Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 200150 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 950 PM EDT Wed Jul 19 2017 .SYNOPSIS... Hot and humid conditions will lead to the chance for thunderstorms at times through the rest of the week and into the weekend. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
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Dry conditions were maintained this evening across the entire ILN CWA minus a tiny area of Wayne/Fayette counties in Indiana. As indicated by the 00Z KILN sounding, an ample cap at 775mb helped keep instability from being realized. Attention will now turn upstream to what is currently a severe MCS moving into the Chicago area. Though this system is currently on a near-direct path toward the ILN forecast area, there is a substantial difference in air mass, in particular with regards to a notable dry wedge over the ILN area between 5kft-10kft. All indications are that this system will weaken after moving into Indiana and encountering a more hostile thermodynamic environment. With this in mind, the previous forecast appears on track with regards to PoPs (increasing tomorrow morning) and temperatures. Still no signs of likely dense fog development overnight, but some minor visibility restrictions are probably going to occur, especially in valley locations. Previous discussion > Main concern of the day is heat and humidity, as afternoon temperatures rise into the upper 80s to around 90. Combining dewpoints in the mid to upper 60s yields heat index values approaching the mid 90s this afternoon. With diurnal heating in mind, isolated thunderstorms can`t be completely ruled out. Capping may be a little weaker over the northwest part of the CWA so that may be where any isolated cells would develop, but with weak forcing even those would be sparse and no severe weather is expected. Cumulus will dissipate this evening, leaving some mid/high clouds for the overnight. Given light winds and the moist low levels, patchy fog is expected to develop particularly near river valleys. Fog should dissipate shortly after sunrise.
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&& .SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/... Some models suggest a MCS from the overnight will drop down toward the CWA Thursday morning, but in a decayed or dissipated state. As such, have left the forecast dry for Thursday morning but there is the potential for some debris clouds to move into the CWA. In addition, depending on the track of the MCS, there may be an outflow boundary that promotes convection for Thursday afternoon ahead of a cold front dropping down from the north that could also serve as a focus for convection that lasts into the evening. The evolution and interaction of both mesoscale and synoptic features is somewhat uncertain, but given the environment we have chance pops for scattered convection spreading south during the afternoon and evening hours, and given the possibility that a front is in place Thursday night have continued low pops for the overnight hours. As far as thunderstorm hazards, precipitable waters will be on the rise and the risk for heavy rainfall and local flash flooding will exist. In addition, high instability combined with better shear than recent days particularly across the northern portion of the CWA, suggests that the strongest storms will be capable of producing damaging wind gusts. Temperatures will also be a little warmer than the last couple of days, with maxima likely reaching or just exceeding 90F in most areas, leading to heat index values in the upper 90s to around 100 degrees. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... Sensible weather on Friday will be determined mostly on where exactly a frontal boundary lays out across our region. Models vary on position, but it most likely will be near the I-70 corridor. It is expected to remain quasi-stationary on Friday, with some scattered convection possible in its vicinity. Dewpoints are expected to pool near the boundary, and with forecast highs in the upper 80s to the lower 90s, we will likely see heat indicies near or slightly above 100 for at least the southern two thirds of our forecast area. This potential will be dependent on any convective debris clouds that may inhibit max heating. Will continue to mention heat indices in the HWO product. Upper level ridge will become suppressed some Friday night through Saturday with northwest flow predominating across the Great Lakes. Embedded disturbances will interact with the aforementioned boundary which models indicate that it may shift a little northward as a warm front. As such, will continue with a chance of showers/storms with the highest chance expected across the northern zones. Temperatures will be similar to Friday`s highs, upper 80s to lower 90s, with heat indicies approaching or surpassing 100 degrees over the southern two thirds of the CWFA once again. Max heating will again be dependent on convective debris clouds. Models concur that amplification in the flow will begin to occur Sunday into Monday. This will push a mid level trough across the Great Lakes and Ohio Valley. This will be associated with a slow moving cold front during this period. The threat for showers/storms will continue ahead of this front until late Monday afternoon and evening until the front is expected to pass southeast of our region. Surface high pressure is forecast to build into the region Monday night into Tuesday and extend into the region on Wednesday. This will bring a period of dry weather, along with drier air and cooler temperatures. && .AVIATION /02Z THURSDAY THROUGH MONDAY/... It appears that the TAF sites will remain dry through the rest of this evening, with winds becoming light and variable. As there is plenty of moisture in the atmosphere, some MVFR visibilities will be possible near daybreak, with the possibility of IFR visibilities at KLUK. VFR conditions are expected to prevail again in Thursday, with a few more clouds, and SW winds of around 10 knots. The bigger question for Thursday is that chances for showers and storms may be increasing, especially at the northern TAF sites (DAY/CMH/LCK). Probabilities for convection remain low, but this will have to be monitored closely for later forecasts. For now, the TAFs will be kept dry through the afternoon. OUTLOOK...Thunderstorms will be possible at times between Friday and Monday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...BPP NEAR TERM...Hatzos/BPP SHORT TERM...BPP LONG TERM...Hickman AVIATION...Hatzos is the U.S. government's official web portal to all federal, state and local government web resources and services.