Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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036 FXUS63 KLOT 231734 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1234 PM CDT Mon Sep 23 2024 .KEY MESSAGES... - Locally heavy rainfall possible tonight into Tuesday with an axis of 1 to locally +2 inches of rain possible. - An unusually high degree of uncertainty exists in the forecast from Friday onwards. && .DISCUSSION... Issued at 329 AM CDT Mon Sep 23 2024 Through Tuesday: Challenging forecast this morning with a seemingly increasing threat of a swath of heavy rainfall tonight into Tuesday across portions of our CWA. Early morning water vapor imagery shows a compact, but very well defined closed mid-upper level low tracking east along the KS/NE border. This closed low is embedded within the broader NE-SW oriented upper trough that`s the remnants of the big cut off low that had been parked over the southwestern U.S. To the southeast of this trough, there is an plume of moisture extending from TX northeast into the lower Ohio Valley associated with the subtropical jet. Meteorologically, a fascinating and complex evolution to the pattern is progged to take place over the next 48 hours as a 140kt+ 250mb jet crashes ashore into western Canada today ahead of a progressive and fairly high amplitude upper ridge that will spread into western North America. The downstream effects of this evolution is progged to cause the positively tilted trough and embedded upper low over the central Plains to get shoved eastward toward the Mississippi Valley tonight into Tuesday. As this occurs, the trough is progged to take on an increasingly negative tilt with the quickly approaching upper ridge and strong Pacific jet expected to lead to this negatively tilted trough eventually closing off into an upper low over the lower Missouri Valley Tuesday. As this is occurring aloft, a weak surface low currently over AR is expected to track to northeast to near KSTL by later this evening, then to southern Lake Michigan by 18z Tuesday. As this low tracks northeast, it should begin to deepen tonight into Tuesday morning in response to an increasingly diffluent flow associated mid-upper trough becoming negatively tilted. In addition, guidance suggests that there will be a coupling of a pair of upper level jet streaks helping maximize upper level divergence tonight into Tuesday morning over the region. In response to this low level cyclogenesis, a low level jet will develop tonight and result in strong northward moisture transport. There is a pretty strong signal in GFS, ECMWF, and RAP that an axis of heavy rainfall will develop near the elevated baroclinic zone where low level moisture convergence will be maximized tonight into Tuesday morning. PWATs are progged to increase to 175% to nearly 200% of normal along this axis of maximized moisture convergence. In addition, the strong ascent and weakest convective stability/instability is progged to be in the low/middle troposphere, below the freezing level. This could result in warm rain processes dominating in this band tonight, potentially enhancing the heavy rainfall threat. Important to briefly note that the NAM and some of the CAMS are much weaker with cyclogenesis and subsequently are not depicting this northern axis of heavy rainfall tonight/Tuesday. A complex synoptic evolution like this seems more likely to be handled better by the global models, so have largely discounted the HREF/NAM solutions and weighted the forecast heavily toward the GFS and ECMWF. This would suggest the axis of heavy rainfall would end up from west central IL east-northeast into the Chicago metro area. Certainly some wiggle room and the location of the highest rainfall totals may need to be adjusted in subsequent forecasts. One last "interesting" aspect to this system is the potential for some lake enhancement to the rainfall. Forecast soundings over southern Lake Michigan certainly would favor some convective lake enhancement potentially resulting in heavier rainfall inland front the southwestern portions of Lake Michigan. Where exactly this lake enhancement sets up will hinge on the precise track of the surface low and where the subsequent lake enhanced convergence zone develops. Given water temps still in the 70s and potential for a rather deep marine convective layer, there could easily be some localized rainfall totals over 2" where lake enhancement occurs. Generally speaking, the antecedent moderate drought-like conditions should limit the flooding threat. However, if confidence increases in the potential for heavier rainfall setting up in the highly urbanized areas of Chicago, then later shifts may need to consider a targeted flood watch for Chicago since moderate drought won`t increase the amount of water concrete can soak up. This would be of particular concern if it looks like lake enhancement will set up over the highly urbanized areas. One final wildcard to keep an eye on tomorrow is the potential for some low topped convection/supercell threat over mainly our northwest Indiana counties. The most likely scenario is that cloud cover and rain should prevent any meaningful destabilization. However, southeastern most CWA is progged to get into the warm sector with the sfc low progged to track northeast to southern Lake Michigan, so if there is any diurnal heating of the boundary layer in this warm sector, the threat for low topped supercells with an attendant low end tornado risk would increase. Right now, this threat appears quite low and am not planning on messaging it in our products, but it is something worth monitoring. - Izzi Tuesday Night through Sunday: After any lingering precipitation from Tuesday`s system clears the area overnight into Wednesday, Wednesday is shaping up to be a pleasant late September day with seasonable highs in the mid 70s as surface high pressure begins to build into the region from the west. In all likelihood, we should remain dry on Thursday as well with the continued influence of the surface anticyclone in the region. Thursday`s highs look like they`ll be a smidge higher than Wednesday`s, so as long as an encroaching mid- to high-level cloud shield from the south holds off long enough on its arrival as the majority of available guidance suggests. Onshore flow on both days will likely result in slightly cooler temperature readings closer to the lakeshore. Aloft, the mid-week weather pattern will be a bit more complex as a trough diving southeastward out of the northern Plains will get pinched off from the jet stream and become a closed-off upper-level low that should center itself somewhere to our south or southwest Wednesday into Thursday. At the same time, a high- amplitude upper- level ridge will be setting up shop over the central third of the continent, effectively yielding a Rex block. Medium-range guidance often struggles to accurately model the evolution of these blocking patterns several days out, and to add further complexity to the weather pattern and forecast, a tropical disturbance (and likely soon-to-be Tropical Storm/Hurricane Helene) will likely be making landfall somewhere along the eastern half of the Gulf Coast on Thursday, setting itself up for an encounter with the aforementioned cut-off low. How these two systems end up interacting will ultimately dictate what weather we will see Friday through early next week, and unfortunately, there remains too large of a spread in outcomes advertised in both deterministic and ensemble guidance to be able to say much aside from that we have low confidence in the forecast for this time period. Thus, have continued to ride with the NBM-delivered output for the long range portion of our forecast, which features model-averaged daily highs in the 70s and very broad-brushed slight chance and chance PoPs from Friday onwards. Refinements to this portion of the forecast will almost certainly be needed as forecast trends become clearer. Ogorek && .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