Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 040836 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 336 AM EST Sun Dec 4 2016 .SYNOPSIS... A weak upper level disturbance will bring light rain to the area this afternoon and evening. Drier conditions are expected on Monday, as high pressure moves into the area. On Tuesday, an area of surface low pressure is expected to move into the Ohio Valley, bringing rain to the area. Behind a cold front on Wednesday night, much colder conditions are expected through the end of the week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... A fairly dry air mass is currently in place over the middle Ohio Valley. Surface dewpoints are generally running in the upper 20s to around 30 in the ILN CWA, and the 00Z KILN sounding recorded a precipitable water value of 0.31 inches, with a significant amount of dry air between 850mb and 500mb. This helps to set up the scenario for the morning, as a large but not-particularly-potent area of lift moves east into the region. A shortwave trough currently moving through the plains states will cross the Mississippi River this afternoon, with only some very unimpressive moisture advection / isentropic ascent ahead of it. High-res models are stingy with their dBZs this morning, and the dry air gives reason for disbelieving the potential for most of the echoes over Kentucky to reach the ground. Most of Kentucky is covered by clouds in the 8kft-10kft range, with no precipitation occurring. Only over the southern and western portions of the state is rain occurring, where clouds are under 7kft. Ultimately, there is little reason to believe that precipitation will occur in the ILN CWA early in the day, outside of a slight chance of a quick sprinkle or flurry sneaking through the dry layer (and a 20 PoP will be maintained to account for this possibility). By afternoon, the overall forcing with this system will be focused in two locations. Right entrance dynamics will help force precipitation over the southeastern states, which may extend into eastern Kentucky, but will largely miss the ILN forecast area. More focused forcing from the 500mb trough will pass through the northern Ohio Valley and southern Great Lakes, and this will be the primary thing to watch. All indications are for a high-PoP but low-accumulation event, with weak ascent and a relatively quick motion resulting in total averaged QPF of only 0.10-0.15 inches. The highest PoPs (categorical for most of the CWA except the far south) will time out from 20Z-02Z from west to east. The only remaining item to discuss is the impact of temperatures on the precipitation type forecast. The diurnal curve will be small today, with highs ranging from around 40 (NW) to 45 (SE), an increase of only around ten degrees from this morning. There is a surface trough that will switch winds to the west and provide cold advection, but it lags the upper trough (and thus the precipitation) by several hours. So, any chance for a mix with snow will largely depend on diurnal or dynamic cooling, and this is not a particularly favorable scenario for either this evening. Discounting a 00Z NAM solution which appeared to dynamically cool the boundary layer much too quickly, it only appears that temperatures briefly get cold enough for some snow to mix into the northern / northwestern ILN counties during the back half of the precipitation. A rain/snow mix has been included this evening north of a line from Richmond to Delaware. If temperatures cool a slightly more than anticipated, some trace or slushy tenth-or-two accumulations might be possible in Mercer/Auglaize/Hardin tier. && .SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY/... Between 03Z-09Z tonight, a weak surface trough is expected to pass east through the region, with a slightly cooler and drier boundary-layer air mass coming in on the high pressure behind it. However, the air mass aloft will be notable warm and dry, creating an unbreakable inversion over the Ohio Valley on Monday. Thus, trapped low-level moisture will be unlikely to break, leading to BKN/OVC stratocumulus remaining in place through the day. A shortwave trough and surface low currently over northern Mexico (as of Sunday morning) will begin their trek northeast on Monday, picking up a decent feed of moisture off the Gulf of Mexico as they approach the Ohio Valley on Tuesday morning. The overall forecast thinking has changed little for the ILN CWA with regards to this system, which still looks much too warm to support a threat for snow. In fact, max temps were raised slightly with this forecast update, primarily in the southeastern CWA, where temperatures may get into the lower 50s. Despite this, there still appears to be no chance for convective activity, due to the strong inversion mentioned above -- which will remain in place as it moistens on Tuesday. Non-diurnal temperature curves were used throughout the short term of the forecast. Still three days out, it is not surprising that there are a few model differences in exact timing and potency of this system. However, it would take a significant change to the surface low track to produce a change in the forecast thinking. Rainfall amounts appear beneficial rather than hazardous -- possibly up to an inch in some locations, with saturated soundings from the surface to about 15kft, and precipitable water values over just over an inch as well. Behind this system, on Wednesday morning, a cold front is forecast to move southeast through the region. The cold advection behind this front is potent, and will signal the start of a period of below normal conditions. Most of Wednesday appears likely to be dry, but there is a rather large spread in possibilities regarding some upper jet forcing that may or may not allow for some snow to develop at some point in the Wednesday / Wednesday night time frame. The going forecast will be left generally unchanged for now. && .LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/... A broad trough will develop over the central United States by mid week and then shift quickly east through through the end of the work week. Short wave energy rounding the base of the trough will lead to an increasing chance of precipitation Wednesday night as an associated strong cold front pushes east across the area. This will usher in a much colder airmass through the end of the week. The 12Z GFS has trended faster with the cold push than the ECMWF and this will ultimately affect temperature trends Wednesday afternoon into Thursday and how fast any pcpn chances over to snow. Highs on Wednesday will range from the upper 30s northwest to the mid 40s southeast. Precipitation Wednesday night may start as a rain/snow mix but as the cooler air begins to move in, it will transition over to all snow. However, the models have trended drier with this feature so would expect any accumulations to be fairly minimal. Much colder air will then settle into the region through the end of the work week as 850 mb temperatures drop down into the -14 to -16 degree celsius range. In somewhat cyclonic low level flow and with possible fetches off of the warm water of the Great Lakes, will hang on to some lower end pops for snow showers Thursday into Friday. Depending on the exact timing of the strong cold push, we may likely have non diurnal type temperatures on Thursday with highs then on Friday only in the 20s. We will quickly transition into a more zonal flow pattern by Saturday, allowing for bit of a moderation in temperatures. && .AVIATION /08Z SUNDAY THROUGH THURSDAY/... Surface high pressure will move off to the east today. This will allow an upper level disturbance to move toward the Great Lakes. Clouds will thicken and lower through the morning hours. Decent warm, moist advection/ascent ahead of the disturbance will bring light rain to the terminals later this afternoon into this evening. As it rains for a period, ceilings will lower into the MVFR category and then eventually the IFR category. Visibilities for the most part will remain MVFR. The upper level disturbance will move east/northeast away from the region tonight. Rain will end, but low clouds will persist in the IFR category. High pressure is then expected to build into the region on Monday. OUTLOOK...MVFR/IFR ceilings to linger into Monday morning. MVFR/IFR ceilings and MVFR visibilities expected Tuesday with rain. MVFR ceilings to linger Tuesday night into Wednesday morning. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...JGL AVIATION...Hickman

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