Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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899 FXUS63 KLOT 272344 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 644 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 /00Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 628 PM CDT Mon May 27 2024 Forecast concerns for the terminals... - Outflow boundary from storm NE of ORD likely shifting winds to NE at ORD toward 0030z; lesser chance at MDW - Showers and isolated thunderstorms crossing the area early to mid evening from the west; may bring northwest wind gusts of 30-40 kt, especially toward RFD - Period of MVFR ceilings late tonight into Tuesday morning - Showers with embedded thunderstorms expected Tuesday afternoon Initial focus is on corridor of isolated thunderstorms moving southeast to the east of ORD and MDW with an outflow boundary being enhanced by Lake Michigan and tracking southward towards ORD. Looks like it will make it to ORD around or not long after 0030z so have added a shift to northeast winds at that time. It`s less clear if the boundary will make it to MDW so have kept northwesterly winds there for now. The other focus is on an approaching upper wave moving into NW IL with an associated band of showers and thunderstorms moving eastward with it. This activity will reach RFD 0030-0100z and the Chicago area terminals towards 02z or so. However, lightning is expected to be on a downward trend with sunset. Have left TEMPO TS in the TAFs for now. In addition, west to northwest wind gusts of 30-40 kt may linger as the activity arrives so have maintained TEMPO gusts as well. The spotty shower activity that trails the main band looks to dissipate with sunset and may be holding onto the shower mention a bit long at each terminal. Scattered to broken stratus is expected overnight in the 2000-3000 ft range. Another trough will approach Tuesday morning and cross the area during the afternoon bringing more shower activity with some embedded thunderstorms. Have hit this a little harder with this forecast update as activity looks likely. Activity will shift off to the east by or into the early evening. MDB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago