Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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650 FXUS63 KLOT 240836 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 336 AM CDT Tue Sep 24 2024 .KEY MESSAGES... - Showers, drizzle, and even a couple of thunderstorms today, with some occasional bursts of heavy rain likely - Lake effect showers could continue tonight into early Wed over NW IN - Chances for showers this weekend into early next week, but confidence in specifics remains low at this time && .DISCUSSION... Issued at 336 AM CDT Tue Sep 24 2024 Through Wednesday: Bottom line up front: Waves of showers, possibly heavy at times, and a couple of thunderstorms are expected today, especially this morning over our western CWA, continuing into the afternoon eastern CWA. In between showers, drizzly conditions are expected today, especially this morning. System rain is expected to move out of the CWA later this afternoon or very early this evening, however lake effect showers threat is expected to continue over northwest Indiana tonight and possibly into Wednesday morning. Details: Surprisingly tricky forecast today with still substantial spread in model guidance in the short term with respect to precipitation trends over the next 18 hours. Currently, a 1008 mb surface low is located just north of STL with an inverted trough/weak warm front extending north from this low to northeast IL and north across eastern Wisconsin. A secondary warm front extends east from the low across central IL into central Indiana. Currently, a northeast-southwest axis of showers, and even a couple of thunderstorms, extends from northeast Missouri to the Rockford area. This activity seems to be tied to the elevated low level baroclinic zone where moisture convergence is maximized. Objective analysis suggests fairly modest elevated instability with just a couple hundred J/kg of MUCAPE. Model guidance is not handling this activity well and recent satellite and radar trends have shown a slow increase in coverage and intensity of this activity over the past few hours. The elevated baroclinic zone should continue to shift slowly eastward this morning and while guidance generally isn`t producing much precip associated with it, it will be moving into equally if not more moist/unstable air mass. Given this, seems reasonable that this axis of showers/isolated storms should hold its own if not show a bit of an uptick this morning as it moves east across northern IL out ahead of the upper trough. Seasonably high PWATs, rather deep warm cloud, and weak low level instability near the 925-850 mb boundary should result in strongest ascent below -10C, which all favor mostly warm rain processes and the potential for particularly efficient rainfall rates from rather benign looking convection on radar. Additional convection is forming near/ahead of the sfc low closer to the surface warm front(s). Model soundings and modified 00z KILX sounding suggests that this convection is likely rooted in the boundary layer with a few hundred J/kg of MLCAPE in objective analysis ahead of this low across central IL. This modestly unstable air mass should continue northeast in conjunction with the upper trough and sfc low overspreading the southeast 1/3rd or so of our CWA early this morning. Stronger synoptic low level wind fields will remain east of our CWA, but given the very low LCLs and with LFCs just barely off the surface, it isn`t out of the question that there could be a brief weak tornado/landspout threat this morning in our SE CWA with any more robust storms that are able to ingest the low level vorticity maximized near the warm front and just ahead of the sfc low. Between the slowly eastward moving axis of showers/isolated storms over our western CWA and the expected increase in convection near/ahead of the sfc low, we should see a couple swaths of widespread beneficial rainfall this morning. The convection near/ahead of the surface low could expand in coverage developing north-northeast along the inverted trough/warm front early this morning. Given the generally favorable set-up for efficient heavy rain producing showers and storms, there is certainly a potential for some isolated 1 to maybe 2 inch rainfall totals this morning into early this afternoon. Outside of the heavily urbanized Chicago metro corridor, the recent drought-like conditions should limit the threat of flooding with most areas able to handle a 1-2" rainfall over a couple-few hour period. If one of the localized heavier rainfall areas sets up over Chicago metro area, this could lead to a urban flooding threat this morning. Final wild card is the potential for the lake to get involved. Most short range guidance suggests that this inverted surface trough will move slowly east this morning and back out over the lake. Any potential lake enhancement to convection and rainfall should be near and east of this surface trough, so close meso- analysis this morning will be need to monitor the evolution of this feature. If it remains near/over the metro area, it is possible that could become a focus for quasi-stationary moderate to heavy rainfall and increase the flood threat. If the trough moves east over the lake early this morning, as a lot of the short range guidance suggests, then that would lessen, though not completely eliminate a Chicago area urban flooding risk. By late morning/midday, the surface low should move east of Chicago`s longitude pulling the trough east of Chicago with it. Thermodynamically, lake induced instability is increasing now and should be sufficient for lake effect/lake enhancement to the rain/convection prior to sunrise. The thermodynamics will remain favorable for lake enhancement to the convection today, tonight and possibly into early Wednesday. Behind the inverted trough, the lake effect threat will focus over the lake and potentially into northwest Indiana this afternoon, tonight, and possibly into Wednesday morning as prevailing flow should push convection into Indiana and keep it east of Chicago. Normalized lake induced CAPE values tonight into Wednesday morning are quite low, with extremely skinny, but potentially fairly deep convective layer. Given the very moist environment in place, cannot rule out a locally heavy rainfall threat into northwest Indiana tonight into Wednesday morning if lake effect convection is on the more robust side. Given the very low normalized CAPE, the margin or error between enough instability for robust LE convection and little if any LE convection is razor thin. There is a very real possibility that lake induced CAPE is just a tiny bit weaker than progged and that could lessen the LE threat dramatically. The extremely large QPF values from lake effect that the HRRR is producing over northwest Indiana seems pretty unlikely, with the more likely scenario being weak/minimal convection tonight and Wednesday. Nevertheless, it will be something we will be watching closely today. While the lake effect/enhanced shower threat should be east of Chicago this afternoon, upper trough taking on a more negative tilt could result in showers lingering across northeastern IL well into the afternoon. Summary: Rain should taper off/end over western CWA later this morning and eastern CWA later this afternoon or early this evening. The exception will be the potential for lake effect showers continuing into northwest Indiana tonight into Wednesday morning. Otherwise, Wednesday should feature decreasing cloudiness and very mild temps. - Izzi Wednesday Night through Monday: A complex evolution to the synoptic weather pattern continues to limit forecast confidence for the long term forecast period. By Thursday, a trough will have been pinched off from the jet stream and become a closed upper-level low centered over or just east of the Ozarks. To its north, the axis of a high-amplitude ridge will be encroaching upon the western Great Lakes, while to its southeast, soon-to-be Hurricane Helene will be on final approach to making landfall along the Florida Gulf Coast. With the influence of the ridge in our region on Thursday, we will, in all likelihood, be treated to dry conditions and near to slightly above normal temperatures with highs in the mid to perhaps upper 70s (coolest near the lake). After making landfall, Helene will encounter the upper low and likely engage in a Fujiwhara-like interaction that will see the tropical cyclone rapidly get torn apart with its remnants then getting slung anticyclonically around the low and eventually absorbed into the low itself. While there is still quite a bit of spread exhibited in the latest suite of 00Z deterministic and ensemble forecast guidance regarding exactly how this interaction will play out, there seems to be growing support for most or all of our forecast area remaining dry on Friday due to either 1.) Helene having a slower forward motion and making landfall later and/or 2.) the continued presence of the ridge suppressing precipitation associated with both cyclones to the south. There`s enough of a QPF signal in the Grand Ensemble to warrant not yanking the NBM`s PoPs for Friday entirely just yet, but did end up lowering them a bit once again. Regardless of exactly how Helene interacts with the upper low, tightening pressure fields in the vicinity of these two interacting systems will yield a period of breezy northeasterly winds Friday into Saturday that will induce high wave action on Lake Michigan, leading to dangerous conditions along the lakeshore. Saturday onwards remains a fairly big question mark with respect to precipitation owing to dependencies pertaining to the aforementioned synoptic-scale interaction. In one possible scenario, we could see rain showers on Saturday as Helene`s remnants get whipped from east to west, then see daily periodic rain showers heading into next week as the upper low continues to meander around nearby. Alternatively, if Helene and the upper low meet up farther to the east, or if the upper-level ridge ends up being stronger than expected and continues to keep everything suppressed to the south, then the upper low could remain bottled up well to our south or southeast through early next week, keeping its associated precipitation away from our forecast area. There are several other possible "middle ground" outcomes as well, but with how difficult it is to model such a complex atmospheric interaction, it will still probably take at least another day or two for us to gain some more clarity as to whether or when it will actually rain this weekend into early next week. For now, have continued to leave the NBM`s broad- brushed slight chance and chance PoPs for this time frame largely untouched. Ogorek && .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