Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 250835 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 435 AM EDT THU AUG 25 2016 .SYNOPSIS...
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Warm and humid conditions will persist today, with mid-level ridging centered over the southeastern states. There will be chances for showers and storms on occasion, with a weak frontal boundary located in the northern Ohio Valley. This front will move south through the region into Friday, bringing slightly drier conditions going into the weekend. However, temperatures are expected to remain above normal into next week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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The last remnants of the Indiana tornado outbreak are currently a disorganized mess of storms affecting central Ohio, producing nothing more than lightning and some heavy downpours. The activity on Wednesday has clearly laid out a boundary across the ILN forecast area. The forward propagating storms exiting the CWA to the east represent the southeasternmost extent of this boundary, which is now beginning to make a pivot across western Ohio and Indiana, connecting all the way back to the developing MCS over northern Illinois. With a moist air mass and plenty of instability available (RAP analysis indicating 500-100 J/kg of MLCAPE even at this early hour) random/scattered development is expected to continue in the warm air advection along this boundary, as it eventually makes a northward motion through the ILN CWA. Over the next few hours, attention will turn to the upstream MCS. Though individual storm motions are to the ENE, more of an eastward propagation into the instability is eventually expected. This is unlikely to lead to a direct hit on the ILN CWA, but the northwestern / northern sections of the CWA will probably be impacted to at least some degree. With this morning activity, severe weather appears unlikely -- probably just some heavy rain within the slightly-elevated convective complex. In the absence of strong low-level forcing, convective potential today will be driven by convergence in the 925mb-850mb flow, combined with whatever mesoscale features in the low levels are able to manifest themselves as a focus for new updrafts. This may very well occur on what remains of the southern flank of the morning convection, which will also help create an instability gradient across northern Indiana/Ohio. Morning model analysis has proven to be challenging with regards to determining timing and placement of new development. The NSSL-WRF is not capturing the current convection, the latest RAP is much too slow with the current convection, and the last several HRRR runs leave a stable environment in the wake of the storms, producing very little in the late afternoon and evening. The building heat and humidity seem like a near-certainty to provide an unstable air mass, so the nearly-dry solution for the afternoon/evening does not seem the most reasonable. It will, however, be a question of where the leftover boundary from the morning convection will eventually set itself up. All in all, the PoP thinking has changed very little from previous forecasts -- 30-40 percent chances focused in the northern CWA during peak diurnal timing (or slightly later), with dry conditions maintained in the far south. Without organized forcing to tilt the scales toward more widespread activity, there isn`t the support in the models or in the pattern for higher PoPs. This convective area of concern is located on the southern periphery of some stronger westerly deep-layer flow, which will combine with slightly-gusty southwesterly surface winds to produce a respectable amount of shear -- and even some turning in the lowest 1 or 2 kilometers. MLCAPE of 2000-2500 J/kg will be possible to the south of the morning convection, which should overlap at least somewhat with the favorable shear near and just north of the forecast area. This will create an environment that will be favorable for supercells, potentially with a tornado threat, under the important distinction that storm development will be highly contingent on how the next six hours proceed. This is no carbon copy of yesterday -- there are several differences: concerns with antecedent convection, no MCV passing through the vicinity during peak convective timing, and surface flow that is a little less backed. Instability should actually be a little stronger, and deep-layer shear is similar. It`s not looking possible to have all the answers in this midnight-shift forecast assessment, so mesoscale analysis through the morning will hopefully shed light on if and when storms will develop -- and their potential severity if they do. With the way the flow is set up, would not be surprised if there is a little bit of echo training, and potentially an isolated flash flood concern. However, would rather not hit that risk too hard with only a 40 PoP in the grids (and the aforementioned coverage / forcing questions). There were signs of that potential already this morning, with 1.00"-1.50" rainfalls along a corridor from Darke County OH to Fairfield County OH. Where dry conditions are expected to be maintained in the far southern sections of the CWA, temperatures in the lower 90s will allow for heat indices to approach 100 degrees -- and maybe even reach that mark for an hour or two. This will be covered via HWO/SPS.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/...
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After any remaining convection dissipates during the overnight hours on Thursday, dry conditions are expected through the rest of the short term. The cold front moving into the area will be ushering in a slightly drier air mass, but the front is expected to be severely lacking in forcing and convergence. With surface high pressure moving into the southern Great Lakes on Friday, then sliding east on Saturday, moisture and convective chances should remain on the southern and western periphery of the Ohio Valley.
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&& .LONG TERM /SUNDAY THROUGH WEDNESDAY/...
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High pressure will exit the region Saturday and surface easterly flow will turn southerly. Cloud cover from convection in the midwest spill into the Ohio Valley. A surface boundary is expected to lay out over the region early Sunday as high pressure builds in the Upper Midwest. This should just provide a modest increase in diurnal thunderstorm activity chances Sunday with a little more increase on Monday, both with subsequent nighttime drops in these chances. Northeast flow on Monday into Tuesday night is expected as the high traverses the Great Lakes region. Another repeat of increased chances of afternoon thunderstorms is expected Tuesday and Wednesday given what will become a nebulous surface pattern. While all of the potential precipitation may not contain thunder, have not tried to minimize the morning and evening chances by calling them showers, even though this may be how the pattern materializes. Just peppered any precip chances in the extended as thunderstorms. 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 increasingly uncomfortable air to the region. Some lower 80s temperatures may be expected in areas where precip develops and skies remain cloudy all day.
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&& .AVIATION /07Z THURSDAY THROUGH MONDAY/... A trailing line of showers and thunderstorms are expected to affect the northern terminals of KDAY, KCMH, and KLCK through 12Z. MVFR ceilings to local IFR/LIFR visibilities can be expected. Otherwise, for later today, a weak cold front will approach the region by late in the day. Latest model guidance, including high resolution/convection allowing models indicate that the region should stay dry before the cold front starts to sag into northern Indiana and northern Ohio between 21Z and 00Z. As this occurs, scattered showers and thunderstorms are expected along and ahead of the frontal boundary. For tonight, the weak cold front is expected to sag south toward the Ohio River. With the loss of daytime heating, and the lack of significant forcing either in the lower and upper levels, scattered showers and thunderstorms should wane this evening/overnight as the front moves south. Given the low chances of precipitation making it to KDAY/KCMH/KLCK at this time, have kept these terminals dry. OUTLOOK...Thunderstorms possible Sunday and Monday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...Franks AVIATION...Hickman

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