FXUS63 KLSX 201120
AFDLSX

Area Forecast Discussion...Updated Aviation
National Weather Service Saint Louis MO
620 AM CDT Wed May 20 2026

.KEY MESSAGES...

- Cooler temperatures are expected through Friday.

- There will be several chances for showers and thunderstorms over 
  the upcoming Memorial Day weekend.  


&&

.SHORT TERM...  (Through Late Thursday Afternoon)
Issued at 332 AM CDT Wed May 20 2026

Temperatures have dropped into the 50s early this morning behind the 
cold front that has now moved south of Missouri and Illinois.  There 
are scattered showers that continue to form to the north of the 
front by low level frontogenesis that will gradually move south of 
the CWA through mid-late morning. The HREF is then showing a 
short wave ridge building over the Midwest later today into 
tonight which will keep the forecast dry into early Thursday. At 
the same time a large surface high will slowly move east across 
the Great Lakes causing the surface winds to slowly shift from 
northerly to easterly by tomorrow. Model guidance is showing that 
we will likely stay mainly cloudy through tomorrow, with lower 
ceilings in the southern half of the CWA today which favors cooler
temperatures. Highs will likely stay in the 60s today and range 
from the mid 60s to mid 70s on Thursday.

Britt

&&

.LONG TERM...  (Thursday Night through Tuesday)
Issued at 332 AM CDT Wed May 20 2026

After the brief dry period on Wednesday night and Thursday, the 
chances for showers and thunderstorms increases over the Memorial 
Day weekend into next week as the LREF is showing southwesterly 
upper flow over Missouri and Illinois.  Model guidance is consistent 
with a trough moving across the area late Thursday night into Friday 
which is when the LREF has 60-90% of its members are producing rain 
over the area. This is also the period when PWATS will be the 
highest (near 2.0") and WPC has highlighted the southern half of the 
CWA in a marginal risk of excessive rainfall.   There are lower 
chances on Friday night, but two or three more shortwaves will move 
across the area over the weekend bringing additional chances for 
showers and thunderstorms the rest of the weekend. At this point, 
the LREF/NBM members are keeping the better chances (40-60%) over
southeast Missouri into southwest Illinois which is closer to 
where a surface front will be during the extended weekend. 

Temperatures will start to warm back up over the weekend, though 
as pointed out by the previous discussion, the NBM is on the 
warmest side of guidance. While upper heights will be building 
over the area which favors the warmer temperatures, they will be 
tempered by the clouds and rain chances which should keep highs in
the 70s.

Britt

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 604 AM CDT Wed May 20 2026

Mainly dry conditions are expected through the period. Ceilings
near 3000 feet at JEF, STL, and CPS are expected to gradually
lift to near 5000 feet by 15Z. Ceilings at UIN are expected to 
stay above 10000 feet through the period. Northerly winds will
veer out of the northeast while staying below 10 knots.

Britt

&&

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

$$

WFO LSX