


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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990 FXUS63 KLOT 080727 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 227 AM CDT Tue Jul 8 2025 .KEY MESSAGES... - There is a medium chance (30 to 50%) for scattered showers and thunderstorms this afternoon, with perhaps highest coverage near and south of Interstate 80 - There is a low chance (20 to 30%) for isolated thunderstorms on Wednesday, primarily near the Lake Breeze. - Showers and thunderstorms are likely Friday into Saturday (50 to 70% chance), though there is still some timing uncertainty. - High and low temperatures through the end of the week will be seasonable and in the mid to upper 80s and low to mid 60s, respectively. && .DISCUSSION... Issued at 226 AM CDT Tue Jul 8 2025 Through Wednesday: Early morning radar and satellite imagery depict a gradually decaying MCS tracking east-southeastward along the IA/MO border. An MCV is evident on the northern side of the MCS, which is similarly tracking east-southeastward toward west-central IL. Meanwhile, isolated showers and even a few thunderstorms continue to percolate within a broad zone stretching from near Davenport, IA to Milwaukee, WI. Evidently, a modest northeastward-pointing 15-20kt 925 to 850mb low- level jet has provided just enough moisture advection to activate an ACCAS plume based at about 6500ft. Thankfully, the environment across northern IL is pretty darn hostile anything beyond weak convection this morning, per paltry mid-level lapse rates less than 6 K/km and a pocket of dry air above 500mb sampled by the 00Z DVN RAOB. Elsewhere, it`s a fairly quiet morning across our area with partly cloudy skies and "cool" temperatures in the low to mid 60s. Toward sunrise, the aforementioned low-level jet should begin to wane. As a result, would expect any remaining showers across northwestern IL to dissipate. Interestingly, most CAM guidance paint a different picture with renewed shower and even storm development west of I-39 by 10AM. Such development appears tied to another, but simulated, MCV emanating out of north-central IA. At this point, don`t really see any signs of another MCV north of the aforementioned circulation in southern Iowa in radar, satellite, or observational data. However, the presence of ACCAS west of the Mississippi River does provide enough justification to maintain 20 to 30% chances for showers through about noon, primarily west of I- 39. This afternoon, rising boundary layer temperatures and dew points will render the atmosphere uncapped by early afternoon. Accordingly, will advertise scattered thunderstorms in our forecast for this afternoon, first developing west of I-39 just after noon and then spreading eastward toward Lake Michigan and northwestern Indiana by late afternoon. Even with convective temperatures being met, coverage of storms remains an item of lower than average forecast confidence given nebulous large- scale forcing. With that said, coverage may end up highest south of I-80 in relative close proximity to the remnant MCV moving through central IL. Will cap PoPs at 50% for now and let the day shift make adjustments upward or downward based on morning observations. Where storms to occur today, 1500-1750J/kg of MLCAPE, 500-700J/kg of DCAPE, PWATs near 1.75", and 20-25 kt of deep- layer shear should relegate cell behavior to the "pulse" variety with loosly-organized upscale growth into eastward-propgating clusters behind surging outflow boundaries. Such a regime should support locally gusty winds (45 to 55 mph) and torrential downpours (1-2"/hr rainfall rates). Will touch base with SPC later this morning to see if a northward extension of the Level 1/5 threat area currently positioned to our south is needed to cover an isolated threat for downed tree limbs. Outside of thunderstorms, highs today should range from the low to upper 80s. Coverage of showers and storms should wane after sunset, though would not be surprised to see a few cells fester near the Wisconsin State line ahead of a very weak frontal zone through about midnight. With a surface high pressure directly overhead to promote nearly calm winds and pockets of wet soil, can envision pockets of shallow ground fog developing during the early morning hours of Wednesday. Overnight lows should fall to the mid to upper 60s. On Wednesday, residual low-level moisture and the warming boundary layer may allow for an isolated shower or storm to develop by early afternoon. However, coverage may be quite limited owing to increasing mid-level subsidence. Like prior shifts, am noting a signal for a stronger than typical lake breeze given a building high pressure system over the northern Great Lakes. At this point, it appears winds and waves may fall under the level for concern for rip currents, but this will be something to monitor closely. Highs will range from the low to mid 80s inland, but only the mid to upper 70s lakeside. Borchardt Wednesday Night through Monday: Primary forecast concern and challenge is thunderstorm potential Friday into Saturday. Weak high pressure is expected to move across the western Great Lakes Wednesday night and shift east of the area on Thursday. This should allow for mainly dry weather through Thursday evening though can`t rule out an isolated shower or storm, especially across central IL. A lake breeze is expected Thursday afternoon with temps a few degrees cooler, with mid 80s expected inland. Confidence is slowly increasing for showers and thunderstorms for the end of the week into the first part of the weekend. An upper trough will be moving across the northern Plains and upper midwest while a wave ejects out of the central Plains. Models develop a surface low over the Lakes region Friday night into Saturday with a cold front moving across the region. Quite a bit of support among the ensembles during this time period as well. However, there still remains uncertainty for timing and the location/track of a surface low. Blended pops have increased a bit with some likely pops Friday evening for the western cwa. Blended pops also continue Saturday night into Sunday, which may end up being too long but no changes for now. If storms were to be moving across the area Friday afternoon/evening with peak heating, there could be the potential for strong/severe storms. Precipitable water values will be in the 2 inch range during this time, with heavy rain and localized flooding also possible. If precip trends/frontal boundary noted above pan out, temps could trend back toward normal values for the weekend, lower to mid 80s for highs, 60s for lows. Trends then support warmer temps going into early next week, with highs perhaps back into the lower 90s. cms && .AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 1231 AM CDT Tue Jul 8 2025 Forecast concerns include... Chance of thunderstorms this afternoon. Low chance for showers through mid morning. Potential for fog early Wednesday morning. Current isolated showers across northwest IL are expected to dissipate over the next few hours before reaching the Chicago terminals, though an isolated shower will remain possible. A few showers across southeast MN/western WI may continue to fester through the overnight hours, bringing a chance for showers to RFD after daybreak. If these were to materialize, they may persist north of the terminals through the mid/late morning hours. There remains a chance of thunderstorms for all of the terminals this afternoon, though coverage remains uncertain and maintained prob mention for now. A large complex of thunderstorms is moving southeast across southwest IA into northern MO. How these storms evolve over the next 12 hours, will impact storm trends locally this afternoon. The most favored areas appear to be west central IL with activity possibly dissipating further to the northeast. Overall, low confidence for trends, which will need to be monitored for possible later forecast changes. After this potential precipitation, the rest of this evening into early Wednesday morning is looking mainly dry. Winds are light and variable and will become light southerly by daybreak then become southwest by mid morning with speeds increasing into the 10-12kt range. The forecast soundings suggest there may be a period in the late morning, when gusts into the 15-20kt range will be possible. Wind directions this evening are uncertain. If there are storms, winds may favor a light southeast direction. Forecast soundings also show hardly any wind aloft early Wednesday morning as weak frontal boundary sags into northern IL. This would easily support fog development. However, fog may be dependent on how widespread any precipitation becomes and the best location for fog appears to be northwest IL, which would be after the current 24 hour RFD taf period. But some fog mention may be needed with later forecasts. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago