Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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922 FXUS63 KLOT 212004 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 304 PM CDT Sat Sep 21 2024 .KEY MESSAGES... - Near record heat expected today, fittingly for the last day of astronomical summer - Scattered showers/t-storms tonight likely to build to a steadier beneficial rain with some embedded thunderstorms Sunday - Another period of showers and possible storms Monday night into Tuesday, highest chances along and east of I-55 && .DISCUSSION... Issued at 304 PM CDT Sat Sep 21 2024 Through Sunday night: Another unseasonably warm afternoon is underway with high temperatures solidly in the upper 80s and lower 90s as of 2 PM CDT. However, the upper ridge that has been supporting the summer-like warmth as of late is starting to break down as it gets shoved east by the approaching upper-low over the southwest CONUS. Ahead of the upper-low, strong warm advection has been interacting with a plume of moisture across IA and western IL which has resulted in some showers and thunderstorms taking shape near the Mississippi River. While these showers and storms have so far been able to maintain themselves, they should begin to interact with the much drier airmass over northern IL and dissipate prior to reaching the I-39 corridor. Though, there is a small (around 15%) chance that an isolated shower or storm could briefly impact portions of western Ogle and/or Lee counties this afternoon before the showers/storms dissipate. A better coverage showers and thunderstorms is expected to move into northern IL overnight as the aforementioned upper-low ejects a couple of vort maxes into the western Great Lakes and forces a cold through the area. While this setup will allow showers and storms to persist throughout the day on Sunday, there still is expected to be some occasional breaks in the rain on Sunday particularly during the late morning and early afternoon hours before the front arrives. Furthermore, the widespread rain will also limit the amount of destabilization that will occur Sunday afternoon which means that the coverage of thunderstorms should gradually wane through the day on Sunday as well. That said, rainfall totals from this system are expected to be solidly in the 0.5 to 1.0 inch range areawide but there is the potential for some pockets of higher rainfall amounts in excess of an inch. Unfortunately, the convective nature of the showers/storms make it difficult to pinpoint the exact location of these higher amounts. Regardless, antecedently dry conditions should allow the ground to soak up the rainfall without issue so the risk for flooding remains very low. The aforementioned cold front will begin to push the bulk of the showers and storms south of I-80 by Sunday evening which may allow most of northern IL to dry out Sunday night. However, the presence of an upper vort overhead in combination with lingering moisture may allow at least isolated showers to persist Sunday night. Aside from the rain, winds will quickly build behind the front Sunday evening especially over the open waters of Lake Michigan with gusts in the 15 to 25 mph possible. These breezy northerly winds will also build waves on the lake and create dangerous swimming conditions and hazardous boating conditions. Though, the front will finally usher in notably cooler air in its wake which will lead to more typical mid- September temperatures for Sunday and Sunday night. Yack Monday through Saturday: Cooler, more typical late September, weather is slatted for least the first half of next week following a period of showers and storms with a cold frontal passage on Sunday. Shower and storm chances with this first initial disturbance and cold front should shift south of the area for Sunday night into Monday. This should thus result in mainly precipitation free conditions in our area for daylight hours of Monday. However, expect temperatures to only top out around 70 as cool northeasterly winds prevail. While much of the day Monday is now looking dry, there is an increasing signal for another period of showers and possible storms Monday night into Tuesday as the central Plains mid/upper trough finally ejects eastward across the Mid-Mississippi Valley. As it does, it should also foster the development and consolidation of a northeastward shifting area of low pressure across IL and IN Monday night into Tuesday. The track of this wave largely looks to favor the southeastern half of the area through Tuesday (roughly areas along and southeast of a line from Waukegan to Mendota, IL). Some low potential (<20%) also exists for a few strong storms Tuesday, primarily across IN in closer proximity to the evolving warm sector. Following the departure of this wave of low pressure late Tuesday, the weather pattern looks to become rather "blocky" across the central CONUS as a building upper ridge overspreads the upper Midwest and western Great Lakes regions. Models often struggle with the evolution of blocky patterns like this, so there is not a lot of confidence in the details. However, it looks like Rex Block type pattern with northern stream ridging and a southern stream upper low somewhere in the central Plains or lower Missouri Valley region. If this Rex Block develops as ECMWF suggests with pretty good ensemble agreement, then dry and warmer than average weather could return later in the week into next weekend. KJB && .AVIATION /18Z TAFS THROUGH 00Z MONDAY/... Issued at 1245 PM CDT Sat Sep 21 2024 Key messages: - A lake breeze is expected to to provide northeast winds at GYY, east winds at MDW, and southeast winds at ORD - though magnitudes should be 10 knots or less - A front will move through from west to east providing chances for showers and thunderstorms around northern Illinois and northwestern Indiana Sunday There is a line of showers and storms over the Mississippi River gradually moving east, but are starting to fall apart as they interact with the drier air in Illinois. The line will be monitored, but given the lower confidence in impacts, TAFs were kept dry through this evening. The main impact in the afternoon will be a lake breeze that develops. Winds ware expected to turn to the northeast at GYY, near east at MDW, and southeast at ORD. However, wind speeds should be at or less than 10 knots. A front is expected to move across the airspace early Sunday morning. Confidence with the onset of rain is low at this time, it was opted to maintain the going PROB30 mention prior to 12Z Sunday morning. Confidence then increases with showers at the terminals after 12Z. While there will be instability present, latest guidance is keeping the threat of storms more isolated and embedded in nature. Therefore, the VCTS mention was maintained in the latest TAF. The chances for thunder are expected to diminish after 18Z, though showers should remain around the terminals. Winds ahead of the boundary will be out of the southeast before flipping to the southwest. Guidance is suggesting that by midday, wind directions will slowly become more westerly and even northwesterly after 21Z. A lower level jet aloft may develop and with proper mixing, occasional stronger gusts up to 20 knots are possible Sunday afternoon. DK && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago