Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 250752 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 352 AM EDT Sun Sep 25 2016 .SYNOPSIS... High pressure centered northeast of the area will provide one more dry and warm day for the region. A cold front will move into the area early on Monday, bringing a chance of precipitation. Behind this front, temperatures will become cooler for the rest of the week, and will be slightly below normal for late September. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Surface high pressure is currently centered somewhere near Ottawa, with enough influence to keep relatively dry and quiet conditions all the way southeast into the Ohio Valley. A frontal boundary previously in the vicinity has stalled out well south of the ILN forecast area, leaving a pattern that will support weak easterly flow over the area through the day. Skies are very clear at this early hour, and cumulus development will likely be somewhat limited by the dry air mass over the area right now -- with dewpoints generally in the 50s. Slightly greater moisture exists near and southwest of Cincinnati, where afternoon cloud development may be a little more robust. A few high-res models even suggest that some weak showers could develop in south-central Indiana, possibly close to the ILN CWA borders. With instability as limited as it is, and no signs of forcing, this forecast will be kept dry. With the front out of the area, the temperature gradient across the forecast area today should be a little more relaxed. Essentially still in the warm sector of the next large-scale weather system, as discussed below, temperatures will remain well above normal for late September. && .SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/... A developing mid-level cyclone is currently located over North Dakota. Along with a surface low, this feature will move ESE over the next few days. Within the regime of positive vorticity advection, a cold front is forecast to clear its way through the Ohio Valley. Model agreement on frontal timing is quite strong, leading to high confidence in the expected frontal passage -- entering the forecast area in the 10Z-12Z time frame on Monday morning, and clearing the CWA by 18Z-20Z during the afternoon. However, confidence in precipitation coverage along and ahead of the front is not as certain. Though convergence is fair and upper support is not really lacking, the front is a long distance from its parent low, and the quality of the deep-layer moisture is suspect (a narrow ribbon of around 1.50" PW). Now at the tail end of the range of a few high-res model runs, there are reasonable expectations that shower activity may be scattered rather than solid. As a final note, instability is also very marginal (only a few hundred J/kg of MLCAPE) owing to the passage during the diurnal minimum. Will keep PoPs only 60 percent at most in the north and northwest, limited to 40-50 percent further south and southeast. There is certainly an expectation that coverage (and quality of convection) may be increasing as the front exits the ILN CWA. SPC has drawn in a marginal risk for Day 2 over WV / western PA / far eastern OH. The ILN CWA looks like it will be in an ideal spot for a minimum in convection, warranting only a slight chance of thunder with this system. With frontal passage occurring during the upswing in the diurnal cycle, the temperature forecast for Monday found its way to being a little bit complicated. Areas in the far southeastern CWA (Lewis KY / Scioto OH) will likely climb back into the 80s one more time, before conditions begin to cool during the early to mid afternoon. Elsewhere, cold advection will already be ongoing by late morning or early afternoon, limiting highs to the 70s. In fact, the far northwestern CWA may struggle just to get to 70. With the tightening pressure gradient and cold advection behind the front, some gusts in the 20-25 knot range will be possible north of I-70. With a new air mass in place, low temperatures on Tuesday and Wednesday morning will be in the middle to upper 40s -- the first time in the 40s since June. Highs will actually come in below normal for the first time in a while, with max temps for Tuesday and Wednesday expected to be in the upper 60s to around 70. With continued subsidence and cold advection, winds are expected to be breezy again on Tuesday, and wind gust grids were increased from the previous forecast. After nearly stalling over the Great Lakes, the mid-level low will gradually lumber its way toward the Ohio Valley on Wednesday. There is still a great deal of uncertainty with this system, but there is enough of a signal to include some 20-30 percent precipitation chances in the northern sections of the CWA on Wednesday -- in addition to dropping temperatures there by a couple degrees. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... A closed upper level low will move across the Great Lakes through mid week. Energy rotating around the low will eventually allow for the trough/low to dig down into the Ohio Valley region. The 12z GFS is more progressive with this feature, keeping it as a trough and shifting it off the East Coast through the end of the week. However, the 12Z ECMWF closes off the low and actually retrogrades slowly west across the Upper Ohio Valley through the end of the week. This results in some uncertainty in the forecast as we head through next week. For now, will allow for a few showers across mainly our north on Wednesday as the better energy transfers down across our area. Will otherwise maintain a dry forecast through the long term, although if the retrograding ECMWF ends up being right, some lower end pops would also be necessary Thursday into Friday but given the uncertainty, think the best option at this point is to trend toward a blend. This will lead to below normal temperatures through mid week with highs mainly in mid to upper 60s. Will then allow for a gradual warming trend through the end of the week with highs into the mid 70s by Saturday. && .AVIATION /07Z SUNDAY THROUGH THURSDAY/... Surface high pressure centered over southeast Canada/Eastern Great Lakes this morning will gradually move east today. Surface ridging will extend back into the Ohio Valley, while a mid level ridge traverses west to east overhead. Clear skies this morning can be expected. The exception will be at KLUK where some IFR conditions are forecast due to river valley fog. This should burn off by 13Z. For this afternoon, some scattered diurnal cumulus clouds will develop over the western terminals. For this evening, mid level ridge will move east. Diurnal cumulus will likely dissipate by sunset, leaving mostly clear skies. For the overnight period, large scale mid level trough will dig southeast into the western Great Lakes and northern/middle Mississippi River Valley. An attendent cold front will accompany this system at the surface. Showers and embedded thunderstorms are expected along and ahead of the front, which should make in roads toward the western terminals between 09Z and 12Z. OUTLOOK...Showers and embedded thunderstorms will bring MVFR conditions Monday morning and early afternoon. Breezy west winds expected on Tuesday with gusts between 25 and 30 knots. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...JGL AVIATION...Hickman

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