Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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149 FXUS63 KLOT 072038 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 338 PM CDT Tue May 7 2024 .KEY MESSAGES... - Threat for any additional severe thunderstorms ending by 5-6 PM CDT today - Another round of thunderstorms may develop late Wednesday into Wednesday night, some of which could be strong to severe, mainly south of I-80 into central Illinois and Indiana - Cooler with periodic showers Thursday and then Friday night into the weekend && .PREVIOUS MESOSCALE UPDATE... Issued at 1259 PM CDT Tue May 7 2024 We`re closely monitoring for an increased severe weather threat within the CWA through the mid to late afternoon. The area of greatest concern is across northwest Indiana where there is a more distinct tornado threat. The threat for damaging hail has also increased further northwest in the CWA, encompassing much of if not the entire Chicago metro area. Our window of severe weather potential is approximately from 1-5PM CDT. 17-18z analysis indicated a deep occluding surface low across the ND/SD border, its warm front lifting into southern Wisconsin, and its cold front back across eastern Iowa. A seasonably warm and moist air mass has developed across the warm sector, with temperatures in the 70s and dew points in the mid 60s, except across our far northwest (upper 60s) and the northeast IL shore (50s and 60s due to onshore winds). Elevated convection is ongoing across northwest Illinois, where a hail and gusty wind threat is present. This area will continue to be monitored for severe hail up to 1" or so in diameter. A warning was recently issued for Ogle County. The main focus for the more appreciable severe threat will be spurred by strong large scale ascent (mid-level PVA and height falls and upper jet support) and cold frontal convergence. Given already weak and eroding MLCINH, isolated cells across west central IL at the top half of the noon hour may be the beginnings of the anticipated convection ahead of the cold frontal zone. While surface winds had been backed, we`re seeing an anticipated veering to southwesterly taking place across northern and central Illinois. Effective bulk shear of 60+ kt, steepening mid-level lapse rates, sizable CAPE in the hail growth zone, and straightening hodographs with decreased low level shear present a favorable setup for splitting supercells with damaging hail as their main threat, some of which could be significant. As such, in the 1630z SPC outlook update, severe hail probs have been spread farther northwest, and include the potential for 2"+ stones. Near and east of I-57 in eastern Illinois and particularly across northwest Indiana is the area most likely to have a ~2-3 hour overlap of more backed southerly breezy southerly surface winds, strong low level shear with decent low-level hodograph curvature, 100-150 J/kg of 0-3 km MLCAPE, and aforementioned very strong deep layer bulk shear and steepening mid-level lapse rates. Supercells in this corridor certainly could be supportive of all hazards, including tornadoes. Given the shorter residence time of the most favorable parameter space, confidence in significant (EF-2+) tornado occurrence within the CWA is on the lower side and appears more favored just east and southeast of the CWA, though given the conditional sig-tor potential, SPC`s 10% hatched tornado probs appear appropriate. Farther north and west, certainly can`t rule out a brief tornado threat, though the shortening and straightening hodographs seem to be a key limiting factor. The cold front passage this afternoon will quickly end the severe threat by 5 PM CDT or so. Castro && .DISCUSSION... Issued at 338 PM CDT Tue May 7 2024 Tonight through Wednesday Evening: A post cold frontal short-wave may touch off a few showers and thunderstorms across portions of far northern Illinois through the early to mid evening, with slight chance (~15-20%) PoPs for this activity. Generally quiet weather is then expected tonight into Wednesday morning, as the mid-level short wave lifts northeast of the area and short wave mid-level ridging develops overhead. Winds will become light to calm tonight as weak surface high pressure slides across the area. Forecast lows tonight are in the mid to upper 50s. The most recent guidance has trended drier through much of Wednesday afternoon, enabling another warm day (mid 70s to lower 80s F) despite increasing cloud cover. The exception to the warmth once to the early afternoon will be lakeshore areas due to winds turning onshore synoptically and aided by lake influence. From late Wednesday afternoon through Wednesday night, another round of showers and thunderstorms is expected to develop. Some scattered showers may develop across far northern Illinois and across the state line into Wisconsin in the late afternoon. More notably, the approach of the next short-wave will result in a gradual uptick in showers and thunderstorms to the south of I-80 prior to sunset, and then spreading northward in the evening and overnight. A surface low pressure wave is expected to develop along the cold/stationary front which will trail across the southern Plains in association with this wave, which will then propagate northeast into the mid-Mississippi Valley by Wednesday evening. Overall, the guidance has come into better agreement in barely returning the front northward into our far southern CWA (south of US-24) as a warm front by Wednesday evening. This is where some potential may exist for surface based severe weather threats, primarily damaging winds and hail. Otherwise, as most of our area will be on the cool side of the front, the main concern will be elevated hail producing thunderstorms , some of which may become marginally severe (hail up to 1" diameter) as mid-level lapse rates steepen amidst supportive effective bulk shear. In addition, seasonably high PWATs may present a localized ponding or even flooding threat, primarily near and south of I-80. Castro/Ratzer Late Wednesday Night through Tuesday: A mid-level trough extending southwest across Quebec and southern Ontario will phase with a mid-level low over the Missouri Valley late Wednesday night. As the phasing wave crosses the area on Thursday, a residual low to mid-level TROWAL combined with PWATs around 1" and a deep layer favoring warm rain processes supports the potential for a low-end localized band of heavy rain (1-2" totals by mid day Thursday) across portions of northern Illinois into southern Wisconsin. Outside of this band, scattered diurnally enhanced showers and perhaps a few storms are expected. Much cooler conditions are in store, with highs only in the upper 50s to lower 60s inland and lower to mid 50s near Lake Michigan. Adding to the cool feel, northerly winds will gust up to 25-35 mph, strongest near the lake. After a quick-moving ridge yields dry and seasonable temps on Friday, another upper-level low is progged to cross the western Great Lakes on Saturday. Anomalously cold mid-level temps shifting across the area through the day will yield scattered showers and storms, with some stronger cores likely producing locally strong wind gusts. Diverging solutions by early next week with regards to handling of the departure of Saturday`s upper low and a building ridge to the west greatly diminishes the forecast by early next week. It appears temperatures will trend back to above normal by the end of the period. Kluber/Castro && .AVIATION /18Z TAFS THROUGH 00Z THURSDAY/... Issued at 1259 PM CDT Tue May 7 2024 Key Aviation Messages: - Scattered TSRA, possibly locally severe this afternoon - SE winds shifting SW and gusting over 20 knots around 19Z Satellite analysis depicts MVFR clouds growing in depth to become more concerning for thunderstorm expansion across NE Illinois in the 18z-19z hour and then expanding into NW Indiana after 19z or so. The storms in around Chicago and points westward do pose a threat for hail along with winds in excess of 35 kt. The main window for storms will continue until about 23-0z, longest in NW Indiana, with the main window for the Chicago terminals through 2230z. VFR conditions will return behind the storms this evening, with initially breezy SW winds easing overnight. Winds will shift southeasterly and then easterly through the late morning into the afternoon Wednesday. Another batch of precipitation will shift toward NE Illinois later in the day. Confidence in thunder toward the tail end of the 30 hr TAF is still below 30 percent. KMD && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago