Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 080854 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 354 AM EST Thu Dec 8 2016 .SYNOPSIS... Cold air will continue to get colder over the next day or two, and temperatures will remain well below normal. Some flurries are expected on Friday morning, before an area of high pressure moves into the region late Friday into Saturday. On Sunday and Monday, a complex low pressure system will through the Ohio Valley, bringing a mix of rain and snow to the region. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A large and lumbering area of stacked low pressure is currently located just south of James Bay, crossing slowly eastward through Canada. This feature is dominating the longwave pattern across much of North America, and is keeping the Ohio Valley in a regime of deep-layer westerly/northwesterly cold advection flow. As is unsurprising, the very weak 700mb disturbance moving through the region right now is producing radar echoes but no precipitation -- the dry air below the mid-level clouds is evaporating the hydrometeors as they fall. A dry forecast will be maintained today, but with some question about cloud cover. There is a considerable amount of upstream stratocumulus, which will likely affect the northern half of the ILN CWA through the day, before expanding across the whole area tonight. Further to the south, drier 925mb/850mb conditions are expected, so sky cover will be more of a question of any persisting mid-level clouds. With some sun sneaking through in the southern / southeastern sections of the forecast area, temperatures might be able to rise to near or just above freezing. The forecast is much less optimistic further north, where cold advection and persistent cloud cover will combine to allow very little rise in temperatures. The far northwestern ILN CWA -- currently in the upper 20s -- may not reach values that high again through the rest of the day. With a boundary layer that will be well-mixed, though shallow, some wind gusts in the 15-20 knot range may occur at any point throughout the day -- even into the evening. && .SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY/...
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The cold advection will continue unabated through Friday and into early Saturday. Aloft, the coldest air is expected to be in place on Friday morning. GFS/GEFS/NAM anomalies indicate that 700mb / 850mb temperatures will be about 2.0-2.5 / 1.5-2.0 standard deviations below normal, respectively. This will easily allow min temps to drop into the upper teens to lower 20s tonight and Friday night, even in conditions that are advective and completely unfavorable for standard radiational cooling. With the continued cooling aloft, a steep (but shallow) mixed layer will continue to exist near the surface, resulting in occasional stratocumulus through this period of the forecast. Given the thermal positioning of the top of the mixed layer through the first half of Friday, some light snow showers appear possible. This has been focused mainly in the northern sections of the CWA, but a chance of flurries will continue to be used for the majority of the forecast area. Though most locations will not receive any accumulation, a quick tenth or two could occur in a snow shower on Friday morning / early afternoon, likely limited to areas north of I-70. Outside of the Great Lakes, most of the eastern half of the US will be dry to start things out on Saturday. This will only be a brief interlude before a shift in the pattern. Though conditions are not expected to warm up too much for Saturday in the Ohio Valley, the signs of the change in pattern are evident, as the surface high and 850mb/700mb ridge axis move east through the Tennessee Valley -- bringing a deep-layer change in wind direction. Dry weather is expected, with an overall diminishing trend in low-level moisture. By Saturday night, attention will focus on developing precipitation, which will affect the Ohio Valley from early Sunday morning through Monday afternoon. Although there is high confidence in precipitation occurring, and high confidence that some accumulating snow will occur, the specifics are not yet possible to predict -- given continued run-to-run / model-to- model changes in the modeling of the thermal fields, surface low track, and overall mid-level flow pattern. As an example, the spread among GFSE members increases significantly between 12Z Saturday and 12Z Sunday, first showing notable inconsistency with the initial low- amplitude / high-speed wave moving into the southern Great Lakes. Combined with some right entrance jet dynamics, this will spark the first wave of precipitation over the area on Sunday. Hard to ignore the operational 00Z ECMWF/GFS runs, which both keep precipitation focused mainly north of the Ohio River through early Sunday, before additional precipitation develops everywhere later on Sunday and Sunday night. Will keep changes minimal given the uncertainty, but PoPs were adjusted based on these trends, and weather grids continue to indicate an expectation for rain and snow to mix during diurnal heating on Sunday. There was enough confidence to start off an HWO mention of accumulating snow for the northern half of the CWA, but no reason to get into details as of yet.
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&& .LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
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This low is forecast to move northeast into/through the Great Lakes, eventually allowing a cold front to pass through Sunday night or Monday morning. Until then, confidence has increased in a period of precipitation Saturday night into Sunday and therefore have likely pops across the CWA. Precipitation type will likely be snow at the onset, but with some warmer air moving in from the south on Sunday, expect snow to mix with/change to rain particularly south of Interstate 70. GFS is colder than the ECMWF, and therefore used a blended approach for p-type transition Sunday. Eventually, any lingering precipitation would change back to snow Sunday night/Monday morning as colder air moves back in behind the cold front. High pressure briefly builds in for Tuesday, but a fast-moving, weak system may bring a chance of light precipitation by Wednesday, although timing this far out is in question. Below normal temperatures are expected for the long term forecast period.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/... The center of a large scale upper level trough was located south of James Bay Canada this morning. This feature will rotate east/northeast through the day. For this morning, considerable mid clouds will exit the region around 12Z. Thereafter, CAA in conjunction with stratocumulus clouds will advect/develop through the day as they move southeast. It appears that the northern terminals will see some MVFR ceilings by 18Z while the southern terminals remain VFR. An increasing pressure gradient along with some diurnal mixing will bring wind gusts into the 20-25 knot range. For tonight, low level flow will shift to a northwest direction, allowing cold air to move across the Great Lakes. This will allow stratocumulus to thicken and lower overnight. There could be some scattered flurries, with a few snow showers possible near the northern terminals. Ceilings will all drop into the MVFR category, with some models suggesting ceilings dropping below 2000 feet in some spots. Winds will remain a little gusty overnight, but perhaps not as high as today with some night time cooling/decoupling. OUTLOOK...MVFR ceilings expected on Friday, which may linger into Friday night. MVFR/IFR ceilings and visibilities possible Sunday into Monday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...BPP AVIATION...Hickman

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