Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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263 FXUS63 KLOT 271942 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 242 PM CDT Mon May 27 2024 .KEY MESSAGES... - Scattered showers and thunderstorms are expected this afternoon, with a low (~5%) chance for locally damaging winds southwest of a Rockford to a Pontiac line - Better chances for numerous showers and thunderstorms Tuesday afternoon && .DISCUSSION... Issued at 242 PM CDT Mon May 27 2024 Through Tuesday Night: Visible satellite imagery shows one wave over western Michigan exiting the region to the east, while another wave over southwestern Minnesota is digging southeastward into Iowa. A weak line of showers out a head of the circulation has exited Iowa and steadily moving into the forecast area, but with recent AMDAR soundings showing a strong cap around 850, there is lower confidence in thunderstorms developing in the next few hours. However, as the circulation moves southeast into the Illinois, better instability will advect into Illinois allowing for showers and thunderstorms to develop. There has already been an uptick in lightning in Northern Iowa. Model soundings are suggesting fairly weak lapse rates with CAPE values less than 500 kg keeping the severe threat fairly marginal. While small hail less than an inch is possible, the main threat for the strongest cells that develop will likely be locally stronger wind gusts, which could locally reach up to 60 mph. Away from the instability to the north and east including the Chicago metro, isolated cells may develop, but the probability remains around 30 percent or less. Once the sun sets, the chance for convection diminishes quickly there after. Another wave is projected to drop south out of Canada on Tuesday. With a upper jet moving the left exit region over northern Illinois, it could allow for better forcing. Wind shear profiles and mid level lapse rates once again do not look overly impressive to merit a strong concern for severe weather specifically; however, models have trended stronger with instability suggesting a higher coverage of showers and thunderstorms. Chances for convection diminish after sunset as the wave moves east. Cold air advection behind the system will allow low temperatures on Wednesday morning to drop down into the low 50s and isolated upper 40s. DK Wednesday through Sunday: The mid/upper-level ridge axis centered across the Rockies early this week will build eastward across the Midwest into the western Great Lakes for the second half of the week. As it does so, the active belt of enhanced northwesterly flow aloft across the Great Lakes region in the near term will get shunted east of our area. This will set the stage for a few day period of dry and quieter weather through the end of the week as surface high pressure sets up shop over the Great Lakes region. While the weather will be quieting down by mid-week, below average temperatures will persist into Thursday before conditions begin to moderate back above normal for the weekend. Therefore, expect high temperatures for Wednesday and Thursday to only be in the 60s to low 70s (coolest lakeside). Overnight low temperatures also look to get downright chilly for the end of May, with readings likely to drop into the mid 40s outside of urban areas. Temperatures then will warm back to around 80 degree highs for the upcoming weekend. With the warming temperatures, comes increasing chances for showers and thunderstorms. It appears the area will gradually transition back into a more active weather pattern sometime this weekend into early next week. Accordingly, our current forecast is carrying some low end chances (20-30%) for showers and storms for the weekend. However, it is worth noting that there is a considerable amount of ensemble spread with how quickly this transition will occur. For this reason, the chances currently highlighted in the forecast are likely to broad-brushed and hence will need refinement as forecast certainty grows. KJB/GI && .AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 1234 PM CDT Mon May 27 2024 Forecast concerns for the terminals... - Scattered showers and thunderstorms late this afternoon through early evening - Period of low-end VFR to MVFR ceilings late tonight into Tuesday morning - Another round of showers and thunderstorms Tuesday afternoon The MVFR ceilings that have blanketed northern IL and northwest IN this morning continue to erode and should be fully scattered out over the next couple of hours. As skies scatter, the atmosphere should quickly mix into stout winds aloft generating 20-25 kt wind gusts through this afternoon before gusts subside late this evening. At the same time, a potent upper-level low pressure system will begin to pivot into northern IL which should allow for the development of scattered showers and thunderstorms. However, instability continues to look rather limited, especially with eastward extent, which makes confidence on thunder coverage at the Chicago terminals low. Despite the lower confidence have still decided to maintain a TEMPO for thunder at the Chicago terminals for a couple of hours this evening in case sufficient instability is realized and storms do form overhead. Additionally, there is still the potential for a few storms to produce locally gusty winds in excess of 30 kts but the coverage of these "severe" storms will likely remain along and west of FEP to IKK line. Showers and storms will come to an end this evening (by 05z at the latest) leaving dry and mostly VFR conditions overnight. Though, there continues to be a signal for some 2000 to 3000 ft ceilings to develop after midnight and persist into Tuesday morning. Heading into Tuesday, any MVFR ceilings that develop are expected to lift through the morning leaving VFR conditions for the rest of the forecast period. However, another upper- level disturbance is forecast to pivot through the Great Lakes Tuesday afternoon which will once again promote the chances for scattered showers and thunderstorms. While instability does look notably better on Tuesday than today, coverage is still somewhat uncertain so have decided to handle with PROB30s at the 30-hour TAF sites for now. Given that the storm timing on Tuesday is forecast to be after 18z I have not added formal mentions to RFD, DPA, and GYY yet but will need to consider it with future issuances. Yack && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago