Area Forecast Discussion
Issued by NWS Chicago, IL

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000 FXUS63 KLOT 272325 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 625 PM CDT Mon Mar 27 2017 .SHORT TERM... 230 PM CDT Through Tonight... The immediate lakefront fog, at times appearing dense, continues with a couple downtown Chicago cameras having shown dropping visibility once again this early afternoon. According to City of Chicago partners, the stratus remains just off the deck on Lake Shore Drive. Will utilize SPS and Graphical NOWcast for messaging of this dense fog into early evening. Dissipating time is challenging to say. Dry air appears to be oozing its way down the lake with improved marine webcams along Wisconsin and Michigan shores, and would expect that trend to inch southward due to increasing northerly wind speeds into the evening. Those stronger north winds do not arrive in the immediate Illinois and Indiana lake front areas until mid-evening, so expecting this to last at least a few hours after dark. The compact upper low over southeast Missouri this afternoon continues to chug eastward. Weak but moist isentropic ascent and transient mid-level frontogenesis is generating lift and some showers over the CWA, mainly along and south of I-80 with a few stragglers north. These showers should shift eastward early to mid evening. Stratus will continue through the night with high confidence. Cannot rule out some patchy drizzle at times beyond the rain showers. The north winds will steer in cooler air, with lows in the upper 30s forecast for many areas north of I-80, and lower 40s south. MTF && .LONG TERM... 230 PM CDT Tuesday through Sunday... Generally cloudy spring weather will prevail through the week a slow moving mid week system bringing more rounds of rain. Flow off the lake will be rather persistent keeping lake shore areas chilly with inland pushes of the chillier lake air varying by day. Tuesday will be precipitation free as tonight`s system departs to the east. Weak high pressure ridging extending southward from central Canada will keep north to northeast winds in place which will keep lake areas in the lower 40s while inland portions of the Chicago metro warm to the upper 40s/50 degrees. Areas beyond the metro will see mid 50s or so. Cloud cover looks pretty persistent through the day though some erosion/breaks may occur later on the afternoon. Another longwave trough, currently swinging across the southwest U.S., will split with a southern stream upper trough cutting off and drifting eastward into the central and southern Plains. This will follow a similar pattern to the systems over the last several days and remain centered south of the forecast area. This system will spread waves of moisture/ascent and resultant rain northward into the area. High clouds increase Wednesday but it is possible some sun makes it through early. Most, if not all, areas remain dry through Wednesday afternoon, though some sprinkles could occur in the far southwest late in the day. Waves of rain arrive Wednesday evening and continue through Thursday. The closed upper low and attendant surface low take their time passing so damp,cloudy and potentially showery weather continue into Friday but coverage of precip will likely decrease over what will have occurred Thursday. Could see some snow mix in with any rain showers far north late Thursday night/early Friday morning. Temps Wednesday will be similar to Tuesday. Thursday will see a larger north- south spread in highs as a warm front nudges northward, though it may not make it very far into the forecast area. Should see upper 40s to lower 50s Friday but low cloud cover may persist as will lake side cooling. Saturday may turn out to be decent for most areas with high pressure expected to move over the area. Light flow and likely some sunshine should allow a lake breeze to make an inland push allowing some areas of the Chicago/NW IN metro to lose at least a few degrees in the afternoon. Still early for these details but something to keep in mind. Guidance suggests a weak northern stream wave passing Sunday bringing clouds and possibly a minimal amount of rain. Another longwave trough will be moving onshore over the western U.S. before splitting allowing another upper low to move across the central/southern Plains and up the Ohio Valley early next week, with a general repeat of recent systems favored. MDB && .AVIATION...
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For the 00Z TAFs... High-end IFR/low end MVFR cigs to persist into Tuesday morning, before steady improvement occurs during the day. Current obs depict cigs rising a bit early this evening, with some 1000-1500 ft noted upstream over portions of NE IL and SE WI. Cameras along the lake continue to depict 800-900 foot bases at times however, thus have maintained a cig just under 1000 ft for ORD/MDW this evening. Light rain associated with weather system passing south of the terminals was eroding along the northern periphery and moving southeast of the terminals, and should not be a factor. MVFR cigs Tuesday look to scatter/rise to VFR by mid-late afternoon. Winds from the northeast 020-040 at/just below 10 kts will become a little more northerly late tonight, before shifting back northeast during the day Tuesday. Ratzer
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&& .MARINE... 245 PM CDT Dense fog continues over the far southern part of the lake this afternoon and that will linger into early this evening. Expect that increasing northerly winds will help to steer slightly drier air down the lake with improvement happening during the evening, though just how quickly is low confidence. The increasing northerly winds after dark will build waves and expecting Small Craft Advisory conditions by daybreak Tuesday. These will continue through the day and likely into the evening given the fetch. The next low pressure passing to the south of the area will increase easterly winds Wednesday night that will turn northerly on Friday. Right now these are forecast to be 25-30 kt through all three of those days, and will be enough to certainly support Small Craft Advisory conditions along the Illinois and Indiana nearshore areas during that time. MTF && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Dense Fog Advisory...nearshore waters until 10 PM Monday. Small Craft Advisory...LMZ743-LMZ744-LMZ745...3 AM Tuesday to 9 PM Tuesday. Small Craft Advisory...LMZ740-LMZ741-LMZ742...3 AM Tuesday to 3 AM Wednesday. Dense Fog Advisory...LMZ777-LMZ779 until 7 PM Monday. && $$ 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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