


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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782 FXUS63 KLOT 301759 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1259 PM CDT Mon Jun 30 2025 .KEY MESSAGES... - Thunderstorm threat late this afternoon 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. && .UPDATE... Issued at 1125 AM CDT Mon Jun 30 2025 The primary forecast concerns today into this evening continues to focus on the threat and coverage of thunderstorms. In the near term, a surface boundary is noted as of this writing extending from near ORD, southwestward into northern La Salle and Kendall counties. A very moist prefrontal airmass (dew points in the lower 70s) has fostered some recent attempts at convective initiation near this boundary, particularly over Chicago. However, some residual capping has kept these attempts short lived over the metro. While a few brief isolated showers and storms will continue to be possible with this boundary for the next couple hours southeast of I-55, it appears an area of subsidence and a slightly drier low-level airmass moving into northern IL will keep the chances of storms under 15% in the wake of this surface boundary (northwest of I-55 and inland from the lake) through at least 2 to 3 pm this afternoon. The main weather feature to watch for potentially driving more scattered storm development over portions of our area later this afternoon (after 3 PM) into this evening is the robust mid-level short wave trough (and an associated mid and upper level speed max) currently dropping southeastward into the Upper Midwest. This feature is slated to swing into southern WI and northern IL into this evening. Also of note, is a remnant MCV disturbance shifting eastward across the southeast IA and northeast MO state line. This feature in combination with the approaching larger scale trough looks to offer the best potential (40-50% chance) for some storms late this afternoon and evening. However, confidence in the coverage of storms continues to be low, particularly along and north of I-80 owing to lingering questions on the magnitude of lower-level convergence. It remains possible the the better coverage of storms later today into this evening largely remains south of I-80. Nevertheless, we will continue with 30-50% chances for showers and thunderstorms this afternoon and evening, with the best chances looking to be in the 4 PM to 10 PM timeframe mentioned previously. Increasing mid and upper-level flow during this period with the approaching mid-level trough could also support a few stronger storms. KJB && .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 /18Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 1259 PM CDT Mon Jun 30 2025 Key Messages: - 30% chance of TSRA this afternoon into early evening. - Lake breeze to turn winds NE at GYY (~19-20Z), MDW (~21-22Z). Earlier concerns of midday isolated to scattered thunderstorm development along the lake breeze has yet to materialize early this afternoon due to lingering mid-level capping. Accordingly, have opted to trend down on the TS mention and shift the PROB30 timing a bit later, focusing it during the mid afternoon to early evening timeframe (~20-02Z) when the approaching cold front paired with a remnant MCV moving toward the area from west central IL could foster isolated to widely scattered thunderstorm development areawide. TS timing at RFD remains earlier (~19-23Z) due to closer proximity to the cold front. Showers and storms decrease in coverage by late evening (2-3Z) as the front begins to exit to the east with dry conditions forecast overnight through Tuesday. Winds remain WNW early this afternoon around 10-12kt. Meanwhile a lake breeze is slowly making inland progress, currently hugging the Illinois shore and is expected to turn winds NE at GYY around 19-20Z, and could reach MDW as early as 21Z, though confidence in that timing is on the lower side. Can`t fully rule out a wind shift at ORD but this would likely require storms to form over the lake with the associated outflow helping to push the lake breeze further inland. Winds return to a northwest direction areawide this evening behind the cold front with a lake breeze expected to swing inland again during the afternoon on Tuesday, with greatest confidence in it reaching MDW. It may reach ORD toward the end of the 30-hr TAF period, though opted to hold off at this range due to lower confidence. Petr && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago