Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 040741 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 241 AM EST Sun Dec 4 2016 .SYNOPSIS...
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A weak upper level disturbance will bring light rain to the area this afternoon and evening. Drier conditions are expected on Monday, as high pressure moves into the area. On Tuesday, an area of surface low pressure is expected to move into the Ohio Valley, bringing rain to the area. Behind a cold front on Wednesday night, much colder conditions are expected through the end of the week.
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&& .NEAR TERM /TODAY/...
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A fairly dry air mass is currently in place over the middle Ohio Valley. Surface dewpoints are generally running in the upper 20s to around 30 in the ILN CWA, and the 00Z KILN sounding recorded a precipitable water value of 0.31 inches, with a significant amount of dry air between 850mb and 500mb. This helps to set up the scenario for the morning, as a large but not-particularly-potent area of lift moves east into the region. A shortwave trough currently moving through the plains states will cross the Mississippi River this afternoon, with only some very unimpressive moisture advection / isentropic ascent ahead of it. High-res models are stingy with their dBZs this morning, and the dry air gives reason for disbelieving the potential for most of the echoes over Kentucky to reach the ground. Most of Kentucky is covered by clouds in the 8kft-10kft range, with no precipitation occurring. Only over the southern and western portions of the state is rain occurring, where clouds are under 7kft. Ultimately, there is little reason to believe that precipitation will occur in the ILN CWA early in the day, outside of a slight chance of a quick sprinkle or flurry sneaking through the dry layer (and a 20 PoP will be maintained to account for this possibility). By afternoon, the overall forcing with this system will be focused in two locations. Right entrance dynamics will help force precipitation over the southeastern states, which may extend into eastern Kentucky, but will largely miss the ILN forecast area. More focused forcing from the 500mb trough will pass through the northern Ohio Valley and southern Great Lakes, and this will be the primary thing to watch. All indications are for a high-PoP but low-accumulation event, with weak ascent and a relatively quick motion resulting in total averaged QPF of only 0.10-0.15 inches. The highest PoPs (categorical for most of the CWA except the far south) will time out from 20Z-02Z from west to east. The only remaining item to discuss is the impact of temperatures on the precipitation type forecast. The diurnal curve will be small today, with highs ranging from around 40 (NW) to 45 (SE), an increase of only around ten degrees from this morning. There is a surface trough that will switch winds to the west and provide cold advection, but it lags the upper trough (and thus the precipitation) by several hours. So, any chance for a mix with snow will largely depend on diurnal or dynamic cooling, and this is not a particularly favorable scenario for either this evening. Discounting a 00Z NAM solution which appeared to dynamically cool the boundary layer much too quickly, it only appears that temperatures briefly get cold enough for some snow to mix into the northern / northwestern ILN counties during the back half of the precipitation. A rain/snow mix has been included this evening north of a line from Richmond to Delaware. If temperatures cool a slightly more than anticipated, some trace or slushy tenth-or-two accumulations might be possible in Mercer/Auglaize/Hardin tier.
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&& .SHORT TERM /TONIGHT THROUGH MONDAY/...
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The short term forecast discussion will be updated shortly. Previous discussion > Monday afternoon, surface high pressure will build back into the region with forecast soundings showing some breaks in the clouds. 850 mb temperatures Monday are around 3 degrees C which would support highs in the mid to upper 40s.
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&& .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
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Mid level short wave energy will eject northeast across the Tennessee Valley through the day on Tuesday as an associated surface low moves up across the upper Ohio valley. There are some model timing differences with this as the 12Z NAM is about 6 hours faster than the 12Z ECMWF while the 12Z GFS is somewhat in between, but closer to the slower ECMWF. Will trend toward the slower solutions and have a decent pop gradient north to south across our area at the start of the long term period. Higher pops will then overspread the rest of the area through the morning hours and then continuing into Tuesday afternoon. Low level thermal profiles appear warm enough to keep precipitation all rain through the day on Tuesday with highs ranging from the lower 40s northwest to the lower 50s in the southeast. Precipitation will taper off quickly from west to east Tuesday evening as the short wave weakens and moves off to the east. Some cooler air will filter in behind the system but the pcpn should be over before it is cold enough to change any of it over to snow. A broad trough will develop over the central United States by mid week and then shift quickly east through through the end of the work week. Short wave energy rounding the base of the trough will lead to an increasing chance of precipitation Wednesday night as an associated strong cold front pushes east across the area. This will usher in a much colder airmass through the end of the week. The 12Z GFS has trended faster with the cold push than the ECMWF and this will ultimately affect temperature trends Wednesday afternoon into Thursday and how fast any pcpn chances over to snow. Highs on Wednesday will range from the upper 30s northwest to the mid 40s southeast. Precipitation Wednesday night may start as a rain/snow mix but as the cooler air begins to move in, it will transition over to all snow. However, the models have trended drier with this feature so would expect any accumulations to be fairly minimal. Much colder air will then settle into the region through the end of the work week as 850 mb temperatures drop down into the -14 to -16 degree celsius range. In somewhat cyclonic low level flow and with possible fetches off of the warm water of the Great Lakes, will hang on to some lower end pops for snow showers Thursday into Friday. Depending on the exact timing of the strong cold push, we may likely have non diurnal type temperatures on Thursday with highs then on Friday only in the 20s. We will quickly transition into a more zonal flow pattern by Saturday, allowing for bit of a moderation in temperatures.
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&& .AVIATION /07Z SUNDAY THROUGH THURSDAY/... Surface high pressure will move off to the east today. This will allow an upper level disturbance to move toward the Great Lakes. Clouds will thicken and lower through the morning hours. Decent warm, moist advection/ascent ahead of the disturbance will bring light rain to the terminals later this afternoon into this evening. As it rains for a period, ceilings will lower into the MVFR category and then eventually the IFR category. Visibilities for the most part will remain MVFR. The upper level disturbance will move east/northeast away from the region tonight. Rain will end, but low clouds will persist in the IFR category. High pressure is then expected to build into the region on Monday. OUTLOOK...MVFR/IFR ceilings to linger into Monday morning. MVFR/IFR ceilings and MVFR visibilities expected Tuesday with rain. MVFR ceilings to linger Tuesday night into Wednesday morning. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Haines LONG TERM...JGL AVIATION...Hickman

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