Area Forecast Discussion
Issued by NWS Wilmington, OH

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371 FXUS61 KILN 201024 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 624 AM EDT WED JUL 20 2016 .SYNOPSIS... High pressure will extend across the Ohio Valley and mid-Atlantic states today, with warm and generally dry conditions expected. As a large ridge builds over the central part of the country, heat and humidity will continue to increase through the weekend, with the warmest conditions expected on Friday and Saturday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A large ridge of high pressure aloft has become well established over the central plains. At the surface, high pressure currently extends from the mid-Atlantic through the Ohio Valley, and will grow as it shifts gradually southward over the next day or two. Near-surface flow over the ILN forecast area remains quite weak, and this will remain the case through the day today, with only light southeasterly winds expected. The air mass has changed only very slightly from yesterday, allowing for a slightly-adjusted persistence forecast for max temperatures today, with no significant change in surface dewpoints either. The only significant challenge for the forecast today is for any convective potential. First and foremost, the cluster of storms currently over southwestern Indiana has continued to defy model projections by maintaining a slow easterly progression. This cluster split off from an earlier MCS, which had been propagating southeast along the 925mb theta-e gradient. The storms, as they currently exist and propagate, are expected to soon outrun their favorable environment. There is a notable dewpoint gradient from southwest Indiana to southeast Indiana, and less favorable thermal conditions just off the surface will also mean that any elevated instability feeding the storms will become weaker with eastward extent. SPC mesoanalysis indicates about 1000 J/kg of MUCAPE where the storms are currently located, but this diminishes to well under 500 J/kg at the border of the IND/ILN forecast areas. If they were to survive, at their current rate, it would take until 7AM for the storms to move into the ILN CWA. Though a few models suggest a chance for some precipitation in the western half of the forecast area this afternoon -- and a very, very isolated shower/storm or two may well occur -- the vast majority of locations should remain dry. The forecast will do the same as of now. && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/... Increasing heat and humidity will be the main focus through the rest of the week, but a challenging forecast for potential convection is also a part of the situation. By Thursday, the center of the surface high will be moving south from the mid-Atlantic region, and the pressure gradient over the Ohio Valley will begin to tighten. As this occurs, heights will rise, as the plains ridge continues to strengthen and expand. This pattern will favor a warming and moistening of the boundary layer on Thursday and Friday. There will be little additional change going into the weekend, as the ridge will flatten somewhat, and 925mb-850mb winds will take a slight northerly component (limiting additional warm advection). Confidence remains high in a very warm period from Friday through Sunday, and the HWO will maintain its mention of possible advisory-criteria heat (heat indices over 100 degrees) for all three days. After a careful analysis of all available information sources -- direct model projections, anomaly analysis, ILN sounding and dewpoint climatologies -- it should be noted that this will be an anomalously warm period, but not at a level approaching the extreme end of the scale. By Friday afternoon, model projections suggest 850mb temperatures of around 22C. ILN sounding climatology would suggest that this is around 2 standard deviations above normal, and GFS/NAM anomaly analyses suggest similar values. Surface dewpoints are forecast to increase significantly between Thursday afternoon and Friday afternoon, with raw model projections reaching as high as the upper 70s to around 80 degrees. It is exceptionally rare to have dewpoints reach 80 degrees -- it has happened on only a few days in the entire CVG period of record. Even dewpoints in the upper 70s are rare, even in mid to late July. With this air mass not looking quite so historic, the dewpoint forecast will be allowed to peak out in the mid 70s on Friday, with the understanding that if mixing is a little deeper than expected, these values could even drop a little more during the afternoon. Further complicating things is the chance of some convection (or at least clouds) on Friday, which (as will be discussed below) appears slightly greater than in previous forecasts. With the above factors considered, the temperature forecast for Friday was actually lowered slightly, allowing for a range from around 91 degrees (central Ohio) to 97 degrees (north-central Kentucky) on Friday. Record highs (well above 100 degrees for all three climate sites on Fri-Sat-Sun) are easily safe, as records like that typically require a drier air mass to challenge. The end result is that Friday -- the day most likely to have the highest heat index values -- appears likely to meet heat advisory criteria for most (if not all) of the ILN forecast area. Warning criteria of 105 degrees or higher will be possible in the southwestern sections of the CWA, especially for the Cincinnati metropolitan area and northern Kentucky. Similar conditions are expected for Saturday and Sunday, but dewpoints may come down by a couple degrees over the weekend. Analyzing the convective potential through the rest of the short term forecast period continues to be a challenge. The air mass is expected to remain quite unstable, but somewhat capped under the ridge, meaning that widespread developmental convection will be unlikely. However, virtually all available computer models develop scattered and generally-disorganized convection across the Great Lakes and Ohio Valley at times through the next few days. In the absence of any larger-scale or well-defined lifting mechanisms, this activity will likely be tied to indeterminate mesoscale features or weak shortwaves rounding the northeastern periphery of the ridge. Through this forecast cycle, precipitation chances will be kept very low through Friday morning. By Friday, a cold front will be moving south into the Great Lakes. Ahead of the front, as was described earlier significant heat and humidity will build into the Ohio Valley. Weak convergence in between (mainly across northern Indiana and Ohio) is expected to lead to a little bit more of an organized chance for showers and storms. PoPs will again be kept very low until the degree of capping (or any signs of further support for lift) can be determined more clearly. As the cold front moves south and pivots, a slightly drier and more stable air mass will enter the southern Great Lakes, shifting the chances for additional convection a little further to the southwest. However, the entire ILN forecast area will still be allowed a low-end precipitation chance in the forecast. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... On Sunday, greater short wave energy entering the northern plains is forecast to develop a surface low that will drag cold front toward the ILN area. A better chance for thunderstorms will exist in increasing moisture and forcing ahead of the front. The chance for convection will continue Monday as the front makes its way across the area. Look for drier conditions Tuesday under weak high pressure behind the front. Temperatures will be hot. Somewhat cooler readings in the mid and upper 80s are expected Monday and Tuesday due to modest cold advection. && .AVIATION /10Z WEDNESDAY THROUGH SUNDAY/...
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A dying MCS over south central Indiana is not expected to make it to the southwestern terminals of KCVG/KLUK this morning. Otherwise, surface high pressure centered over the eastern Great Lakes this morning will continue to extended southwest into the mid and upper Ohio Valley through tonight. Some cirrus can be expected from time to time in the northwest flow aloft along with some scattered diurnal cumulus clouds. Can not completely rule out an isolated pop up shower/storm in the heat of the day near the southwestern terminals where instability will be the highest, but this is obviously too small to place in the TAFs. Winds will be calm or light easterly this morning, light and variable this afternoon, then back to a light east or southeast wind tonight. OUTLOOK...Thunderstorms possible Sunday.
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&& .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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