


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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024 FXUS63 KLOT 031119 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 619 AM CDT Thu Jul 3 2025 .KEY MESSAGES... - Isolated storms possible this afternoon with a locally gusty wind threat. Coverage of storms looks quite low today. - Monitoring a low chance (20 percent) for additional showers and storms late tonight into Friday morning. - Hot July 4th and Saturday but not as oppressively humid as the previous rounds of heat recently (~95-100F heat indices). - More appreciable thunderstorm chances return Saturday night and particularly Sunday with a slowly approaching cold front. There may be a day or two of quieter weather to start next work week before thunderstorm chances return midweek. && .DISCUSSION... Issued at 250 AM CDT Thu Jul 3 2025 Through Friday Night: Mid-level ridging will continue to build in across the region today. Thicknesses and 925-850 mb temperatures will increase a bit from where they were yesterday, and resultantly, expecting temperatures to end up 2 to 5 degrees warmer, except near the lake where winds will again turn onshore late in the morning. While some threat for afternoon and evening showers/storms exists today, coverage continues to look pretty sparse. While surface dewpoints probably won`t mix out as much as recent runs of the HRRR suggest (based on how things evolved on Wednesday), boundary layer moisture doesn`t look particularly deep. In addition, forecast soundings universally show a fairly persistent area of mid-level warming (near 600 mb) which should result in capping through early to mid afternoon, along with notably drier air than earlier progged from 700-500 mb. All of this, combined with a lack of well-defined upper level features, suggests that convection should remain pretty limited in coverage. The inland-moving lake breeze could serve as a focus for slightly increased chances as dewpoints are forecast to increase into the upper 60s behind it, but the signal remains too nebulous for anything more than slight chances at this time. Deeply mixed boundary layer and the presence of dry air will support a localized downburst wind threat in the vicinity of any taller cores, should they manage to develop at all. Any convection that develops during the day could linger a bit into the evening, but should gradually diminish with the loss of heating and attendant instability. Through the late evening and overnight hours, southwesterly 925-580 mb flow is forecast to increase a bit on the northern flanks of the surface high. While flow isn`t forecast to be particularly strong (25-30 knots or less), this will tend to cut across more tightly packed pressure surfaces leading to increasing isentropic ascent with time, especially near and north of the IL/WI state line. As is typical in these scenarios, guidance is all over the place regarding the degree of mid-level saturation, but at least some signal for nocturnal elevated convection is present. Have introduced some slight chance PoPs overnight and into Friday morning to account for this, but the signal remains too unclear to justify anything higher at this time. The potential for overnight and morning convection muddies the forecast a bit for Friday. If robust convection persists into the morning, outflow boundaries and remnant MCVs could focus additional convection in the afternoon, and debris cloud cover may also impact temperature trends. At this point, this seems like a less likely outcome. With little/no morning cloud cover, temperatures on Friday should push into the low to mid 90s, except for northshore locales in Illinois. Would not be surprised if a few upper 90s materialized, particularly near the O`Hare/Midway/Gary vicinity. As it stands right now, the threat for additional afternoon and evening storms, while non-zero, remains under 15 percent and have continued with a dry forecast. Carlaw Saturday: The continued trend for Saturday on a majority of the guidance has been a slight slowing of the anticipated flattening out of the 500 mb ridge (heat dome), meaning capping remains likely to hold through much of if not the entire daytime period. Still can`t completely rule out an isolated thunderstorm in the far northwest CWA late in the day as a slow moving trough approaches the mid and upper MS Valley. 850 mb and 925 mb temps look to even be a smidge warmer than on Friday, along with breezy southwest winds gusting to 30 mph. Finally, the signal for dew points to mix out at peak heating is stronger than on Friday on non-NCEP guidance (which has been a consistent feature of the non-NCEP suites). With this in mind, nudged down dew points into the low-mid 60s over the Chicago metro, and a bit lower than the NBM elsewhere. Taking all these factors into consideration for Saturday`s temps, there`s a distinct potential for highs to reach the upper 90s in spots, especially in the Chicago urban heat island northward to eastern Lake County IL. At the least, mid 90s should be more prevalent than on Friday, especially near and east of I-39 where any incoming thicker high cloud cover should arrive after peak heating. With a bit warmer warmer temps and a bit lower dew point temps forecast for Saturday, the end result should again be peak heat indices in the ~95-100F range. As a final note, if the more aggressively mixed out guidance (particularly ECMWF/EPS with 50s dew points over Chicagoland) verifies, then localized ~100F highs would be in play. In this scenario, the heat indices could even be a bit lower than the actual temps. However, temps that hot (upper 90s to ~100F), still increase the risk for heat related illnesses. Keep heat safety in mind if you`ll be spending prolonged time outdoors. Castro Saturday Evening through Wednesday: The increasingly sheared nature of the approaching trough(s) while it encroaches upon the departing h5 ridge will slow the progress of the cold front associated with the weak surface low embedded in the trough. Low-level isentropic ascent may allow for sustenance of more organized storms over eastern Iowa late Saturday afternoon to reach the northwest 1/3 or 1/2 of the CWA in a decaying phase Saturday evening and overnight. Parched mid-levels with southeastward extent, weak shear, and unfavorable timing will likely keep locales near and southeast of I-55 dry through the night. It will be another warm and muggy night aside from lower end convective chances, particularly in Chicago (upper 70s). The aforementioned cold front will drift south-southeast across the area through Sunday/Sunday evening while encountering pronounced moisture pooling (low-locally mid 70s dew points). While temps will be as much as 10 degrees cooler, it will be an uncomfortably humid day on Sunday. With weak deep layer shear and generally unidirectional flow parallel to the front, we`ll need to monitor for scattered slow-moving storms and chances of localized corridors of heavy rainfall and resultant flooding Saturday afternoon and evening. The moist environment and marginal (6C/km or less) mid-level lapse rates will likely mean capping erodes prior to noon, so it may be an early start for the scattered convection (30-50% PoPs). Localized gusty winds associated with wet downbursts may be possible in this setup, but overall the severe threat appears to be low. It currently appears more likely than not that the front will clear most of if not the entire area on Monday, which would keep most locations dry. A slight slowing of the front would reintroduce a concern for additional scattered convection, hence the low (20-30%) PoPs near/south of the Kankakee River appear reasonable. Seasonable temps and less humid conditions should then prevail into the start of Tuesday. Shower and thunderstorm chances will increase again late Tuesday into Wednesday with a larger trough axis crossing the region. Castro/Kluber && .AVIATION /12Z TAFS THROUGH 18Z FRIDAY/... Issued at 619 AM CDT Thu Jul 3 2025 Main Concerns: - Timing of lake breeze wind shift at MDW and ORD this afternoon. - Isolated SHRA and maybe a TS possible on the lake breeze this afternoon. Much lighter winds aloft today will be more conducive to the lake breeze pushing steadily inland and likely moving across both MDW and ORD. Expect northeast winds to develop at GYY between 15z and 16z. The easterly wind shift at MDW could occur as early as the 17z hour, and as late as the 21z hour. ORD`s timing looks to be as early as the 19z hour and as late as the 22z hour. The lake breeze should be one of the primary foci for attempts at convection this afternoon into the early evening. However, forecast soundings are prohibitive enough for anything more than very sparse TS coverage to continue to keep the chances below threshold for PROB30 mention. If a TS does develop and directly impact a Chicago metro terminal, strong/gusty downburst winds will be possible. In the currently less likely scenario of higher SHRA and TS coverage on the lake breeze, variable outflow winds would make for a challenging wind forecast. Finally, can`t rule out a few TS nearby overnight tonight and decaying convection (non-zero TS chance) Friday morning. While chances again are too low for any TS mention in the TAFs, the setup Friday morning will need to be monitored for gusty outflow winds even with just showers (or in the absence of precip). Added a PROB30 into the ORD and MDW TAFs for this potential. Castro && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago