


Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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700 FXUS63 KLOT 191406 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 906 AM CDT Thu Jun 19 2025 .KEY MESSAGES... - Scattered showers/isolated thunderstorms end by mid-morning today, though could see an isolated storm or two in the vicinity of the lake breeze late this afternoon in NE IL and NW IN. Additional scattered thunderstorm potential exists Friday. - Stretch of hot and humid conditions with peak afternoon heat indices possibly over 100 degrees this weekend into early next week. - Chances for rain return middle of next week && .UPDATE... Issued at 906 AM CDT Thu Jun 19 2025 Recent satellite imagery depicts clearing skies from northwest to southeast across the area in the wake of a departing upper- level shortwave. Current temperatures around 70 will hence be poised to rise 10 to 15 degrees by this afternoon leading to highs in the lower 80s. As a surface high pressure system currently centered near the KS/MO/OK/AR border regions shifts southeastward this afternoon, surface winds across the Great Lakes will modestly back westerly. Meanwhile, winds over Lake Michigan will turn south to southeasterly in response to the thermal gradient along the shoreline. The net result should be the development of a low- level confluence zone along the shoreline. With forecast soundings depicting largely uncapped temperature profiles by mid-afternoon, a pocket of dry air above 850mb in tandem with mid-level subsidence may act to effectively squash any convective attempts this afternoon. With that said, if any updraft were to become established, effective shear on the order of 25 to 30 kt and relatively low freezing levels near 12kft would encourage storm- scale organization with attendant threats for gusty winds and hail. At this point, this seems to be a fairly low- probability scenario (20% chance a storm forms, followed by a 5% chance it becomes severe, effectively leading to a 1% joint probability for a severe storm. That`s not zero, but certainly quite a bit lower than yesterday). Updated products will be sent soon. Borchardt && .DISCUSSION... Issued at 356 AM CDT Thu Jun 19 2025 Through Friday Night: Early morning GOES vapor imagery depicts a mid-level trough centered across the western Great Lakes and Midwest. Scattered showers and a few isolated thunderstorms persisted per regional radar mosaics, mainly associated with two sheared mid-level vorticity axes, extending as far west as southwest WI/northwest IL. These scattered showers and isolated storms will continue to shift east across the region over the next several hours, exiting the forecast area by around mid-morning. Guidance indicates substantial subsidence developing in the wake of the departing trough this afternoon, with 50-60 meter 500 mb height rises noted. This, combined with a modest decrease in low-level moisture, would appear to be at least somewhat unfavorable for any diurnal convective redevelopment today. However, several CAMs depict isolated to widely-scattered convective cells during peak diurnal heating later this afternoon across far northeast IL and northwest IN, in a region of weak convergence in the vicinity of a weak lake breeze boundary (which looks to stay near the IL shore then extend southeast into IN). Even through forecast soundings do show some subsident warming in the 600-700 mb layer, afternoon thermodynamic profiles suggest little/no capping with perhaps 500-700 J/kg of MLCAPE for a time. Surface dew points are likely too high and thus convection likely overdone in the 4km NAM, ARW and NSSL WRF guidance, though can`t rule out a couple of isolated late afternoon SHRA/TSRA similar to the drier HRRR output. Have included a slight (15%) pop within the aforementioned convergence zone for this late this afternoon. Any isolated showers or storms should dissipate quickly by sunset this evening, with dry and quiet conditions expected overnight. Well to our northwest however, guidance has trended more bullish with the development convection across the eastern Dakotas/Minnesota which several models develop into an organized MCS. Several CAMs propagate this MCS southeast into the upper Mississippi Valley early Friday morning, potentially moving into northwest/northern IL after sunrise in a decaying form. While confidence is not extremely high in this, did add chance pops for thunderstorms across northern IL Friday morning. SPC`s Day 1/2 severe outlooks include far northern IL along the southern periphery of the greater severe threat across MN/WI for this possibility, with wind the main concern if the MCS were to maintain itself this far southeast. If the decaying MCS and/or remnant outflow boundary were to push into the forecast area Friday, redevelopment of thunderstorms may occur during the midday/afternoon hours with diurnal heating. At the same time however, a well-advetised synoptic upper level ridge begins to build in earnest toward the Mississippi River Valley. Warming aloft (noted in the 450-650 mb layer in forecast soundings) associated with the development of the ridge may provide more substantial capping which could limit the redevelopment of afternoon convection, especially if the earlier outflow limits surface temps a bit. Have maintained some slight chance (<25%) pops north of I-80 for this at this time, though confidence is fairly low in that level of detail at this time. Strengthening of ridge and increasing capping looks to shift thunderstorm threat north of the area Friday night. Temperature-wise, highs this afternoon should rebound into the low- mid 80s (coolest into northwest IN) with some sunshine. Solid mid- upper 80s are reachable Friday as low-level warm advection begins to ramp up, though the potential effects of any morning MCS outflow lends lower confidence to widespread upper 80s just yet. Ratzer Saturday through Wednesday: A trough over the western CONUS is expected to deepen, further amplifying the upper level ridge over the eastern CONUS on Saturday. Southwesterly steering flow will continue to advect in warmer temperatures, driving 850 mb temperatures into the 20 to 25C range over the weekend. As dew point temperatures climb into the low 70s, it is setting up quite the hot and humid weekend across the forecast area where heat indices will reach into the mid 100s (low 100s in Chicago). However, there is lower confidence in dew point trends. The NBM is coming lower than other models, particularly for areas outside of the Chicago Metro. If higher dew point temperatures are realized, that could drive up heat index values above the current forecast. In addition to the uncertainty of the dew points, ensembles are suggesting a middle level shortwave will move from the Dakotas toward Lake Superior and potentially send a MCS west to east just north of the forecast area over Wisconsin. While both Saturday and Sunday are expected to be dry, there is still uncertainty on this storms track and its influence in northern Illinois. For now, it was decided not to issue any excessive heat products for the weekend. The upper level trough out west is expected to phase with another upper level low that drops southward over Saskatchewan Sunday into Monday. Models are suggesting the lows aloft break down into more of an open wave and move east on Monday into Tuesday. The upper level ridge over the Mid-Atlantic will gradually compress and drift slightly more over the southeast through midweek. As the ridge sags southward, temperatures over northern Illinois will slowly diminish from the 90s on Monday and Tuesday down into the 80s on Wednesday. As a new surface high pressure grows early next week in Central Canada, a weak surface baroclinic zone is expected to develop along a roughly Denver to Milwaukee line. There is still some discrepancies in the models regarding the boundaries exact location, but many of the ensembles are suggesting it could become quasi- stationary through the middle of next week. Meanwhile, persistent south-southwest mid level winds (from the high over the southeast) will advect Gulf moisture northward into this area driving precipitable water amounts up to around two inches. With such nebulous forcing, rain rates should be weak enough that inhibit flooding concerns for the moment. And given the uncertainty on the exact placement of the boundary and northern Illinois sitting just on its southern periphery, there could be periods without rain that will need to be ironed out. But the signal is growing that the middle of next week could be a bit wet. DK && .AVIATION /12Z TAFS THROUGH 18Z FRIDAY/... Issued at 633 AM CDT Thu Jun 19 2025 Key Aviation Messages for the current TAF period: - Lingering showers are expected to move east of terminals by 14Z - Less than 20 percent chance for an isolated storm to develop along a lake breeze this afternoon. VFR cigs and vis have returned to the airspace and are maintained through the current TAF period. The last bit of showers are still over northern Illinois and moving east- southeast. Most terminals will start to dry out by 13Z, but high confidence for showers to be east of the terminals by 14Z. Drier conditions are expected today. Models are suggesting a weak lake breeze that will remain east of airfields this the afternoon. Confidence is low in an isolated storm developing along it so the TAF was kept dry. Winds will remain light and out of the northwest and slowly become west by midday. Lighter south-southwest winds are expected this evening and through the overnight. There is also the potential for another chance for thunderstorms around and after 18Z on Friday, but the probability was too low to consider it at this TAF issuance. DK && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago