Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 201324 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 924 AM EDT Fri Apr 20 2018 .SYNOPSIS... High pressure over Minnesota and Iowa will shift east into the Great Lakes and Ohio Valley over the next few days, bringing dry weather and below normal temperatures. Some frost and freezing temperatures will be possible again tonight especially north of the Ohio River. The high will move into New England over the weekend as low pressure slowly organizes over the lower Mississippi River Valley. This low pressure system will move very slowly northeast and increase the threat of rain across the Ohio Valley Monday night and Tuesday as temperatures remain below normal. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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A very dry air mass is in place over the Ohio Valley, with the 12Z KILN sounding recording a precipitable water value of just 0.11". An expansive area of surface high pressure extends from the northern plains through the Carolinas, centered over southern Wisconsin / northern Illinois this morning. The going forecast remains largely reasonable, with a continued expectation of higher temps / lower dewpoints than indicated by a typical blend of model data. Seeing that this is already well-represented in the grids, few changes were necessary. Previous discussion > Ample sunshine today as height rises/shortwave ridging develop across the Ohio Valley, and surface high pressure builds through the Great Lakes and Ohio Valley. Yet another day of below normal temperatures on tap, though very dry lower troposphere will aid warming of the boundary layer with full sun. Mid-upper 50s for all with light north/northeast winds as the high drifts by to the north, keeping weak low-level cold advection across the area. Will allow dewpoints to mix out this afternoon in this favorable setup for afternoon moisture mixing as typically occurs in April with north/northeast boundary layer flow.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... Collaborated the need for another set of frost/freeze headlines with adjacent offices and determined there`s enough uncertainty in a few areas to hold off for now, though feel pretty certain at least Frost Advisories will be needed for the Scioto Valley of south-central Ohio westward into portions of southwest Ohio. A worst case scenario allows for very strong radiational cooling with the very dry boundary layer tonight, allowing potential for a freeze again in the Scioto Valley. However, there are few concerns: 1) Clear model signal of increasing cirrus/high level clouds as the night wears on which may be enough to keep that near-sunrise temp plummet from occurring and 2) a persistent but weak easterly flow that maintains over the area as the high drifts by to the north and then northeast which may be enough to keep frost from being widespread and 3) the very dry boundary layer may not be moist enough to allow for a heavier frost potential. Adjacent offices were either uncertain on temperatures cold enough for frost, or uncertain on wind/cloud/dry boundary layer to keep frost from forming. So will hold off on new set of advisories/warnings for right now, but can see the Scioto Valley /at a minimum/ needing some level of messaging tonight as the best overlap of lighter winds/coldest temperatures/thinnest cirrus is here. Ran with a consensus of bias-corrected statistical MOS for low temps in this stable temp regime. On Saturday and Saturday night, deep closed low across KS/OK drifts slowly east/southeast into the lower Mississippi River Valley. Mid/high level isentropic ascent to bring a thickening shield of mid/high clouds over the area during this period /esp north of the Ohio River/ while low level flow remains extremely dry and sourced from the east. This will keep any precipitation tied to the approaching upper low pinned very far to the west across the mid Mississippi River Valley, and more below normal temperatures on the cool easterly flow. && .LONG TERM /SUNDAY THROUGH THURSDAY/... Surface high pressure over the eastern Great Lakes will shift off to the east through the day on Sunday. Meanwhile, a closed 500 mb low over the southern Mississippi Valley will drift slowly east through the day on Sunday before lifting northeast across the Tennessee Valley and opening up Monday into Tuesday. There are some model differences with the northward extent of the pcpn shield associated with this with the 00Z GFS somewhat farther north than the 00Z ECMWF and CMC. However, the 00Z GEFS is somewhat more in line with keeping things a bit farther south so will trend forecast accordingly. With this in mind, will allow for some lower chance pops to work into our southern areas through Monday afternoon. Will then gradually overspread the remainder of the area with slight chance to chance pops Monday night into Tuesday with the highest pops being across our southeast. The GFS is a bit more impressive than the ECMWF with some additional mid level energy dropping down from the northwest Wednesday into Thursday. Either way though, it looks like this will be enough to linger at least a chance of showers across the area through mid week. Daytime temperatures will remain near or slightly below normal through the long term period. Highs will generally be in the low to mid 60s while overnight lows will be mainly in the 40s. && .AVIATION /13Z FRIDAY THROUGH TUESDAY/... VFR conditions continue across the terminals as 12Z approaches. Clear skies are abundant with light north to northeast flow. A few wisps of cirrus are possible over the next 18 hours but skies will largely be clear. Winds will remain light, mostly below 8kts out of the north/northeast but will tend to back to an easterly direction into tonight as pressure passes by to the north. There will be slow increase in high-level cloud cover later in the night into Saturday morning, starting out in the 20-25 kt layer and thickening a little as it lowers toward 15kft on Saturday morning. OUTLOOK...MVFR ceilings possible on Monday into Tuesday. && .ILN WATCHES/WARNINGS/ADVISORIES...
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OH...None. KY...None. IN...None.
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