Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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799 FXUS63 KLOT 140518 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1218 AM CDT Sun Jul 14 2024 .KEY MESSAGES... - Scattered showers and storms are expected this afternoon, mainly across the western portion of the CWA. These could produce strong to severe downburst winds. - A MCS may dive southeast from MN/WI into the area this evening into the overnight hours. Strong to severe winds are possible with this system. - Additional rounds of convection, including the threat for severe thunderstorms and flash flooding, are probable late Sunday-Sunday night and late Monday-Monday night, with scattered thunderstorms possible for some on Tuesday. - Hot and humid conditions are expected Sunday and Monday with peak heat indices over 100 degrees. Monday looks to be the hotter day with peak heat indices approaching 110F. A cold front passage on Tuesday will end the heat and humidity. && .DISCUSSION... Issued at 314 PM CDT Sat Jul 13 2024 Through Sunday Night: Current satellite imagery depicts a shortwave digging southeast from MN/WI towards the area away from an ongoing MCV in northern WI. Steeper mid-level lapse rates (7+ C/km) have been advecting north/northeastward from northern MO through the morning and early afternoon. The shortwave has provided enough lift to overcome the dry mid-levels and some showers and storms have begun across northwestern IL and southwestern WI. These storms are expected to spread east through the afternoon, but should remain confined to the western half of the CWA until this evening. Given the 2000-3000 J/kg of MLCAPE that has developed overhead, these storms could produce strong to severe downburst winds and hail this afternoon despite the low-end bulk shear values (20-25 kts). Given these shear values, storms should remain scattered in nature and are not expected to grow upscale. An impressive EML across the Northern Plains has allowed convection to fire in MN is expected to congeal into a MCS this afternoon. This MCS looks poised to dive southeast towards the area this evening into the overnight hours. The most likely timing for this system looks to be after sunset. Given this, convection should be in a weakening phase as it reaches the northern parts of the area. However, storms will still be capable of producing strong to severe wind gusts. Uncertainty still remains on how the exact timing and progression of this system as the 12 and 18 Z guidance has not been in agreement with the handling of the multiple rounds of storms and MCS`s today. One rather worrisome outcome for the storms overnight is outflow setting up a scenario for convection to regenerate along the nose of a 30-40 kt LLJ. This would lead to a quasi-training event and the same areas being hit by storms multiple times. With PWATs expected to be approaching 2", this leads to a threat for localized flash flooding. Confidence in this actually occurring is quite low at the current time, but the pattern is favorable given a properly oriented outflow boundary. Convection should be clearing out of the area by tomorrow morning, setting the way for a hot and humid day on Sunday. Temperatures are expected to climb into the low 90s with dew points in the low 70s leading to peak heat indices pushing 100 degrees. Although hot, this falls below the criteria for a Heat Advisory (peak heat indices of 105 degrees). However, if convection carries over into the morning hours, this could augment the high temperatures for the day, leading to cooler peak heat indices. Tomorrow afternoon, a warm nose around 900 mb should cap convection across the area despite a reservoir of 2000-3000 J/kg of MLCAPE. Some guidance has a weaker capping inversion and has convection being able to initiate. Although this scenario looks unlikely, this too would work to limit high temperatures tomorrow. Sunday night, yet another round of storms looks possible with an embedded shortwave / vort max expected to move across the area. This wave should move overhead through the late afternoon and early evening. This sets the stage for more nocturnal showers and storms overnight. Given the steep mid-level lapse rates (7-8 C/km) and ample low-level moisture another large reservoir of MUCAPE (2500- 3000 J/kg) will build. A nocturnal low-level inversion could keep storms from forming or could at least limit the strength of these storms, however, the progression of convection tomorrow night is still very unclear. Carothers Monday through Saturday: There is a scenario during the daytime on Monday, currently the most likely one, in which the main story weather impacts wise is the heat and oppressive humidity. That being said, our area being precariously positioned on the northeast periphery of the mid-level ridge with moderately brisk west-northwest flow means "convective surprises" certainly can`t be ruled out. Whether any surprises would be in the form of remnant overnight convection holding on longer or additional subtle impulses interacting with the reservoir of extreme potential instability later in the morning and afternoon is of very low confidence. Thus, we`re only featuring slight chance (~20%) PoPs, for the northeast 3/4 of the CWA Monday morning, and for areas near and north of the IL and Kankakee Rivers in the afternoon. If heating can be maximized, progged 850 and 925 mb temps support highs in the lower to mid 90s, likely highest over the heart of the Chicago urban corridor, and a bit lower over the farming areas, due to the effects of evapotranspiration (ET) wringing out moisture from the crops at the expense of slightly lower temps. Anticipating dew points at peak heating in the lower-mid 70s over the Chicago metro (low 70s over the city) and mid-upper 70s (possibly locally near 80F) over the farming areas due to aforementioned ET effects. Forecast peak heat indices in the 100-110F range suggest much of the area flirting with or exceeding local heat advisory (105+F heat indices) criteria. However, given the lingering uncertainty regarding possible convective effects, will handle heat headlines on a day by day basis. The signal for more widespread convection is much stronger by early Monday evening into the overnight. There has been decent run to run consistency (and ensemble member and mean signal) for a coherent and stronger mid-level impulse and speed max crossing the mid and upper MS Valley intercepting the strongly unstable (3-5k J/kg of MLCAPE) air mass in place. In addition, looking at 40+ kt of deep layer bulk shear supporting storm organization. Conceptually speaking, the pattern seems to point toward mode favoring MCS, possibly a severe forward propagating MCS, though the differences between MCS and supercell/embedded supercell mode can be subtle, and both modes are plausible as well. Most of the area is in a level 2 of 5 (slight risk) severe threat per the SPC Day 3 Outlook, and this appears reasonable. Damaging winds are the preferred main hazard, with large instability and steep mid-level lapse rates supporting large hail, and combo of strengthening low-level shear and very moist profiles/low LCLs making tornadoes a possibility as well. Felt comfortable with the corridor of likely (~60%) PoPs near and north of I-80 Monday evening sagging a bit south overnight. Very high column moisture (PWATs 150-200% of normal) and the potential for training/back-building convection also make flash flooding a distinct threat to monitor for. Looking ahead into Tuesday, as is common, there will be question marks on the position of the effective frontal boundary given likely MCS activity overnight. While the warm and humid air mass won`t yet be dislodged, a cold front will also be progressing southward and bringing in slightly drier air from the north. Suspect that following the dissipation of any lingering morning showers and thunderstorms, the best chance for afternoon redevelopment near the cold front will be for areas near and southeast of I-55. Forecast highs are in the mid- upper 80s, except lower 80s near the lake from winds turning onshore, and peak heat indices from the I-88 corridor in the Chicago metro and points south and east in the mid-upper 90s. Following the cold front clearing our southern CWA Tuesday night into Wednesday morning, mean troughing will set up over northeastern North America while powerhouse 500 mb ridging rebuilds over the western US. This should bring an extended period of dry, cooler and less humid conditions Wednesday PM onward through next weekend. Castro && .AVIATION /06Z TAFS THROUGH 12Z MONDAY/... Issued at 1218 AM CDT Sun Jul 14 2024 The back edge of thunderstorms will steadily work southeast from RFD to DPA/ORD/MDW/ORD and then GYY over the next 2-3 hours giving way to a dry period. Trends in winds overnight are an item of low confidence as a residual cold pool, though a general southwesterly direction is expected by daybreak. Attention then turns to the next convective system developing over eastern Minnesota, which is poised to dive southeastward into Wisconsin this morning. At this point, the expectation is for the next complex to miss the terminals to the north, but felt introducing PROB30 groups at all but RFD from about 15-17Z would be prudent just to be safe. Notwithstanding the threat for the Minnesota complex to impact the terminals tomorrow morning, most daylight hours are expected to be dry tomorrow. Yet another convective episode may occur during the evening, for which the inherited TEMPO groups were maintained. Borchardt && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago