Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 240221 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 1021 PM EDT Tue May 23 2017 .SYNOPSIS... As high pressure exits the region to the east, a large upper level low will move into the middle Mississippi Valley. As this low slowly moves east over the next few days, surface low pressure will move north into the Ohio Valley. This will bring occasional precipitation to the region through Thursday. After the upper low moves east by Friday, additional chances for rain will enter the forecast over the weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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A few showers/storms had developed along our southern CWFA this evening. This was along an instability axis where deeper moisture exists. Any thunder should wane overnight, leaving only a slight chance/low chance of showers mainly in our eastern CWFA. With models indicating low temp/dewpoint spread in our southeast, along with light to calm winds, have placed some patchy mist/fog here as well. Otherwise, increasing overall moisture and clouds will allow for a warmer and more uniform distribution in min temps tonight into Wednesday morning, generally in the middle to upper 50s.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/... By Wednesday morning, upper level low pressure will be centered over Missouri. As this low continues to move southeast, surface cyclogenesis is expected to occur ahead of a shortwave moving northeast through the Tennessee Valley, and eventually into the Ohio Valley. The interactions of these complex low pressure systems will drive the chances for precipitation on Wednesday. What remains a disappointing part of this forecast cycle is that the 12Z models have still not come into perfect alignment in not only the intensity of the surface low, but also the path it takes (which is dependent in large part on how the upper trough is handled). Furthermore, while there is better agreement in timing between 12Z runs, the swing slower in timing from the 00Z/06Z runs is also somewhat concerning. Thus, this forecast remains below average in terms of confidence in specifics. As an example, the 12Z run of the GFS has a low position at 00Z Wednesday evening near Zanesville OH, and the 12Z WRF-ARW has the low position at 00Z Wednesday evening near Georgetown KY. What remains mostly certain is that precipitation is expected to develop ahead of the shortwave and surface low, rotating generally NNE through the ILN CWA on Wednesday. As mentioned, there is slower timing with the 12Z runs, suggesting that the greatest timing for precipitation across the ILN forecast area will be from 15Z (southwest) to 00Z (northeast). PoPs have been capped at 80 percent owing to an expectation that at least some of the activity will be cellular / convective, lending some uncertainty to whether or not every grid point will pick up precipitation. Models have remained in fair agreement on developing some instability -- despite the unimpressive boundary layer conditions (temps in the mid to upper 60s and dewpoints in the upper 50s). There are two factors to consider here. First, the cooler temperatures aloft associated with the upper low will lead to a slight steepening of the mid-level lapse rates. Second, the later onset timing of precipitation may help with the boundary layer thermodynamics. Regardless, the instability is not expected to be too significant, especially with unfocused near-surface warm advection. Slight chances for thunder will be included across the CWA during the late morning through early evening hours, though in reality, the thunder chances might end up more focused than this. Any chances for hazardous weather will be tied closely to the path of the surface low, which is a low-confidence forecast as previously mentioned. It is a given that somewhere near the path of the low, there will be backed surface flow (roughly ESE) that juxtaposes with slightly stronger southwesterly flow aloft. Unfortunately, with low position being uncertain, specifics on the wind fields are even more so. It is always worth taking a close look at the scenario when a deepening surface low moves through the region in the spring, but as for now, there is not enough forecast confidence to define the magnitude or location of a potential threat. The concern is that a favorable turning profile could combine with the weak instability to allow for a conditional threat of organized/rotating storms. As occlusion starts to occur on Wednesday night, most of the forcing will be rotating north of the ILN CWA, with additional precipitation more likely to occur going into Thursday in association with the upper level system. PoPs will be lowered from south to north from Wednesday evening through the overnight hours. Min temps are currently forecast to be in the lower to middle 50s, but if the 12Z runs are correct in their cloudier solutions going through Thursday morning, these values may be too low. && .LONG TERM /THURSDAY THROUGH TUESDAY/... The upper level low will slowly pull east of the region through the day on Thursday. As it does, some additional moisture will wrap around the back side of the low. The combination of this and some continued cyclonic low level flow should lead to a decent chance of some lingering showers through much of the day on Thursday. With a cool airmass in place, temperatures will be several degrees below normal with highs on Thursday mainly in the mid 60s. As the upper level low moves off to the east, some brief mid level ridging will work across our area through the day on Friday. This will allow for any lingering pcpn to taper off Thursday night with dry conditions expected during the day on Friday. Temperatures will moderate with highs on Friday mainly in the mid 70s. We will then transition into a southwesterly flow pattern across our region as an upper level low builds into the northern Great Lakes. The combination of the moist southwesterly flow and a series of mid level short waves rotating around the upper level low will lead to chances of showers and thunderstorms starting Friday night and continuing through the weekend. PWs will push up in excess of 1.5 inches as we progress through the weekend so depending on exactly how things end up lining up, some locally heavy rainfall could eventually become an issue. Highs both Saturday and Sunday are expected to be in the mid to upper 70s. The deeper moisture will shift off to our east Monday into Tuesday. Will however hang on to some lower chances for precipitation through the end of the long term period as some additional weaker mid level short waves will rotate down across our region. Highs for Monday and Tuesday will be in the low to mid 70s. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... An upper level closed low centered over Iowa this evening will dig south overnight, then southeast toward the middle Mississippi River Valley on Wednesday. As this occurs, low pressure will develop over the Ohio Valley region, pushing northward through are region late tomorrow afternoon and evening. This low will be accompanied by a frontal boundary which will undergo the occlusion process. In the very near term, it will be a complex ceiling and visibility forecast for the next 12 to 15 hours before precipitation arrives. Models suggest that weak, moist low level warm air advection from the southeast will develop overnight. The region to our southeast has had precipitation today, and already is a source of lower level moisture. It remains unclear on the extent and the height level of potential stratus overnight into the first part of Wednesday morning. As such, have kept ceilings in the MVFR category, although some of these could dip below 2000 feet. In addition, it is unclear if patchy fog/mist may form, especially in areas that have a considerable temperature/dewpoint depression at this time. For now, have kept patchy mist/fog development just to the southeast of the terminals since this area has the highest chance of seeing such conditions. On Wednesday, as the low and frontal boundary approach from the southwest, rain showers and embedded thunderstorms will move into our region. There could be a brief window of VFR conditions at some of the terminals before the main precipitation moves in. Otherwise, MVFR visibilities and eventually MVFR ceilings can be expected with and behind frontal boundary/low passage. Thunderstorms chances are low enough at this time as to not mention. For Wednesday night, precipitation will have moved north of the region, leaving only a chance of lingering showers in the cyclonic wrap around of the surface low to our north. With plenty of low level moisture, ceilings will likely drop into the IFR category overnight. OUTLOOK...MVFR to IFR ceilings likely Wednesday night into Thursday with MVFR visibilities possible. MVFR ceilings, visibilities, along with a chance of thunderstorms possible Friday night into Sunday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos/Hickman SHORT TERM...Hatzos LONG TERM...JGL AVIATION...Hickman is the U.S. government's official web portal to all federal, state and local government web resources and services.