Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 240759 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 359 AM EDT WED AUG 24 2016 .SYNOPSIS... With upper level ridging over the southeastern states, warm and moist air will begin to flow into the region today. Embedded waves in the mid-level flow will bring chances for showers and storms today and Thursday, in the northern Ohio Valley and southern Great Lakes. High pressure over the Great Lakes will bring drier conditions for Friday and Saturday, before chances for storms increase slightly on Sunday and into the new week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... As surface high pressure continues to move away to the east, an increasing gradient is setting up over the Ohio Valley. Ridging over the southeastern states is centered due south of the Ohio Valley, meaning that the mid-level flow is more westerly than southwesterly. Nonetheless, flow from 850mb and lower will have a significant southwesterly (theta-e advection) component, with both temperatures and dewpoints forecast to increase today. The atmosphere will be increasingly moist (precipitable water of around 1.8-1.9 inches) and unstable (MLCAPE of 750-1250 J/kg). Forcing is the clear limiting factor in terms of convective potential today. Warm frontal processes are failing to generate much upstream activity early this morning, though a few showers may still be possible in the northwestern CWA between 09Z-15Z. With no discernible surface features in the area today, attention will turn toward forcing slightly further aloft, though notably focused to the north and west of the ILN forecast area. A shortwave is forecast to move across southern Michigan during the late afternoon, with convergence in an area of enhanced 925mb-850mb flow pushing across northern Indiana. This should spark convection upstream of the region, which may eventually propagate into the forecast area toward the greater instability. With storms following this pattern ending up separated from the stronger flow and the source of lift, it is not surprising that high-resolution models have been fairly sparse in forecast coverage this far south. Will keep PoPs in a similar range as the previous forecast -- 20 percent in the south to 40 percent in the north. In terms of hazards, hard to see more than perhaps a threat for some gusty winds as the isolated/scattered storms propagate forward, but the severe threat over the ILN CWA appears pretty low without higher-end instability or a better-organized source of forcing. GFS/NAM projections indicate a 3C-4C rise in 925mb/850mb temperatures from yesterday to today, and max temps have been adjusted accordingly. This should allow for rises from yesterday of a few degrees in the northern CWA (where clouds will be a little more prevalent) and as much as 4-6 degrees in the south (where temps are likely to top out in the upper 80s). && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/...
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The thermodynamic environment over the Ohio Valley will continue to modify from Wednesday into Thursday, with a steady stream of theta-e advection to the north of the upper ridge. 850mb temperatures are forecast to gain another 3C-4C by 18Z Thursday, and forecasts for surface temperatures have responded in kind -- no doubt, also helped by the slightly drier forecast, as will be discussed later. Interestingly, SREF plumes do not seem to be totally picking up on the warmer forecasts, and the new max temp grid for Thursday is way at the high end of the SREF spread (but in line with raw GFS/NAM projections). This allows for lower 90s in the southern third of the ILN CWA, with upper 80s elsewhere. The zone of quick WSW winds aloft will remain in place just northwest of the forecast area on Thursday, and will actually become even better established. Still, the forcing along with this feature (and a weak shortwave) will remain focused too far north for a direct impact to the middle Ohio Valley, and if anything, models have been a little more explicit in keeping drier conditions in place for Thursday. Though this shift means a lesser chance of convective debris affecting the area on Thursday morning, it`s still a possibility. That would really be the only limiting factor for instability, with SREF means (running on boundary layer temps that may actually be too low) indicating MLCAPE values of up to 2500 J/kg. Overall, the kinematic and forcing pattern looks quite similar to Wednesday, but with greater instability, and a little bit stronger shear as well. That will mean that if storms are able to propagate this far south and east, the threat for some strong storms will exist, with a primary threat of wind damage. Coverage is really the big question mark, and PoPs remain quite conservative. Until this comes into better view, it seems premature to add the threat to the HWO. As the dampening upper level wave moves east into Quebec, a very weak surface front will make some progress into the ILN forecast area on Friday, though it may never clear through the area before stalling (or washing out). The weak forcing warrants nothing more than a 20-percent chance during diurnal timing in the southern third of the forecast area, with only the northern half of the forecast area seeing a notable drop in temperatures from Thursday to Friday (and even that will only be by about 5 degrees). A slightly drier air mass, courtesy of a large area of high pressure, will settle in over the Great Lakes region on Saturday. Dry and mostly sunny conditions are expected, allowing for temperatures to increase by a degree or two from Friday, despite light wind flow and a fairly neutral advection pattern.
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&& .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/...
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For Sunday, moisture and energy spilling southeastward from Great Lakes low pressure will provide a chance of thunderstorms. Additional thunderstorms will be possible Monday when the low is forecast to drag a cold front across Ohio. A few thunderstorms may persist on Tuesday as the boundary sags gradually south to Kentucky. With a large and slow moving high in the upper atmosphere centered from the Central Appalachians to the Mid-Atlantic coast, and with mainly warm advection in the lower levels, a return to mid-summer heat is indicated. High temperatures ranging from the mid 80s to the low 90s will couple with dew points near 70 to make outdoor activities uncomfortable at best.
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&& .AVIATION /08Z WEDNESDAY THROUGH SUNDAY/... A mid level ridge over the lower Gulf Coast states will slowly build northward through the TAF period. At the same time, embedded disturbances will rotate eastward through the Great Lakes. Return moist flow, along with the proximity of the disturbances, will bring at least a chance of showers and thunderstorms to the terminals today. Chances do not appear high at this time, with the best chance likely occurring near the KDAY terminal this afternoon. Have maintained a VCTS/CB at this location. Southeast to south surface flow will become south later this morning around 10 to 12 knots with some local gusts in the 15 to 20 knot range by afternoon. For tonight, the latest high resolution/convection allowing models indicate that the threat for showers/storms overnight will likely be closer to the lower Great Lakes, or mainly north or northwest of the terminals. As a result, have just VFR conditions at this time. OUTLOOK...Thunderstorms possible from Thursday into Thursday night, mainly near the northern terminals. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...Coniglio AVIATION...Hickman

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