


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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114 FXUS63 KLOT 300804 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 304 AM CDT Mon Jun 30 2025 .KEY MESSAGES... - Thunderstorm threat late this morning through the evening. The strongest storms will be capable of localized damaging winds and heavy rainfall and an isolated flood threat. - Hot weather for 4th of July festivities Thursday-Sunday, with increasing humidity and periodic thunderstorm chances, especially over the weekend. && .DISCUSSION... Issued at 303 AM CDT Mon Jun 30 2025 Through Tuesday: Fairly complex and lower-confidence forecast for today, with generally limited large scale forcing mechanisms (at least until late in the afternoon) but with several meso-alpha scale surface boundaries on which to force storms later this morning and afternoon. There is some potential for more organized strong- severe convection very late this afternoon and evening as a robust shortwave trough presses through, but this still isn`t necessarily a "lock" yet. Nighttime microphysics RGB imagery reveals mid-level stratus along and east of about I-55, within a corridor of enhanced low to mid-level moisture. Modest isentropic ascent within this region may yield some intermittent showers/storms early this morning, with the focus eventually shifting east of the region through daybreak. While a subtle outflow boundary (exhausted from earlier storms in southern Wisconsin) has settled near to just north of I-80, forecast guidance suggests that the low-level wind field will homogenize this morning with mixing as a broad surface trough shifts across the region. This may end up generally limiting surface convergence, and with neutral mid-level height tendencies persisting through early afternoon, suggesting additional storm coverage could end up remaining somewhat subdued through midday. The exception to this will be near a sharpening lake breeze, and perhaps in the vicinity of any locally enhanced convergence near the aforementioned outflow. We continue to suspect that the HRRR is too aggressively mixing dewpoints out today, which is likely playing some role in its more limited coverage of showers/storms, and we`re largely discounting its recent solutions as a result. With dewpoints expected to remain around 70, convective temperatures will be reached quickly this morning with heating, and the general expectation is for scattered shower/storm development to occur through late-morning/midday, focusing across parts of NE Illinois and NW Indiana. Through early afternoon, mid-level flow isn`t expected to be all that significant, resulting in limited deep layer shear. As a result, any storms that develop during this period will mainly be pulse/multi-cell clusters, carrying a gusty downburst and locally heavy rainfall threat. Through the afternoon, a robust shortwave trough will begin to sweep into southern Wisconsin, shifting a mid/upper-level wind max into northern Illinois. Modest (20-40 m/12 hour) height falls will coincide with the approach of this trough, with a gradual increase in overall larger-scale forcing. Interestingly, not seeing a universal signal in the guidance suite of increasing thunderstorm coverage during this period, potentially due to the limited nature of near-surface convergence, and lower/mixed-out dewpoints. That said, the potential for one or several remnant MCVs from convection currently developing across central Nebraska to pivot across southern Iowa as the main shortwave shifts overhead lends some concern for higher storm coverage late this afternoon and evening, although perhaps not quite as far north as the 00z NAMNest advertised. Continue with scattered thunderstorm chances during this period, and this will be the main time to watch (roughly 4-10 pm) for more organized strong-severe convection in our area as deep layer shear increases. If the mid-level flow increases as much as the NAMNest indicates, an embedded supercell and attendant large hail threat could also materialize, but this is not the most favored outcome at this point. Storm chances may linger through the late evening, but will come to an end during the early overnight hours as a cold front pushes through. Tuesday will feature more seasonable temperatures with lower humidity. Carlaw Tuesday Night through Sunday: Lower humidity and more pleasant nighttime conditions for a few days will revert right back to uncomfortable heat and humidity in time for the peak of Independence Day related celebrations Friday through the weekend. As is common during the heart of summer in hot, humid, and unstable patterns, thunderstorm chances will also increase with time, especially Friday-Saturday, though expect plenty of dry time as well. Following a seasonable and quiet Tuesday night, drier northwest flow but still warm low-level thermal fields (ie. mid-upper teens C at 850 mb and low-mid 20s C at 925 mb) will result in a very warm to hot Wednesday away from lake breeze cooling. Dew points appear poised to mix out into the low 60s if not lower, which with plenty of sun should translate to highs in the upper 80s to around 90F. A lake breeze should work onto the Chicago and northwest Indiana shore, keeping highs in the lower-mid 80s there. We`re currently carrying a dry forecast on Wednesday PM, though can`t completely rule out (~10% chance) a gusty thunderstorm near the lake breeze convergence axis, particularly if dew points are high enough on the cool side of the boundary. Thursday will be fairly similar to Wednesday, though likely a couple degrees warmer away from the lake (solidly upper 80s to lower 90s) and dew points a tick higher. Despite a fairly parched air mass still in place aloft and likely some mixing out of dew points at peak heating, anticipate little/no capping and moderate instability in the afternoon. This could present a threat for isolated "airmass" thunderstorms with downbursts (~20% PoPs near and north of I-80 for now). The lake breeze convergence zone could again be favored focus area, if convection indeed initiates. We`ll then have to watch for nocturnal convection nearby Thursday night or even just outflow effects from convection to the north on the edge of the EML plume. This will be as pronounced 500 mb height rises edge eastward while we`re still in northwest flow locally (steering flow toward southeast). Barring a prohibitively large convective outflow footprint into Friday, the 4th this year may be in the upper echelon of recent hot July 4ths. It doesn`t currently look record threatening, but mid 90s are a distinct possibility as 590+ DaM 500 mb heights crest the southwestern Great Lakes and mid-MS Valley region. If mid 90s occur at ORD and RFD, it would be the warmest 7/4 since 2012`s record setting 102F highs at both sites. Conceptually, the 500 mb setup should lend itself to increased capping and limited if any convective chances. However, it`s too early to say this for sure, so periodic slight chance PoPs for the PM hours through the overnight into early Saturday appear warranted. The 500 mb ridge will remain in place but flatten out over the weekend, opening the door for occasional scattered thunderstorm chances and perhaps even semi-organized MCSs. Stronger large scale forcing will remain tied near and north of the Canadian border, though envision a myriad of convectively modulated impulses, MCVs, along with fronts from northern stream disturbances could all serve as triggers for bouts of storms. Mid and upper level winds (and corresponding deep layer wind shear) don`t currently look supportive for widespread organized severe storms over the weekend and beyond, though high PWAT and high DCAPE air masses (as we`ve seen this past week) can compensate for isolated severe threats. The high PWATs may also translate to periodic localized flash flooding episodes. If coverage Saturday daytime is lower and/or convective initiation (CI) holds off until after peak heating, highs again look quite toasty, into the lower-mid 90s with some upside potential. Temps Sunday may be marginally cooler, but then there`s a stronger signal for dew points to reach the lower-mid 70s. It`s impossible at this range to predict any of the specific details aside from the general pattern, so please stay tuned for updates through the week as we assess and refine the Thursday- Sunday (fireworks festivities prime time) period. Castro && .AVIATION /06Z TAFS THROUGH 12Z TUESDAY/... Issued at 1238 AM CDT Mon Jun 30 2025 Main Concerns: - Scattered TS with gusty, variable winds early this afternoon through the late afternoon, with another round of TS possible into the early evening. - Lake breeze wind shift likely at MDW and GYY, and possible at ORD, with TS trends likely to influence wind directions. A potentially impactful flying day looks to be in store later today, albeit one fraught with forecast uncertainty. As of this writing, an outflow boundary is sagging south early this morning, bringing a brief period of north to northeast winds to the terminals. The potential for TS appears to have ended for the overnight period. Similar to today, a very warm and humid air mass and lake breeze convergence should quickly foster the development of TCU and then TS over the Chicago metro. It`s a bit more unclear out by RFD. Any storms causing direct terminal impacts will bring strong, VRB downburst winds and brief heavy rainfall. Had enough confidence to maintain VCTS mention (starting at 17z) with PROB30s for direct impacts due to coverage uncertainty. The biggest question mark into this evening is whether a second round of more organized TS will impact the terminals, or develop farther south. Added in one last PROB30 for this potential. Finally, owing to the convective uncertainty plus lake breeze proximity, forecast confidence in exact wind trends is on the lower side. It does appear likely that GYY and then MDW will shift to northeast, but then the boundary may stay just east of ORD, unless it`s ushered through by TS outflow. Observational trends will need to be closely monitored. Winds will then shift back to west-northwest in the late evening and overnight as conditions quiet down. Castro && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago