FXUS63 KLSX 261939

AFDLSX



Area Forecast Discussion

National Weather Service Saint Louis MO

239 PM CDT Thu Jun 26 2025



.KEY MESSAGES...



- Higher chances of showers and thunderstorms will exist tonight 

  through Monday, which, along with associated clouds, casts some 

  uncertainty in exact high temperatures and heat index values 

  each day.



- More seasonable temperatures and humidity along with drier

  conditions will prevail Tuesday through mid-next week.





&&



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

Issued at 238 PM CDT Thu Jun 26 2025



There has been little change in the weather pattern across the Mid-

Mississippi River Valley in the last 24 hours with the region at the 

northwestern periphery of an upper-level ridge and a hot and humid 

airmass residing below with heat index values in the upper 90s to 

around 100 F across much of the CWA. Due to slightly warmer mid-

level temperatures/weaker mid-level lapse rates, diurnal showers and 

thunderstorms have been more isolated and struggling to produce as 

much lightning as Friday.



Although the aforementioned showers and thunderstorms will dissipate 

around sunset, an additional cluster of showers and thunderstorms 

across southeastern IA/northwestern MO this afternoon will track 

southeastward this evening, reaching northeastern MO around 10 pm. 

This cluster will likely be weakening by that time as instability 

decreases and it becomes outflow dominant with weak wind shear (deep-

layer 10 to 20 kt). However, showers and thunderstorms will take the 

longest to weaken across west-central to central MO where moisture 

convergence will be maximized at the nose of a 40 kt southwesterly 

LLJ. This evolution is also supported by most CAMs, hanging on to 

this southwestern flank of the cluster until the LLJ weakens Friday 

morning.



Following a relative lull during the morning on Friday, additional 

scattered development of showers and thunderstorms will take place 

mainly across east-central, southeastern MO and southwestern IL 

during midday into afternoon near remnant outflow as instability 

increases and an upper-level shortwave trough passes. Although 

MLCAPE will reach 1500 to 3000 J/kg, weak deep-layer wind shear of 

only 10 to 15 kt decreases confidence that any thunderstorms will 

become severe, but a few weak microbursts could lead to gusty winds. 

Cloud debris and greater coverage of showers and thunderstorms will 

both lead to cooler high temperatures on Friday, the main limiting 

factor for the going stretch of advisory level heat to continue. 

That being said, afternoon heat index values will still approach 100 

F in the St. Louis metro and southwestern IL, least likely to be 

significantly impacted by early day outflow or precipitation/clouds 

with temperatures reaching around 90 F. Showers and thunderstorms 

will dissipate and/or depart to the east Friday evening with 

propagation and decreasing instability.



Pfahler



&&



.LONG TERM...  (Saturday through Next Thursday)

Issued at 238 PM CDT Thu Jun 26 2025



Over the weekend, model guidance is in agreement that upper-level 

flow over the Mid-Mississippi River Valley will become quasi-zonal 

and be navigated by one or more perturbations/shortwave troughs. The 

presence of these features will be important in determining when/if 

showers and thunderstorms over the weekend will be more than just 

scattered and diurnal in nature. For this reason, despite low-level 

WAA and southwesterly flow leading to warmer temperatures, there is 

slightly lower confidence in high temperatures Saturday and Sunday, 

with the NBM interquartile range varying 5 F from the upper 80s to 

the mid-90s F. With dewpoints also continuing to be in the 70s F, 

afternoon heat index values could reach around 100 F, especially in 

the St. Louis metro.



A pattern change will take place early next week as an upper-level 

trough deepens across the Upper Midwest and Great Lakes, accompanied 

by passage of a shortwave trough and cold front through the Mid-

Mississippi River Valley. Between the weekend and Monday, ensemble 

model guidance have the greatest membership with measurable rain on 

Monday which aligns with when the greatest large-scale forcing will 

be present. Deep-layer wind shear will remain weak but current 

indications are that it will be slightly stronger and 15 to 20 kt, 

which suggests the potential for at least a few strong thunderstorms 

with 50 to 70 percent probabilities of 2000+ J/kg of MUCAPE. Another 

impact of the pattern change will be post-frontal low-level 

northwesterly CAA cooling temperatures closer to average and 

seasonably lower humidity by Tuesday along with drier conditions.



Around the middle of next week into the 4th of July Holiday, model 

guidance suggests that upper-level northwesterly flow in the wake of 

the trough may evolve into more quasi-zonal flow once again. This 

process will transition the seasonable, tranquil conditions back to 

warmer and more humid conditions as low-level flow becomes southerly 

with the a warm front eventually lifting through the region, 

accompanied by opportunities for showers and thunderstorms.



Pfahler



&&



.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

