Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 180724 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 324 AM EDT Fri Aug 18 2017 .SYNOPSIS... A weak cold front will exit the area this morning taking any remaining showers and low clouds with it. A noticeable drop in humidity is expected but temperatures will remain warm through the weekend as high pressure builds across the Ohio Valley. A weakening weather system will cross the area on Saturday with a threat for showers and thunderstorms, particularly north of the Ohio River. Dry weather is then expected Sunday and into early next week as high pressure reasserts itself. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... GOES 16 water vapor loops and RAP analyses this morning indicated a shortwave trough over eastern Wisconsin with the mid-Ohio Valley still in modest southwesterly mid level flow. This has led to little airmass change despite pre-frontal convection overturning the boundary layer to some degree. Surface analysis indicated that the primary surface cold front still lagged west of the area, but will be entering the forecast area around sunrise as the shortwave over Wisconsin spreads downstream into Michigan. The trough axis will cross the area this morning, so residual shower activity mainly over the lower Scioto Valley as of 07Z will slowly clear to the east, though it may take until after sunrise for the last bit of precipitation to exit due to the lack of boundary layer push through about 12Z. Once this push occurs, expect a pretty decent drop in PWAT but not much low level cold advection. Thus, expect highs to push back toward the middle 80s, perhaps even a few upper 80s over northern KY with central OH lagging back in the lower 80s. Clouds will be somewhat problematic - but there is enough consensus in forecast soundings that current low stratus which has formed in the I-71 corridor will begin pushing east as it mixes out with increasing westerly flow after sunrise. By afternoon, there should be a healthy amount fair weather cumulus forming due to residual boundary layer moisture /dewpoints holding firm in the 60s/ and insolation. So while the day may start rather cloudy in some areas, think sunshine will win out by afternoon. && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... Attention quickly turns to a rather sharp/impressive mid/upper level shortwave trough which amplifies/digs into the western Great Lakes this evening, and continues to sharpen/amplify /especially in 500mb-250mb layer/ as it crosses the area Saturday morning into early afternoon. Kinematic forcing is strong aloft, but due to short wavelength with departing wave from today, and relative PWAT minimum in front of the wave with little chance for low level trajectories to enhance a flow of moisture, system seems moisture starved. To that end, despite a rather impressive mid/upper level presentation to this wave, lower level cyclogenesis/frontal development is slow to occur which decreases convergence and necessary boundary layer lift where instability is adequate. Expect an area of showers and thunderstorms to be crossing Indiana later tonight into Saturday morning, likely weakening to some degree through Saturday morning as it moves through the forecast area /higher chances along/north of I-70/. Given strong forcing aloft, don/t expect this precipitation to die off Saturday morning so this may hamper destabilization to some degree west of I-71. There does appear to be a small window in the afternoon in the Scioto Valley where destabilization may overlap the best forcing for a threat of strong/severe storms. Forecast soundings at ILN/CMH at midday Saturday certainly would support organized/strong updrafts with MLCAPE > 1000 J/kg and effective shears approaching 40kts with broad turning from swly boundary layer winds to strong west-northwesterly upper tropospheric flow with sharp upper jet. So there remains uncertainty on timing and overlap of best diurnal destabilization and ongoing clouds/rain chances on Saturday that casts some doubt degree of severe threat. Kept rain chances in the 40-55% range especially focusing on central Ohio closest to the better dynamical lift with this system. In cloud/rain free areas, it will be quite warm again with highs pushing toward the middle/upper 80s. Expect the wave to quickly transition east of the area Saturday evening with rain chances ending from west to east as drier air filters in, with cooler overnight lows from previous nights. && .LONG TERM /SUNDAY THROUGH THURSDAY/... Guidance is in fairly good agreement for the start of the long term period, showing surface high pressure parked right over the heart of the Ohio Valley region in the wake of Saturday/s system. With ample subsidence, expect dry conditions Sunday as mid level ridging begins to nudge back northward into the area. With mostly sunny skies and a strengthening upper level ridge over the southeastern U.S., trended temps up a degree or two across the board -- with highs in the mid to upper 80s expected. By Monday, the high will have shifted east of the immediate Ohio Valley region, with return flow once again becoming established in the area. With this, and the midlevel ridging aloft, highs will top out several degrees warmer than Sunday -- with the potential for highs near 90 degrees for the southern half of the FA Monday afternoon. The main item of interest for Monday will be extent /or lackthereof/ of cloud cover for the impending solar eclipse. While the GFS is currently showing some cirrus working into western parts of the FA during the afternoon, the ECMWF shows all thick cirrus remaining well to the north/west of the immediate area. Even with the uncertainty regarding coverage or thickness of cirrus, it appears likely that Monday will be pcpn-free for the entire area. Even with some thin cirrus, diurnally-driven FEW to SCT Cu are possible during the eclipse timeframe. With increasing southerly flow and the corresponding increase in low level moisture, expect a warm Monday night period, with lows generally around 70 degrees. By early Tuesday, increasing low level moisture and the slow approach of a cold front will allow for the introduction of slight chance PoPs, especially for northwestern portions of the FA. PoPs will slowly increase during the day on Tuesday as low level destabilization occurs in the pre-frontal environment. Even with this, model solutions are in remarkable agreement (this far out) showing the front (and corresponding highest PoPs) moving through Tuesday night (NW to SE). Although it currently appears that best forcing will hold off until early Tuesday night, any shift in timing may act to increase or further decrease alignment/overlap with peak diurnal instability Tuesday. Nevertheless, any mid level disturbances ahead of the surface frontal boundary may be enough to initiate storm activity during the afternoon on Tuesday before the arrival of the front itself. Behind the front, a big change in airmass will occur as upper level troughing becomes more amplified east of the Great Lakes region. Unseasonably chilly temperatures aloft are forecast midweek and beyond, suggesting a period of below normal surface temperatures for the second part of the long term period. It appears probable that the area will see at least one or two days where temperatures do not make it out of the 70s Thursday and/or Friday. && .AVIATION /07Z FRIDAY THROUGH TUESDAY/... In the wake of showers which have largely pushed away from the TAF sites, an area of IFR/MVFR stratus has formed along I-71 and is impacting DAY/ILN/CVG/LUK, and am expecting this to spread/develop into the CMH/LCK terminals as well soon. This area of stratus on the back edge of shield of mid/higher level cloud but there is a back edge that will be progressing east /albeit slowly/ given still high boundary layer moisture as weak cold front works through the area. So through sunrise have hit low clouds and minor reductions to visibility hard in the newest TAF issuance. With sunrise, vertical mixing will increase and winds will become gusty especially at wind prone sites like DAY. This will serve to break up/lift any residual low clouds and expect a scattered cumulus deck to form, which may be broken in coverage for a time at some sites, but for now running scattered. Expect VFR conditions past about 13 or 14Z. Next weather system will spread mid/high level clouds toward the TAF sites on Friday evening. OUTLOOK...Thunderstorms are possible on Saturday, and again on Tuesday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Binau NEAR TERM...Binau SHORT TERM...Binau LONG TERM...KC AVIATION...Binau

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