Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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398 FXUS63 KLOT 120833 CCA AFDLOT Area Forecast Discussion...CORRECTED National Weather Service Chicago/Romeoville, IL 333 AM CDT Wed Jun 12 2024 .KEY MESSAGES... - Decaying showers/storms may push into parts of far northern Illinois late tonight/early Thursday morning. - 40-50% chance for thunderstorms Thursday afternoon and evening. Any storms that develop would be capable of damaging wind and hail and localized flooding. - Hot and humid conditions are expected early next week, though thunderstorms may occur at times and modulate the heat. && .DISCUSSION... Issued at 315 AM CDT Wed Jun 12 2024 Through Thursday Night: Generally zonal flow is in place across the northern Plains with a west northwest flow pattern across the Upper Midwest. This will set the stage for potential MCS development across the region the next few days. As is often the case in these scenarios, the forecast for each subsequent day is highly dependent on thunderstorm evolution the day prior and results in lower than normal confidence. Today has the potential to be the warmest day of the year so far as southerly warm and moist air advection remains established over the region. Highs are currently forecast to reach the upper 80s to around 90. An attempt at a lake breeze this afternoon may hold high temperatures in the low to mid 80s near the immediate lakeshore, however. Heat indices today look to remain mostly in check with dew points mixing into the 50s and potentially as low as mid 40s over the Chicago metro. Several thunderstorm complexes will likely be underway across portions of Minnesota/Wisconsin/Iowa by this evening and overnight. Model guidance continues to favor any upstream storms being in a decaying phase as they approach the area thanks to strong capping and dry low levels which should keep most of the area dry through tonight, with the best chance (20-30%) for a shower/storm mainly across northwest Illinois and toward the WI/IL stateline. As for the severe thunderstorm potential late Thursday afternoon into the evening, it remains highly conditional on whether thunderstorms, in fact, develop. Questions remain regarding how early Thursday AM decaying storms and resulting surface boundary locations influence warming during the day on Thursday, and accordingly impact the thunderstorm potential. As a result, a few general scenarios of how Thursday might play out are becoming apparent: Scenario 1: Greater shower/cloud coverage persisting through the morning hours and/or earlier surface frontal passage which would limit our warming, keeping highs in the 80s and below the convective temp needed to break the stout capping in place (per 00Z NAM/NAMNest/ECMWF). This would keep a large portion of the local area free from convection altogether, including the Chicago metro. This scenario does include the potential for storms that develop across southeast Iowa to progress east southeast across the southern third of the forecast area (generally south of a Peru, IL to Rensselaer, IN line) with a damaging wind threat. Scenario 2: Less cloud cover and/or a quicker recovery of the warm airmass would result in temperatures warming into the lower 90s across the area. This would likely be warm enough to break the capping inversion prior to sunset resulting in numerous thunderstorms across the area (per 0Z HRRR, ARW, RAP, GFS, and 6Z NAMNest). If this is the case, there is ample instability in place areawide with 2000+ J/kg of MUCAPE. This combined with 35-40kts of deep layer shear would be supportive of damaging wind and hail. Scenario 3: The 0Z NSSL WRF is a bit of a combination of the two above scenarios with storms developing generally along/south of I-88 mid afternoon along the effective front as the mid-level wave approaches, with less coverage (isolated to widely scattered), any of which could be severe. At this range, this may be the more likely of the three scenarios. With all of the above said, the SPC severe threat level 2 of 5 for areas generally south of I-80 seems appropriate with a Level 1 of 5 risk for the rest of the area. Any storms that do develop will also be capable of high rainfall rates and localized flooding given PWATS potentially upwards of 2 inches. Continue to monitor the latest forecast trends as it may not be until Thursday morning that we get a decent handle on the thunderstorm potential. Petr Friday through Tuesday: The two primary forecast concerns and challenges in the extended are high temps and thunderstorm chances. Friday and Saturday are expected to be dry with temps in the lower/mid 80s inland, cooler near the lake. An upper ridge is expected to build across the region Sunday through the middle of next week and if this materializes, high temps Sunday through Wednesday next week could be in the lower to middle 90s each day. However, precip chances have crept back into most periods of the extended and from this distance, there is little skill in trying to make changes. If thunderstorm clusters are able to form close enough to or in the local area, they will impact temperatures and their outflows/lingering boundaries will affect later possible rounds of thunderstorms. There are two time periods of note, the first is Sunday as the ridge is building north. There does appear to be some chance for thunderstorms across northern IL or perhaps just north of the area. In addition to the uncertainty if precip will occur, there is also uncertainty of when it might occur. The second time period is at the end of the period, next Wednesday. There has appeared to be some consensus for a frontal boundary moving across the area for the middle of next week, which would bring a break from the heat and perhaps a better chance of showers/thunderstorms when/if it moves across the area. cms && .AVIATION /06Z TAFS THROUGH 12Z THURSDAY/... Issued at 1223 AM CDT Wed Jun 12 2024 Only forecast concern is a chance of showers, possibly with isolated thunder, early Thursday morning, at the end of the 30 hour ORD/MDW tafs. A weak disturbance will move across the area early this morning with a mid deck expected to develop across the area. There is a small chance of sprinkles or a few showers toward daybreak this morning but confidence is too low to include any mention with this forecast and trends will need to be monitored over the next few hours. There is another chance for showers and possibly isolated thunderstorms early Thursday morning. This activity will be dependent on how widespread convection becomes northwest of the area Wednesday night and how far it can persist before it dissipates Thursday morning, with a better chance for RFD and northwest IL. Confidence is too low to include anything for the new ORD/MDW 30 hour tafs. Light southerly winds to start will turn more southwest by daybreak with speeds increasing to 10-12kt later this morning with a few gusts possible. Winds are expected to turn more westerly this afternoon then back southwesterly this evening. Low level winds will increase tonight, though they appear to remain below low level wind shear criteria. However, Its possible some gusts into the 15-20kt range may develop tonight. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago