NZUS93 KLOT 250745 LLLLOT ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-010-011-019- 251945- Public Information Statement National Weather Service Chicago IL 245 AM CDT Fri Apr 25 2025 Area Forecast Discussion National Weather Service Chicago/Romeoville, IL Issued by National Weather Service Lincoln IL 230 AM CDT Fri Apr 25 2025 .Key messages… -Scattered showers and storms are likely (50-70%) this afternoon, particularly south of I-80 this afternoon. Severe weather is not anticipated. - Seasonably cool Saturday, followed by a return to much above average temps early next week. - A chance for strong to potentially severe thunderstorms as a cold front moves across the region Monday into Tuesday next week. && .DISCUSSION… Issued at 230 AM CDT Fri Apr 25 2025 Early this morning, an area of showers and t-storms are ongoing across central portions of IA, with a few additional widely scattered warm air advection showers also noted across northwestern IL. Much of this early morning activity is associated with a few subtle eastward shifting shortwave impulses. It appears likely that much of this storm activity will weaken as it shifts eastward towards the Mississippi River into a less favorable thermodynamic environment during the predawn hours this morning. Accordingly, we are not expecting much more than some widely scattered remnant showers across far northern IL later this morning. As we head into this afternoon, a weak surface low will track eastward along a quasi-stationary frontal boundary in place across central portions of IL. Along and south of this surface boundary ample low-level moisture (low 60s dew points) is expected to result in at least modest early afternoon destabilization across southern sections of the area, especially from the Kankakee River southward. This should thus result in scattered shower and thunderstorm development in this region this afternoon, particularly between 12 and 4 PM this afternoon. Fortunately, rather poor mid-level lapse rates and overall modest mid-level flow should curtail the threat for any severe storms. Temperatures today will be cooler than those observed on Thursday, but with the much cooler airmass expected to lag well north of the surface boundary today, readings are expected to top out in the low to mid 60s far north, and near 70 in the south. The only exception will be near the Lake Michigan shore, where an onshore wind component will keep temperatures in the 50s. Temperatures will drop off back into the 40s tonight as the cooler airmass shifts into the area on northerly surface winds in advance of a surface high building across the Upper Midwest. This surface high will shift across the western Great Lakes on Saturday. The associated onshore northeasterly winds will result in a much cooler day Saturday, albeit a sunny one. Inland sections of northern IL look to top out around 60, while areas along the lakeshore likely struggle to get out of the 40s. This then sets the stage for a rather chilly Saturday night, as light winds and mainly clear skies under the surface high will allow temperatures to radiate back into the mid to upper 30s, especially outside the Chicago urban heat island. Also, cannot rule out some patchy frost in some of the coldest spots late Saturday night. Mid-upper level ridging will begin to shift east into the Mississippi Valley on Sunday, with surface winds turning south-southeasterly following the eastward departure of the Great Lakes surface high. Accordingly, temperatures are expected to begin a warming trend on Sunday, as inland highs top out in the 60s. The slight easterly component to the flow will likely result in an IL shore lake breeze and temperatures several degrees cooler. KJB Next Week.... An upper level low that will sit off the California Coast this weekend will move onshore and begin to bifurcate as it approaches the Rocky Mountains on Monday. The dominant shortwave will become positively tilted with a strengthening upper-level jet whose left exit region will move over Iowa into western Wisconsin late in the day. With a deepening upper-level jet streak over the region, deep layer shear will increase to over 50 knots Monday night into Tuesday. As the trough moves across the Rockies, it will trigger lee cyclogenesis over eastern Wyoming which will also move over the northern Plains with a developing cold front extended from the central low. Low-level southwesterly flow will draw mid 60s dew points into southern Wisconsin on Monday with a warmer air mass being advected into the region. High temperatures on Monday could easily reach into the upper 70s with a 10 to 20 percent chance for areas south of I-80 to potentially touch the low 80s. Latest model runs have slowed the progression of the upper level wave/surface low slightly. Model soundings are showing steep mid-level lapse rates and MLCAPE values approaching 1500 J/kg. Most soundings do show a fairly robust cap through much of Monday which prompts the probability for SBCAPE greater than 500 J/kg to lower from 70-80 percent down to 50 to 70 percent. Nevertheless, soundings show the cap weakening as the front progresses through the area overnight. As the front moves through into Tuesday morning, the upper-level jet streak will slowly meander into the Great Lakes placing the right entrance region over the eastern portion of the CWA. Speed and timing of the front will be critical to monitor with this system as there is a non-zero chance that the front is east of the area by Tuesday morning, in which case the threat of storms would be minimal. Given all these environmental parameters, we will likely remain in the SPCs extended outlook for severe weather chances on Monday into Tuesday. Based on recent model trends. It will be important for people to stay tuned to forecast updates, because as of right now it appears the prime window for severe weather with this system could be at night for much of the area north and west of I-57. A brief ridge will pass over the area late Tuesday into Wednesday to provide a brief window of surface high pressure and quieter conditions with temperatures in the low to mid 60s. However, the next upper-level wave will likely eject out of the southern Rockies on Wednesday with a developing surface low over the southern Plains that will take a northeasterly track toward the Great Lakes. There is still a lot of uncertainty on the exact track of the low. The Euro ensemble has it hugging the Ohio River Valley, while the GEFS has a slightly more northern track toward our forecast area. This results in the Euro keeping us mainly dry, whereas the GEFS has showers with a murky chance of storms. With it still over a week away, confidence remains low but enough to keep a lazy eye on. -DK $$