Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 261027 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 627 AM EDT Fri May 26 2017 .SYNOPSIS... Low pressure will move east of the region this morning, allowing for an area of high pressure to settle over the Ohio Valley today. However, an unsettled weather pattern is expected for the weekend, with several upper level disturbances helping to bring occasional chances of showers and storms to the area. Behind a cold front on Sunday night, drier conditions are expected on Monday. && .NEAR TERM /THROUGH TONIGHT/... While the middle Ohio Valley remains on the back side of low pressure over the Mid-Atlantic region, heights are beginning to rise, and the air mass is gradually drying. Some persistent but patchy stratocumulus remains in place across the eastern two-thirds of Ohio, with some lower clouds (and possibly some fog) also advecting through NE Indiana early this morning. Aside from these items of interest (which should cease to be an issue by mid to late morning) a fairly tranquil day is expected. A flattening of the upper level pattern will allow for more westerly winds, eventually advecting in some high clouds, without allowing any sustained ridging to build in. At the same time, a weak surface ridge will extent into the area from the southeastern states, but glancing more than encompassing the Ohio Valley. 5-10 knot westerly flow will thus be in place at the surface, with max temps in the upper 70s to near 80 (southwestern CWA) to lower 70s (northeastern CWA). && .SHORT TERM /SATURDAY/... An active weather pattern is setting up for the weekend, with several chances for convection that will be discussed here and in the Long Term AFD section below. As of 3AM EDT on Friday morning, radar and satellite data indicate an area of convection over northeastern Kansas, associated with a shortwave trough that is tailing a fairly sharp mid-level ridge. This shortwave is expected to drive convection eastward over the next 24 hours, and there are indications (for reasons discussed below) that the convection may be strong as it eventually reaches these longitudes late tonight. It is the latitude of this shortwave that will provide the most challenging part of the forecast. The key will be in how quickly (and to what degree) the ridge ahead of the shortwave is able to dampen. The GFS/GEFS are an interesting example to look at for this case, with (for a short-range forecast) an unusual amount of spread in 24HR GEFS 500mb height contours as the shortwave is expected to be moving into the region. Using QPF swaths as a first-guess, the operational runs show a great deal of spread in the location of the axis of convection, with a range of possibilities that is not even clustered around specific model types. The operational GFS is at the far northern end of the solutions (mainly outside the ILN CWA), joined closely by the NCAR WRF and WRF-ARW. Slightly further south (affecting the northern half of the ILN CWA) are the WRF-NMM and ECMWF. The NAM/WRF-NMM/NSSL WRF are further south, focusing storms along the Ohio River. These specifics are presented as an example -- obviously, using any single one of these model runs verbatim would not be a wise choice. However, given the tendency for convection to propagate along instability gradients, the northernmost solutions can probably be discounted. As mentioned, there is reason for some concern with this system, particularly for a threat of heavy rainfall. Along with the shortwave, the convection will be fueled by a low-level jet, providing a source of convergence and moisture transport -- in addition to an axis of precipitable water values of over an inch and a half. This will allow for deep convection to develop, with the potential for heavy rainfall rates. If there was forecast confidence in the waterlogged southwestern quarter of the ILN CWA being affected, there would already be justification for a flash flood watch. As it is, one might eventually be needed, but not for an event 24 hours out with this kind of placement uncertainty. In addition, do not want to totally negate a potential (though conditional) severe threat, with sufficient deep-layer westerly shear to support storm organization. Instability will likely need to be analyzed by way of MUCAPE (elevated instability) rather than SBCAPE, owing to the pattern and the time of day. Thus, while storms may not be surface based, a chance for large hail could exist. After the shortwave has moved through the region, it is somewhat unclear on whether or not additional storms will be able to develop during the day on Saturday. There will definitely be increasing low-level moisture and steepening mid-level lapse rates, but capping and subsidence behind the shortwave may limit this potential, at least through the afternoon. Will keep lower-end PoPs in place, and mainly focused in the southern sections of the CWA. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... A frontal boundary will be laying near the Ohio River Saturday night, and then it is forecast to pivot northeast as a warm front on Sunday ahead of a surface wave of low pressure. Uncertainty still exists at this juncture on the potential timing and coverage of an MCS to eject out of the mid Mississippi Valley into the Ohio Valley Saturday night into Sunday morning. Have broad brushed PoPs, with likely showers and thunderstorms currently favoring locations along and south of the Ohio River where the axis of instability will be favored. Damaging winds, large hail, and locally heavy rainfall will all be a concern during this window of convection. Later Sunday afternoon and night, surface wave of low pressure will move northeast to the southern Great Lakes, which will eventually allow a cold front to push eastward through our region. Again, will be concerned with severe weather potential ahead of the cold front along with locally heavy rainfall. Will mentioned hazardous weather threats in the HWO. A large upper level low will slowly track across Ontario for the first half of the week and then weaken and lift Thursday. This will result in generally cool conditions across the region through the rest of the period. A couple of embedded disturbances rotating around the upper low may bring some showers to northern zones Monday afternoon, and again on Tuesday. Models then diverge at the end of the week as to how fast a cold front will push through the region. As such, have kept a low threat for showers/storms on Thursday. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/... MVFR clouds have now almost completely cleared out from the TAF sites, and some stratus / mist over northwestern Ohio is unlikely to make it to any of the airports. Thus, aside from perhaps a brief MVFR ceiling or visibility in the first hour of the valid TAFs, VFR conditions are expected through the day today, with WSW winds generally at or below 10 knots. Clouds will begin to lower and thicken tonight. It is appearing likely that thunderstorms will move into the area some time after 00Z, though it is not yet clear exactly where they will move. This means that thunder (and lowered conditions) will not yet be included in the TAFS, but at some point it will likely need to be, once it becomes clear which sites will be impacted. Where storms do occur, MVFR/IFR conditions are expected to occur. After the storms move out, some showers may persist through the overnight and going into the day on Saturday. Ceilings are likely to drop to MVFR, and IFR ceilings are a possibility. OUTLOOK...MVFR ceilings and visibilities are possible Saturday, along with a chance of thunderstorms. Intermittent chances of thunderstorms will remain possible from Saturday through Monday morning. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...Hickman AVIATION...Hatzos

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