Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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735 FXUS63 KLOT 220851 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 351 AM CDT Sun Sep 22 2024 .KEY MESSAGES... - Beneficial rainfall today with a 30-50% chance for isolated to widely scattered thunderstorms. - Second wave of rain and possibly some thunderstorms is possible later Monday night through early Tuesday, especially southeast of I-55. && .DISCUSSION... Issued at 351 AM CDT Sun Sep 22 2024 Through Monday: An upper-level trough centered over Colorado is in the process of getting sheared out this morning. Out ahead of it to the east, a baroclinic zone extends northeastward into the Great Lakes with multiple convectively-enhanced disturbances noted within this zone on GOES water vapor imagery. Unsurprisingly, there are multiple regions of shower and thunderstorm activity present across this corridor, and some of this activity will work its way across our forecast area today. While some locations could see rain prior to daybreak, the more widespread rainfall likely won`t arrive until after sunrise, when richer, deeper column moisture (precipitable water values between 1.6 and 1.8 inches -- near-record values for this time of year, per DVN/ILX sounding climatology) is slated to roll in from the southwest. In response to the increasing large-scale ascent, mid-level lapse rates should steepen enough to support the potential for at least isolated thunderstorms today. The main window for today`s thunderstorm potential appears to be this morning with the nose of an impinging mid-level jet and/or outflow boundaries from early morning convection in southern Wisconsin serving as potential foci for convective development. However, if this morning`s rainfall ends up being less widespread (or just less detrimental to destabilization efforts) than expected, then thunderstorms may also develop later this afternoon along an incoming cold front. The overall degree of available instability in either case looks to be far too meager to support a threat for severe weather, especially in the absence of stronger deep- layer shear. Additionally, despite the high level of moisture content for this time of year, progged rainfall rates and storm motion do not appear to be overly supportive of a threat for flooding (especially when considering the antecedent dryness), though suppose that some localized ponding could be seen in poor drainage areas wherever any convective elements and associated heavier rainfall rates are able to persist for a relatively longer period of time. For most though, this should be a beneficial rainfall that puts a dent in our rainfall deficits for this month. Most of the rain should come to an end after the passage of the cold front this afternoon/evening, though light post- frontal showers or drizzle could linger longer into the night. Today`s rain and cloud cover will bring an end to our recent spell of above normal and near-record warm temperatures, though today`s high temperatures still look like they`ll end up being near-normal for this time of year (low to mid 70s). Slightly cooler, though still similar, temperature readings are expected on Monday in the wake of today`s cold frontal passage. Behind the front, winds will turn northerly and become blustery over and near Lake Michigan late today through Monday morning. These winds will whip up some decent waves over the lake that will cause dangerous swimming and boating conditions through Monday night. Ogorek Monday Night through Saturday: Final slug of vorticity associated with the Plains weakening upper low is progged to move eastward across the Mississippi Valley Monday night into Tuesday. Some differences in guidance in how amplified this wave will be as it moves across the region, meaning some varying solutions in how far north final wave of rain will spread. NBM pops did a respectable job representing that uncertainty with highest pops southeast of I-55 into northwest Indiana. Northwestern CWA may end up seeing little if any rain from this second wave if the more suppressed solutions verify. It looks like there will be at least a couple of days of dry weather with seasonable temperatures Wed and Thu. After that confidence is very low in the forecast. As mentioned previously, medium range guidance continues to develop a very high amplitude and blocky pattern across North America. Model predictability tends to be much lower in these type of pattern, particularly with smaller scale details. The general idea for late this week into the weekend is for Rex Block type pattern to develop with strong positive height anomalies over the upper Great Lakes into Canada with a potential cut off low developing south of this ridge across the lower or middle Mississippi Valley. Further lowering the model predictability is a potential large tropical cyclone spinning up in the Gulf of Mexico later this week before possibly interacting with the cut off low. The spectrum of possible outcomes for later this week is huge with our whether late this week into the weekend ranging from potential flooding rains to sunshine and unseasonably warm temps, and everything in between. It will hinge on if and where a cut off mid-upper level low develops and then if a tropical cyclone develops, how it interacts with the possible upper low. We continue to see significant model to model and run to run variability in the details of these key features and wouldn`t be surprised to see additional large inter and intra model variability over the coming few days. Given this, made no changes to the NBM forecast Thursday night onward, which does now include some very low pops. - Izzi && .AVIATION /06Z TAFS THROUGH 12Z MONDAY/... Issued at 1243 AM CDT Sun Sep 22 2024 - SHRA/embedded TSRA likely after sunrise into the afternoon - Wind shift to northerly late this afternoon/very early evening with cold front Showers and thunderstorms over southern IA and northern MO will move east and begin to impact the terminals later this morning. Guidance is aggressively in developing MVFR/IFR CIGS this morning once the rain begins. Opted to hang onto timing from previous TAFs for MVFR and IFR, however, guidance is often too quick with developing CIGs, so may need to push back timing in later updates based on observational trends. Timing for the heaviest SHRA/embedded TSRA still looks to be mid morning into the early afternoon with coverage of rain expected to decrease late in the afternoon before ending around sunset. Southwesterly winds expected much of today until a cold front results in an abrupt wind shift to northerly late in the afternoon. Period of gusty northerly winds is expected at the terminals closer to the lake this evening. After a potential period of IFR CIGS, expected CIGS to lift a bit to MVFR late in the afternoon with a tendency toward scattering out tonight. Timing of clearing may be delayed a bit by some lake stratocumulus at GYY and potentially ORD and MDW. - Izzi && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Beach Hazards Statement from this afternoon through Monday evening for ILZ006-ILZ103-ILZ104. IN...Beach Hazards Statement from this evening through late Monday night for INZ001-INZ002. LM...Small Craft Advisory from 4 PM this afternoon to 7 PM CDT Monday for the IL nearshore waters. Small Craft Advisory from 7 PM this evening to 10 PM CDT Monday for the IN nearshore waters. && $$ Visit us at weather.gov/chicago