Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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943 FXUS63 KLOT 280537 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1237 AM CDT Tue May 28 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 && .UPDATE... Issued at 1008 PM CDT Mon May 27 2024 Isolated to widely scattered thunderstorms will continue to decrease in coverage through midnight, with a continued gradual decrease in diurnal instability. As of 10 pm, scattered showers and isolated thunderstorms continue to decrease in coverage, with most along and northeast of a Madison WI to Valparaiso line, as well as south of a Galesburg to Kankakee and Rensselaer line. Some of these may persist through midnight as a mid-level short wave and its associated mid-level cold pool (-20C at 500 mb across SW WI and NW IL) move across the area, though diurnal cooling and decreasing low-level instability will allow these to continue to weaken and dissipate overnight. Some gusty surface winds to 40 mph or so may linger for another hour or so south of the Kankakee River, however no strong or severe thunderstorms are expected. Trimmed pops/cloud cover from the west per current radar and satellite trends, and adjusted winds per obs trends to be more northerly across the Chicago area/lakeshore in association with earlier outflow boundaries. Otherwise, no other significant changes made to the going forecast through Tuesday AM. Ratzer && .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 /06Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 1236 AM CDT Tue May 28 2024 The primary aviation weather concerns through the 06Z TAF period include: * Showers and thunderstorms Tuesday afternoon into the evening with a corresponding potential for MVFR cigs/vsbys. * A chance for additional MVFR cigs late Tuesday night into Wednesday morning. Light westerly winds, at times variable, will take us through the night underneath a SCT 3-5 kft deck moving into northern IL from the northwest. Most of the morning will see WNW winds below 10 kt before ramping up to around 10-12 kt for the better part of the day. Scattered showers and thunderstorms will move over the airfields from the north and northwest Tuesday afternoon. The onset time continues to trend later in model guidance, now looking like 18Z at the earliest across all sites but could wind up being closer to 19 or 20Z before we see rain. There is a solid potential for some embedded thunderstorms as well, primarily through only about 00Z. While we should remain predominantly VFR, we could see periods of MVFR cigs or vsbys while storms are in the area. Scattered rain showers will persist likely only through about mid-evening, although there is a growing signal for light, non- impactful showers to continue later into the night, especially at the Chicagoland sites. With the arrival of the rain, winds are expected to veer to NNW. It`s very possible that we see brief, if not prolonged, periods of a NNE direction during the afternoon, but overall confidence in a NNE at any given time is shaky. Following the rain, near- northerly winds below 10 kt can be expected through the rest of the period. There is a moderate signal for MVFR cigs to develop late Tuesday night into Wednesday morning, although best bet looks like continued VFR. Doom && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago