Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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330 FXUS63 KLOT 231934 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 234 PM CDT Mon Sep 23 2024 .KEY MESSAGES... - Waves of showers will move through the region tonight through tomorrow. - An unusually high degree of uncertainty exists in the forecast from Friday onwards. && .DISCUSSION... Issued at 234 PM CDT Mon Sep 23 2024 Through Tuesday night: Early-afternoon GOES-16 water vapor imagery depicts an elongated positively-tilted trough over the Lower Mississippi River Valley downstream of a secondary trough racing southeastward across the northern Plains. Meanwhile, a recent surface analysis reveals a broad surface low over the Ozarks within a broad baroclinic zone that arcs northeastward toward the Lower Great Lakes. Mid to upper-level clouds continue to spread over our area, which will continue to limit temperatures through the afternoon. In all, the table is set for another period of wet weather for parts of our area. Over the next 24 hours, the Lower Mississippi River Valley and incoming northern Plains troughs will phase, causing the surface low to deepen while lifting northeastward along the baroclinic zone and into the Lower Great Lakes. While a northwestward and stronger trend in the path and strength of the low, respectively, was noted overnight, a reverse trend of a weaker and further southeast evolution of the low has been exhibited by 12Z/morning guidance. Regardless of where the low goes, increasing diffluence in weakly coupled and eventually the left exit region of an upper- level jet acting upon the broad baroclinic zone should allow for waxing and waning areas of deformation and frontogenesis, altogether supporting waves of showers to parade across the general region tonight through tomorrow. The one fly in the ointment for tomorrow remains the threat for a lake-induced warm-core mesolow to develop beneath the phasing troughs, which may lead to a focused area of low-level confluence and a persistent band of rain somewhere near the Lake Michigan shoreline. With PWATs near 1.5" and weak tropical-like lapse rates through the troposphere, rainfall rates in any lake-induced confluence bands can quickly get out of hand (that is, >2"/hr), leading to a very localized but high-impact area of flooding. Forecasting the development of lake-induced mesolows, let alone stationary confluence bands, is quite tricky especially when phasing upper-level troughs are involved. Given the trend in model guidance is for a weaker synoptic-scale low with a further southeastward path, think the threat for a stalled mesolow near the shoreline now looks lower than before. With that said, observational trends in the track and strength of the surface low will be compared against model guidance overnight to help inform any increase in messaging for a localized threat for flooding. Mesolow or not, on-and-off lake effect showers are still expected along the Lake Michigan shoreline all the way through daybreak Wednesday. Borchardt Wednesday through Monday: Quieter and seasonably warm (highs in the 70s, warmest away from the lake) conditions are expected Wednesday and Thursday. It will be relatively cool Wednesday night, in the upper 40s to mid 50s outside of Chicago, thanks to favorable radiational cooling conditions. Our attention will then turn to the remnants of expected tropical cyclone Helene (still PTC-9 as of this writing), for the Friday onward forecast. While there is a good deal of uncertainty at this lead time, the mid and upper pattern over the region does exhibit resemblance to the approach of Francine`s remnants a week and a half ago. A strong upstream blocking ridge (near 590 DaM 500 mb heights) looks poised to develop and extend across the northern Great Lakes in the Friday-Friday night period. For this reason, the farther north guidance with Helene`s remnants and QPF footprint, namely the Canadian global and its ensemble, may not be suppressing the remnants enough given the upstream blocking. The upper pattern is also a bit complex even in the more suppressed guidance through Friday night into early Saturday, with interaction and possible eventual merging between Helene`s remnant closed low and a larger upper low over the mid South/mid MS Valley. As this occurs, the remnant moist plume may eventually yield an occasionally showery regime Saturday through Monday. With the above thinking in mind, collaboratively lowered PoPs through Friday night, especially with northward extent, where primarily dry conditions appear quite likely, and then maintained 30-50% PoPs Saturday onward for the CWA. Aside from precip. chances, expect a prolonged period of breezy northeasterly winds Friday-Saturday as Helene`s remnants and expansive surface high pressure locked to our north yield a tight pressure gradient. This will likely bring another round of dangerous lakeshore conditions. Temperatures will average near to above normal into early next week as we (already) turn the page into October. Castro && .AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 1234 PM CDT Mon Sep 23 2024 The main aviation weather concerns are: - Deteriorating cigs/vsbys tonight into Tuesday morning with the next batch of rain. - Uncertainty remains in the exact storm track, with resultant uncertainties regarding cigs, vsbys, and where the steadiest/heaviest rain sets up. The next system will arrive late tonight into Tuesday. Cigs will build down to low-MVFR, potentially IFR, at the Chicago- area terminals. A swath of steady rain is expected to develop, but uncertainty in the exact placement of the steadiest/heaviest precipitation remains. For now, have relegated the lowest vsbys (1sm) within a PROB30 group. The lowest cigs/vsbys are expected through about late-morning Tuesday. At RFD, latest indications are that most of the precipitation and lowest cigs/vsbys may remain just off to the east of the airfield. Activity should tend to ease/diminish through midday and into the afternoon hours. A secondary push of showers may develop Tuesday evening, beyond the current extended ORD/MDW TAF windows. There is low confidence in wind directions after about 12z Tuesday, with some potential for winds to become northwesterly earlier than advertised, and highly dependent on the low track. For now, have maintained consistency from the previous forecast which remains inline with most guidance at this time, but note that alterations to wind directions may be necessary in future updates. Carlaw && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Beach Hazards Statement until 10 PM CDT this evening for ILZ006-ILZ103-ILZ104. IN...Beach Hazards Statement until 10 PM CDT this evening for INZ001-INZ002. LM...Small Craft Advisory until 7 PM CDT this evening for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago