367
FXUS63 KLSX 171046
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
546 AM CDT Sat May 17 2025

.KEY MESSAGES...

- Dry weather is expected today, with a chance for rain late
  tonight into Sunday. Temperatures will be as much as 10-15
  degrees cooler than Friday today.

- Periods of rain and thunderstorms are expected Sunday night
  through Tuesday night. Locally heavy rain is possible at times,
  and some storms could be severe Sunday night and Monday
  afternoon/evening.


&&

.SHORT TERM...  (Through Late Sunday Afternoon)
Issued at 330 AM CDT Sat May 17 2025

West-northwest flow will bring cooler and drier air into the region
today.  This combined with weak high pressure behind yesterday`s
cold front will bring the region a much needed quiet weather day.
Temperatures will be 10-15 degrees  cooler than Friday with dew
point temperatures dropping from yesterday`s 60s to the 40s and 50s
today.

The next upstream short wave trough will dig into Rockies tonight.
This will begin the process of cyclogenesis over the western Plains
and turn the low level flow back to the south-southwest over the
Plains and Lower Mississippi Valley.  Models show storms developing
Saturday night over Oklahoma on due to moderate moisture convergence
the nose of a modest low level jet.  CAMs show the storms growing
upscale into an MCS which moves into southwest Missouri late in the
evening then into parts of central and eastern Missouri overnight.
Moisture convergence is forecast to be weakening by the time the MCS
moves into our area, and instability will be limited by that time as
well, so the MCS should be on a downward trend, and may be little
more than scattered showers with a few rumbles of thunder after 06Z.
Regardless, chance PoPs are still appropriate generally along and
south of the Missouri river and west of the Mississippi.  Some
showers linger into Sunday morning before dissipating as the low
level jet refocuses out over the eastern Plains.

The warm front associated with the surface low over the Plains will
is forecast to lift from  Arkansas to near the I-70 corridor in
Missouri and the I-64 corridor in Illinois during the day Sunday.
Moisture convergence along the front kicks off another round of
precip over eastern Kansas/northwest Oklahoma/southwest Missouri
during the late morning.  Storms should migrate northeast as the
front lifts through the afternoon, though the strongest forcing and
instability will be confined to southwest Missouri according to the
RAP and GFS.  This is supported by weakening convection in the CAMs
through the afternoon.  Therefore little change in the low
chance/slight chance PoPs for Sunday west of the Mississippi.

Carney

&&

.LONG TERM...  (Sunday Night through Friday)
Issued at 330 AM CDT Sat May 17 2025

The forecast continues to look wet for Sunday night through at
least Tuesday/Tuesday night. The trough over the Rockies cuts off
into a slow moving closed low which tracks across the Plains
Monday and Tuesday, ending up over Great Lakes Region by 12Z
Wednesday. Waves of vorticity will eject northeast from the Plains
around the base of the low as it moves east through the period.
South-southwest low level flow ahead of the stacked low pressure
system will bring a continuous flow of warm moist air from The
Gulf into Missouri and Illinois resulting in nearly continuous low
level moisture convergence. While there will doubtless be breaks
in the rain, it will be difficult to find a 6 hour period when
there aren`t showers over the area at least part of that time.
Current thinking is the heaviest rainfall will be Sunday night and
again Monday night when the low level jet will be strongest,
therefore providing the strongest moisture convergence. Latest
storm total QPF from WPC is in the 2-5 inch range for Sunday night
through Tuesday night, with the higher end of that range focused
over parts of central and northeast Missouri into west central
Illinois. The U.S. Drought Monitor is showing parts of these areas
in D0-D1 (abnormally dry to moderate drought), so much of this
rain will be beneficial. That said, some flooding on small rivers
and streams is possible so hydrologic conditions will need to be
monitored closely.

Also of note, there will be at least two more chances for severe
thunderstorms Sunday night and again Monday afternoon into Monday
night.  Deterministic guidance shows 1000-1500 J/Kg MUCAPE over
parts of central Missouri Sunday night with 40-50kts of 0-6km shear.
 This would be enough to produce rotating updrafts, though the best
chance for storms looks to be late evening/overnight when they would
most likely be elevated.  Large hail would be the primary threat and
a secondary wind threat is possible though less likely.  Some of the
storms would be in the vicinity of the surface warm front and if
they remain rooted in the boundary layer there could be a tornado
threat as well, but I think the potential for that is very low at
this time.  Another severe threat develops Monday afternoon/evening
in the warm humid airmass south of the warm front which by that time
is forecast to be across far northern Missouri or southern Iowa. The
GFS shows MLCAPE increasing to 3000+ J/Kg in 40-50kts of 0-6km
shear.  Forecast soundings are showing steep mid level lapse rates,
wide CAPE profiles, and plenty of instability in the hail growth
zone.  Rotating updrafts are again likely, adding to large to very
large hail potential, and while hodographs are initially very
linear, some low level curvature develops when the low level jet
increases in the evening.  There will therefore be a tornado threat
in addition to the hail and damaging wind threat at least during the
afternoon and evening.

Wednesday through Friday continues to look relatively cool and dry,
although there could be some lingering showers on Wednesday as the
tail end of the mid-upper trough swings through the Mississippi
Valley.  Wednesday will be the coolest day with clouds and
potentially some light showers and highs struggling to reach the low
to mid 60s.  Should see more sun Thursday and Friday allowing
temperatures to rise into the upper 60s to low and mid 70s by Friday
afternoon.

Carney

&&

.AVIATION...  (For the 12z TAFs through 12z Sunday Morning)
Issued at 539 AM CDT Sat May 17 2025

VFR flight conditions and west-northwest flow is expected to
prevail today. Some wind may gust up to 25-30 kts at times,
particularly this morning across northeast Missouri and west
central Illinois. Expect wind gusts to diminish late this
afternoon into early evening. A weakening complex of showers and
thunderstorms may affect parts of central and southeast Missouri,
possibly as far north as the St. Louis Metro Area. However,
confidence in precipitation at the central Missouri and Metro TAF
is not high enough to mention in the forecasts at this time.

Carney

&&

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

$$

WFO LSX