Area Forecast Discussion
Issued by NWS Wilmington, OH

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365 FXUS61 KILN 270744 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 344 AM EDT Thu Apr 27 2017 .SYNOPSIS... A cold front will push east across the region today. High pressure will briefly build in behind the front tonight. A frontal boundary will develop and then linger across the area through the first half of the weekend. This will result in unsettled conditions. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Mosaic radar early this morning continues to show a decreasing trend in echo coverage and lightning activity to our west. Precipitation to our west is within a prefrontal convergence area. High resolution/convection allowing models indicate that by the time the precipitation reaches our western zones toward sunrise, coverage should be scattered with little in the way of thunder. For today, as low pressure moves north toward eastern Lake Superior, a cold front will advance east across the forecast area. There should be enough forcing and some diurnal instability for a chance of showers and thunderstorms for roughly the eastern half of the CWFA. There is a low chance for an isolated strong/severe storm (wind) as the front pushes east. It will be breezy given fairly decent pressure gradient and winds mixing down from aloft. Wind gusts will be the highest in the climatologically favored Whitewater/Miami Valleys and west central Ohio where wind gusts maybe as high as 35 mph. High temperatures will be somewhat muted in terms of a normal diurnal trend due to clouds and some CAA behind frontal passage. Will forecast highs from the mid 60s far west to the mid 70s far east. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... The cold front will have cleared our area by this evening, allowing weak high pressure to build into the middle Ohio Valley overnight. Post frontal stratocumulus clouds should scatter, eventually being replaced by some mid/high clouds streaming in from the southwest aloft. Winds will diminish fairly quickly near or after sunset. Lows will fall into the mid and upper 40s toward sunrise Friday morning. On Friday, weak high pressure will move off to the east. In a southwest flow aloft, a weak disturbance and developing isentropic lift will increase clouds from west to east through the day. A few showers may develop across the western CWFA late in the day. There will be a decent temperature gradient from northwest to southeast due to differences in the thickness of cloud cover and the overall low level thermal fields. Highs will range from the upper 60s northwest to near 80 along and south of the Ohio River. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
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On Friday evening, ridging off the southeast coast will set up a pattern of deep-layer warm and moist flow across and north of the southeastern states. Height rises over the Ohio Valley will also be occurring, as a frontal zone begins to develop from the middle Mississippi Valley into the southern Great Lakes, well in advance of a big upper wave developing in the central plains. As moisture advects into the region from the Gulf, models are also consistent in depicting a plume of steep mid-level lapse rates moving northeast into the lower and middle Ohio Valley on Friday night. As these factors combine, nocturnal elevated convection is expected to develop, in a regime with moderate to strong shear. The threat for severe weather will be contingent on how much instability develops, though confidence is a little higher than it was yesterday, at least in terms of elevated instability. Surface-based convection is appearing somewhat unlikely, but elevated cells capable of large hail and heavy rain appear to be an increasing threat, especially in the southwestern ILN CWA between 02Z-10Z. While a more significant heavy rain threat is developing west of the region, current indications are that while storms in the ILN CWA Friday night (and into Saturday) may produce heavy rain, a prolonged / widespread event is less likely. Thus, the chance for flooding (like the chance for severe) will probably be somewhat localized. As frontogenesis continues to occur into Saturday, the surface boundary appears likely to set up directly over the ILN CWA. This will result in a significant gradient in both temperatures and weather, with continued chances for showers and thunderstorms gradually shifting north, but a very warm and likely capped air mass further south. Additional precipitation thus looks like more of a concern in the northern half of the ILN CWA on Saturday, then perhaps only with disorganized activity in the northwestern CWA by Saturday night and into Sunday. Temperatures are expected to range by as much as 20 degrees across the ILN CWA on Saturday, with mid 80s in the southeast and mid 60s in the northwest. The very sharp temperature gradient currently forecast here results in lower-than-normal confidence in the exact numbers, however. There remains strong agreement in the general pattern for Sunday into Sunday night, with deepening low pressure at the surface and aloft ejecting from the plains into the Great Lakes, driving a strong cold front through the Ohio Valley. GEFS clustering of the mid-level low has improved, suggesting that timing confidence is becoming a little more certain, but the 00Z ECMWF came in significantly slower than its previous run, functionally negating the preceding statement. One other item of note is that the 00Z models suggest a slight change to the pattern, which appears a little more occluded than depicted before. It looks mostly certain that the ILN CWA will end up completely in the warm sector within a regime of deep and strengthening southerly flow on Sunday, before precipitation becomes more likely on Sunday evening from west to east. With a slight bit of increased confidence in later timing and drier conditions in the warm sector, the temperature forecast has been increased slightly, allowing for mid to possibly upper 80s in the far southeast. The gradient will not be as strong as Saturday, so the northwest ILN CWA should still get at least into the mid 70s, unless pre-frontal precipitation has greater coverage than expected. The front on Sunday night will be strong and moving fast, ushering in a pretty significant change in air mass. Thus, low-level forcing is not expected to be an issue, and convective development along the front should be solid. That will combine with a fairly strong magnitude of shear to allow for some level of severe threat to exist. Waning instability, unfavorable timing, and mostly-meridional flow (questionable directional shear favorability) will serve as limiting factors to the severe threat. As this is still several days away, for an event with several question marks, it is probably too early to make a specific forecast for hazards. With that said, some threat for heavy rain and severe weather could occur. Behind this front, there will be a notable change in air mass -- and strong cold advection that will lead to strong wind gusts, into the 35-40 MPH range. There will also be a significant drop in temperatures -- a difference of around 20 degrees from Sunday. This forecast will continue to undercut the general model blend, coming closer to raw GFS/ECMWF numbers for Monday. No signs of significant weather on Tuesday, with a slight bit of warming in pseudo-zonal flow. By Wednesday, another developing frontal zone will combine with an approaching wave to allow for increases in precipitation chances through the middle of the week.
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&& .AVIATION /08Z THURSDAY THROUGH MONDAY/... As low pressure lifts north toward eastern Lake Superior today, a cold front will advance east across the region. Before any precipitation arrives toward the western terminals by sunrise, we first must deal with winds increasing along with a strong low level jet aloft. The strong low level jet requires at least a period of non-convective LLWS at the terminals roughly between 06Z and 12Z. For today, high resolution models/convection allowing models are really backing off on precipitation coverage ahead of the front across our western forecast area. Also, instability is expected to be fairly low this morning. As a result, just have a chance of showers for western terminals this morning. It is unclear if ceilings will lower into the MVFR category west due to lack of widespread rain. Have kept ceilings VFR. As the front moves east this afternoon, there may be enough diurnal instability for a chance of thunderstorms toward the KCMH/KLCK. Winds will be gusty with a subtle shift in wind direction from south to southwest behind frontal passage. Expect wind gusts in the 25 to 30 knot range. For tonight, front will be east of the area. Weak surface high pressure will build briefly into the middle Ohio Valley. Post frontal stratocumulus clouds should scatter through the evening with some mid and high level clouds arriving from the southwest aloft. Gusty winds should diminish fairly quick after sunset. OUTLOOK...MVFR ceilings and visibilities with a chance of thunderstorms are possible Friday night into Monday. Wind gusts to 30 kt possible Sunday through Monday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hickman NEAR TERM...Hickman SHORT TERM...Hickman LONG TERM...Hatzos AVIATION...Hickman

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