Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 151048 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 548 AM EST Fri Dec 15 2017 .AVIATION...
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Low pressure system over Lake Superior will bring another period of active weather across SE MI today and tonight. A swath of light snow will work across the area this morning with winds turning westerly and increasing later this morning in its wake. This will bring lake effect snow showers east from Lake MI which may cause rapid fluctuations in vsby and cigs. A weak cold front will then drop through the area turning winds to the northwest this evening pushing the lake effect bands south of the area. All in all it looks like a low VFR type day with periods of MVFR or IFR in the heavier showers. Best chance of the heaviest snow bands will be in the 16-21Z timeframe. MBS will be in the 16-19Z window with Detroit area terminals around 18-21Z. Snow showers may linger through the night across the Detroit terminals as the front slows its progress. For DTW...Varying conditions through the period and especially this afternoon as lake effect snow bands or snow squalls set up over lower MI. Amendments to timing will be made as the bands start to present themselves, but best timing looks to be from 18-21Z. VSBYs could drop to IFR or possibly even LIFR in the most intense showers. //DTW THRESHOLD PROBABILITIES... * High confidence in cigs aob 5000 ft agl through the period. * Low for ceilings below 200 ft or visibilities below 1/2SM this afternoon in the heaviest snow showers.
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&& .PREV DISCUSSION...
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Issued at 423 AM EST Fri Dec 15 2017 DISCUSSION... Some perpetuation of very light snow showers/flurries ongoing early this morning, the result of renewed eastward penetration of the lake Michigan moisture plume. This process a response to a period of backed west/southwest low level flow occurring in advance of an inbound shortwave now translating across northeast Wisconsin. Attendant surface trough leading in the main height falls with this system, with an extensive region of light snow ongoing tied to both an increase in mid level ascent and low level convergence. This forcing on track to lift across southeast Michigan during the daylight period. A noted increase in moisture quality as respectable synoptic moisture taps the resident lake Michigan moisture plume will yield a period of deep layer saturation with respect to ice. This improvement in microphysics under the background of increasing large scale forcing will subsequently support a fairly widespread region of light snow/flurries locally. Highest coverage centered late morning and early afternoon. Accumulation of less than an inch. Late day transition toward potential lake effect snow showers/ squalls, as large scale support sweeps east and westerly low level flow deepens. The overlake thermodynamic profile certainly supportive, yielding a convective depth near 10 kft and upwards of 100 J/KG of cape within the outward projecting lake bands. The underlying mean low level wind field will favor some degree of downstream penetration of this lake convection. This setup favors multi-bands or segments focusing somewhere between the I-69 and I-94 corridors, but with no clear convergence signal to suggest one particularly will be more susceptible in this case. Inbound lake bands likely be quite squally, given expected gusts to 30 mph under modest late day mixing. This will lead to brief, localized high intensity snowfall rates, with potential for a quick dusting to an inch of additional accumulation. Weak frontal boundary will remain in the vicinity of southeast Michigan tonight and Saturday. The combination of weak fgen forcing and subtle mid level warm air advection will maintain a narrow ribbon of ascent over or near the region during this time. Mixed signal across the model spectrum in terms of placement and magnitude of this forcing, but recent probability guidance suggests at least modest potential for another period of light snow to develop locally during this time. Existing boundary will provide a slightly greater thermal gradient on Saturday, as the resident air mass begins the slow moderation process under increasing mean thicknesses. Highs ranging from lower 20s north to lower 30s south. Pattern of warm air advection will strengthen Saturday night and Sunday, in advance of a southern stream wave ejecting out of the plains. High degree of uncertainty yet in terms of the possible track and strength of this system, which will ultimately determine potential coverage/timing and ptype of any associated precipitation. Forecast will continue to conservatively call for a rain/snow mix at this stage. Potential for all locations to inch above the freezing mark on Sunday. MARINE... High pressure is in place across the southern lakes while the next clipper begins to track across Lake Superior. This clipper will bring gusty southwesterly winds and elevated waves back to Lake Huron this afternoon. Though gusts should stay below gale force they may reach 30 knots this afternoon before turning northwest behind the departing system this evening. The winds will result in elevated waves around the tip of the Thumb this afternoon through tonight necessitating a small craft advisory for a portion of the nearshore waters. Winds decrease and turn out of the east this weekend as a high pressure system drifts just north of the Great Lakes.
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&& .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...Small Craft Advisory from 4 PM this afternoon to 4 AM EST Saturday for LHZ421-441. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....DRK DISCUSSION...MR MARINE.......DRK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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