Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 271044 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 644 AM 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 is currently between a cold front (which passed through the area yesterday morning) and a sprawling area of stacked low pressure (which is currently centered just north of Lake Superior). In between the two systems, a fairly dry air mass is in place, with the 00Z KILN sounding recording a precipitable water value of just 0.35 inches. This dry air mass will present little in the way of weather concerns for today -- perhaps just a few cumulus, but even that is not a certainty. Winds may actually be the most interesting part of the weather story for the next 12-18 hours, as the pressure gradient is still strengthening, as the Great Lakes low gradually moves south. The west-southwesterly flow is fairly deep, and with sunny conditions in a dry air mass, deep mixing is expected to occur. This will fairly easily result in wind gusts of around 25 knots (perhaps up near 30 knots) in the northwestern corner of the forecast area. There should actually be a fairly quick drop-off in wind speeds with southeastward extent, as the pressure gradient (especially in the 925mb-850mb layer) weakens considerably closer to Kentucky / SE Ohio / West Virginia. 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. && .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 MONDAY/... 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 /12Z TUESDAY THROUGH SATURDAY/... Skies will remain clear with generally light winds from the south/southwest through mid morning or 15Z. Otherwise, our region will find itself between a departing cold front to our east and a large scale upper level low dropping south into the western Great Lakes today. An increasing pressure gradient, combined with fairly decent diurnal mixing, will allow winds to increase and become gusty from the west/southwest between 15Z and 17Z. Wind gusts will range between 20 and 28 knots with the highest gusts expected at KDAY. At the same time, models indicate a subtle moisture channel in the low levels, which should result in a few high based diurnal cumulus clouds by 17Z. For tonight, cumulus will flatten to some low altocumulus and will be mixed with some cirrus through early morning. Upper level low pressure will continue to move toward the southeast. Models indicate that a cold front will begin developing to our west. The combination of this approaching front and dynamic lift associated with energy rotating around the upper level low will cause clouds to increase/thicken between 09Z and 12Z with a few light showers/sprinkles possible by 12Z. OUTLOOK...Local MVFR ceilings and visibilities possible with showers on Wednesday. Widespread MVFR ceilings with local MVFR visibilities possible with showers Thursday through Saturday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...JGL AVIATION...Hickman

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