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FXUS63 KLOT 222348 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 548 PM CST Sun Dec 22 2024 .KEY MESSAGES... - Freezing drizzle may develop (30-40% chance) very late tonight into Monday morning, potentially resulting in icy road conditions and slippery travel on untreated or elevated surfaces. - Turning mild with on-and-off chances for rain during the Holiday week. && .DISCUSSION... Issued at 300 PM CST Sun Dec 22 2024 Through Monday Night: The main weather concern revolves around the potential for a glaze of ice from freezing drizzle late tonight into Monday morning. Initially this afternoon, a small mid-level wave shifting ESE across southern Wisconsin is producing virga from a 10-12kft cloud layer, but with a decent sub-cloud layer present, any snowflakes reaching the surface should remain north of the state line. A strong trough will then cross the Upper Mississippi Valley and western Great Lakes on Monday as an associated surface cold front crosses the area late in the afternoon. Low-level moisture advection ongoing across northern Texas and into southern Oklahoma has been evident through stratus attempting to expand northeastward. An elongated corridor of 40-50 knot low-level flow developing this evening will quickly advect this airmass toward the forecast area very late tonight into Monday morning. Broad but persistent isentropic ascent will promote quick low-level saturation and growing cloud depths capable of generating an expanding area of drizzle. In line with observational trends today, guidance has trended slightly slower with the arrival of deeper saturation over the area, with increasing chances of drizzle developing after 5am and especially closer to sunrise for areas around and east of I-55. Though temps will rise above freezing by late morning (latest in northwest Indiana), there may be a 2-4 hour period where icy conditions develop on any untreated or elevated surfaces. Additionally, patches of drizzle may develop farther north toward the Wisconsin state line in response to some lift on the southern periphery of the approaching trough. Given the timing associated with the Monday morning commute and lower probability/higher impact set-up, have opted to issue a Special Weather Statement highlighting for the chance of slippery conditions through the morning with the knowledge that a Winter Weather Advisory could be required with a short lead- time if the drizzle plume blossoms earlier than expected. Light rain or drizzle may continue for several hours Monday afternoon before the cold front advects in somewhat drier air by the early evening hours. Kluber Tuesday through Sunday: The extended forecast period will feature mostly cloudy conditions, periodic rain chances, and above normal to potentially well above normal temperatures through the end of the week. The expected milder conditions are in response to the northern stream upper jet lifting back into Canada. While confidence in above normal temperatures occurring remains quite high, there is still some uncertainty with the degree of warmth. This will ultimately depend on the paths of a series of waves and associated disturbances as they eject off the Rockies and move across the broader region beneath the Polar jet. These systems will also bring multiple chances for rain to the area through the end of the week. The messy upper level pattern lends itself toward lower than typical forecast confidence in the timing and placement of each system and accordingly leads to a more broad brushed approach to precipitation chances. There will be dry periods in between systems but pinpointing when these occur remains challenging at this range given continued model differences. After Monday's wintry precip, the next potential for precipitation arrives sometime Tuesday (Christmas Eve) night into Christmas Day. The GFS remains the furthest north and quickest in bringing rain into the area by Tuesday evening, whereas the ECMWF has trended much slower and keeps the rain mainly south of the area, with the NAM and Canadian somewhere in between. There remains a small chance (<15%) that a slower arrival of light rain could allow for a brief period of light icing to occur before surface temperatures warm above freezing. A faster arrival Tuesday evening (i.e. GFS) would likely limit the degree of cooling after sunset and hold temperatures near to above freezing. Given confidence remains low in rain actually lifting into areas where sub-freezing temps may occur, opted to maintain all rain in the forecast. Any precipitation that does fall would remain quite light, with QPF through the day on Thursday less than 0.3", highest south of I-80. Looking beyond into early January, long-range ensemble guidance continues to point toward a pattern shift occurring around the New Year with upper troughing setting up across the eastern half of the CONUS which would allow for arctic air intrusions into the region. This matches up well with the latest CPC 3-4 Week Outlook which has a 55-60% chance of below normal temperatures occurring during the first half of January across northeast Illinois and northwest Indiana. Petr && .AVIATION /00Z TAFS THROUGH 06Z TUESDAY/... Issued at 548 PM CST Sun Dec 22 2024 Aviation Forecast Concerns for the 00Z TAFs: - Breezy south winds with occasional gusts around 25 kts for ORD/MDW tonight. - Less frequent gusts at GYY/DPA/RFD, supporting LLWS conditions with 45 kt SSW low level jet above 1500-2000 ft AGL. - Potential for a brief period of -FZDZ early Monday morning. - Lingering IFR/low MVFR ceilings Monday/Monday night. Early evening surface analysis places low pressure over north- central Nebraska, with a tightening southerly surface pressure gradient developing eastward into the Midwest. South-southeast winds will increase across the forecast area later this evening as the low tracks east-northeast toward the region in response to an approaching mid-level disturbance. Already starting to sere some 20+ kt gusts at ORD/MDW which will likely persist and increase to around 25 kts overnight, while other TAF sites which typically decouple more strongly (RFD/DPA/GYY) likely see more sporadic less frequent surface gusts with LLWS conditions developing beneath a 45 kt SSW low level jet above 1500-2000 ft AGL later this evening. Winds will eventually shift to the southwest Monday morning as the surface low tracks across WI, then west late in the day with the weak trailing cold frontal passage. While regional radars currently indicate returns across parts of MN/IA/WI and IL early this evening, a deep, dry low-level air mass is resulting in this precip remaining aloft with cloud bases generally 10-12 kft. Low level moisture currently noted as MVFR/VFR stratus spreading into OK/AR from TX is what we'll have to watch overnight for the potential to result in development of -FZDZ/DZ here toward morning. Still some variability in the degree of saturation between various models with some depicting higher MVFR cigs and more spotty precip potential Monday morning, and others depicting IFR conditions with more widespread -FZDZ/DZ potential. This lends low confidence in deviating much from inherited TAFs at this point, but will continue to monitor upstream trends overnight. It does appear that the threat of -FZDZ would be short lived, only a few hours at the most, before temperatures rise above freezing. Low confidence extends to ceiling trends Monday as well, with guidance again split between IFR and low- MVFR prevailing ceilings. Ratzer && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory until 9 AM CST Monday for the IL nearshore waters. Small Craft Advisory until noon CST Monday for the IN nearshore waters. && $$ Visit us at weather.gov/chicago








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