Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 171145 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 645 AM EST Sun Dec 17 2017 .SYNOPSIS... Weak weather disturbances crossing the Ohio Valley today and tomorrow will bring clouds and a little rain or drizzle to much of the area as temperatures remain mild in the 40s. While clouds will likely hang on into Tuesday, temperatures will warm into the 50s ahead of the next weak cold front which will slip through the area rather quietly on Tuesday evening. This front will bring cooler temperatures and a little sunshine for Wednesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Early morning GOES 16 infrared/water vapor loops clearly indicated a shortwave trough over KS/MO moving quickly east atop subtropical ridging which was slowly building over the Caribbean. As the shortwave moves east/northeast atop the ridge into a confluent airstream over the eastern U.S., the wave will substantially shear/weaken as it outpaces boundary layer moisture will lag significantly back to the southwest. By early this evening, there will be little semblance of the wave in the mid/upper level height fields, but attendant moisture /especially low level moisture/ will be slow to scour out. There`s enough vertical motion field via weak isentropic ascent and q/g forcing to allow an axis of light rain to move/develop across much the area through the day. While low levels be stubborn to saturate initially, this should be won over by noon or early afternoon with bands of virga initially on radar congealing into a east-northeast advancing band of light rain through the afternoon hours, mainly along/south of I-70 and north of the Ohio River, but expect some light rain to fall on just about everyone today, although some locations will have difficulty measuring. Maintained 60-75% rain chances at the high end in the afternoon given a continued decent spectral/mesoscale model very light QPF signal. With thickening clouds, did drop temps down just a degree or two from previous highs as onset of rain/lowering ceilings will retard diabatic heating. && .SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY NIGHT/... Wave departs this evening with scattered light rain quickly ending but clouds continuing to thicken/lower as weak warm advection on swly boundary layer flow continues in the wake of the wave. In fact, forecast soundings suggest low level moisture will continue to deepen past 2km this evening as next subtle ripple in midlevel height field approaches. Combination of weak vertical motion in/above this deeper low level moisture field will induce patches/areas of drizzle and fog, particularly later tonight into much of Monday. Introduced a PoP for measurable drizzle or even very fine/light rain given maintenance of low level moisture field in excess of 1.5km into Monday evening. With saturated surface temp/dewpoint traces, think the temp curves will be muted on both Monday afternoon and Monday night, so tightened those up considerably. Monday night may not see any fall, and despite current forecast indicating steady temps overnight or perhaps dropping a degree, would not be at all surprised if temps Monday night actually went up a degree or two given most recent data trends. Bottom line - the Monday and Monday night periods will see plenty of stratus, light fog, and patches/areas of drizzle - most of which will diminish with time through Monday night. On Tuesday, height falls dropping through the northern Great Lakes will allow a weak cold front to slip toward the area. Continued southwesterly low level flow will allow temperatures to respond into the 50s, particularly with a few breaks of sunshine possible on Tuesday afternoon, though still expect mostly cloudy skies with any breaks confined to the later part of the afternoon. Front slips through quietly /dry/ on Tuesday evening with modest low-level cold advection signal and temps back below freezing Tuesday night. Wednesday will feature high pressure moving directly over the area with good subsidence/drying which should allow for more sunshine although it will be cooler, with highs back in the lower 40s. Will need to watch southern stream shortwave trough/surface low coming out of the southern branch of the western CONUS split flow regime. Bulk of deterministic/ensemble data has taken this system and its rain shield south of the area, though enough members grazing nrn KY with a bit of light rain to keep an eye on this system into mid-week. && .LONG TERM /THURSDAY THROUGH SATURDAY/... While Thursday is a relatively high confidence dry forecast as high pressure pulls away into New England and a large upper trough advances eastward from the central Plains, confidence in this portion of the forecast degrades very quickly. GEFS/EPS plumes continue to indicate a tremendous amount of spread in the details despite a slowly increasing signal of a cold front/low pressure shifting through the Great Lakes Friday afternoon through Saturday morning. There`s a great deal of variety in the timing of the front, orientation of the boundary, and whether a frontal wave will form on the boundary or a dominant surface low will deepen through the Great Lakes and swing the cold front through with much more brute force. There`s solutions ranging from a very potent surface low developing through WI/MI which allows highs on Friday well into the 50s with nearly similar dewpoints, and then a strong frontal surge with hints of CAPE and need for thunder consideration. Other solutions continue to be seen of a much slower/strung out frontal passing that takes far longer and entrains very deep moisture from the Gulf of Mexico, and hints at a heavy rain/flooding threat Friday night into Saturday. Still further, there have been members/runs showing a rain-to- wet snow scenario for Friday night/Saturday morning, or a band of rain passing quietly through and then the front clearing for a quiet/dry Saturday. Just not seeing any ensemble movement to a preferred solution, so will hit rain chances highest on Friday night, with a trend to cooler temperatures for Saturday and perhaps some rain/snow mix or brief wet snow. Given ensemble spread that exceeds the natural variability, continue to expect a very low confidence forecast in this time period for a few more days. && .AVIATION /12Z SUNDAY THROUGH THURSDAY/...
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Warm frontal processes north of the area will help usher in more moist air on southerly winds today. A mid level shortwave will enter this afternoon and flatten out the weak h5 ridge that is currently in place over the Ohio Valley. As it shears out crossing Ohio, a few showers or area of light rain is expected to affect the region. Models continue to indicate a wsw-ene orientation of the band of rain, which would hit Dayton and then Columbus areas with a higher potential for rain for a few hours this afternoon. HRRR and NAM12 are showing another southern maxima to the rain today, so have included some showers with MVFR cigs and VFR vsbys at just about everywhere with a slightly pessimistic forecast with lower vsbys from DAY to CMH This h5 shortwave moves east of the region tonight and with it the threat for rain is removed. Increasing moisture in the lower levels will be found in the region overnight, and an area of enhanced upward motion may initiate some patchy drizzle or a few sprinkles late overnight or early Monday morning. The sprinkles and/or drizzle does not have a feature to key off of, and the h7 upward motion fields are as splotchy in coverage as the QPF fields being generated by the NAM12. The better threat of any drizzle or sprinkles will occur towards 12z Monday. Models continue to show saturation of the low levels overnight so ceilings should lower overnight and trend toward IFR category. Have kept vsbys VFR for the overnight hours given the latest guidance is holding the onset of any precip until after 12z. OUTLOOK...IFR/LIFR ceilings and MVFR/IFR visibilities continuing into Monday. MVFR/IFR ceilings and MVFR visibilities possible Monday night into Tuesday morning.
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&& .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Binau NEAR TERM...Binau SHORT TERM...Binau LONG TERM...Binau AVIATION...Franks

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