Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 291345 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 945 AM EDT Thu Sep 29 2016 .SYNOPSIS... A large area of low pressure will remain in place over the Ohio Valley through the rest of the week, providing cool and occasionally rainy conditions. Chances for rain will begin to diminish on Saturday, as the low gradually begins to move to the northeast. Drier and warmer conditions are expected for Sunday and Monday, with surface high pressure building in from the west. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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950 AM Update > Observational platforms /radar and satellite loops/ place center of upper low near Louisville...drifting SSE. A very broad/weak surface low is just south of CVG and also dropping SSE. Low clouds/fog are very slowly breaking up/lifting this morning with vis imagery showing a far breaks in cntl OH. Going fcst logic /read below/ largely on track. Isold/sct showers are currently pestering the Scioto Valley in plume of deeper SSELY flow on the ern flank of the anomalous closed low. These will increase in number and intensity through the peak of the diurnal cycle...and with the upper low settling SSE with time...curvature of flow will tend to steer these into the Scioto Valley/Central Ohio and areas of SW OH. Least chances of rain today will be west of I-75 in wcntl OH. Localized heavier amounts seem targeted for the lower Scioto Valley where an 1"+ could fall from several bands/arcs of showers and storms. Severe threat is quite marginal and discussed well below. Northeast boundary layer flow /weak/ underneath southeast mid level flow /moderate/ promotes enough effective shear for modest storm organization as instby peaks mid-late afternoon. Hail the most likely threat given cold mid level temps...but wind is not out of the question if some localized heating can push temps toward the upper 60s in the Scioto Valley. 29.12Z RAP/HRRR 0-1km shear is weak the bulk of the day...briefly strengthening >20kts over Scioto Valley in the 22Z-23Z timeframe...so a rotating storm or two can`t be discounted early this evening before instby wanes. 513 AM Update > Based on current observations, and expectations for the next hour or two, the dense fog advisory has been shifted to the west. As the wind flow has shifted slightly, areas along the eastern edge of the original advisory have turned more to a low stratus deck, with visibilities improving significantly. However, with KDAY/KVES recently coming in with 1/4SM observations, a few counties also had to be added. 355 AM Discussion > An impressively lengthy curved band of precipitation currently exists along a cyclonic path from near Knoxville to west of Indianapolis. As of this writing, the entire band is wrapping around the ILN forecast area, with little to no precipitation actually occurring within the ILN CWA. Of foremost importance is the issue of fog and stratus, which has developed in earnest over the past several hours. Ground conditions are a precarious situation, with ceiling height differences of just a couple hundred feet making the difference between only modest instability reductions and full-on dense fog. The current dense fog advisory was issued where the majority of the 1/4SM / OVC001 observations have come in -- roughly between Wilmington and Wapakoneta, with a county or so on either side. This has, as of now, largely spared the DAY/CMH metro areas -- which are covered solidly by stratus decks a few hundred extra feet off the ground. While there is still a chance that dense fog could develop outside of the current advisory, the current placement does appear to be where conditions are most favorable -- the calmest of winds near the center of the weak surface low, and a dearth of mid-level moisture. Going into the afternoon, attention will turn toward convective potential. The center of the mid-level low will gradually move south today, shifting from central Indiana to south-central Kentucky by evening. The current band of precipitation is representative of upper-level forcing and positive vorticity advection along the inner edge of the 500mb-300mb jet max, and this feature will gradually shift westward over the course of the day, allowing precipitation to spread back into the ILN forecast area from the east. This looks like one of those strange days in which the cloud-bearing wind / storm motion vector may well be southeasterly. In addition to the upper forcing, convergence associated with low-level features will be on the increase as well, though primarily southeast of the ILN CWA. Combined with an increase in instability with a little bit of surface heating -- especially across the expected genesis region of West Virginia / eastern Ohio / southwest Pennsylvania -- some marginal instability is expected to develop. Steep lapse rates will be present thanks to the cold air aloft, but although some speed shear will be present, it is tough to say if the directional shear will be favorable for any storm organization -- flow backs from northeast to southeast with height, which is a fairly unusual orientation. Nonetheless, the mesoscale setup is somewhat favorable for some organization. Yesterday, any chance at boundary layer destabilization was done in by the giant blob of rain that developed very quickly and moved eastward across the area. This is a little less likely to occur in the storm genesis region today, but how much strength holds together as the convection moves toward central Ohio is questionable. As such, the SPC Day 1 outlook only allows for a small extension of the marginal risk area into the ILN CWA, which appears reasonable. Chances for convection do exist across the entire forecast area during the peak of the diurnal cycle, but the greatest chances for both rain and thunder will be in the northeastern third of the CWA. With some potential for slow motions and training, and ground conditions that are now more saturated than before, heavy rain is a concern that will have to be monitored.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... Overnight, a swath of showers (associated with the upper forcing and whatever is leftover from afternoon convection) is forecast to move westward through the ILN CWA. If nothing else, this forecast for rain makes the potential for fog tomorrow morning a little more uncertain, even if conditions would otherwise be favorable. Patchy fog wording will be included in the forecast, along with 50-70 percent chances of precipitation. As the mid-level low starts to swing northward again on Friday, there is reasonable model agreement in a relatively dry wedge entering the south-central section of the ILN CWA during the afternoon, with little in the way of forcing. PoPs on Friday afternoon and evening will range from 20-50 percent, with the highest values in the northern and northeastern sections of the forecast area. With systems like this, and still plenty of room for error in timing out the vorticity maxima rotating around, this PoP forecast should still not be seen as an explicit forecast of specific timing and placement. Precipitation chances will begin a more significant decrease on Saturday, as the low starts a quicker motion out of the Ohio Valley and back into the Great Lakes from whence it originated. Early on Saturday, the western half of the ILN CWA will be most likely to receive additional precipitation, but a gradual drying from south to north is expected by afternoon. The relatively small diurnal temperature ranges will begin to expand a little by Friday, and especially by Saturday, as slightly drier overall conditions allow for a little more heating in the afternoons. In fact, the forecasted highs on Saturday (upper 60s to lower 70s) are only a little bit off from normals for early October. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... This period will be characterized by a much warmer drier/period vs the short term. On Sunday morning...the closed low will be drifting northeast...weakening...and opening up into a more progressive shortwave trough over New England by Monday. Thus...there is still a small threat of showers into Sunday but right now the timing/placement of this threat is very tough but seems focused north of I-70 closer to mid level cold pool. Large scale ridging ahead of the next longwave trough over the west will allow for dry weather Monday thru Wed as high pressure at the surface moves across the eastern U.S. Temperatures will be warming steadily and by the middle of next week should be well into 70s if not the lower 80s with dewpoints hanging tough in the 50s. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... Large scale upper level low centered over south central Indiana this morning will drift south toward south central Kentucky through the day. The low will stall tonight, then it will slowly move back to the north and northwest on Friday. Daytime heating will allow ceilings and visibilities to improve, such that western terminals will likely see predominate VFR conditions (BKN-OVC cumuliform clouds) by 19Z. The exception will be the eastern terminals (KCMH, KLCK) where energy rotating around the upper level low will bring showers and perhaps a few embedded thunderstorms. This should keep conditions at least in the MVFR category after 19Z. How far west and southwest this precipitation gets is still in question at this time, but have brought some light showers to KILN/KDAY with a VCSH at KCVG/KLUK. For tonight, operational and high resolution models indicate that low level convergence will increase which should increase the threat for showers as the night wears on. In addition, ceilings will likely lower back down below a 1000 feet with visibilities fluctuating between 1 and 4 miles in showers and mist. Threat for any embedded thunder should diminish after 04Z. OUTLOOK...IFR/LIFR ceilings and IFR visibilities possible Friday morning. MVFR ceilings and visibilities possible with showers Friday afternoon through Saturday. && .ILN WATCHES/WARNINGS/ADVISORIES...
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OH...None. KY...None. IN...None.
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&& $$ SYNOPSIS...Hatzos NEAR TERM...Binau/Hatzos SHORT TERM...Hatzos LONG TERM...Binau AVIATION...Binau/Hickman

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