Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 161914 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 314 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 /THROUGH TONIGHT/... Surface low pressure is currently located in the central plains, and will be moving gradually northeast into the northern Great Lakes by tomorrow. The Ohio Valley is still sufficiently ahead of this system, such that stronger southerly flow has yet to envelop the region. Modest instability has allowed for some showers to develop this afternoon, though absent any notable low-level forcing, there should not be significant coverage of precipitation. Given the diurnal nature of this activity, and signals from recent high-res model runs, PoPs through the overnight hours have been decreased (would not be surprised if much/all of the CWA was dry through most of the 04Z-12Z time frame). With surface dewpoints remaining high -- upper 60s / lower 70s -- min temps will likely be close to the same. && .SHORT TERM /THURSDAY 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 TUESDAY/... 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 /19Z WEDNESDAY THROUGH MONDAY/... Isolated/scattered and disorganized showers are developing over the region today, with a slight chance of a thunderstorm in the Cincinnati area this afternoon. It still appears this will be too isolated to include specifically in any of the TAFs, outside of running a VCSH at KCVG/KLUK. With mid-level clouds moving into the region overnight, it is unclear how much fog will be able to form. However, given the past few nights and the abundant low-level moisture, some MVFR visibilities appear possible by morning. Storms will become more widespread tomorrow afternoon, though the most likely timing will be after the end of the TAF period. Nonetheless, some rain or storms may be possible in the late morning and early afternoon. Widespread MVFR or lower conditions are not expected as of now. Wind gusts of 15-20 knots will be possible tomorrow afternoon. OUTLOOK...Thunderstorms are possible Thursday afternoon and night. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...Sites AVIATION...Hatzos

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