303 FXUS63 KLOT 091152 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 652 AM CDT Mon Jun 9 2025 .KEY MESSAGES... - A few instances of penny to quarter size hail, and 50+ mph winds may accompany some of the strongest storms in northeastern IL and northwestern IN this afternoon. - A period of dry and warmer weather is expected midweek. - Shower and thunderstorm chances return late in the week into next weekend. && .DISCUSSION... Issued at 320 AM CDT Mon Jun 9 2025 Through Tuesday: Recent GOES-19 water vapor imagery depicts a couple of small scale impulses across MN rotating around the western and southern periphery of an upper low centered over the Arrowhead region of MN early this morning. The first, is currently shifting across southwestern MN, while the second is noted digging southward across western parts of MN at the time of this writing. Our interest today lies with the northernmost impulse digging across western MN. This feature, along with a secondary surface cold front, is expected to be the focus for scattered showers and thunderstorms as it swings southeastward right across northern IL and northwestern IN during the early to mid afternoon hours today (roughly 12-3pm). An unseasonably cold mid-level airmass is associated with this robust mid/upper low. In fact, the 00Z RAOBs from yesterday evening across the Dakotas into MN sampled 500 mb temperatures of -18 to -22C, which based on the SPC`s sounding climatology rank within the coldest 10% for this time of year. Our main concern with this cold airmass is that as it shifts overhead during peak heating today steepening low to mid-level lapse rates will support an uptick in thunderstorm chances along the front with eastward extent across northeastern IL into IN and Lower MI. The presence of relatively low freezing levels around 7,000 ft AGL and a strengthening low to mid-level wind field will allow the strongest storms to produce some instances of hail (3/4"+ in diameter) along with localized strong wind gusts up to 60 mph. The main threat area for these stronger storms will primarily be across northwestern IN and points east-northeast into Lower MI this afternoon. However, the threat for strong storms as far west as the Fox Valley in northern IL cannot be ruled out. Expect the primary storm threat to end later this afternoon as the front shifts east of the area. Some isolated showers and possibly a storm could linger behind the front late this afternoon across far northern IL, but this activity would also end prior to sunset early this evening. Outside of showers and storms today, highs will range from the near 70 (northwest) to the mid 70s (southeast). Winds will also be breezy through the afternoon, with west winds expected to gust up around 30 mph. These breezy conditions will abate around sunset early this evening. The upper low will shift across southeastern Ontario on Tuesday. in its wake, we are expecting a rather quiet and dry weather day. High temperatures will rebound a few degrees, with readings generally in the mid to upper 70s. Skies during the day look to be partly cloudy, and with more wildfire smoke potentially moving back into the area, we could end up with some hazy conditions once again. KJB Tuesday Night through Sunday: Mid-level ridging will be building into the Mississippi and Ohio River Valleys Tuesday night into Wednesday which will promote a period of dry weather. Beneath the ridging skies look to remain mostly sunny with stout south-southwesterly flow both at the surface and in the mid-levels which will set up a period of above normal warmth as well with highs in the mid to upper 80s. In fact, with 850 mb temperatures forecast to be in the +16 to +20C range, there is a good chance for temperatures to flirt with the lower 90s Wednesday afternoon. From Wednesday night/Thursday onward the forecast becomes much trickier due to a complicated upper pattern overhead. Essentially an area of zonal (west-east oriented) upper-level flow is forecast to set up across the central CONUS with modest ridging remaining to the south and a more amplified pattern (characterized with several shortwaves pivoting through it) across the northern CONUS and southern Canada. With northern IL and northwest IN currently forecast to be smack in the middle of these patterns, which side we will ultimately be under is very uncertain. As if that wasn`t enough, a low-level baroclinic zone is also forecast to set up somewhere in the Mississippi Valley and Western Great Lakes region serving as the focus for periods of showers and thunderstorms. That said, there are a couple of note worthy scenarios that are worth discussing. For the Wednesday night through Thursday night timeframe: There is a rather pronounced signal in the 00z ensemble guidance (nearly 60% of the members) that the aforementioned baroclinic zone may stay just north of our area and keep the bulk of any shower and thunderstorm activity into WI. If this scenario plays out, then Thursday could very well be similar to Wednesday with highs once again in the upper 80s to near 90, depending on the degree of cloud cover, and mostly dry conditions. However, if the zone sets up farther south (as 40% of members suggest) then at least the northern third to half of our area could see showers and thunderstorms along with cooler temperatures due to more easterly winds. Additionally, there could also be a conditional flood threat with this baroclinic zone given that mid-level flow (which steers showers and storms) will parallel the boundary resulting in training activity over the same areas for an extended period of time. For now, have maintained the POPs offered by the NBM which has 20-40% chances for showers and storms across northern IL and northwest IN (highest near the IL-WI line) which seems reasonable based on the described uncertainties. For the Friday through Sunday timeframe: This period of the forecast will likely be dependent on how far north the baroclinic zone is on Thursday and whether or not it gets pushed southward by the showers/storms forecast to develop upon it. Furthermore, there is also an upper low that guidance is developing across the southern Plains and lifting into the Ohio River Valley and southern Great Lakes late Friday into Saturday. That said, the ensemble consensus currently favors (60-70% of the members) the baroclinic zone getting pushed into our area on Friday brining with it showers and thunderstorms for Friday afternoon. Then another round of showers and possible storms looks to move into the area with the aforementioned upper low Friday night with wet weather lingering through Sunday. Now there is still a chance (around 30%) that the upper low stays far enough south that the baroclinic zone is able to push further south on Saturday and focus the showers south of our area on Sunday. So at this point the weekend does not look like a complete washout just yet, but something we will be watching closely over the coming days. Regardless of how the rain plays out, Friday through the weekend does look to have more seasonable temperatures with highs in the mid-70s to lower 80s inland and cooler conditions near the lake due to onshore winds. Finally, rain does look to get pushed out of our area Sunday night into Monday as broad ridging begins to build into the western and central CONUS for next week. While this pattern will favor another period of above normal temperatures for mid-June, current consensus is for the ridge axis to be centered more into the central Plains which should keep our area on its eastern periphery. This means that we could see periods of showers and storms if shortwaves round the ridge next week. Yack && .AVIATION /12Z TAFS THROUGH 18Z TUESDAY/... Issued at 652 AM CDT Mon Jun 9 2025 Forecast concerns for the terminals... - Scattered showers and thunderstorms late morning through the afternoon (17z-22z). A few storms could be severe with gusty winds and hail. - Breezy non-thunderstorm winds this afternoon gusting 25-28 kts. Another period of breezy winds expected on Tuesday. An upper low continues to spiral into the western Great Lakes this morning with an associated cold front stretching from central WI to eastern NE. While there are a few showers along the cold front in WI, additional scattered showers and thunderstorms are expected to develop into eastern IA and northern IL later this morning and then move into northwest IN this afternoon. Despite the limited moisture, sufficient instability and deep layer shear are expected to be present this afternoon which may allow a few storms to become severe with gusty winds up to 50 kts and hail up to quarter size as the main threats. The scattered nature of the showers/storms should limit their time over any location to 2-3 hours at most, but the broad forcing of the upper low will support the potential for off and on showers through 00z this evening. Outside of the showers/storms, winds will gradually increase this morning as mixing commences resulting in west-southwest gusts in the 25-28 kt range this afternoon. While gusts will subside this evening, the 25-30 kt low-level jet overhead may allow for occasional 20 kt gusts to reach the surface overnight especially at the Chicago terminals. Regardless, winds will once again become breezy on Tuesday but with gusts in the lower 20 kt range from the west. Otherwise, expect VFR conditions to prevail through the TAF period, however, there is a chance (<20%) that some 2500-3500 ft ceilings drift into northern IL early Tuesday morning. Since the MVFR ceilings are lower confidence have opted to tease this potential with a FEW035 mention for now. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory from 1 PM this afternoon to 8 PM CDT this evening for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago