


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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072 FXUS63 KLOT 200555 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1255 AM CDT Fri Jun 20 2025 .KEY MESSAGES... - A localized area of strong winds may develop in the wake of showers/storms late this morning into early afternoon. - First heat wave of the summer expected this weekend into early next week with multiple day of highs well into the 90s and peak afternoon heat indices of 100-105F likely. - Extreme heat watch 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. && .DISCUSSION... Issued at 215 PM CDT Thu Jun 19 2025 Through Friday Night: Regional water vapor imagery depicts implied northwesterly flow from the northern Plains to the Great Lakes along the backside of a departing upper-level trough. Partly sunny skies in tandem with a residually moist boundary layer have allowed for cumulus to develop this afternoon amidst temperatures in the lower 80s and a westerly breeze. Over the past 30 minutes or so, have noted pockets of increasing cumulus growth including attempts for sustained convection across DuPage/Cook counties. Suspect this type of behavior will continue through the afternoon with a 20% chance of a storm at any given location across northern Illinois and northwestern Indiana. With some 50kt+ of flow at 500mb and relatively low freezing levels near 12kft, cannot rule out a storm managing to become strong and producing gusty winds at hail. Such a threat should be quite isolated (5% or less chance at any given spot). Will also have to keep an eye on the loosely-orgnaized cluster of storms moving through central Wisconsin to graze northeastern IL this evening with a threat for gusty winds. With our area on the northern side of a broad surface high centered to our south, tonight looks quiet locally with mostly clear skies and overnight lows in the 60s. With that said, it will be a busy night upstream of our area. Late tonight, a subtle upper-level wave (currently progressing through the Dakotas) should excite the development of a low-level jet across the upper Mississippi River Valley beneath an eastward- advecting plume of steep mid-level lapse rates. With time, moistening at the base of the EML should allow for explosive thunderstorm development across northern Minnesota but also possible northern Iowa. Thanks to the aforementioned pocket of dry air positioned across the Great Lakes, the orientation of the instability axis should be more or less north to south along the Mississippi River into early tomorrow morning. As a result, the upscale growth (or development of GWAC) from overnight convection should be guided toward eastern Iowa tomorrow morning. With that said, west to northwest mid-level flow would orient the stratiform region of any convection downstream into southern Wisconsin and eventually northern Illinois after daybreak. While stratiform rain is usually not overly impactful, do have to note a notable signal in model guidance for the development of a pronounced wake low/gravity wave train on the eastern side of whatever convective complex races into Iowa. If such a gravity wave were to develop, strong descent in the downward branch of the wave would allow for the low-level jet reach ground, altogether supporting a "pop" of strong southerly winds from late morning to early afternoon. HRRR guidance is most bullish in depicting a nearly 8mb/100mile surface pressure gradient from I-80 to the Wisconsin state line by noon tomorrow with associated southerly wind gusts of 50 to 60 mph. Other guidance, such as the HRDPS and experimental NSSL MPAS CAMs, appear more reasonable with southerly wind gusts of 30 to 40 mph. This will be something to monitor closely tomorrow morning as the overall character of the convection and associated stratiform region can be assessed. Outside any funny business with winds tomorrow, residual debris cloud cover will limit the onset of diurnal heating and keep temperatures in the mid 80s. At this point, don`t see much of a threat for additional convection to develop in the afternoon owing to a pronounced dry layer just above the ground (which itself would be generated by the wake low/gravity wave in the morning, should it occur). Borchardt Saturday through Thursday: Medium range guidance is in excellent agreement on developing an anomalously strong early season upper ridge over the Ohio Valley into the mid-Atlantic region this weekend into early next week. By Saturday night and Sunday, 500mb heights are forecast to peak around 597-598dm, which is 2+ standard deviations above normal for mid-late June. The strength and placement of this mid-upper level ridge will shunt the jet stream well to our northwest, from the northern High Plains east-northeast across Ontario and Quebec. This should keep convection and associated ring of fire 100s of miles to our north and west this weekend, which does increase confidence in the expected heat, as convection often disrupts forecast heat near along the periphery of heat domes like this one. By Monday, northwestern flanks of the upper ridge look to get beat down just a bit by northern stream trough, but still appears as though convection should remain well north of our CWA, but will be a bit closer than over the weekend and worth keeping an eye on in coming days. ECMWF and GFS progged 925mb temps to climb into the upper 20s to near 30C each day, which based on local 925mb temp climo would support high temperatures in the mid to upper 90s. With very little cloud cover expected Saturday-Monday, exactly how hot temps get may be dictated a bit by how deeply mixed the boundary layer gets during the afternoon. On Saturday, GFS and ECMWF both keep boundary layer depth capped off around 5kft, which would prevent afternoon dewpoints from mixing out during the afternoon. Low level trajectory from the southwest is from a region of the country and has experienced abnormally wet conditions over the past 30 days. So, despite limited contribution from evapotranspiration this early in the season, the mid-upper 70s degree dewpoints being forecast by guidance Saturday afternoon actually may not be all that unreasonable. Tight pressure gradient between low-mid 990s mb low pressure over the northern Plains and strengthening high pressure to our east should result in strong southwesterly winds, likely gusting over 30 to perhaps nearly 40 mph during the day Saturday. Given the temps in the 90s and forecasted oppressive dewpoints, the wind will not provide much relief from the heat, but more likely act likely a blast furnace for anyone venturing out during the afternoon. While confidence isn`t high in this, by Sunday and especially Monday GFS and ECMWF suggest boundary layer looks to become more deeply mixed. If this occurs, it could result in dewpoints mixing out and potentially ending up lower than currently forecast. While this would likely result in slightly lower peak afternoon heat indices, it would also likely mean ambient temps could get a couple degrees warmer. Not even out of the question that a couple locations could make a run at triple digit air temps if mixing is really maximized. Regardless of the exact high temperature, exact dewpoints, and resultant heat indices, the forecast message is the same: several days of potentially dangerous heat and humidity are expected this weekend through at least Monday. Have issued an extreme heat watch for Cook County and the city of Chicago where extreme heat warning criteria is lower due to the combination of Urban heat island effects and higher vulnerability to heat in more urbanized area. For Chicago and Cook County, it appears likely that the extreme heat warning criteria of 3 consecutive days of 100F+ heat indices will be met, which combined with oppressively warm conditions at night will offer minimal relief from the heat. Outside of Cook County, most areas will likely end up needing a heat advisory this weekend. Upper ridge is forecast to weaken Tuesday into the middle portions of next week which should allow the ring of fire convection to work its way into our area, likely bringing relief from more extreme temperatures heading into the middle of next week. - Izzi && .AVIATION /06Z TAFS THROUGH 12Z SATURDAY/... Issued at 1255 AM CDT Fri Jun 20 2025 Key Messages: - Period of -SHRA this morning into early afternoon (30% chance of TSRA at RFD). - Strong SSW winds may develop in "wake" of -SHRA with localized gusts >40 kt possible. Light southwest winds and dry conditions are forecast through the overnight hours. Thunderstorms currently across Minnesota and Iowa will continue to progress east and southeast toward the area early this morning. This activity will decrease in intensity as it approaches with the majority of the lightning activity expected to remain largely west of the area (30% chance near RFD and much lower into the Chicago metro). While in a decaying phase, confidence was still high enough to convert the PROB30 groups for VFR -SHRA to TEMPOs for the metro area terminals mid morning into early afternoon and prevail -SHRA at RFD through much of the morning. We will also be closely monitoring surface observations in case strong winds develop in the wake of these showers (and possible isolated embedded storms). Hi-res guidance has been consistent in such a scenario occurring and current storms across ND/SD/NE have already exhibited characteristics of wake low wind signatures. Wind gusts upwards of 35-45 kts would be possible, with a narrow corridor of wind gusts >45 kt not out of the question. This will likely have to be handled tactically, similar to severe thunderstorms. Regardless if these winds develop, winds will remain breezy out of the southwest through the afternoon with gusts in the mid to upper 20 kt range. 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. At least sporadic gustiness is expected overnight at the surface with prevailing gusts >20 kt expected by daybreak Saturday. 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...Extreme Heat Watch from Saturday morning through Monday evening for ILZ103-ILZ104-ILZ105. IN...None. LM...None. && $$ Visit us at weather.gov/chicago