320 FXUS63 KDTX 211148 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 648 AM EST Fri Dec 21 2018 .AVIATION... Widespread rain associated with sprawling eastern US low pressure system will become more scattered in nature as the morning progresses as the best deformation forcing shifts north of the airspace. Drier and cooler air will filter in across the region this afternoon resulting in brisk northwest winds gusting around 20 knots. Local probabilistic and hi-res guidance continues to indicate a decreasing trend in precip potential this afternoon, with dry conditions increasingly likely as secondary low pressure tracks east of the region. IFR/LIFR ceilings will attempt to slowly lift to MVFR by this afternoon, although periodic low stratus will remain possible in lingering moist low-level environment. Despite drier air working in especially by 00z, MVFR ceilings likely to persist into tonight as moisture remains trapped underneath sharpening inversion. For DTW...Rain exists by 14z with generally dry conditions anticipated heading into the afternoon. Remnant moist low-levels and arrival of secondary low center in general sprawling low pressure system may be enough for sporadic drizzle, and possibly a wet snowflake towards 00z, but coverage too low to include in the TAFs at this time. IFR/MVFR ceilings likely persist into the afternoon along with slowly improving MVFR visibilities. .DTW THRESHOLD PROBABILITIES... * High in ceilings at or below 5000 ft through the period. * High that precipitation will fall as all rain. * Very low in cigs at 200 feet and/or vsbys of 1/2sm or less through 15z. && .PREV DISCUSSION... Issued at 357 AM EST Fri Dec 21 2018 DISCUSSION... Light rain continues across SE Michigan during the early morning with relatively mild air firmly in place across the region. Surface wind has gradually shifted to the north during the night but temperatures remain above freezing as far north as the Straits and north shore of Lake Huron leading up to sunrise. The lack of low level cold advection and a degree or two of daytime warming, such as it is on this first day of winter (solstice=523 pm est), maintains rain as the precip type through the day. Radar does confirm model soundings that show a low freezing level hovering around 1000 ft, deep enough for complete melting of hydrometeors before reaching the ground, although not impossible for a few snowflakes to make it while rate is higher. The primary support for the rain pattern is a low pressure system moving east of the area at the surface with a distinct wave aloft producing isentropic lift transitioning to deformation as the system moves quickly northward. The latest 00Z and rapid update model data then show a lengthy break between systems, other than some pockets of drizzle, from late morning through the afternoon while temperatures remain above freezing. Prospects for a rain/snow mix changing to snow this evening are tied to the potential westward extent of moisture from the next low pressure system. Early morning satellite imagery shows this second circulation moving northward from the Gulf coast within the deep southerly flow downstream of the full latitude trough. It is considerably stronger than the morning system over the eastern Great Lakes and has access to Atlantic moisture that could leak westward during late afternoon and early evening. It appears to help maintain layer RH in model data deep enough for precipitation over SE Michigan while model QPF is minimal despite mid level deformation lingering within the trough axis. Forecast soundings across model solutions do show some concerns with the thoroughness of mid level saturation for ice nuclei which naturally leads to drizzle consideration. The limiting factor for drizzle will be decreasing RH below 850 mb as the cooler air arrives during the evening. Considering these complicating factors, plan to just maintain a chance POP for a light rain/snow mix during the evening followed by dry and colder conditions overnight. High pressure builds into the central Great Lakes Saturday and Saturday night to maintain dry conditions and temperatures closer to normal for mid/late December. This is followed by the next low pressure system arriving Sunday in the quick westerly flow aloft. The system is shown to be somewhat of a hybrid clipper phasing with Pacific upper jet energy over the Plains and producing a weak surface reflection over the western Great Lakes by Sunday morning. It attempts to displace cP high pressure sprawled across the eastern half of the country extending all the way through the western Gulf which strongly limits low level moisture. Dry air is also firmly in place over the central Great Lakes ahead of the surface low track as an additional limiting factor before the system moves quickly eastward Sunday evening. The Christmas holiday period then begins with another stage down on temperatures cold enough for possible lake effect snow showers Christmas Eve. MARINE... A sprawling and deepening low pressure system will lift north into the eastern Great Lakes today and into Quebec tonight. Winds will continue to back to the north northwest this morning and strengthen as the pressure gradient tightens. A Gale Warning will remain in effect for much of the open Lake Huron waters beginning late this morning and continuing into tonight. A prolonged period of gusts to marginal gales around 35 knots will occur during this time, with a long and favorable northerly fetch resulting in building wave heights across much of the Lake Huron basin exceeding 10 feet at times. Elevated waves along the nearshore waters will also lead to hazardous conditions for small craft from outer Saginaw Bay to Port Huron. Northwest winds and waves will slowly decrease during the day Saturday as the pressure gradient weakens across the Great Lakes. Generally light winds will prevail Sunday as a weak upper-level disturbance moves across the region. HYDROLOGY... Widespread rainfall associated with a deep and sprawling low pressure system encompassing much of the eastern United States will continue through the morning hours. Rain will gradually become more scattered in nature this afternoon before ending this evening as drier and colder air works in from the west. As colder air filters in, rain transitioning to snow will be possible with no accumulation expected. Additional rainfall amounts up to a quarter of an inch will be possible, and is not expected to cause any flooding concerns. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...Gale Warning from 10 AM this morning to 4 AM EST Saturday for LHZ362-363-462-463. Small Craft Advisory until 3 PM EST Saturday for LHZ421-441>443. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....IRL DISCUSSION...BT MARINE.......IRL HYDROLOGY....IRL You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.