Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 211038 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 638 AM EDT THU JUL 21 2016 .SYNOPSIS... A large ridge of high pressure over the central portion of the United States will bring hot and humid conditions to the Ohio Valley through the weekend. Temperatures will be at their warmest from Friday through Sunday, with at least some chance of showers and storms as well. A cold front is forecast to bring a better chance of storms on Monday, along with a slight drop in temperatures through the first half of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The well-advertised warm pattern is continuing to set up over the CONUS, with an expansive ridge over the central part of the country, and surface high pressure sliding into the southeastern states. The surface pressure pattern will remain somewhat weak over the Ohio Valley today, but there will be some light southwesterly flow introduced to the region -- one change from the very weak flow yesterday. With dry and mostly sunny conditions expected across the entire CWA today, there is every reason to believe that temperatures should climb another degree or two from the values reached yesterday. That would put the entire forecast area in the lower 90s. This is also unlikely to be inhibited by convection, as there is a reasonable amount of confidence that dry conditions will be maintained through the entire day -- a zone of weak convergence will remain west of the ILN forecast area. Dewpoints will still be only slowly increasing, thanks to the weak boundary layer advection. This will allow for a dewpoint increase into the mid 60s to around 70 degrees, producing heat indices that will range from the lower to upper 90s. While below advisory criteria, these values are still quite warm, and a Special Weather Statement will address it. && .SHORT TERM /6 PM THIS EVENING THROUGH SUNDAY/... Heat and humidity will be the main focus through Friday and the weekend, but a challenging forecast for potential convection is also a part of the situation. With surface high pressure moving into the southeastern states, and the plains ridge continuing to strengthen and expand, the pattern over the Ohio Valley will favor increasing boundary layer theta-e advection on 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. What has changed in this forecast cycle is that confidence in reaching advisory criteria has diminished just slightly from before, thanks to a number of factors. Looking at several sources of information -- direct model projections, anomaly analysis, ILN sounding and dewpoint climatologies -- it should be noted that while this will be an anomalously warm period, it will not be at a level approaching the extreme end of the scale. By Friday afternoon, model projections suggest 850mb temperatures of around 21C-22C. The local ILN sounding climatology would suggest that this is around 2 standard deviations above normal. 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. When factoring in a little bit of boundary layer mixing, forecasting dewpoints this high does not seem like a prudent choice -- instead, the forecast will allow dewpoints to peak out in the mid 70s on Friday, close to SREF mean values. As for temperatures, even assuming an unimpeded path to heating, it would be difficult to fight the model consensus too much in a pattern like this one (anomalous but not extreme). Guidance numbers for Cincinnati are 93 (MAV/MET) with an SREF mean of 90. Even the warmest member of the SREF at CVG reaches only 93 degrees. Further complicating things is the chance of convection and clouds on Friday, which remains a low-confidence forecast, as will be discussed below. This forecast will stay at the high end of the SREF plumes and near the guidance numbers, allowing for max temps on Friday that will range from 90 (far northern CWA) to 93 (northern Kentucky). 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. In terms of convective potential for Friday, the larger-scale models are of no help with regard to storm timing and placement, simply printing QPF across wide swaths of the region. High-resolution models (WRF-ARW/WRF-NMM/NSSL-WRF Realtime) appear mostly lost once beyond their depictions of Thursday evening / Friday morning convection upstream, and even those forecasts have been incredibly jumpy from run to run. All in all, the convective forecast for Friday is extremely low in confidence, and at this point the best forecast may be just to focus on the pattern -- weak boundary layer convergence ahead of a front, with the possibility of a shortwave rounding the northeastern periphery of the ridge, within a very warm/moist but somewhat capped air mass. Hazards from storms that develop on Friday will likely be limited to very heavy rainfall (with a very moist air mass and slow storm motions) and gusty winds (either from downbursts or cold pool development). Nonetheless, the overall severe threat appears fairly low. On Saturday, the convergent zone ahead of the front will shift to the southwest, and precipitation chances will be limited to the southwestern half of the ILN forecast area (with some slightly drier air advecting into central and northern Ohio). Convective chances will increase again on Sunday for the entire CWA. Potential headlines for heat are the final item to discuss. With the slight forecast reduction in max temps for Friday, heat indices are only expected to reach 100 degrees for a little less than half of the ILN forecast area. Confidence in a CWA-wide heat advisory for Friday is not high at all. Given the potential for upstream convection impacting the situation on Friday, confidence is only moderate even for the expected-to-be-warmest southwestern corner of the forecast area. After collaborating with neighboring offices, and noting that Friday remains the third period of the forecast, it was decided to wait one more forecast cycle before making a decision on initial headline placement. Regardless, this will still be an abnormally hot period, wherever advisory headlines end up becoming necessary. As mentioned earlier, a Special Weather Statement will be issued to discuss the heat on Thursday, and also to indicate the concerns for Friday through Sunday. && .LONG TERM /SUNDAY NIGHT THROUGH WEDNESDAY/... On Sunday night/Monday...the MT shortwave mentioned above should be crossing to the north of the Great Lakes and will likely be the impetus for better storm coverage/development but timing is concerning which may bring better rain chances Sunday night. A little better flow with the wave may allow for a little better storm coverage/organization/severity if a Monday frontal passage occurs at peak heating. On Tues/Wed...some relief though height falls will be rather modest and cooling too. Expect temps/dwpts to fall back to more seasonable summertime values during this time. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... The center of surface high pressure will settle over the mid Atlantic region today, then it will drift off the coast tonight. For today, return flow around the high will bring an increase in surface moisture. This will result in some FEW-SCT diurnal cumulus clouds along with southerly winds in the 7 to 10 knot range. For tonight, models are still having difficulty in discerning where upstream convective complexes may head toward as they dive south/southeast around the periphery of an expansive mid level ridge from the upper Mississippi River Valley/upper Midwest. Certainly, there will be some debris clouds from the convection, but it is unclear how much of this convection will make it into our forecast area by 12Z Friday. So although there may be a risk of a thunderstorm late tonight, low confidence in location and coverage support keeping the terminals dry. OUTLOOK...Thunderstorms possible Friday through Monday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...Binau AVIATION...Hickman

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