Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 271107 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 707 AM EDT Sat May 27 2017 .SYNOPSIS... With warm and moist air over the Ohio Valley, there will be occasional chances for showers and thunderstorms through the weekend. After a cold front moves through the region Sunday night, drier conditions are expected for Monday. && .NEAR TERM /THROUGH TONIGHT/... Pseudo-zonal / slightly ridged mid level flow currently exists over the Ohio Valley. The latest surface observations indicate that there is a weak surface low over central Indiana. One shortwave has helped spark the convection over the ILN CWA early this morning, and GOES-16 (preliminary / non- operational) water vapor (CH09/CH10) imagery suggests there is another weak vorticity maxima over northern Illinois, likely somewhere in the 925mb-850mb layer. At the surface, there is a boundary stretching across the region roughly from west-to-east, though it is somewhat wavy (in part due to the surface low). A well advertised low level jet has been helping to bring increased theta-e and moisture convergence just off the surface, but this influence is going to continue to shift ESE with time. Thus, as the lead shortwave continues to move east as well, convective potential will wane over the next few hours, leaving most of the ILN CWA dry by daybreak. Over the boundary that will remain in place, isentropic ascent may lead to some showers continuing for the next few hours, primarily in the northern half of the CWA. However, the stable near-surface air north of the boundary is obvious, as the convection moving across the northern tier of ILN counties has produced several ripples / gravity waves that are evident on mosaic radar imagery. As the Indiana surface low moves east today, the surface boundary will remain in place across the ILN CWA, causing a gradient in theta-e across the forecast area. In subsidence behind this early morning shortwave, generally dry conditions are expected for the first part of the day. The forecast is less clear going into the afternoon. What appears fairly certain is that increasing heat (temperatures near 80 in the southern CWA) and humidity (dewpoints in the mid to upper 60s in the southern CWA) will combine with steepening mid-level lapse rates (eastward-advecting EML) to produce significant instability -- CAPE values as high as 2000-3000 J/kg in the southern half of the forecast area. As is usually the case, big numbers are not the whole story, as the biggest negative factor for convection appears to be the presence of a 750mb-800mb cap. It seems that the consensus of model soundings would support this cap largely holding north of the boundary, but there is more of a range of possibilities near and south of the boundary during peak heating. Forcing is also a concern, as the boundary can certainly act as a focus of convergence, but mid/upper support appears to be lacking. Nonetheless, the environment will be supportive of strong to severe storms, based almost on the instability values alone. There does appear to be sufficient deep-layer WNW shear to allow for some organization as well, but wind speeds aloft actually begin to decrease after 21Z. That lays out the conditional threat -- if the cap breaks, and if the mid-level flow has not begun to weaken too much, some organized severe storms will be possible (mainly in the southern third of the CWA). Looks mainly like it would be a wind/hail/rain threat -- even though supercell structures could be possible, low level flow is too weak for more than a very isolated tornado threat. However, as the upper flow is kind of parallel to the boundary, there is at least some concern for training storms and maybe flash flooding. A flash flood watch was considered, but would like more confidence in the cap breaking and storm coverage before issuing. && .SHORT TERM /SUNDAY/... After any storms along the boundary have dissipated or moved SE of the ILN CWA this evening, another break in the action is expected. From here, attention will turn to the severe (potentially very severe) convection occurring well upstream across Missouri and southern Illinois. This activity is not even expected to reach the lower Ohio Valley until late evening, and thus, any impacts on the ILN CWA will be well into the overnight hours. 00Z runs across the suite of models have been fairly consistent in depicting this activity propagating into the instability / greater theta-e air mass, roughly along an ESE trajectory. The ILN CWA will be on the northern fringe of this, still in a favorable area for convection to develop ahead of a mid-level shortwave, but removed from the best thermodynamic environment at a sub-optimal time of day. A severe threat will exist -- mainly a damaging wind threat -- but should not be as significant as areas further south and west. The main focus here will be in the overnight / early Sunday morning hours. Once the morning activity has passed east, another break is expected through the first half of Sunday. Additional storms associated with the cold front will be discussed in the Long Term AFD section below. With less certainty in the pattern, will not allow a completely dry forecast for this time period, as there should still be some isentropic ascent with continued warm advection aloft. The temperature forecast through the first three periods of the forecast is one that is largely dependent on convection and clouds, so perhaps it would be better read as a mid-point of the expectations -- get some clearing and it will be too low, get some convection and it will be too high. There will be a general SW-to-NE gradient in temperatures across the ILN CWA through the period, and depending on the position of the surface boundaries, a tighter gradient may exist than what is currently in the forecast. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... A cold front will move east through the region Sunday night. Scattered showers and thunderstorms are expected along and ahead of the front. A few strong to severe storms will be possible late Sunday afternoon into the evening hours with damaging winds and large hail the primary severe weather threats. Precipitation should exit east of the eastern CWFA after midnight, with clearing expected behind the front. The first half of the week will feature a large closed upper level low which is forecast to rotate slowly east across Ontario. Embedded disturbances rotating around the low, along with weak sfc trof axes, will bring a low chance threat of a few showers from time to time, mainly during the peak heating of the day. Otherwise, partly cloudy conditions with seasonable temperatures in the 70s are expected. As the upper level low finally rotates farther east by the end of the work week, we should finally see a distinct dry period as surface high pressure settles over the Ohio Valley. Again, temperatures will be near seasonal normals. && .AVIATION /11Z SATURDAY THROUGH WEDNESDAY/... Some patchy MVFR ceilings remain in place across the region, but these should lift to VFR over the next few hours, and VFR conditions are expected to prevail through the day today. Chances for precipitation will be limited to passing light showers this morning, before dry conditions are expected late morning into the afternoon. Winds will be westerly to northwesterly at generally around 10 knots. During the mid to late afternoon, there will be a chance of some showers and storms developing near Cincinnati. However, confidence remains too low to include specifically, so a VCSH will be used in the TAFs. Additional chances for showers and storms will be possible late Saturday night into Sunday morning, along with MVFR ceilings. OUTLOOK...Intermittent chances of thunderstorms along with MVFR ceilings will be possible from Saturday night 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 is the U.S. government's official web portal to all federal, state and local government web resources and services.