DISCUSSION
Issued at 212 AM CDT Sat Jul 27 2024

Through Sunday:

Today will be dry with seasonable high temps in the lower/mid 80s. A lake breeze is expected to move inland across northeast IL with cooler temps there.

Increasing clouds are expected late this afternoon into tonight ahead of an upper level low that will move across the area on Sunday. Models seem in fair agreement with a chance of showers in the southwest quarter or so of the cwa in the predawn hours Sunday morning and the coverage is expected to increase Sunday morning. Best coverage may be along/south of a Rockford to Aurora to Watseka line. Thunder coverage looks rather low for Sunday and maintained slight chance in the morning and chance in the afternoon. Though increased pops to likely for the entire area. Main threat on Sunday will be slow moving cells producing heavy rain with a low potential for localized flooding. Could also be the potential for a few funnel clouds on Sunday.

With the expected shower coverage and mostly cloudy skies, lowered high temps for Sunday. Temps still a bit tricky as any several hour break in the precip with partly cloudy skies may allow temps to jump well into the 80s. But for now have lowered highs into the upper 70s to around 80. Dewpoints will steadily increase into the lower 70s Sunday, allowing for more humid conditions. cms

Sunday Night through Friday:

Lingering showers and storms should tend to diurnally diminish through Sunday evening, although a still relatively moist airmass with several mid-level perturbations overhead may keep some shower activity going on a spotty basis. Towards daybreak on Monday, most extended hires guidance suggests a decaying thunderstorm complex may be marching eastward across Iowa and towards the Mississippi River. Mid and upper-level flow looks to weaken considerably with eastward extent, and with the main core of the EML plume still well to our south and west, any activity that maintains into our forecast area should be predominantly behind any outflow, elevated, and weakening.

Through the day on Monday, west to southwesterly lower- tropospheric flow will begin to push the eastern fringes of a more conditionally unstable airmass our way, with 700-500 mb lapse rates incrementally steepening with time. Depending on how any lingering morning convective activity evolves, it's possible that debris cloud cover blunts insolation to some degree which would also cut down on afternoon instability a bit. Still, with temperatures expected to warm into the mid and upper 80s amidst lower 70s dewpoints, MLCIN is largely forecast to erode through the day. Model guidance suggests warm air advection will increase Monday evening in response to an intensifying southwesterly low- level jet across northern Missouri and central Illinois while a weak surface low/reflection scoots eastward into Wisconsin. The past few runs of the ECMWF have consistently tried to resolve a forward-propagating MCS initiating to our north and dropping south to southeastward down the instability gradient Monday evening and overnight. With an improved thermodynamic and kinematic environment compared to Sunday, this is one possibility we'll have to monitor, although--as is typical with these scenarios-- predictability is extremely low. If this were to materialize, some strong-severe weather threat would exist.

Most guidance continues to push a cold front/composite outflow boundary through much or all of our forecast area through Tuesday which would largely limit additional shower and thunderstorm chances. This period was still a bit too far out to justify cutting inherited blended PoPs, but if trends continue, this may be supported. Moisture pooling immediately around this boundary could yield locally increased dewpoints in the mid to upper 70s, but at this time, it looks like the highest dewpoints may focus immediately south of the CWA, with the latest peak heat indices generally in the mid to upper 90s in our forecast area.

At some point towards the middle of the week, this effective boundary should meander back northward as a warm front. Increased dewpoints will be overtopped by a plume of greatly-steepened mid- level lapse rates during this timeframe, resulting in a renewed threat for additional MCS advances into/towards our region. The Wednesday/Thursday period continues to look a bit more interesting in this regard, as a belt of faster northwesterly 500 mb flow sets up. Given the synoptic scale setup, any MCS activity would carry an attendant severe weather threat, as well as a potential for heavy rain and flash flooding given a supportive NW-SE orientation along the frontal zone. If Monday evening's predictability is low, confidence during this time frame is even lower, though. We'll continue to keep an eye on this time period, with the active stretch possibly continuing into the upcoming weekend.

Carlaw

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/
Issued at 619 AM CDT Sat Jul 27 2024

The only aviation weather concerns are:

- A potential lake breeze this afternoon

- Increasing chance for showers Sunday morning

Light and variable winds early this morning will become predominantly south to south-southeasterly. Some occasional south-southwesterly winds will be possible this afternoon as well. Latest guidance has trended towards holding the lake breeze immediately east of ORD/MDW this afternoon, but it's going to be close. The latest Chicago-area TAFs will show prevailing SSE winds, but some variability will be possible later today.

Otherwise, VFR conditions will continue today and tonight. Cloud cover will thicken up tonight as a disturbance approaches from the south. This feature will bring an increased chance for showers to the region Sunday morning. Some spotty showers will be possible as early as about 12z, but it currently looks like the better chances will hold off until mid-morning. There's a chance for MVFR cigs as well, but chances currently are a bit too low to introduce within prevailing or TEMPO groups.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...None.