Area Forecast Discussion
Issued by NWS Chicago, IL

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170 FXUS63 KLOT 012344 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 644 PM CDT FRI JUL 1 2016 .SHORT TERM... 211 PM CDT Through Saturday... High pressure is centered across the eastern Dakotas and Minnesota this afternoon providing fair conditions along with a modest northerly breeze across much of northern Illinois and northwest Indiana. A little bit of a lake shadow can be seen on visible satellite imagery along with some convergence in the winds stretching from the middle of Lake County, IL south across Cook County. Northeast flow in place on the east side of the boundary has kept temperatures in the low to mid 60s immediately along the lake front with mid 70s in place farther inland. Otherwise, it has been a quiet day across the region with some afternoon fair weather cumulus continuing to erode as dry air advects into the region. Expect skies to go mostly clear this evening and tonight once the cumulus dissipates. The high center is progged to build southeast into northern Illinois by the predawn hours and the ridge axis should settle just to our south during the day Saturday. Favorable conditions for radiational cooling will be in place overnight and leaned towards some of the cooler bias corrected guidance to try to hit the typical cold spots in the CWA which should cool to around the 50 mark and wouldn`t be surprised if a couple spots even dipped into the 40s. Most of the Chicago metro and urban heat island area should hold onto mid even upper 50s though. Expect fair conditions to continue through the day Saturday with light flow allowing a lake breeze to develop. Temperatures should warm into the mid to upper 70s with increasing cirrus streaming overhead. Deubelbeiss && .LONG TERM... 311 PM CDT Saturday night through Friday... The main forecast concerns will center around the potential of moderate to heavy rainfall over portions of the area Sunday into Sunday evening. A series of upper level disturbances are expected to track eastward across the Mid-Mississippi valley later in the weekend within a semi- zonal upper-level flow pattern. Model guidance continues to suggest that this will help induce and drive an area of surface low pressure from the Central and Southern Plains across the Mid-Mississippi valley by Sunday evening. All in all this is shaping up to be a impressive rain event for some in Illinois and Indiana as a 40+ KT low-level jet noses into the region by Sunday. With high Precipitable water values up to around 2 inches, heavy rain will certainly be an issue Sunday. The main question, however, continues to be how far north will this rain get into northern Illinois/northwest Indiana. This is especially true given there will be relatively dry easterly flow that will be in place into Sunday as the surface ridge axis gradually shifts over the eastern Great Lakes region. Ultimately this could play havoc in how far north the rain does proceed into Sunday, possibly even resulting in a sharp gradient between areas with rain and areas with little to no rain. In spite of this, the latest 12 UTC guidance has shifted the focus a bit farther north into at least my southern areas (south of I80) Sunday and Sunday evening. If this pans out, it could be a very rainy and cool Sunday across my southern areas, with the potential of 1 to 3 inches of rainfall. Thunderstorms chances look minimal, and likely confined over far southern sections of the area, if not south of the area all together, in closer proximity to the nose of the low-level jet. Following this system it appears that the 4th will be drying out area-wide and could shape up to be a pretty nice day. High temps on the 4th look to be around 80, though onshore flow will result in cooler conditions along the lake. As we head into next week it still appears that a board flat upper level ridge will begin to set up over the CONUS. This should help set the stage for a warming trend through the week as the flow turns southerly again by Tuesday night. The air mass expected to set up over the area looks to support temperatures well into the 80s by mid- week, with some 90s even possible by Thursday and Friday. If this higher heat pans out, we could also be flirting with heat index values near 105. However, I have some concerns that this more extreme heat in the 90s could fail to occur. First, it appears that the flat nature to the upper ridge will favor a potentially active period of weather over the Northern Plains and Upper Midwest and is not out of the question that this could spawn an MCS or two, which could impact the area later in the week, and impact the magnitude of high temperatures. Secondly, it appears that a cold front will be approaching the area by late week, and depending on its timing, (which could depend on the amount of convective activity) it may push through and push the heat to our south. All in all, our current forecast mentions highs near 90, but confidence at this time is a bit lower than average. KJB && .AVIATION... For the 00Z TAFs... Modest lake induced NE winds will ease at sunset and become light/variable overnight through midday Saturday before another lake breeze causes a modest easterly wind again Sat afternoon. VFR conditions expected through the period. Izzi && .MARINE... 326 PM CDT The small craft advisory will continue into tonight as northerly winds continue to produce waves above 4 ft. Conditions will improve on Saturday, however, as a surface ridge of high pressure builds over the southern part of the lake. Light winds are expected for a couple of days as this high dominates. The only system of interest this weekend is an area of low pressure, which is expected to track south of the lake Sunday night. This low could result in a period of modest easterly winds (~10-20 KT) along southern Lake Michigan. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Beach Hazards Statement...ILZ006-ILZ014 UNTIL 9 PM Friday. IN...Beach Hazards Statement...INZ001-INZ002 UNTIL MIDNIGHT Saturday. LM...Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 10 PM Friday. Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 4 AM Saturday. && $$ 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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