Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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436 FXUS63 KLOT 240625 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 125 AM CDT Tue Sep 24 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 /06Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 125 AM CDT Tue Sep 24 2024 Key Messages: - Rain showers and drizzle at times through the TAF period. - Ceilings as low as LIFR and visibilities as low as IFR expected at times through this afternoon. A baggy low pressure system centered near KSTL at press time will continue to lift northeastward throughout the day today and bring periodic showers through the area today. During the overnight hours, drizzle may be observed at times and produce IFR/MVFR visibilities and LIFR/IFR ceilings. Towards and especially after daybreak, precipitation should become more of a true rain, with the potential for a few-hour long period of steady rainfall to produce visibility and ceiling reductions that will be comparable to what the overnight drizzle has been producing. RFD may be far enough removed to the northwest to avoid seeing the more impactful ceiling and visibility reductions that are expected in the Chicago metro, but could still nevertheless experience MVFR conditions at times. A few lightning strikes could not be ruled out at/near the Chicago metro terminals during the daytime today, but it appears now that the better chances for thunderstorms will remain southeast of the terminals. Most of the rain should come to an end by this evening with visibilities and ceilings likely to improve by this time as well, but at least isolated showers and MVFR ceilings will likely linger around in the vicinity of Lake Michigan Tuesday night into Wednesday morning. The wind direction forecast for today is a bit tricky with the low pressure center passing by and the expected rain, but guidance has generally trended towards winds turning northerly and northwesterly a little earlier compared to previous model runs, so have adjusted the wind directions in the TAFs a bit to account for these trends. Ogorek && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago