Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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153 FXUS63 KLOT 170532 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1232 AM CDT Sat Aug 17 2024 .KEY MESSAGES... - Scattered thunderstorms late this afternoon and evening, mainly near and north of I-80. A few storms may become strong to severe with a threat for damaging winds and large hail. - Scattered showers and isolated to widely scattered t-storms on Saturday, then isolated to widely scattered showers, and perhaps a few isolated t-storms on Sunday. - Long duration of dangerous swimming conditions and hazardous boating conditions on southern Lake Michigan Sunday-Monday. && .UPDATE... Issued at 1040 PM CDT Fri Aug 16 2024 Thunderstorms have cleared most of the forecast area as of late evening, with the exception of our far south and southeast counties roughly southeast of a Pontiac to Valparaiso line. These too, should exit the area by or shortly after midnight tonight. The threat of severe weather has decreased where storms remain, thanks to low- level diurnal cooling and convective outflow, though locally heavy downpours and isolated wind gusts to 50 mph remain possible. Otherwise, skies have become partly cloudy where storms have exited through increasing cloud cover is expected to develop from the north overnight. Ratzer && .DISCUSSION... Issued at 322 PM CDT Fri Aug 16 2024 Through Saturday Night: A clear "swirl" pattern is evident in GOES water vapor and visible satellite imagery this afternoon over the Upper Midwest, where a vertically-stacked low pressure system is centered at this time. Along the southern periphery of the low pressure region, an embedded vorticity maximum can be made out. Immediately downstream of this vort max, scattered showers and thunderstorms have blossomed across northeast Iowa and southern Wisconsin, and the expectation is for this convection to continue to increase in coverage and intensity as the vort max steers it our way into an uncapped and increasingly unstable air mass. Quality low-level moisture has largely remained in tact here into this afternoon with dew points generally in the upper 60s to low 70s as of this writing, so feel pretty confident that we`ll get at least a few storms in our forecast area late this afternoon into this evening, mainly in areas near and north of I-80, which are positioned closer to the core of the mid/upper-level low and better large-scale forcing for ascent. Dry mid-level air has thus far caused our inbound convection to struggle to take full advantage of the 1000-1500 J/kg of MLCAPE analyzed in RAP objective mesoanalysis, but if storms can eventually manage to overcome this dry air, then increasing mid- level flow going into this evening could enhance shear profiles enough to support some storm organization, perhaps allowing for maybe one or two strong to marginally severe storms to materialize and track across the northern half of our CWA. Most of this evening`s showers and storms should then depart our area by midnight, though some straggling spotty showers could hang around for up to a few hours longer. The mid/upper-level low will still be in our vicinity come tomorrow, likely centered over the Lower Peninsula of Michigan. Its close proximity to our forecast area in conjunction with steepening low- and mid-level lapse rates should foster another period of diurnal convective showers across our CWA beginning as early as the late morning, with peak coverage not expected until the mid-late afternoon. While lapse rates will not get particularly steep above 650 mb (where forecast soundings indicate that a warm nose will be present), equilibrium levels should still easily exceed the -20C level, so the deepest convective cells tomorrow will likely contain lightning for a relatively brief period of time before their updrafts come crashing down amidst an overall lack of deep-layer shear. Overall shower coverage should be scattered and generally greatest across the eastern half of our forecast area, but with us being only on the periphery of the stronger large-scale forcing for ascent, the factors inhibiting convective growth and storm longevity should keep thunderstorm coverage isolated to widely scattered at best. Shower/storm coverage should then wane after sunset with the loss of diurnal instability, though lake effect processes will likely lend a hand in keeping a lingering stream of broken rain showers going into northwest Indiana through the night. The daytime showers, associated cloud cover, and northwest winds should keep high temperatures capped at about 80F tomorrow. Ogorek Sunday through Friday: The mid-upper level low pressure that will bring diurnally driven showers and some storms on Saturday will shift into the eastern Great Lakes on Sunday. While large scale forcing will be steadily diminishing (mid-level height rises), weak embedded impulses on the west side of the upper low, lingering mid-level moisture, and modest destabilization inland of an advancing lake shadow may yield the development of isolated to widely scattered showers (highest PoPs 20-40%). Shower activity should favor inland areas east of I-39 in the early to mid afternoon. Mid-level moisture will likely quickly dry out by the late afternoon, causing lingering showers to dissipate. Regarding thunder chances, generally poor mid-level lapse rates (5.5C/km or less) are forecast, so with diminishing forcing and warmer air aloft, expectation is a few isolated thunderstorms (~20%/slight chance) at most inland. Notably, the instantaneous lightning flash density product from the 12z ECMWF kept almost our entire area devoid of any implied lightning. The slow movement eastward of the weak surface low pressure associated with the upper low, seasonably strong expansive high pressure building in from the north and northwest, and relatively cool air over the warm lake (modest lake induced instability) will result in an extended period of full fetch brisk northerly winds Sunday through Monday. Expecting to need a Beach Hazards Statement and Small Craft Advisory for this period barring a significant forecast change. A pseudo-blocky pattern will develop next week, keeping us in dry northerly mid-level flow aloft between strong ridging over the southwest and pronounced troughing over northeastern North America. This will bring dry weather, comfortable temperatures (slightly cooler by day lakeside), and low humidity through the work week. Late summer heat may finally make inroads into the region during the last work week of August. Castro && .AVIATION /06Z TAFS THROUGH 12Z SUNDAY/... Issued at 1232 AM CDT Sat Aug 17 2024 Aviation Forecast Concerns: - MVFR ceilings developing around daybreak and continuing through midday. - Isolated to widely scattered showers and thunderstorms again this afternoon into the early evening, especially for Chicago metro terminals. - Low-confidence potential for winds to shift northeast for Chicago terminals later this afternoon/evening. While all is quiet and VFR across the terminal airspace early this morning, there is a fairly extensive deck of MVFR CIGs currently noted across southeastern MN and into adjacent areas of northeastern IA and west central WI. This area of cloud cover is expected to develop southeastward over the next several hours, before overspreading the terminial airspace across northern IL by daybreak this morning. These MVFR CIGs then are likely to persist overhead through much of morning hours, before diurnal improvement gradually pushes bases back into the VFR category sometime around midday. As we head into this afternoon, an upper level low will continue to drift slowly east-southeast across the western Great Lakes. As it does, a couple of smaller mid-level impulses wrapping around its western periphery will be steered right across northeastern IL later today. These should each help foster at least isolated to widely scattered diurnal showers and thunderstorms again this afternoon, especially across eastern WI/northeast IL including the Chicago terminals. While TSRA coverage may not be very high, forecast soundings do indicate enough instability during the afternoon and early evening hours to continue the going message of a PROB30 for TS and a prevailing VCSH. Westerly winds tonight are expected to become northwest to north-northwest during the day as the surface low gradually progresses eastward into towards Lake Huron. A couple of models suggest a shift to northeast is possible for KORD/KMDW/KGYY later this afternoon as a lake breeze tries to develop and push inland. However, confidence in this remains very low at this distance. It appears the ability of this wind shift to make it all the way inland to ORD and MDW may ultimately become dependent upon the ability for any outflow from this afternoons convection to drive the lake breeze boundary inland. Given the low confidence with this wind shift at this stage, we have opted to leave a formal mention out of the TAF at this time. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago