Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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081 FXUS63 KLOT 170822 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 322 AM CDT Sat Aug 17 2024 .KEY MESSAGES... - Scattered showers and thunderstorms again this afternoon followed by another chance of scattered showers on Sunday. - Long duration of dangerous swimming conditions and hazardous boating conditions on southern Lake Michigan Sunday-Monday. && .DISCUSSION... Issued at 321 AM CDT Sat Aug 17 2024 Through Sunday: The upper-level low continues to slowly progress eastward across the northern Great Lakes this morning with a pair of shortwave disturbances pivoting across WI. As of this writing, the shortwaves are not generating much more than widespread cloud cover across WI which will gradually spill into northern IL and northwest IN through the morning. As diurnal heating commences after sunrise instability is expected to increase with most guidance in good agreement on at least 500 to 700 J/kg of MLCAPE developing this afternoon despite the cloud cover. This instability in combination with the broad forcing from the aforementioned waves passing overhead will be sufficient to kick off some scattered showers and isolated thunderstorms, mainly during the afternoon and early evening hours. Otherwise, expect mostly cloudy skies and seasonably cool temperatures in the upper 70s to around 80 degrees. Heading into tonight, showers and any thunderstorms should taper shortly after sunset as the diurnal instability wanes. However, guidance has began to latch onto the idea that some lake effect showers (and perhaps a storm or two) may persist through the night. While instability profiles are rather modest, ample moisture will be in place along with lingering ascent from the upper low. Therefore, have decided to introduce some 20-30% POPs into the forecast for tonight, primarily for Lake, IL and Porter counties, where this potential seems to be maximized. Elsewhere, dry conditions are expected for tonight with overnight lows dipping into the lower to mid-60s areawide. The upper low will continue to meander across Lower Michigan and into the eastern Great Lakes on Sunday as a surface high begins to build into the Midwest. While the potential for another round of scattered showers is possible Sunday afternoon, the drying mid-levels should limit the longevity and coverage of showers to mainly areas near Lake Michigan. Aside form the linger rain chance, more sunshine is expected for our Sunday with high temperatures remaining in the upper 70s to around 80. Additionally, the approaching high will also increase winds, especially over Lake Michigan, as the pressure gradient tightens. These breezy northerly winds (gusts of 20 to 25 mph) will act to quickly build waves at area beaches throughout the day on Sunday resulting in a high risk for rip currents and dangerous swimming conditions. Given that the signal for these elevated winds and waves has been very persistent in guidance for several forecast cycles, have decided to hoist a Beach Hazard Statement with this forecast package for Lake IL, Cook, Lake IN, and Porter counties beginning Sunday morning through at least Monday night. Thus, any beach goers this weekend should remain out of the water and obey any beach restrictions. Yack Sunday Night through Friday: An expansive surface ridge of high pressure extending from the Midwest northward across Ontario early next week is expected to foster a persistent period of enhanced northerly winds down the length of Lake Michigan Sunday through late Monday. The net result of this will be the development of dangerous, to potentially life threatening swimming and boating conditions along the southern shores of Lake Michigan on Sunday as waves build in excess of 5 feet. Expect these conditions to persist through at least Monday night before abating winds and waves result in gradually improving conditions into Tuesday. Outside of the dangerous swimming and boating conditions on Lake Michigan through Monday night, a few day stretch of quiet weather is likely next week. This as our area ends up locked along the eastern periphery (western periphery) of an upper- level ridge (trough) axis centered across the Plains (eastern CONUS). With this pattern expected to favor northerly lower- level flow and large scale subsidence our area through mid-week, seasonably cool weather conditions (highs 70s, lows in the 50s) can be expected through midweek. Later next week, there are signs that the upper-level ridge axis over the Plains will begin to migrate eastward towards the western Great Lakes. As this occurs, it appears temperatures are expected to be on a warming trend (readings back into the 80s again) late in the period. Ensemble forecast guidance even hints that conditions could turn downright hot (temps in the 90s) in our area for the final week of August under this upper ridge. KJB && .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