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000
FXUS63 KLSX 180407
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
1107 PM CDT Wed Apr 17 2024

.UPDATE...
Issued at 950 PM CDT Wed Apr 17 2024

The previous forecast trends remain on target and incoming 00Z
guidance doesn`t resolve the questions we have regarding the
severe weather potential on Thursday. It still appears a loosely
organized MCS should evolve in the vicinity of NE KS and NW MO
overnight attendant with returning low-level moisture and lift via
the LLJ. There is already some scattered convection in NE KS and near
the KS/NE border. This MCS should then track eastward into the
northeast MO (and perhaps central MO if it is a bit more
elongated) by mid-morning. It could then completely weaken with a
new MCS evolving along the cold front in central MO by early
afternoon. Another possible solution is the dying MCS could become
re-energized on the SW flank in central MO around midday. Either
way the anticipated MCS would evolve into a severe MCS as it
tracks east-southeast into east-central and southeast MO and
southwest IL through the afternoon and early evening tapping
potentially quite unstable air owing to increasing low-level
moisture/sfc dew point and heating. Some of the CAM/HRRR runs have
also depicted supercells developing out ahead of the MCS, and
nearly all of the CAMS indicate updraft helicity tracks with the
QLCS indicative of mesovortices. Adding to the uncertainty is
there are signs that mixing off the Ozarks could reduce the
instability for a time from SE MO into EC MO in the afternoon,
modulating the SVR weather intensity to some degree. Despite these
varying solutions and uncertainties, the region across east-
central and southeast MO and southwest IL still appears to be
quite favored for severe weather on Thursday afternoon and early
evening with potential for at the very least, damaging winds and
possibly a few QLCS tornadoes.

Glass

&&

.KEY MESSAGES...

- A broken line of thunderstorms will move into the area from the
  northwest on Thursday morning. The line will continue moving
  from the northwest to southeast through the day. Storms are
  expected to strengthen and become severe during the late morning
  and afternoon. The highest potential for severe storms will be
  across east central and southeast Missouri as well as southwest
  and south central Illinois. Damaging winds up to 70 mph will be
  the primary threat. The storms could also produce large hail and
  possibly a tornado or two.

- Dry weather and below normal temperatures are expected for
  Friday through at least Sunday night.


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 240 PM CDT Wed Apr 17 2024

.KEY MESSAGES...

A short wave digging into the northern Plains is causing lee-side
cyclogenesis over eastern Colorado this afternoon.  This system will
move into the central Plains tonight, and cause the low level jet to
strengthen over Oklahoma and Kansas.  GFS/RAP/NAM all show moderate
to strong low level moisture convergence on the nose of the jet
overnight across northern Kansas and southern Nebraska, and all
guidance subsequently spits out a broad area of precipitation
associated with this moisture convergence.  CAMs show this
developing into an MCS overnight, which moves into northeast
Missouri after 12Z Thursday.  Guidance moves the surface low across
Missouri into Illinois Thursday, and southerly flow ahead of the low
brings 65-70 degree dew points temperatures up into the area ahead
of the MCS.  This rich low level moisture pushes CAPE values up to
2500-3000 J/Kg in the afternoon ahead of MCS and cold front
associated with this low.  0-6km shear is forecast to be in the 40-
50kt range, and 0-3km shear will be around 30-40kts.  With this kind
of CAPE and shear, the storms should be able to strengthen and
become severe during the late morning or early afternoon as they
move into east central Missouri.  The shear vectors aren`t quite
parallel to the line/cold front, but they`re close enough that I
think discrete cells are unlikely, and a severe MCS with damaging
winds and some large hail is much more likely.  0-1km helicity is
100-200 m2/s2 so a tornado or two isn`t out of the question.  There
are a couple of caveats to this scenario: 1) moisture return is
uncertain as the Ozarks may block the higher dew point air from
moving into our area from the south 2) cloud cover ahead of the
front and line of storms may limit insolation.  Either of these
issues could significantly affect the instability available in the
afternoon, and cause the storms to be weaker than expected. However,
with all deterministic guidance as well as the CAMs showing strong
instability resulting in an afternoon MCS, I feel pretty confident
in this solution, and the SPC SWODY2 upgrade to "enhanced" across
the southeastern 1/2 of our forecast area.

Carney

&&

.LONG TERM...  (Thursday Night through Next Wednesday)
Issued at 240 PM CDT Wed Apr 17 2024

Cooler and quieter weather is in store for the Mid-Mississippi
Valley for Friday through at least Monday.  A relatively cold
surface high spills into the region behind the cold front Thursday
night into Friday; and another short wave trough digs from the
weathernorthern Plains into the Ohio Valley on Saturday which
drives a reinforcing shot of cold air into the Mississippi Valley.
The latest guidance shows temperatures for Friday through Monday
morning dipping to between 5 to 10 degrees below normal for mid-
late April with highs ranging from the mid 50s in northeast
Missouri to in the mid 60s across the eastern Ozarks. The LREF is
showing an 850mb temperature spread between the 25th and 75th
percentiles of only a couple of degrees, and the surface
temperature spread is only 3-5 degrees, so confidence in this
cooler weather is high. Temperatures warm back up to around normal
for Monday afternoon as the surface ridge axis finally moves east
of the Mississippi Valley and low level flow swings back around
to a southerly direction. The pattern turns more active again for
Tuesday and Wednesday as another short wave digs from the northern
Plains into the Ohio Valley. Current indications are that the
return flow west of the surface ridge will bring enough moisture
back into the Mid-Mississippi Valley for a round of showers, and
possibly some thunderstorms. Latest deterministic guidance is
showing relatively low CAPE values, and even max values on the
LREF don`t break 500 J/Kg on Tuesday and only scrape 1000 J/Kg on
Wednesday, so current indications are that severe storms are
unlikely. Of course, there`s a long way to go until we get to next
Tuesday/Wednesday, and the deep-layer shear looks pretty stout at
50+ kts, so this system will bear watching.

Carney

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1048 PM CDT Wed Apr 17 2024

VFR flight conditions are expected overnight with surface winds
becoming light easterly.

Showers and storms now over NE KS could impact KUIN as early as
12z, and KCOU/KJEF as early as 15z if they hold together and
become more organized as they move east. We are still expecting
the main thrust of showers and storms at KUIN and KCOU/KJEF in the
17-20z time frame with flight conditions deteriorating to MVFR.
Flight conditions could lower to IFR for a brief period with a
stronger storm/heavy rain and could have gusty winds as well.
Given the uncertainty in timing, these later conditions have not
been included in this TAF issuance. A cold front will bring gusty
northwest winds mid-late afternoon.

For the St. Louis metro terminals, showers and thunderstorms are
expected in the 20-23z time frame with potential for strong
storms. Flight conditions could lower to IFR for a brief period
with stronger storms/heavy rain and could have gusty winds as
well. Given the uncertainty in timing, these later conditions have
not been included in this TAF issuance. A cold front will bring
gusty northwest winds by early evening.

Glass

&&

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

$$

WFO LSX