Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 281847 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 247 PM EDT Wed Sep 28 2016 .SYNOPSIS... A large low pressure system over northern Indiana will drift very slowly south into Kentucky...before reversing course and drifting back to the northwest toward Chicago...before very slowly weakening and losing grip over the Ohio Valley by the end of the weekend. Moist southerly to southeasterly flow around this low will keep rain and scattered thunderstorms in the picture all way into Sunday...with the better chances of rain focused on Thursday and Friday. Temperatures will remain below normal underneath frequent clouds and showers...before warmer and drier weather spreads through the Ohio Valley early next week. && .NEAR TERM /THROUGH TONIGHT/... Lots of weather to look at today thanks to anomalous mid/upper level trough and closed low which is now directly impacting CWA. At 18Z...center of mid-level circulation was almost stationary in NW Indiana...perhaps very slowly drifting SSE. Core of moderate to strong mid/upper level flow existed around the periphery of this system...into and through the basal portion of the longwave trough into southern IND and western KY...and across much of the WFO ILN CWA. Attendant to this mid-level circulation...a discombobulated and weak surface flow existed with several weak surface lows able to be identified in surface observations to the southeast of the cold core...with the primary /weakening/ 1009mb surface cyclone just south of Indianapolis moving east into sern IND. A large shield of rain/embedded thunder was moving across much/all of the ILN CWA due to very strong ascent via PVA ahead of the upper trough and on the immediate poleward/cyclonic shear side if the mid/upper level jet. Back to west where a few breaks existed in the cloud cover /allowing sfc temps into the lower-middle 60s/ discrete cellular convection recently developed as expected where steep mid level lapse rates and the diabatic heating has combined to allow SBCAPEs to reach 500 J/Kg. These storms have been producing many reports of small hail. Through tonight...expect current band of rain/storms over ILN CWA to move east slowly while more scattered convection will continue to develop and move in from the west. Current rain will have tempered destabilization to some degree...but an hour or two of sunshine in southeast Indiana to the Ohio border...will allow storms to remain intact at least to the Ohio border thru 6 PM if not a little later. As with out west...expect sub-severe hail to be the primary threat...with lesser threat of wind. As eluded to in previous discussion...brief/weak tornado threat is non-zero. Despite boundary layer flow over the area being weak/messy...there is a decided backed/southeast component to the flow thanks to the weakening surface low approaching. Thus...given ample/robust mid/upper level flow and respectable instability...effective shears will support storm organization /brief supercellular structures/ and if the right juxtaposition of backed low level flow near/northeast of the surface low and an organized updraft...could see the threat for a rotating storm. An unlikely threat...again given that 0-1km shear values on the whole are < 20kts...but one that bears mentioning/monitoring. There have been many CAMS today with simulated updraft helicity /25m2/s2/ streaks across western OH/ern IND so will be watching discrete storms closely in the 4PM to 7PM window. Another concern...and always a concern on periphery of compact/closed lows will be pivoting rain/convective bands for potential flooding. Widespread rains today have dropped mostly <1" but some areas in ecntl IND/wcntl OH have seen up to 1.25 to 1.5"...and this area in particular could see pivoting band of convection late this afternoon/evening which could cause some runoff issues. Ambient PWAT is not impressive /near to even just below seasonal norms...so environment isn`t primed...but stationary/slowly moving convective elements will still cause concern. Expect weakening of all activity after 7 PM...but rain will linger through the evening in the Scioto Valley into CMH area much of the evening...wrapping back westward into northwest Ohio...but dry slot wrapping into the area from south to north should effectively shut most precipitation down save for a few showers from later this evening through the overnight. && .SHORT TERM /THURSDAY THROUGH SATURDAY NIGHT/... Pesky closed low...with 500mb standardized anomalies to 2 or 2.5 sigma below climo...will drop south into southern KY by Thursday evening...before stalling. As it fills...this system will begin drifting aimlessly back into northern Indian by Saturday morning and there is strong agreement in the suite of NWP on this solution. This will guarantee a continued threat of episodic showers and storms Thursday...Friday...and Saturday...though with time the large scale forcing and cold-core of the system will weaken to the point that precipitation will become less organized and lighter...especially Saturday. Still-strong flow around this system will advect deep moisture off the Atlantic and into/across the Appalachian mountains and into the Ohio Valley through the end of the week. A particular area of enhanced rain threat will be across the northern/eastern CWA in closest proximity to deeper moisture and enhanced isentropic ascent. Given cold pool aloft...and slowly enhanced moisture in the boundary layer on both days...instability is sufficient enough that storms are possible...and will be drifting from SE to NW - a decidedly uncharacteristic storm motion. Both days will have a threat for an anchored or slowly moving arc of rain/storms...so the heavy rain/minor flood threat is not completely out of the question. But as with all precipitation around closed upper lows...there is a wide disagreement in locations/timing of the individual vort lobes rotating around the central circulation...thus rain chances are not yet ramped up as high as they otherwise will likely be in coming forecasts. 60s by day and 50s by night in this reduced diurnal temperature range thanks to plenty of clouds. && .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 /18Z WEDNESDAY THROUGH MONDAY/... A large upper level low will continue to dig south/southeast across the Ohio Valley through Thursday, providing active weather through the TAF period. Have seen a steady increase in coverage of SHRA with embedded TS in the late morning and early afternoon. This trend will continue through the afternoon and early evening, and as such, expect activity to continue to pivot ENE through terminals as a weak sfc low moves E into area. Prevailing VFR conditions are expected with TEMPO MVFR VSBYs and even CIGs in heaviest pockets of pcpn. Expect shield of SHRA/TS to move E into eastern half of area after 21z, with additional redevelopment likely farther west near/coincident with the sfc low. This secondary activity will likely impact western terminals of KCVG, KLUK, KDAY, and KILN before weakening past 00z as sfc low begins to stall/retrograde and instby begins to wane. Expect SHRA to be ongoing between 03z-06z just east of KCMH and KLCK as sfc low settles into area. With this, winds will become light and variable overnight. Do expect that IFR CIGs/VSBYs will develop area-wide after 03-06z, with potential for LIFR conditions in 08z-15z time frame, especially for higher-elevation terminals of KDAY and KILN. IFR/LIFR CIGs will be slow to lift Thursday morning, but do expect MVFR and eventually VFR conditions to return towards the very end of the TAF period. Past 18z Thursday for KCVG, do expect scattered SHRA redevelopment, initially across eastern parts of the area, then translating to the northwest. Instby will be marginal, but cannot rule out isolated TS after 18z. Confidence on potential coverage of TS precludes inclusion in the TAF at this time. OUTLOOK... MVFR ceilings and visibilities possible with showers Thursday afternoon through Saturday. IFR/LIFR ceilings and visibilities possible Friday morning, with IFR ceilings and visibilities again possible on Saturday morning. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Binau NEAR TERM...Binau SHORT TERM...Binau LONG TERM...Binau AVIATION...KC

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