Area Forecast Discussion
Issued by NWS Wilmington, OH

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000 FXUS61 KILN 210536 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 136 AM EDT THU JUL 21 2016 .SYNOPSIS... High pressure will extend across the region through Friday. A weakening front will push through Friday night. The high will build back in for the weekend with very warm and humid air remaining in place. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Cirrus stretching from west-central Indiana to the southern tip of Ohio is rapidly thinning out, and skies will become clear for most of the area overnight. At this hour, dewpoints remain muggy in the lower 70s west of I-75 and will keep temperatures a bit warmer across our west overnight. Winds will remain calm or very light out of the SSE. Could see some patchy fog develop in areas west of I-75 late, but it appears the better potential will be just west of our forecast area where low level moisture is more abundant from rainfall over the past 24 hours. Generally nudged low temps up a degree or so from the previous forecast, though knocked lows down a tad for counties east of Columbus given lower dewpoints in place over central Ohio. && .SHORT TERM /6 AM THIS MORNING THROUGH FRIDAY NIGHT/... Increasing heat and humidity will be the main focus with first heat headlines of the summer possible for Friday. High pressure ridge will keep any convection minimal through Friday morning. Temperatures are certainly heating up along with increasing dew points. This will lead to Heat indices in the mid and upper 90s for Thursday and should top out around and just above the 100 degree mark by Friday. Will hold off any heat headlines for now but continue to mention in HWO. Bigger question mark is amount of convection for Friday afternoon into Friday night. With weak front pushing down expect scattered coverage at least for the late afternoon and evening with diminishing trend through the night Friday night. Again temps near or just above guidance through period. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... At 12Z on Saturday...the ILN forecast area will be situated on the nern periphery of a massive flat/strong ridge centered from the ern Pacific all the way across to the mid-atlantic coast. Two anticyclone centers /~595dm at 500mb/ will anchor this ridge over Missouri and southern California. Several shortwave troughs will be found at 12Z in the modestly strong subtropical jet stream atop this flat ridge...a broad/weak trough crossing sern Canada/New England and a stronger/sharper trough in Montana. The story through this entire period will be 1) heat and humidity and 2) low predictability threats for storms. The impacts will most definitely be felt on a more widespread basis from heat and humidity...as everyone will get in on this action. In particular,Saturday/Sunday will pose the most heat threat although Monday may be oppressive depending on frontal timing. Impacts from storms will be fairly typical for mid/late July...expect slow moving storms with potential for downbursts and localized flooding via excessive rain rates. Details/predictability issues...right off the bat there`s a heat/humidity issue given all global models wanting to slip a weak front (attendant to southeast Canada/New England shortwave mentioned above) into Ohio/Indiana. There will be good drying north of this boundary...and some recent runs /06Z GEFS mean and GFS deterministic/ blow this boundary all the way into KY suggesting a respite from oppressive 70+ dwpts. Not buying it -- think recent 20.12z runs of NAM/CMC/ECMWF/GFS showing front hanging up over northern OH to be more realistic...thus expect heat advisory level impacts to continue across all but perhaps the far north/east on Saturday...and for sure into Sunday. Given 925mb temps and blend of 2m Td...seems reasonable to expect low/mid 90s and low/mid 70s dwpts with heat indices 100-105F most locations. This front will likely spark isold/sct convection Sat aftn/night given strong instability. Given expected weakness in tropospheric flow /effective bulk shears < 20kts both days/ these storms will slow moving...prodigious rain producers with a higher overall threat for localized flash flooding and localized downbursts or small periods/swaths of wind damage if cold pools can merge. On Sunday night/Monday...the MT shortwave mentioned above should be crossing to the north of the Great Lakes and will likely be the impetus for better storm coverage/development but timing is concerning which may bring better rain chances Sunday night. A little better flow with the wave may allow for a little better storm coverage/organization/severity if a Monday frontal passage occurs at peak heating. On Tues/Wed...some relief though height falls will be rather modest and cooling too. Expect temps/dwpts to fall back to more seasonable summertime values during this time. && .AVIATION /06Z THURSDAY THROUGH MONDAY/...
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The center of surface high pressure will settle over the mid Atlantic region today, then it will drift off the coast tonight. For today, return flow around the high will bring an increase in surface moisture. This will result in some diurnal cumulus clouds along with southerly winds in the 7 to 10 knot range. For tonight, models are still having difficulty in discerning where upstream convective complexes may head toward as they dive south/southeast around the periphery of an expansive mid level ridge from the upper Mississippi River Valley/upper Midwest. Certainly, there will be some debris clouds from the convection, but it is unclear how much of this convection will make it into our forecast area by 12Z Friday. So although there may be a risk of a thunderstorm late tonight, low confidence in location and coverage support keeping the terminals dry. OUTLOOK...Thunderstorms possible Friday through Monday.
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&& .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...Padgett NEAR TERM...Kurz SHORT TERM...Padgett LONG TERM...Binau AVIATION...Hickman

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