Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 281732 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 132 PM 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 WEDNESDAY/... 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 /18Z WEDNESDAY THROUGH MONDAY/...
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A large upper level low will continue to dig south/southeast across the Ohio Valley through Thursday, providing active weather through the TAF period. Have seen a steady increase in coverage of SHRA with embedded TS in the late morning and early afternoon. This trend will continue through the afternoon and early evening, and as such, expect activity to continue to pivot ENE through terminals as a weak sfc low moves E into area. Prevailing VFR conditions are expected with TEMPO MVFR VSBYs and even CIGs in heaviest pockets of pcpn. Expect shield of SHRA/TS to move E into eastern half of area after 21z, with additional redevelopment likely farther west near/coincident with the sfc low. This secondary activity will likely impact western terminals of KCVG, KLUK, KDAY, and KILN before weakening past 00z as sfc low begins to stall/retrograde and instby begins to wane. Expect SHRA to be ongoing between 03z-06z just east of KCMH and KLCK as sfc low settles into area. With this, winds will become light and variable overnight. Do expect that IFR CIGs/VSBYs will develop area-wide after 03-06z, with potential for LIFR conditions in 08z-15z time frame, especially for higher-elevation terminals of KDAY and KILN. IFR/LIFR CIGs will be slow to lift Thursday morning, but do expect MVFR and eventually VFR conditions to return towards the very end of the TAF period. Past 18z Thursday for KCVG, do expect scattered SHRA redevelopment, initially across eastern parts of the area, then translating to the northwest. Instby will be marginal, but cannot rule out isolated TS after 18z. Confidence on potential coverage of TS precludes inclusion in the TAF at this time. OUTLOOK... MVFR ceilings and visibilities possible with showers Thursday afternoon through Saturday. IFR/LIFR ceilings and visibilities possible Friday morning, with IFR ceilings and visibilities again possible on Saturday morning.
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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...KC

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