Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 060845 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 345 AM EST Tue Dec 6 2016 .SYNOPSIS... As an area of low pressure moves northeast through the Ohio Valley, rain is expected for much of the morning and afternoon today. Drier conditions will return on Tuesday, but temperatures will drop for the second half of the week after a cold front moves through on Wednesday. && .NEAR TERM /THROUGH TODAY/... Current surface observations and MSAS analysis indicate the presence of a roughly-1003mb surface low over the state of Mississippi, with a low pressure trough that extends northeast and abuts the appalachian mountains in eastern Kentucky and West Virginia. This low is forecast to move northeast today, roughly following the aforementioned trough, with a compact 500mb shortwave trailing just behind. Both the shortwave and surface low are expected to gradually weaken through the day today, but there is plenty enough of a combination of features -- a feed of gulf moisture, upper jet support, and mid-level vorticity advection -- to lead to the development of widespread rain. This current expanse of radar echoes covers the Tennessee Valley, and is moving northward into the Ohio Valley -- just beginning to enter the ILN forecast area as of this writing. This rain will overspread the entire CWA between 08Z and 14Z. Surface observations across the ILN CWA are all reading 32 degrees or higher, and with temperatures continuing a very slow rise overnight, there should be no concern for freezing rain. With the strength of warm layer (max T of +4C to +6C between 950mb-700mb) the vast majority of hydrometeors should be well melted before reaching the ground, so this update will continue the going trend of having only rain as a precipitation type in the weather grids. Because this layer is very dry, however, an extremely limited chance for a brief period of ice pellets is possible -- limited to the first band or two of precipitation. By 09Z at KCVG, a plethora of model soundings indicate that even the wet bulb temperatures will be at or above freezing through the lowest 10kft of the atmosphere. Upstream reports will continue to be monitored, but this will not be included in the current forecast. PoP amounts have been increased to 100 percent for the entire CWA. Though there may be a quick break or two in the rain shield as it transposes itself northeastward with time, the overall amount and duration of the expected rain warrants this, rather than getting bogged in grid semantics and putting in lower numbers for an hour or two. Precipitable water amounts are expected to range from about 0.90" in the northwestern CWA to 1.20" in the southeastern CWA, give or take a tenth or so depending on which model is used. A blend of NAM/GFS/ECMWF numbers would suggest a widespread rainfall of around 0.25" in the northwestern CWA to around 0.75" in the southeastern CWA. WRF-ARW /WRF-NMM QPF fields illustrate the potential for some isolated totals of around an inch or slightly higher, particularly in the southeastern ILN CWA. Overall model timing agreement is strong, and the grids will also show a sharp cut-off to the rainfall on the back side of the shortwave -- ending from 20Z-00Z from west to east. The slight wrinkle with regards to the end time for precipitation is that a shallow layer of moisture appears likely to persist behind the deep moisture ahead of the shortwave. Model soundings suggest that a well-mixed boundary layer will exist this evening, leading to a low stratocumulus deck and the potential for some drizzle. Drizzle has been included in the forecast for a few hours after the main rain has ended. Current expectations suggest that the drizzle will likely end before surface temperatures drop below freezing. Aside from the rising temperatures early this morning, temperatures through the rest of the day should generally approximate a diurnal curve, though with a significant gradient from NW (upper 30s) to SE (near 50). && .SHORT TERM /TONIGHT THROUGH FRIDAY/... Aside from the low clouds, the rest of the air mass in place over the Ohio Valley tonight will be quite dry. On the southeastern periphery of a stacked low over northern Minnesota, a dry cold front is expected to move through the ILN CWA early Wednesday morning, with winds switching to the WNW. The surface reflection of this front is not overly easy to pick out, and in fact, surface winds are expected to back to the SW again by Wednesday afternoon. The cold advection will do its best to stunt the diurnal cycle, and much cooler conditions are expected going into Wednesday night. By Thursday, the slowly-moving longwave pattern feature troughing setting up across the western Great Lakes, with very quick WSW flow aloft over the Ohio Valley and mid-Atlantic. Previous forecasts have mentioned the possibility of snow on Thursday morning, but models continue to back away from this proposition, with both the 00Z GFS / 00Z ECMWF keeping conditions dry across the ILN CWA. While a 20 PoP for a slight chance of light snow has been maintained, a mention of flurries might eventually cover the scenario. There are no signs of forcing in the low levels at all, with only some weak/transient upper support on the south side of the 300mb jet, and this no longer appears to be enough to generate any reasonable chance of accumulating snow. The continued cold advection just off the surface will keep the shallow boundary layer well mixed through Thursday and Friday, which does present the possibility of a persistent (though maybe broken) stratocumulus deck. As the top of this mixed layer intersects the lower end of the DGZ on Friday, some flurries may be possible (particularly in the northern half of the ILN CWA). This has been included in the forecast. Until the advection pattern really starts to adjust itself over the weekend, temperatures will only continue to cool each day this week. Model blends for temperatures looked generally reasonable, but NAM min temps appeared too high for the latter half of the week, and were not used. && .LONG TERM /FRIDAY NIGHT THROUGH MONDAY/... With the upcoming weekend, a warm front will cross northeast through the Ohio Valley and increase the threat for rain on Sunday, possibly a rain/snow mix as the warm air overspreads the cooler air in place. Models continue to disagree in how this system evolves with the ECMWF notably drier and further west than the more progressive and much wetter GFS. Modelers from WPC also seem to have split the difference here noting the warm frontal passage similar to the GFS solution on Sunday, then bows to the ECMWF on Monday morning with the surface low east of Lake Superior. GFS occludes the front off of the east coast on Monday morning and pushes in drier air. Meanwhile the ECMWF continues to keep a strong baroclinic zone in the midwest with continued threat for precipitation over the Ohio Valley during this same time frame. Tried to trend lower on the threat for precipitation on day 6 and 7 and culminated the pops on the high end category Sunday afternoon, in line with the GFS. Against better judgment and nodding towards ECMWF the pops have been lingered into Monday and Monday night to account for lower confidence and natural variability of the models on day 7. && .AVIATION /08Z TUESDAY THROUGH SATURDAY/... TAFs are currently VFR as mid and high clouds stream n ahead of low pressure in the deep south. The low will track ne up the wrn side of the Appalachians today. Mesoscale models continue to delay the onset of the rain ahead of the system, now showing a 09Z-10Z time frame for it to reach the srn tafs. The low is forecast to reach srn KY by 12Z, then it will fill as it moves farther n. Despite this, there is plenty of convergence at H9 and H8 across the region, so the the rain is expected to overspread TAF sites from south to north quickly after 12Z. Ceilings are forecast to drop to IFR as the boundary layer saturates due to the steady rain. As the low tracks off to the east after 18Z, a cold front will sweep through the TAF sites. Rain will end late around 20-21Z for the wrn tafs and a few hours later for the ern tafs. Behind the cdfnt, winds will turn to the w and will increase, with gusts of 20 kts possible. Cigs will try to lift to MVFR in the w behind the front, but the should linger in the IFR range further e. Some drizzle will be possible during the late afternoon and evening hours of Tuesday as the forecast soundings are showing drier air aloft coming in over the trapped low level moisture. OUTLOOK...MVFR ceilings will persist into Wednesday morning. MVFR ceilings are possible again on Friday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Hatzos NEAR TERM...Hatzos SHORT TERM...Hatzos LONG TERM...Franks AVIATION...Sites

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