. There continues to be many factors working against any
convection from developing, including: 1) strong capping between 800
and 875 hPa, 2) weak surface convergence along the Pacific cold
front, 3) stronger mid/upper level forcing for ascent staying well
to our north, and 4) lack of mid/upper level height falls. These
factors all cast a substantial amount of doubt on if any
thunderstorms will develop along the front in our area Thursday
afternoon/early evening. Most CAMs are very quiet, though a couple
of 12Z or 0Z CAMs from last night have shown a couple of supercells
in south-central Illinois. There is potential for severe weather
given very high CAPE (a whopping 3500-5000 J/kg) and 35-45 knots of
deep-layer shear. Supercells with very large hail, damaging winds,
and perhaps even a tornado are all possible with any storms if they
develop in our area. The threat looks very narrow in time and space,
with this highly conditional threat likely confined to portions of
south-central Illinois over a 1-2 hour period (likely sometime
between 4 and 8 PM). CAMs and even coarser resolution guidance
have some activity in southeast Missouri and southwest Illinois
during the afternoon/early evening hours, but this appears to be
disorganized weak convection rooted above the boundary layer
(elevated in nature). Low PoPs (~15-30%) continue into the early
overnight hours Thursday night in far southeast Missouri and
southwest Illinois as guidance suggests these locations may get
grazed by showers and weaker thunderstorms in the proximity of the
front.


Gosselin

&&

.LONG TERM...  (Friday through Next Wednesday)
Issued at 205 PM CDT Wed May 14 2025

(Friday - Friday Night)

The Pacific cold front stalls out late Thursday night into early
Friday morning and largely washes out, with little/no semblance of
it on pressure/temperature/wind fields. There is however a
dewpoint gradient that remains and this remnant boundary is
forecast to push northward early on Friday, bringing higher
dewpoint (and more unstable) air further north with time. There
remains some uncertainty with how far north/northwest the high
instability will get by early Friday afternoon, but indications
are at least a majority of the area will be very warm/unstable
ahead of an approaching cold front from the west. Very strong
(40-55+ knots) deep-layer shear will also be in place. Unlike
Thursday however, this approaching front has more to work with and
less inhibiting factors. The cap in place is quite a bit weaker,
and surface convergence along the boundary is moderately strong.
In addition, there is a midlevel shortwave trough moving across
the mid-Mississippi Valley. In other words, convective initiation
will not be an issue. A widespread threat for severe
thunderstorms appears in the offing over much of the region, with
the best chances across southeast and east-central Missouri along
with south central and southwest Illinois during the
afternoon/very early evening hours. All threats are on the table,
but it appears that very large (2.75+") hail may be the
predominant threat given a likely mode of supercells (rotating
updrafts) and very steep midlevel lapse rates (anomalously high
hail CAPE). The tornado threat is not particularly high, inhibited
by high LCL values and modest 0-1 km shear/helicity values.
Guidance seems to point toward more of a line of storms later on
Friday evening focused on the lower Ohio Valley. The threat for
organized straight-line winds would obviously increase once this
happens, but as alluded to above, this higher wind threat should
stay to our south and east. The current expectation is for severe
convection to exit parts of southeast Missouri/southwest Illinois
on or just after 0000 UTC, with drier and cooler air moving into
the area behind the cold front.


(Saturday - Saturday Night)

A brief period of dry weather is forecast to start off the weekend
with near seasonable temperatures thanks to a weak surface ridge of
high pressure moving into the area. Highs in the upper 70s to low
80s are forecast on Saturday, with mid 50s to low 60s expected for
lows Saturday night.


(Sunday - Next Wednesday)

After that brief break, a return to a more active pattern commences
to end the weekend heading into next week. While mid/upper level
ridging builds into the Mississippi Valley Sunday into Monday, it is
a "dirty" ridge as guidance is in good agreement with multiple
"ridge runners" interacting with a slow-moving warm front. A well-
defined MCS is possible both Sunday and Monday nights, potentially
providing widespread rainfall. Early indications are that the
Monday night round will be further north than its predecessor,
which could limit 2-day totals from getting high enough to cause
too much in the way of river and/or flash flooding. Probabilities
for 2+" of rain on the LREF are still pretty low, only topping out
in the 10-30% range. Parts of northeast Missouri and west-central
Illinois have dried out again and could certainly use some
beneficial rain as well.

While there is high confidence in low temperatures heading into
early next week, high temperatures Sunday through Tuesday may be
heavily impacted by convection and debris clouds. Not surprisingly,
the inter-quartile range of the NBM are quite during this time
period, largely on the order of 7-12 degrees.

Clusters of the 500-hPa height pattern show more spread heading into
Tuesday/next Wednesday, as the evolution of the closed low across
the desert southwest is highly uncertain. The timing of the
ejection, as well as strength/track all diverge substantially within
the envelope of ensemble guidance. There should be an area of low
pressure track to our north sometime late Tuesday/Tuesday night,
bringing a cold front through the region. Another round of showers
and thunderstorms is possible with this front. There also should be
quite a bit of shear across our region, but instability could be a
big question mark given antecedent rounds of convection.


Gosselin

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1029 PM CDT Wed May 14 2025

Dry and VFR conditions are expected through the period. The only
exception is that there still is a low chance (15-20%) for
thunderstorms at STL/SUS/CPS on Thursday afternoon, mainly between
20-23Z. If thunderstorms do develop, they will likely have MVFR
or IFR visibilities and ceilings and could possibly produce hail
and wind gusts over 35 knots. Otherwise, winds will gust at times
up to 25 knots during the day on Thursday outside of
thunderstorms.

Britt

&&

.CLIMATE...
Issued at 227 PM CDT Mon May 12 2025

Record maximum temperatures and warmest minimum temperatures
for May 15.

                MAY 15

ST Louis:
MAXIMUM         94 1944
WARMEST MIN     72 2013


Columbia:
MAXIMUM         90 1944
WARMEST MIN     71 1941


Quincy:
MAXIMUM         93 1944
WARMEST MIN     69 1962


&&

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

$$

WFO LSX