Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 131045 AFDILN AREA FORECAST DISCUSSION National Weather Service Wilmington OH 545 AM EST Thu Dec 13 2018 .SYNOPSIS... High pressure will move off the east coast today, with increasing moisture moving into the Ohio Valley. Chances for precipitation will increase on Friday and Saturday, as a slow- moving area of low pressure moves east through the region. Drier conditions are expected for Sunday and into next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Surface analysis indicates that a weak frontal boundary has stalled along a WSW-to-ENE orientation somewhere near the southern border of the ILN CWA. Warm flow just off the surface is bringing moisture just north of the boundary, resulting in increasing low level saturation under an inversion near 875mb. Scattered clouds have increased in coverage, and will likely continue to do so through the rest of the morning. It should be noted that so far, the cloud development has been a little less robust than expected, so timing trends with regards to the sky grids have been backed off a bit. Fog potential is also looking a little less certain, so only some patchy fog will be kept in the grids. Visibilities around the area are pretty much 10SM across the board, but visibility projections from some of the models are still indicating the potential for reductions in west-central Ohio and surrounding areas. The axis of a low-amplitude ridge currently extends from the southeastern states through IN/IL, and this ridge axis will move east through the forecast area this morning. Once it has passed, winds off the surface will switch to the south, and surface winds will shift to the southeast (backed from the flow aloft). This is definitely a warm advection signal, though not at its strongest in the boundary layer. Nonetheless, there will be enough moisture transport and isentropic ascent to allow for some showers to develop along and near the elevated boundary, though chances for this will be greatest further to the west, where deep-layer moisture is more prevalent. PoPs were kept pretty similar to the last forecast, focusing mainly on the western half of the ILN CWA, and only with lower-end chances for anything measurable. Despite the increasing clouds, there should be a reasonable jump in temperatures today compared to yesterday. Where conditions will be driest and most susceptible to the weak warm advection flow in the low levels -- the southeastern portions of the ILN CWA -- temperatures may approach 50 degrees. Middle to upper 40s appear likely for the rest of the forecast area. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Any warm frontal precipitation chances will likely be exiting the northwestern CWA this evening, leading to dry conditions through the overnight hours. If anything, the recent model runs have been a little more pronounced with this expected break in the action, waiting generally until after 12Z Friday morning to bring the next precipitation chances to the area. In fact, there may even be some partial clearing in the cloud deck overnight, especially in southern Ohio / northeast Kentucky. During the day, the long-advertised upper low will finally begin its northeasterly track toward the Ohio Valley region. For Friday, it will remain quite a ways away, only making it into Mississippi by the afternoon. Ahead of the low, deep-layer southerly flow will become more well defined, with an especially notable increase in southerly 500mb flow on its forward flank. With the ILN CWA remaining north of the low, there is a clear signal for a weakening in the flow near where the wind flow begins to spread out in a deformation pattern. This deformation will likely provide the bulk of the forcing, with widespread precipitation expected to overspread / develop in the forecast area in the 12Z-18Z time frame. The greatest chances appear to be in the southeastern sections of the CWA, and there should be a clear northward limit to precipitation chances -- though the absolute end of this will likely be north of the CWA boundaries. Though warm advection is expected to continue on Friday, the temperature forecast was kept somewhat conservative, given that precipitation and clouds will be the story of the day. Mid to upper 40s -- similar to Thursday but with a flatter gradient -- will be used in this forecast. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Operational models continue to vary on the timing and the position of a mid level closed low to affect the area during at least the first part of the upcoming weekend. The ECMWF and CMC are similar with a southern track, the GFS closer to the Ohio River, and the NAM the farthest north. Have continued to side with the ECMWF which takes the closed circulation from the lower Mississippi River Valley to the Tennessee Valley Friday night into Saturday (a more northerly track does not seem to fit the pattern given the location and strength of the upstream "kicker" energy). The low is then forecast to move east from the Tennessee Valley to a position off the Carolina coast by Sunday evening. A low to mid level deformation axis is still advertised, and this will likely make PoPs a little tricky in that there could very well be a sharp cut off to pcpn. Higher probabilities still favor southern sections Friday night into the first part of Saturday with an west to east deformation axis. By Saturday night, the axis may orient itself more southwest to northeast before exiting the region Sunday morning. It will be too warm for any frozen pcpn, so rain showers are forecast. Lows Friday night will range from the upper 30s to the lower 40s. Highs on Saturday will range from the lower 40s northwest to near 50 southeast. On Sunday, partial clearing is expected to occur from west to east. Highs will range from 45 to 50. For the period Monday into Monday night, models differ on the strength (how much digging) of upper level energy to move into the eastern Great Lakes and New England. The GFS is the most aggressive, the ECMWF the least, and the CMC somewhere in between. A blended model approach has been used, which pushes a cold front through the region on Monday with only a glancing blow of cooler air. It should remain dry with just some CAA stratocumulus clouds expected, particularly across the northern and eastern zones. Highs will range from the upper 30s to the lower 40s. Lows Monday night will drop into the mid and upper 20s. For Tuesday into Wednesday, surface high pressure will build into the region on Tuesday, then move east of the area on Wednesday. Dry conditions are expected to influence the region`s weather with temperatures near or slightly above normal. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... This set of TAFs has been trended a little bit more optimistic with regards to ceilings today, now expecting generally only some occasional MVFR ceilings, with IFR looking less likely to occur. An expanse of MVFR stratocumulus clouds is spreading north through the area this morning, and these clouds will likely persist for a while at KCVG/KLUK/KDAY/KILN, though it is less certain when (or for how long) they will get to KCMH. Occasional MVFR ceilings will be possible through the afternoon, and there is also a slight chance of some showers moving through the Cincinnati and Dayton TAF sites during the afternoon as well. A VCSH has been included for KCVG/KLUK but the chances for rain look too slim to be included anywhere else. VFR conditions are expected heading into the overnight hours, as winds back to the southeast and eventually the east. The next chance for rain -- which looks a little more certain to occur than today -- will be on Friday morning. OUTLOOK...MVFR/IFR conditions will be possible at times from Friday morning through Sunday. && .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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