


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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283 FXUS63 KLOT 201552 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1052 AM CDT Fri Jun 20 2025 .KEY MESSAGES... - A localized area of strong winds will occur in the wake of late morning/early afternoon showers, mainly across interior portions of northern Illinois. - First heat wave of the summer expected this weekend into early next week with multiple days of highs well into the 90s and peak afternoon heat indices of 100-105F likely. - Extreme Heat Warning issued for the city of Chicago and Cook Co. where special 3 day extreme heat warning criteria of peak heat indices of 100F+ are likely to be reached. - Heat Advisory issued issued the rest of the area Saturday-Monday. && .UPDATE... Issued at 1051 AM CDT Fri Jun 20 2025 A somewhat disorganized mesoscale convective complex continues to slowly chug southward across central/eastern parts of Iowa late this morning. The outflow that this convection is developing along will continue to push southwestward, and with the associated anvil crawler lightning bolts struggling to make it to the Mississippi River, it now seems highly probable that our forecast area should remain lightning-free through the remainder of the day. Most areas will still see a period of light rain through mid-afternoon, though. We`ll also need to monitor for new shower development along the northern edge of the nose of the low-level jet/theta-e plume to our west as it slides eastward later this afternoon, but the stabilizing effects of the earlier rain/convection should mute the coverage of any showers that manage to develop later in the day. The main weather item of note for us with this convection will actually be the winds. Multiple personal weather stations and ASOS/AWOS sites in northeast Iowa, southwest Wisconsin, and northwest Illinois have gusted into the 40-50 mph range in the past hour or so due to mesoscale subsidence/drying and the development of a wake low along the northern periphery of the rain shield. These stronger winds should be relatively brief in duration (likely lasting no more than 1-2 hours at any one location) and should weaken with time and southward and eastward extent. However, with the increased likelihood of 40+ mph wind gusts briefly being observed in our western CWA through the early afternoon, will be hoisting a Special Weather Statement for our western counties to highlight this. Ogorek && .DISCUSSION... Issued at 331 AM CDT Fri Jun 20 2025 Through Saturday: Early this morning, we find a complex of showers and embedded thunderstorms moving across IA and southern MN and on a trajectory toward northern IL. These storms had developed on the nose of a moisture-rich low level jet, but they should weaken prior to arrival and continue to decay through the morning as they outrun the better forcing and deeper moisture tied to that jet. While there remains a couple of healthier updrafts embedded in the cluster, we`re already seeing an overall downward trend in rain intensity and lightning activity out west. We should start to see rain approach our CWA around mid-morning and spread eastward into the early afternoon, all while expected to be dropping in intensity and coverage. It`s possible that a couple of storms maintain themselves long enough to reach our western CWA, but thunder coverage should be little to none into the Chicago metro this morning. A few isolated light showers may linger through mid-afternoon or so, but a big majority of the area will be rain-free following this morning. A handful of high-res model camps are resolving a period of gusty winds this morning and afternoon as a result of a wake low behind this morning`s system of showers. Already we`re seeing a handful of 30 to 35 mph gust reports well behind the rain in parts of IA and MN, and models suggest this mass response will only strengthen in the next several hours on its way here. So a period of 30 to 40+ mph gusts is looking exceedingly probable with time, especially for areas west of the Fox Valley, and it would be here during the late morning and afternoon. Winds should come down a few steps during the evening after the boundary layer decouples. However, we`ll then see a ripe low level jet move just overhead Friday night with upwards of 45 kt of flow at 925mb by early Saturday. This will bring gusty conditions to our Saturday, because while daytime mixing does not look overly deep on model soundings, it should be more than enough to tap into 35 to 40 kt of low level flow and pull some of that down to the surface. The windiest conditions should be found near and north of I-80 beneath the core of the jet. Conditions will subside but will remain breezy during the night. As far as the heat goes, today will certainly be a warm summer day, but not nearly as oppressive as what`s expected over the next several days. Temperatures this afternoon are forecast in the mid 80s. Dewpoints in the mid 60s should make conditions feel a few degrees hotter but the heat index should be capped at around 90 today. Tonight, a formidable upper high will meander from the Plains into the Mississippi Valley centering a 590 dam 500mb ridge right over the Midwest. By Saturday evening, the NAEFS and ENS ensembles are forecasting 500mb heights and 850mb temps in the 99th percentile for mid-late June over northern IL and climatological max 700mb temps! Temperatures are forecast to climb into the mid 90s for Saturday afternoon. Peak heat indices should easily surpass 100F around just about the whole area. Exact heat indices are still to be determined with uncertainty regarding afternoon dewpoints, with the few degree spread meaning a several degree difference in the heat index. But given that heat indices to around 105F are looking probable around a majority of the area with more of the same expected in the couple of days to follow, made the decision to issue a Heat Advisory around the CWA from Saturday morning through Monday evening. The only exception is Cook County where the Extreme Heat Watch was upgraded to an Extreme Heat Warning for the high likelihood of three consecutive days of 100+ heat indices, a threshold exclusive to the heavily-developed and densely- populated Cook County where the impacts of extreme heat are generally greater. See the long term discussion below for more details on the heat continuing into next week. Doom Saturday Night through Thursday: There are no major changes in our thinking for the multi-day stretch of oppressive heat which is expected to persist through at least Monday as an impressive upper level ridge for this early in the summer becomes established across the eastern half of the CONUS (peak 500mb heights approaching 600dm!). While the overall message remains the same, in contrast to Saturday, heat indices >100F will more likely be achieved Sunday and Monday via deeper mixing leading to warmer temperatures and slightly lower dewpoints. This should keeping heat indices in check and below the local Extreme Heat Warning criteria of 110F (for areas outside of Cook County). Potentially record warm overnight temperatures will lead to little in the way of relief, especially in Chicago where low temperatures may only drop into the upper 70s. This limits the ability to cool down before the next hot day, especially any buildings without air conditioning. Be sure to check on vulnerable friends, family, and neighbors over the next few days. The upper ridge begins to break down toward the middle of next week which would place the area on the northern periphery of the ridge and along a more favorable storm track. Accordingly, the blended guidance has shower/storm chances (20-40%) Tuesday through the end of the upcoming workweek. Strong to severe storms would certainly be a possibility in this regime. It is also worth noting that a slower weakening of the ridge would lead to oppressive heat indices >100F potentially continuing longer into the work week and lead to extensions of the current heat headlines. Petr && .AVIATION /12Z TAFS THROUGH 18Z SATURDAY/... Issued at 645 AM CDT Fri Jun 20 2025 Key Messages: - Period of -SHRA this morning into early afternoon (30% chance of TSRA at RFD 17-20Z). - Strong SSW winds may develop in "wake" of -SHRA with localized gusts >35 kt possible. A band of VFR rain is beginning to move into far northwest Illinois this morning. This is expected to gradually drift across the area through the morning hours. Associated lightning activity remains well to the west over Iowa and Minnesota. While thunder is not currently expected to reach the Chicago area terminals (<15% chance), a few trailing storms across south central MN may hold together long enough to reach RFD by early afternoon as depicted in some of the latest HRRR runs. Accordingly, shifted the PROB30 group for -TSRA to 17-20Z. We will also be closely monitoring surface observations in case stronger winds develop in the wake of these showers. Hi-res guidance has been consistent in such a scenario occurring though confidence in them developing as far east as the Chicago metro terminals has decreased. Southwesterly wind gusts upwards of 35 kts would be possible, with a narrow corridor of wind gusts >40 kt not out of the question near RFD. In the event these stronger winds don`t materialize, wind directions could end up variable for a couple hours behind the rain (some upstream observations ranging from SE to NW to SW around 10kt). By this afternoon, steady southwest winds in the mid 20kt range are expected. A period of low-level wind shear will be possible this evening and overnight as a 45-50 kt southwesterly low-level jet sets up overhead and surface gusts ease. However, at least sporadic gustiness will continue overnight at the surface with prevailing gusts >20 kt expected by daybreak Saturday then increasing to near 30kt by the afternoon. Petr && .CLIMATE... Issued at 114 PM CDT Thu Jun 19 2025 Temperatures this weekend into early next week could potentially threaten record highs and record warm lows for Chicago and Rockford. Here are the current records: Chicago High Warm Low Saturday 6/21 101 (1988) 74 (1923) Sunday 6/22 97 (1988) 76 (1923) Monday 6/23 97 (1930) 79 (1923) Rockford High Warm Low Saturday 6/21 100 (2022) 71 (1995) Sunday 6/22 97 (1923) 73 (1908) Monday 6/23 97 (1923) 74 (1908) NWS Chicago && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Heat Advisory from 10 AM Saturday to midnight CDT Monday night for ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-ILZ011- ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-ILZ032-ILZ033- ILZ039-ILZ106-ILZ107-ILZ108. Extreme Heat Warning from 10 AM Saturday to midnight CDT Monday night for ILZ103-ILZ104-ILZ105. IN...Heat Advisory from 10 AM CDT /11 AM EDT/ Saturday to midnight CDT /1 AM EDT/ Monday night for INZ001-INZ002-INZ010- INZ011-INZ019. LM...None. && $$ Visit us at weather.gov/chicago