Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 301056 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 556 AM CDT MON MAY 30 2016 .SHORT TERM... 337 AM CDT Today through tonight... The main forecast concern on this Memorial Day is with the potential for a few isolated showers or thunderstorms. Above normal warmth will continue through the holiday weekend. The modest cold front which pushed through the area Sunday has settled across the Ohio Valley early this morning...and was already lifting back north as a warm front across the Central Plains. Well north of the surface front...a few isolated showers had developed over northern IL...in a small region of moist ascent associated with a short wave trough propagating eastward across MN/WI. High resolution short range guidance (RAP...HRRR and 4km NAM) forecasts indicate forcing will weaken through sunrise... resulting in these showers dissipating. Recent radar trends support this with a decrease in both coverage and intensity noted. Large scale subsidence is then expected to strengthen in the wake of the early morning short wave passage...as indicated by rising model mid-level heights of building short wave ridge over the region. This induces weak surface high pressure to spread across the area through early afternoon...with light synoptic west winds expected to allow lake breeze development. 950-925 mb temperatures fields support highs reaching the low-mid 80s across the forecast area...with mid-upper 70s in onshore flow along the lake. While a mix of high cirrus and scattered daytime cu is expected...partly to mostly sunny skies are anticipated. Turning attention to our west...a strong short wave noted over southern Alberta in early morning water vapor imagery is progged to dig southeast into the Northern Plains through tonight. Surface low pressure will deepen across the Dakotas later today in response...strengthening the southerly return flow across the Central Plains. In response...the surface warm front will lift north across the mid-Missouri Valley this afternoon and tonight... with the potential for isolated to scattered thunderstorm development occurring as far east as parts of western/northwestern IL as indicated by several convection allowing models. While the aforementioned synoptic pattern across the cwa today in not particularly conducive for widespread or organized convective development...forecast soundings from the WRF and GFS do depict a conditionally unstable weakly capped thermodynamic profile across portions of the forecast area with 500-600 J/kg of MLCAPE this afternoon. Thus while initiation is not expected in the forecast area...there is a low chance potential for isolated storms over eastern IA or western IL to propagate east into our western cwa counties late this afternoon or early this evening. Even into tonight however...the focus for additional thunderstorm development should remain largely west/northwest of the forecast area where the low level jet axis develops into northern IA/MN and WI. Ratzer && .LONG TERM... 355 AM CDT Tuesday through Sunday... Primary concerns in the extended forecast focus on a period of unsettled weather mid-week. The strong short wave over the Dakotas will deepen and drift to the east-northeast Tuesday through early Thursday...with a slow moving cold front trailing through the Plains. A somewhat narrow south-north oriented axis of deep moisture and warm/unstable air will exist along and east of the front...with the late Tuesday through Wednesday night period looking active with the potential for thunderstorms with slow front-parallel movement. This slow movement and high precipitable water values progged at around 1.5 inches would be supportive of a locally heavy rainfall threat. The potential for severe weather will increase by Wednesday as well...as the upper level jet streak associated with the upper trough spreads across the western and northern Great Lakes region. SPC has included the forecast area in a marginal severe thunderstorm risk in their day 3 outlook. Though clouds and precipitation will likely have some effect on temperatures...above normal warmth will continue through mid-week with afternoon highs in the upper 70s to mid 80s. The cold front moves through the area by early Thursday...with drier and cooler air moving in as surface high pressure develops across the Midwest. While cooler...temperatures are progged to remain close to average for early June with highs in the 70s and lows in the 50s/low 60s. A significant shift in the upper pattern develops late in the week as an upper ridge amplifies over the western CONUS...with a corresponding deep east coast upper trough. Northwest flow will persist across the area through next weekend...with a series of disturbances bringing periodic chances of showers. Ratzer && .AVIATION...
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For the 12Z TAFs... No major changes from the 06Z TAFS. The only aviation weather concern today will be the timing of a lake breeze boundary this afternoon...otherwise VFR conditions are expected. Light westerly winds will start out the day. However...a lake breeze is likely to develop by midday...then shift inland over the area through the afternoon. Currently it appears that it could reach ORD and MDW as early as 20-21 UTC...with an associated easterly wind shift expected. Wind speeds could be up around 10KT for a period with the passage of the boundary...but should gradually abate into the evening. KJB
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&& .MARINE... 116 AM CDT South to southwest winds will continue over the northern half of the lake today. However...a surface ridge of high pressure setting up over the southern half of the lake today will allow for lake breeze development...and onshore flow across the southern end of the lake this afternoon. Expect the winds to become east- southeast and begin to increase on Tuesday as a storm system begins to approach upper Midwest. It appears that winds up around 20 KT will occur late Tuesday into early Wednesday in response to this storm system...which is forecast to shift eastward into southern Ontario by late Wednesday. This will push a cold front over the lake Wednesday night. A period of thunderstorms looks likely Wednesday into Wednesday evening as this front approaches. Following the frontal passage expect lighter westerly flow to set up for Thursday. Late in the period...eighter late Friday or Saturday another storm system could impact the western Great Lakes region with another possible period of storms and higher winds. However...confidence with the timing and track of this system is low at this time. KJB && .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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