Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 271751 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 151 PM EDT Tue Sep 27 2016 .SYNOPSIS... A large area of low pressure will gradually move south from the Great Lakes over the next day or so, eventually remaining in place over the Ohio Valley through the end of the week. After a dry and breezy day today, cooler and rainier conditions are expected from Wednesday through Friday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The Ohio Valley remains situated between a cold front which passed east of the region Monday morning and a large stacked area of low pressure across the northern Great Lakes. In between these two features, extremely dry air has filtered into the area, as evidenced by the 0.31 inches of PWAT sampled on this morning`s 12z ILN sounding. As such, expect mostly clear skies today except for perhaps a few high based cumulus during peak diurnal heating. Still do expect that breezy conditions will develop early this afternoon with a tightening pressure gradient, especially for northwestern parts of the ILN FA. Expect that fairly efficient mixing should allow for gusts between 25-30 kts in far NW part of CWA during the middle of the afternoon. Gradient will be weakest in far SE, so expect that winds may only gust to 15 kts across far southern OH this afternoon. Boundary layer RH values are probably not quite low enough today to run the risk of fire weather concerns, but with the gusty winds in place, it is something that should at least be kept in mind. Thankfully, much of the CWA saw at least some light rain during the past 24 hours, with the heaviest rain occurring in the northwest -- where winds today are going to be the strongest. With dry air in place and abundant sunshine, surface temperatures are rebounding nicely from this morning`s start in the 40s. Highs are still on track for lower to mid 70s across FA. && .SHORT TERM /6 PM THIS EVENING THROUGH FRIDAY/... As model agreement strengthens in the overall scenario for Wednesday through Friday, the forecast will continue to be adjusted as has been done tonight -- slightly cooler temperatures, slightly greater cloud cover, and slightly higher chances of precipitation. For the most part, it still looks well too early to try to time out the vorticity maxima rotating around the huge upper low, especially as model agreement in the exact position of the system is still only marginal at best. With that said, attention will focus to the first such area of upper support, which will begin to develop directly over the forecast area early on Wednesday. This will occur as the overall envelope of the low is just getting into the area from the north. NAM/GFS projections show a jet streak rotating around the southern periphery of the low, with its left exit region tracking directly through the ILN CWA between 12Z-18Z. Combined with a weak surface boundary (and a boundary layer wind shift to the west) there is enough evidence to allow for an increase to likely PoPs in the eastern half of the forecast area for Wednesday (late morning through mid-afternoon). Owing more to the cooling temperatures aloft than anything else, some marginal surface-based instability is expected to develop, and a few low-topped thunderstorms will likely result from this -- especially if any breaks in the clouds end up allowing for some slight surface heating. Beyond Wednesday, it is much less certain where precipitation chances will be favored -- or even which quadrant of the upper low the ILN CWA will fall under. PoPs will be kept in the 20-40 percent range all the way into Saturday, favoring the diurnal peak periods slightly. Though a little more questionable than Wednesday, a slight chance of thunder was included for Thursday as well. Max temps will be held in the middle 60s through the period, owing to considerable uncertainty in potential insolation, and the expectation of occasional rain. This forecast is about 4-8 degrees below normal for max temps. The air mass would support values near 70 if the upswing of the diurnal cycle could coincide with dry and partly sunny conditions, but confidence in a window like that occurring is fairly low. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... The GFS has continued to trend more in line with the ECMWF, lingering the upper level low across the region through at least the end of the week. As a result, have continued to nudge up pops each day through Saturday and tweaked down temperatures a bit. Beyond that, both the GFS and ECMWF are trying to lift the low northeastward and out of the area, although the ECMWF is still slower in that process. At this point, will keep Sunday and Monday dry but again, if things continue to trend toward the slower ECMWF, we may need to introduce some low end pops into early next week and lower temperatures a bit more toward the end of the long term period. && .AVIATION /18Z TUESDAY THROUGH SUNDAY/...
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Mostly clear skies will continue through the afternoon across the area, with just a few VFR Cu for northern sites. A large and pronounced stacked low pressure system will continue to drift south from the Great Lakes area into the Ohio Valley through the TAF period. For this afternoon, a tightening of the pressure gradient will combine with efficient mixing to allow winds to gust to 25-30kts for northern sites of KDAY, KCMH, and KLCK, with gusts to 20-25kts possible elsewhere. These west/southwesterly gusts will be diurnally-driven, with a fast drop off expected towards 23z as temperatures begin to cool with the approach of sunset. Between 00z-12z Wednesday, moisture will gradually be on the increase as the low moves south from the Great Lakes. Past 12z, dynamic lift will increase in association with upper- level energy rotating around the low pressure system. As such, expect showers will develop and move west to east from mid- morning through early afternoon. Expect a quasi- linear cluster of showers to move east through terminal sites toward 18z, with best chance of showers for eastern sites of KCMH and KLCK. Additional (re)development is possible during the afternoon and early evening hours behind (farther west of) initial activity. While VFR conditions are anticipated through the majority of the TAF period, some brief MVFR CIGs/VSBYs are possible in heaviest shower activity Wednesday. Slight model differences on location and timing of activity precludes anything other than a VCSH at this time. While instby will be on the increase after 18z, expect potential isolated TS development to be fairly limited. OUTLOOK...Local MVFR ceilings and visibilities possible with showers on Wednesday. An isolated thunderstorm is also possible. Widespread MVFR ceilings with local MVFR visibilities possible with showers Thursday through Saturday.
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&& .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...KC/Hatzos SHORT TERM...Hatzos LONG TERM...JGL AVIATION...KC

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