Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 270001 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 701 PM CDT Fri May 26 2017 .MESOSCALE DISCUSSION... 518 PM CDT The rotation area of the supercell in southern Ford County, which has re-strengthened since 500 pm, looks to move just south of Iroquois County. The hail threat from the forward flank will move over southern Iroquois County though, and have had a recent report of half dollar size hail, so have issued a Severe Thunderstorm Warning. The warm front continues draped right along the southern CWA edge, with slightly elevated updrafts developing further to its north. There will continue some severe threat with these, mainly hail. New updrafts further to the west will have around 1000 J/kg of MUCAPE and still modest shear due to the MCV to work with. So the watch for some wind and hail will continue, though the tornado threat is diminishing over Livingston and Kankakee Counties owing to some suppression aloft and somewhat worked over low-level air. MTF && .SHORT TERM... 300 PM CDT Tonight through Saturday... For near term convective/severe thunderstorm trends, check for the latest Mesoscale Discussions above. The weather will quiet down tonight with the effective warm front shunted well south behind the MCV. Showers and thunderstorms should exit quickly by the early to mid evening hours. This will set the stage for a quiet and pleasant start to the Memorial Day weekend, at least for the daylight period. Strong heating will take place away from lake influence on Saturday with plenty of sun followed by increasing mid and high clouds during the afternoon. Low level thermal progs support highs in the mid 70s to around 80 away from the lakeshore and few miles inland, which will be kept in the 60s. Castro && .LONG TERM... 300 PM CDT Saturday night through Friday... The effective warm front will gradually lift north in advance of a likely robust MCS, but mainly stay confined to the southern CWA or points south. There is uncertainty on timing and trajectory of this MCS, with the an increasing likelihood for most significant impacts to remain well south of I-80 or even south of the CWA counties. Portions of the area are likely to remain for outdoor activities during the evening hours with likely PoPs for most widespread shower and thunder coverage after midnight (and highest thunder coverage along/south of I-80). Deep layer shear and MUCAPE may be sufficient to support an isolated hail threat and perhaps localized strong to damaging winds. A surface low will take shape and drag a cold front across the area on Sunday with perhaps an isolated risk for gusty to damaging thunderstorm winds and marginally severe hail southeast of I-55. There is plenty of uncertainty with the convective scenario on Sunday and much of the afternoon could certainly be dry northwest of I-55 or I-57. Expecting breezy west-northwest winds in the afternoon behind the front, with only modest cool advection so highs should reach the lower to mid 70s. Memorial Day Monday could be fairly similar to Sunday temperature wise. Main feature of note is a large upper low over the northern Great Lakes, from which some isolated diurnally driven showers and possibly thunderstorms could form during the afternoon. Again, most of the day should be dry, so no need to cancel outdoor plans- just keep an eye on the forecast and the radar. The upper level low pressure will only slowly slide east through the week, keeping area under northwest flow aloft and generally slightly below normal temperatures for the end of May and beginning of June. Can`t completely rule out any shower activity in the mid to late week, but it looks to be an overall drier regime. Castro && .AVIATION...
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For the 00Z TAFs... 701 pm...Main forecast concerns are scattered showers this evening and cigs through Saturday morning. Scattered showers will likely continue across northeast IL through mid evening and confidence regarding how long these may persist is low. Weak low pressure will move across the region later tonight and mvfr cigs are expected this evening and these may persist overnight and possibly lower into ifr but confidence is low and trends will need to be monitored this evening and through the night. East/northeast winds around 10kt will slowly diminish later this evening and possibly turn more northerly overnight...though speeds will be light. Winds are expected to turn more easterly Saturday morning and increase up to 10kts Saturday afternoon with a possible lake breeze. cms
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&& .MARINE... 227 PM CDT Will keep the marine fog advisory going as web cams indicate haze/fog remains over the open waters. A high pressure ridge over the lake will continue east. A weak surface low over the plains will pass south of the lake tonight and continue across the Ohio Valley Saturday. The low may cause wind speeds to increase to 10-20 kt over the far southern end of the lake this evening. One model would suggest gusts up to 25 kt, but do not have enough confidence to go that high. High pressure passes over the lake early Saturday morning and light winds will vary in direction across the lake. Variable winds are expected over the southern end and southerly winds over the northern end. The next low forms over the plains Saturday and reaches the Great Lakes region Sunday morning. The low moves over Michigan Sunday afternoon and winds become west 10-20 kt behind the low. The low over Michigan merges with another Canadian low to the north over Ontario Sunday night. The low is forecast to remain relatively stationary over Ontario early next week which may lead to a an extended period of westerly winds of 15-25 kt. High pressure moves in behind the low mid to late next week. JEE && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ VISIT US AT HTTP://WEATHER.GOV/CHICAGO (ALL LOWERCASE) FOLLOW US ON FACEBOOK...TWITTER...AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSCHICAGO WWW.TWITTER.COM/NWSCHICAGO WWW.YOUTUBE.COM/NWSCHICAGO

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