Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 170007 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 807 PM EDT Wed Aug 16 2017 .SYNOPSIS... As surface low pressure moves into the Great Lakes, increasing southerly flow will bring warmth and moisture into the Ohio Valley tonight into Thursday. A cold front will move into the area later on Thursday, bringing a chance of showers and storms during the afternoon. Drier and cooler conditions are expected on Friday, as high pressure moves into the area. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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Isolated showers will not be strong enough to reach levels for thunderstorm development this early evening, but more showers could sprout through midnight. After this time, the focus will fall on the convective complex currently in southern Illinois that will skirt into northeast Ohio late tonight, more towards daybreak. Overnight lows will be in the lower 70s, close to current dewpoints that are running in the upper 60s to low 70s.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... The low pressure system moving into the northern Great Lakes is forecast to more or less hold its intensity, rather than deepening or weakening, as it progresses forward. Mean WSW flow aloft will align over the Ohio Valley as this low moves northwest of the area, with increasing southerly flow building into the region. This will lead most notably to a further increase in moisture -- with surface dewpoints climbing into the lower to middle 70s, and precipitable water values increasing to near or slightly above 2". Conditions to start the morning will likely be more dry than wet (despite some solutions that appear overdone from the 12Z GFS/NAM) but convection should begin to develop relatively freely by late morning and into the early afternoon. It would be easy to say that the incoming cold front will drive the convective forecast for the day, but it is not that simple. Since the surface low is not deepening, the orientation of the front actually becomes slightly less favorable with time, and there is no hard gradient in wind direction or theta-e to pin a timing forecast on. Rather, a pre-frontal trough or general convergence will help to organize (or semi-organize) storms before and during peak heating, before downstream propagation takes over later in the day. With the actual cold front not moving through until late, a few showers may persist into the late night, though waning (or used-up) instability will limit the strength of this convection. In terms of potential hazards during the afternoon storms, both shear and instability look decent but not spectacular. While there is some turning in the low levels, overall bulk shear values are around 30-35 knots at 0-6km, closer to the higher end of that range in the northwestern ILN CWA. MLCAPE values should be able to reach 1500 J/kg, at least in pockets, but the increasing moisture aloft raises cloud concerns that could limit stronger heating. While this scenario could support a few strong to severe storms, the threat appears marginal overall. The going HWO mention appears sufficient, so no changes will be made. The amount of moisture in the atmosphere will support heavy rain, but the generally-progressive flow should keep flash flood issues localized rather than systemic. PoPs were increased to 70 percent for the entire CWA, timed across the CWA from 18Z to 00Z, with somewhat-generous gradients of lesser PoP values on either side of the peak. No big changes to the temperature forecasts for tomorrow or tomorrow night, allowing for mid 80s during the afternoon and upper 60s to around 70 by early Friday morning. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Models continue to adjust their timing of cold frontal passage and convection Thursday night into Friday morning. Latest runs have come into good agreement on a slightly quicker solution, which now leaves the fa dry at 12Z Friday as the pcpn is shunted off just e of the area. Wly flow behind the front should bring lower dewpoints and more stable air, so dried out Friday. H5 s/w is still advertised to swing through the Great Lakes on Saturday, but the amount of pcpn it brings is not certain. Went with chance PoPs across the north, tapering down to slight chance in the south. As the s/w pulls away Saturday night, high pressure builds in at the surface. It will keep the region dry both days. By Tuesday, models drop energy into the region from the Great Lakes, bringing scattered thunderstorms. The GFS is quicker with the return of the pcpn than the ECMWF or the Canadian hemispheric as it brings QPF in by 12Z Tuesday. Decided to go with a blend of the extended, so brought 20 PoPs into the west. Convection should develop ahead of the front on Tuesday into Tuesday evening. PoPs should be decreasing from the nw to se on Wednesday. && .AVIATION /00Z THURSDAY THROUGH MONDAY/...
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Very isolated showers over the region will likely not affect the TAF sites this evening but will also continue to fire through the evening hours and will need to be monitored. Few cu clouds 5-6kft this evening will have a higher cirrus and then dissipate as mid level AS deck around 10kft precedes the next round of showers that are currently in southern Illinois. Warm front lifting north through the Upper Midwest will have this area of showers with embedded thunderstorms clip the Dayton area late tonight. As the focus of the showers shears out, activity will be scattered and then re-gain focus in the afternoon along a convergent boundary underneath a mid level vorticity maxima. More storms will be found tomorrow afternoon and could affect all TAF sites. Until storms physically reach a terminal, maximizing in the afternoon tomorrow, conditions will remain VFR. OUTLOOK...Thunderstorms are possible Thursday afternoon and night.
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&& .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Franks SHORT TERM...Hatzos LONG TERM...Sites AVIATION...Franks

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