Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 171047 AFDILN AREA FORECAST DISCUSSION National Weather Service Wilmington OH 547 AM EST Thu Jan 17 2019 .SYNOPSIS... As low pressure moves across the Ohio Valley today, a mix of rain and snow will occur. High pressure is expected to move into the area for Friday, bringing dry conditions. A significant winter weather event is expected over the weekend, as a strong area of low pressure moves northeast through the area. This will bring a mix of precipitation types to the area. Very cold air will move into the region for Sunday and next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Current surface analysis indicates that low pressure is located over northeastern Oklahoma, with a weak trough extending northeastward into southern Illinois. The surface low is expected to move off to the northeast into the Ohio Valley today. A mid-level wave is also crossing Missouri as of this writing, with a subtle swirl visible on GOES-E water vapor imagery, and an obvious area of favorable ascent out ahead of it. The combination of these features will bring precipitation to the ILN CWA today, with accumulating snow as the main concern. Though light radar echoes have been showing up for a few hours across the southern and southwestern sections of the forecast area, observations of precipitation have been sparse thus far. Ceilings are still around 2000 feet in this area, with thicker low-level moisture still upstream. As the boundary layer saturates, a brief period of drizzle does appear possible. Depending on surface temperatures, this could be freezing drizzle, as is being observed at KIND as of 0754Z. As moisture becomes deeper (through about 10kft) the chances for measurable precipitation will rapidly increase in the 10Z-14Z time frame. This activity appears initially to be forced well aloft, with vorticity advection ahead of the mid-level wave, and the first push of theta-e advection noted at 850mb and 700mb at this time. As the surface low gets a little closer, eventually the boundary layer flow will also switch to more of a southerly orientation, with temperatures warming through the saturated layer. This will also increase the strength of the forcing, with precipitation absolutely certain to occur (at least intermittently) during the 14Z-20Z time frame across the ILN forecast area. PoPs were increased to 100 percent CWA-wide. The precipitation type question is a little bit trickier, though certainly not to the extent of the weekend system that will be discussed in the Long Term AFD section below. There continues to be very little evidence of any sort of traditional mixed precipitation (outside of the freezing drizzle possibility near onset) thanks to low level conditions that are isothermal or cooling slowly with height. Without any signs of a strong inversion, sleet or freezing rain will be very unlikely. It is true that there will be an inversion in place initially, but this inversion will weaken considerably as moisture advection occurs. Precipitation should generally begin as snow, gradually mixing with rain from south to north as the event progresses. If there is any chance of sleet as the column saturates near precipitation onset, it might be across the far western sections of the CWA. On a similar subject, the strength of the warm advection near the surface is not expected to be overly impressive, and the antecedent conditions will still be relatively cool with the presence of a snow pack. Because of this, there are concerns about just how warm it will get. This also leads to a precipitation type question, because the nearly-isothermal temperature profiles will be very sensitive with respect to which type of precipitation (rain or snow) is favored. It could be a day where wet snowflakes and cold rain mix back and forth from time to time. Nonetheless, there should be enough time with pure snow to allow for most of the CWA to at least receive minor accumulations (perhaps excepting the far south / southwest). In general, forecast amounts are around an inch in the northern half of the CWA, tapering off to near zero in the far south. Two notes about 00Z models with regards to snowfall. First, GEFS plumes have generally come down in amounts over the past couple runs, and the going forecast numbers (as described above) are actually well on the high side of the spread. With that said, operational models (including the 00Z ECMWF) still support totals of around an inch, especially along the convenient Interstate 70 corridor. Second, a few higher-resolution runs (00Z 3km NAM / 06Z HRRR) are indicating the potential for up to an inch in south central Ohio with the initial push of precipitation. Given that it is a small window for temperatures to remain cold enough for snow in this part of the CWA for any sort of lengthy period of time, the 00Z 3km NAM projection in particular seems a bit unlikely. Nonetheless, numbers were increased slightly in this area to account for the possibility. Temperatures should have a fairly small range today, with highs in the lower to upper 30s. Given the location of the low, temperatures may not begin to fall for a while on Thursday evening -- especially in the southeastern sections of the forecast area. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... Precipitation should become notably lighter as forcing weakens by mid to late afternoon today, though some very light rain/snow/drizzle mix may continue into the evening. However, once the axis of the shortwave has passed the area, the surface low will also be moving over the region -- ending any isentropic ascent from the previous warm advection. Thus, there should be a lack of forcing heading into the overnight hours, cutting off chances of precipitation. The remaining low level moisture will, however, have nowhere to go and no means to mix out. There is very high confidence (SREF probabilities running quite high) in thick stratus remaining in place through the overnight hours, with fog and mist also developing. Dense fog is not out of the question. The line between drizzle and mist is probably a fine one, but it would be preferred to have some sort of lift in place to put any actual precipitation into the wx grids. Fog, however, will be included -- along with 100% sky cover. Min temps are expected to range from the upper 20s to mid 30s, representing a drop of only around 3-5 degrees from max temps during the day. Outside of the thick clouds and possibly some persisting fog, there is very little to discuss with regards to the weather on Friday. As high pressure moves into the forecast area, advection will remain weak, and temperatures will gain only a few degrees from overnight lows. Max temps in the lower to upper 30s are actually fairly close to normal -- and /normal/ is not a word that will be used too frequently to describe the weather from Saturday and beyond. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Low pressure over the southern Plains Friday night will track northeast to the middle Mississippi River Valley by Saturday morning. This system will bring a multiple set of hazards for the region through the weekend. The biggest challenge as we head into Saturday and Saturday night will be the exact track and strength of the low pressure system. Operational model wise, the NAM is the fast outlier, followed by the GFS, then the slower CMC and ECMWF. The first interesting thing to note, as pointed out by WPC, is that the mid level trough associated with the low is not undergoing a strong negative tilt as it passes across the Ohio Valley region. That being said, this may give support to the operational ECMWF and its ensemble members which take the low a little farther south then the other operational models. There is a noted pause for concern as low pressure systems passing through the Ohio Valley tend to trend farther north than expected as one closes in on an event. But, this low is not winding up as it passes to our south, so the ECMWF may be on to something. Regardless, we should hopefully have a better idea Thursday night into Friday as the energy will be fully inland from the West Coast. The track of the low and its thermal fields will be key to ptype as it affects our area with considerable moisture. A blended model approach that puts some weight to the ECMWF solution now favors a more narrow ribbon of mixed pcpn for locations between I-70 and the Ohio River. This should result in a transition zone of snow, sleet, and freezing rain that pivots slowly northward during the first part of the day. Given the potential for heavier rainfall rates (more runoff) and temperatures near freezing, it appears that ice accumulations may be kept down in this zone, and also due to other ptypes being mixed in. That being said, current forecasts of 0.10" to perhaps 0.20" of ice accumulation (planar flat ice) will be possible on Saturday. To the north, ensemble probabilities look likely for a significant snowfall, six inches or more, with the potential for amounts peaking in the 8"-10" range near and just north of the I-70 corridor. For points along and south of the Ohio River, a brief wintry mix or just rain can be expected for much of the day. Rain may be moderate to locally heavy at times, which may result in some localized flooding issues. The low will be pushed east/northeast Saturday night by an upstream mid level trough of Arctic origin dropping into the Great Lakes. As this occurs, colder air will filter southeast, allowing ongoing pcpn to change from rain to snow. Another thing to note at this juncture is the the GFS and its faster and more northerly solution places the southern CWFA in a dry slot where as the southern track and slower motion continues deformation pcpn going longer into the night. Have sided with WPC at this time which allows accumulating snow to affect the remainder of the region. Pcpn will gradual taper off overnight as the low pulls away. As this occurs, the pressure gradient will have tighten between low pressure to the east and Arctic high pressure to the northwest. This will increase winds, which will become gusty at times, perhaps up to 35 mph. This will likely result in some blowing and drifting of snow where much of it fell in generous amounts. In addition, with falling temperatures, wind chill temperatures will start to become a concern, especially across our northwest zones. All in all, the first stab at total accumulations (some interrupted by mixed pcpn) looks like 6 inches to perhaps up to 10 inches along and north of the I-70 corridor, 4 to 6 inches south of I-70 and north of a line oriented southwest to northeast between the Tri- State region to Hocking County, with 2 to 4 inches elsewhere. Certainly, winter headlines will have to be issued at some point where confidence increases, which will probably happen on the day shift. Will mention all the hazard possibilities in the HWO. Lows Friday night will range from the mid 20 north to the lower 30s south. Highs on Saturday will range from the upper 20s northwest to the lower/mid 40s southeast. Lows Saturday night will range from near 10 northwest to the lower 20s east/southeast. For Sunday into Sunday night, Arctic high pressure will build into our region. There will be a chance of light snow east early on, otherwise, skies will be partly cloudy to cloudy, with perhaps a chance of CAA flurries. Stiff and gusty northerly winds will allow temperatures to slowly fall through the day. By Sunday night, it should be dry with clearing skies. This will set the stage for cold temperatures, especially where an appreciable snow cover results. Lows will range 5 below to 10 below northwest to 0 to 10 above elsewhere. Winds will stay up, resulting in cold wind chill temperatures from 10 below to 20 below north, with 10 below to near zero south. Arctic high pressure will move east on Monday with a dry day expected. After cold lows, highs will struggle, especially north. Highs will range from near 10 north to the lower 20s far south. There is quite a bit of timing uncertainty with the approach of the next area of low pressure Tuesday into Wednesday. This could also also have an impact on temperatures and thus precipitation type. Leaned towards an ensemble mean for now. && .AVIATION /12Z THURSDAY THROUGH MONDAY/... Deteriorating conditions are expected through the TAF period. Currently, ceilings are MVFR/VFR across the region, but this will change over the next couple hours. Precipitation is expected to spread into the region from southwest to northeast. There may be a brief period of drizzle or freezing drizzle near onset, but snow is expected to be the primary precipitation type through the morning at KDAY/KILN/KCMH/KLCK, with a mix with rain starting at KCVG/KLUK. Rain will eventually mix in northward to the other TAF sites, though not until later in the day. Where snow falls, IFR ceilings and visibilities are both possible, with accumulations of an inch or so at KDAY/KCMH/KLCK. There may be a slight improvement to conditions once the snow switches to rain, but no better than the pessimistic end of the MVFR category. Overnight, precipitation will come to an end, but ceilings will continue to lower into the 200-600 foot range. Some drizzle or fog is also expected, reducing visibilities significantly as well. OUTLOOK...IFR conditions are expected into Friday morning, with MVFR/IFR conditions will continuing through the day. IFR conditions are likely with precipitation on Saturday and Saturday night, with MVFR/IFR conditions and gusty winds on Sunday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Franks LONG TERM...Hickman AVIATION...Hatzos is the U.S. government's official web portal to all federal, state and local government web resources and services.