Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 281054 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 654 AM EDT Wed Sep 28 2016 .SYNOPSIS... A large area of low pressure will move into the Ohio Valley today, moving very little throughout the rest of the week. This will result in cloudy and cool conditions, with occasional chances for rain through Saturday. By later in the weekend, warmer and drier conditions will return, as the area of low pressure moves away to the northeast. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The weather through the next several days will be dominated by a large upper low, currently moving southward into the Ohio Valley from the Great Lakes. Mid-level clouds associated with this system are already encroaching on the ILN forecast area, with chances for precipitation expected to increase over the next few hours. Though it is easy to describe this system as a stacked low, in reality, the surface low is fairly poorly organized. This will result in an muddled boundary layer wind field. It also means that forcing for precipitation will largely be driven by features above the surface, especially during the morning. NAM/GFS projections show a jet streak rotating around the southern periphery of the low, with its left exit region tracking directly through the ILN CWA between 12Z-18Z. This is expected to allow for the rapid development of precipitation by 12Z, expanding in coverage over the northwestern corner of the forecast area. As positive vorticity advection forcing increases going into the afternoon, and a weak surface boundary does manage to work its way in from the west, convective coverage is expected to expand considerably by 18Z. This appears to warrant categorical PoPs for nearly the entire forecast area, with the expectation of one or several curved bands of broken convection developing and moving east. As the center of the upper low moves into southeast Indiana this evening, precipitation chances appear to lessen considerably after 21Z. Trying to forecast mesoscale details in a complex low pressure scenario like this one is often difficult (if not impossible) at distance in time. Now that this event is in the first period of the forecast, and is being resolved by an array of higher-resolution models, the specifics are coming better into view -- and they are somewhat interesting. Steep lapse rates (both low-level and mid-level) have always been expected, with cold air associated with the upper low moving into the Ohio Valley. 850mb temps are expected to range from 7C-12C from northwest to southeast by afternoon, with freezing level heights of around 8000-9000 feet. A significant tropopause fold exists, reaching down to about 450mb. One interesting way to visualize this is to compare a 300mb RH plot -- which is in the stratosphere, and thus is bone dry -- to a saturated 500mb RH plot near the center of the low. All of these factors work to decrease atmospheric stability, even without an expectation for significant surface heating. However, with some southerly low-level flow remaining in place ahead of the surface boundary, some heating may indeed occur (especially in the southern sections of the forecast area). A conservative instability forecast would suggest MLCAPE values of around 300-600 J/kg, but if temperatures can get into the lower 70s, values could perhaps approach 800 J/kg. These numbers may sound small, but this is nowhere near a typical summertime convective situation -- with steep lapse rates, this CAPE will be favorably located at temperatures cold enough to support the development of both lightning and hail. Thus, coverage of thunder in the grids has been expanded significantly. Though steep lapse rates and marginal instability were always expected, the presence of favorable wind shear was much less clear, as it had a lot to do with the juxtaposition of the surface boundary and the larger-scale flow around the low. It now appears that shear will be rather favorable across the ILN forecast area, with 45-55 knots of deep layer southwesterly shear expected during the afternoon (though values are a bit lower north and west of Dayton). Recent HRRR/NCAR Ensemble/NAM runs are also suggesting a brief period of time in which low-level shear may become favorable. As was mentioned earlier, the low-level wind flow is somewhat disorganized. However, if some modest southerly flow can establish itself ahead of the surface boundary, there will be a decent bit of turning in the lowest 1-2 kilometers. This is one mesoscale aspect to the forecast that remains uncertain, and will need to be examined in real time, as it could enhance the severe threat if it occurs. Ultimately, these parameters support the development of a few low-topped severe storms, possibly with supercellular characteristics. Hail appears to be the greatest threat, with organized updrafts in a cold environment. While some small hail may be possible with the morning activity, large hail appears more likely with storms during the afternoon. A chance for gusty winds will probably be limited to storms near the surface boundary. While a tornado threat is not completely out of the question, it would really require all the shear/instability to come together just right, especially with LCLs looking rather VFR-ish and not altogether favorable. For temperatures, 3-hourly values were drawn in through the day, in order to capture the slightly non-diurnal trends expected. In the northwestern CWA, where rain will develop earliest, temperatures will not rise significantly -- only into the lower to middle 60s. However, in the far southeastern part of the forecast area, temperatures may climb significantly through about 18Z -- possibly as high as the mid 70s. && .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY/... Through the rest of the short term forecast, the center of the upper low will gradually move southward (Thursday) before beginning a northward progression (Friday) on its way out of the Ohio Valley (Saturday). The short version of the forecast is that continued cool, cloudy, and occasionally rainy conditions can be expected. through the end of the week. As is unsurprising, the mesoscale features from Thursday through Saturday are a little tougher to resolve with certainty, as small differences in the model runs can result in changes in the placement and timing of shortwaves. In addition, when the center of the upper low is close to the ILN CWA, shortwave/PVA forcing will be next-to-zero -- resulting in occasional periods where widespread or heavier rainfall will be very unlikely to occur. As such, PoPs through the rest of the week generally follow a model compromise in forcing timing and placement, with expectations that this will need to be refined in the coming days. Likely PoPs were used on Thursday afternoon for the northeastern two-thirds of the ILN CWA, as there is reasonable confidence in this being a favored area for ascent, with the low center about 200 miles to the southwest over central Kentucky. Instability appears marginal, but nonetheless, a slight chance of Thunder will be re-introduced for Thursday -- as lapse rates will remain steep enough to allow for CAPE to reach high enough to allow for electrification of at least a few of the deeper cells. Some small hail could be possible also, but marginal instability and much poorer shear than Wednesday will make a severe threat unlikely. Owing to the aforementioned uncertainty, PoPs through the rest of the week have been kept in the 20-40 percent range. && .LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... When the low lifts farther to the northeast on Sunday, decreasing moisture and forcing will be evident. There will be a slight chance of showers in shallow residual moisture. Dry weather appears in the forecast for Monday and Tuesday under high pressure at the surface and aloft. Expect below normal highs in the 60s Friday and Saturday under clouds, precip, and low geopotential heights. A rebound back into the 70s should be noted by Tuesday in a regime of warm advection and insolation. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
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A large scale upper level low will dig southeast into the Great Lakes and Ohio Valley through the TAF period. Jet energy rotating around the digging upper level low will combine with some low level convergence above the surface to generate showers and thunderstorms. Given the current dry conditions across the region, and the elevated nature of the thunderstorms, ceilings should stay VFR for the most part. However, moderate to perhaps local heavy showers/storms will lower visibilities at least into the MVFR category with some pockets of IFR visibilities possible where heavier showers/storms occur. Best chance for thunderstorms this morning will occur at the KDAY, KCMH, and KLCK terminals. This is also where a brief period of IFR visibilities may occur. KCVG, KLUK, and KILN will also see some showers/MVFR visibilities, but the threat for thunder may be a little less. Will make a last minute decision if VCTS/CB will be need at these locations through the morning. Operational models and high resolution models indicate that there will be at least two waves of precipitation today. The first wave will rotate east/northeast through early afternoon. Thereafter, surface low pressure and a cold front is forecast to push east through the region this afternoon and evening. It now looks like the second round of precipitation will be associated with another piece of energy aloft along with a prefrontal trof axis. This feature will bring another round of showers and thunderstorms. Again, ceilings should remain mostly VFR but visibilities will fluctuate between MVFR and IFR in heavier showers/storms. The actually frontal boundary will work its way into the forecast area late this afternoon and evening as surface low pressure begins to translate toward the southeast. There could be a few shower/storms along this boundary before precipitation shows a decreasing trend in coverage. For tonight, center of the upper level low is expected to drift southward toward KSDF while the surface low rotates east/southeast toward south central Ohio/eastern Kentucky. With winds becoming light to calm, and with cooling, low clouds and fog will develop especially between 06Z and 12Z time frame. Have gone with IFR/LIFR conditions. OUTLOOK...IFR/LIFR ceilings and visibilities possible Thursday morning. MVFR ceilings and visibilities possible with showers Thursday afternoon through Saturday.
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&& .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...Coniglio AVIATION...Hickman

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