FXUS63 KLSX 261713

AFDLSX



Area Forecast Discussion...Updated Aviation

National Weather Service Saint Louis MO

1213 PM CDT Thu Jun 26 2025



.KEY MESSAGES...



- Another hot and humid day is forecast today with peak heat index 

  values of 100-105 degrees in the heat advisory. 



- Increasing clouds and thunderstorm chances cast doubt on 

  temperatures and associated heat index values Friday - Sunday.



- There is a stronger signal for cooler and less humid conditions

  for early/mid next week. 





&&



.SHORT TERM...  (Through Late Friday Afternoon)

Issued at 327 AM CDT Thu Jun 26 2025



Persistence is the theme again for today's weather with little/no

change in the ambient air mass. There should be a bit less 

overall cloud cover and only isolated showers and weak 

thunderstorms developing this afternoon. As a result, most 

locations should be ever so slightly warmer than yesterday 

afternoon. Highs in the low to mid 90s with peak heat index values

in the 100-105F range are forecast this afternoon.



Widespread showers and thunderstorms are forecast along a cold front 

this afternoon/early evening stretching from the south-central 

Plains northeast into the Upper Midwest. This activity is forecast 

to attempt to move into parts of northeast Missouri after about 9 or 

10 PM. Thunderstorms are expected to weaken and decrease in 

coverage due to several factors including: 1) convergence along 

the surface boundary abating, 2) diurnal instability decreasing 

after sunset, and propensity for thunderstorms to become more 

outflow dominant as they encounter a very low shear environment 

(~10-15 knots). In addition, the stronger moisture convergence on 

the nose of a 20-30 knot low-level jet weakens overnight and is 

mainly focused in western Missouri. All that being said, scattered

showers and weak thunderstorms entering northeast Missouri late 

this evening should slowly fade as they attempt to move off to the

south and east. Much of the activity after 0600 UTC may not even 

have lightning.



Temperatures and associated heat index values are the main concern 

on Friday. The expectation is for morning activity to gradually 

increase in coverage across east central and southeast Missouri as 

well as southwest Illinois by midday. Showers and thunderstorms 

should be focused along a remnant outflow boundary from Thursday 

night's convection. A midlevel shortwave trough is also forecast to 

help aid in convective development as it heads across the 

Mississippi River by early afternoon. This of course all casts doubt 

on exactly how warm it will get Friday afternoon. The high humidity 

aided by dewpoints in the low 70s are a near certainty, so it will 

really come down to the temperatures. Given earlier development of 

showers and thunderstorms (by late morning) and convective debris 

clouds, not to mention a possible outflow boundary, highs may stay 

in the upper 80s to near 90 degrees. Given all of the possible 

negating factors, did not extend the heat advisory anywhere. 

However, there is at least a low chance that heat index values 

around 100 degrees are possible in/around metropolitan St. Louis and 

adjacent southwest Illinois. This would be more likely if any 

outflow boundary does not move through and convection develops 

either further northwest than anticipated or waits until mid 

afternoon to develop.





Gosselin



&&



.LONG TERM...  (Friday Night through Wednesday)

Issued at 327 AM CDT Thu Jun 26 2025



(Friday Night - Sunday Night)



Trends are coming into better agreement through the weekend in terms 

of thunderstorm chances and temperatures. Confidence is increasing 

that there will be more dry time on Saturday, with rising mid/upper 

level heights and increasing low-level warm air advection. Any 

thunderstorms on Saturday looks to be diurnally driven, and mainlyduring the afternoon/early evening hours. In other words, convection 

and any debris clouds really should not negatively impact 

temperatures on Saturday. Highs in the low 90s with peak heat index 

values around 100 degrees are expected. 



Sunday looks very similar to Friday, with a midlevel shortwave 

trough moving across the mid-Mississippi Valley. This feature should 

help develop more widespread convection, which may help at least 

limit temperatures that afternoon. Highs in the upper 80s to low 90s 

are forecast, but if confidence increases in more widespread 

convection early enough in the day, temperatures may need to be 

dropped another couple of degrees.   





(Monday - Wednesday)



Confidence is also increasing in a change in the synoptic pattern 

across much of the CONUS early next week. Trends have shown a 

slightly stronger mid/upper level wave crossing the Great 

Lakes/southeast Canada, with a cold front moving through our region 

late Monday/Monday night. Widespread showers and thunderstorms are 

expected with the frontal passage. Cannot totally rule out a few 

stronger storms with around 20 knots of deep-layer shear. The bigger 

deal though with this front will be the cooler and less humid air 

moving into the region. Temperatures and humidity levels are 

expected to at least drop back to normal levels for early July, if 

not a touch below. Dry weather is also favored behind this front. 

All in all, it looks like at least a 2-3 day period leading up to 

the 4th with nice weather.





Gosselin



&&



.AVIATION...  (For the 18z TAFs through 18z Friday Afternoon)

Issued at 1213 PM CDT Thu Jun 26 2025



Predominantly VFR flight conditions are expected through the TAF 

period, but there are chances of showers and thunderstorms at times. 

Through this afternoon showers and weak thunderstorms are expected 

to be isolated, limiting confidence in any direct impacts. Tonight 

into early Friday morning, a weakening cluster of thunderstorms is 

forecast traverse northeastern MO, potentially surviving long enough 

to reach KUIN, KCOU, and KJEF. Following a lull in precipitation, 

redevelopment of showers and thunderstorms will take place midday 

into afternoon on Friday with the greatest chances at St. Louis 

metro terminals. Thunderstorms could contain brief downpours with 

MVFR to IFR flight conditions and gusty winds.



Pfahler



&&



.LSX WATCHES/WARNINGS/ADVISORIES...

MO...Heat Advisory until 9 PM CDT this evening for Callaway MO-Cole 

     MO-Crawford MO-Franklin MO-Gasconade MO-Iron MO-Jefferson 

     MO-Lincoln MO-Madison MO-Montgomery MO-Osage MO-Pike MO-

     Reynolds MO-Saint Charles MO-Saint Francois MO-Saint Louis 

     City MO-Saint Louis MO-Sainte Genevieve MO-Warren MO-

     Washington MO.



IL...Heat Advisory until 9 PM CDT this evening for Adams IL-Bond IL-

     Brown IL-Calhoun IL-Clinton IL-Fayette IL-Greene IL-Jersey 

     IL-Macoupin IL-Madison IL-Marion IL-Monroe IL-Montgomery IL-

     Pike IL-Randolph IL-Saint Clair IL-Washington IL.



&&



$$



WFO LSX

