Area Forecast Discussion
Issued by NWS Wilmington, OH

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458 FXUS61 KILN 201527 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 1127 AM EDT Thu Jul 20 2017 .SYNOPSIS... A frontal boundary will sag slowly south across the region through the weekend. This will combine with a warm and humid airmass to produce occasional thunderstorms through the rest of the week and into the weekend. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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MCS has continued to progress southeastward through parts of IL/IN this morning. Over the last couple of hours, infrared satellite imagery has shown an overall warming of cloud tops, however convection has been persistent as it has moved into an area which has experienced some insolation (despite debris clouds) and some moisture advection. Presence of an MCV may also providing an influence. So the first forecast challenge of the day is to determine the fate of this initial MCS. Most of the recent high resolution guidance has suggested that it will weaken with time, perhaps even completely dissipating by the time it would make into the CWA. However, the 13Z HRRR run now holds this together longer, essentially crossing much of the IN/OH portion of the CWA through the afternoon. Based on radar trends, the completely dissipating solution is becoming more unlikely and have already increased pops across the northwest CWA to account for this. May wind up bumping them up further based on radar trends. Am also thinking that even if this decays over the CWA, it would put out an outflow boundary which justifies chance pops for convection through the afternoon encompassing much of eastern IN into OH. At this time am not expecting severe weather with the initial convection, but locally strong storms with gusty winds will be possible with this system. Will also be watching a cold front drop south into northern Ohio by early evening. If the MCS holds together for this afternoon, this will likely delay new convection until later this evening. But with the cold front, instability and increasing shear, a new round of convection is still expected later this evening with both damaging wind and flash flood threats mainly across the northern half of the CWA. Uncertainties with exact timing and coverage still exist. As for temperatures today, it will be a similarly warm day as yesterday, but with debris clouds and potential afternoon convection, maxima will likely only rise to near 90. This means that combined with lower 70 dewpoints, heat index values should remain just below Heat Advisory criteria. Thus, HWO/SPS will continue to discuss the heat.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/... As we head into this evening, the airmass across our area will still be unstable. Thunderstorms are expected to develop along the boundary to our north through late afternoon and then start to push/advect southward into the unstable airmass across our area. This will lead to an increasing chance of thunderstorms across our north this evening, and then likely pushing south across much of the area and weakening through the overnight hours. Deep layer shear will be on the increase from the north from late afternoon into early evening and this would support a severe threat for mainly damaging winds and hail across northern portions of our fa this evening. The boundary will likely lay out somewhere across our area on Friday but there is and has been quite a bit of variation among the models on its exact placement. As a result, will carry more of a broad brush chance of thunderstorms during the day on Friday. This will also make for a tricky temperature forecast for Friday as low level thermal fields remain similar to what is forecast for today. With perhaps a better chance of pcpn on Friday and the boundary possibly across our area, will nudge temps down a bit through the day on Friday, particularly across our north. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... The main question Friday night into Saturday will be where a stalled out stationary front eventually lays out. The overall synoptic pattern will be a mid level ridge at 500 mb with an upper level low retrograding west underneath. A potent upper level low will also be located just east of the Hudson Bay with a cold front extending northwest to southeast across central Ohio. Overall, models have trended slightly further south with the boundary but still have the boundary stalling out across our northeastern zones. This boundary will be the focus for precipitation and have nudged PoPs up across our northeastern zones. Friday night into Saturday a potent short wave will be racing east towards Manitoba with pressure falls occurring across the midwestern United States. As this happens the stationary front that was across our northeastern zones will lift north as a warm front. Even though the weak surface convergence is lost Saturday afternoon the mid level ridge collapses. As this happens PWATs remain around 1.90" (above the 90th percentile for this time of year) and forecast soundings destabilize. ML CAPE values approach 1500 J/kg and k indicies move into the lower 30s. Also Saturday afternoon, a short wave and its associated wedge of PV will dive southeast and wash across the forecast area. Given the upper level lift, moisture, and instability have raised PoPs to likely across the north Saturday afternoon while nudging high temperatures downwards a bit. Sunday into Monday the upper level short wave will dive southeast with an area of surface low pressure tracking across Michigan. The chance of showers and thunderstorms will start to slowly decrease Sunday into Monday as drier air wraps around the low and across the region. Precipitation chances wont come to a complete end though until Monday night when a cold front crosses the region. The latest run of the ECMWF washes the front out around the Ohio river, but this seems to be the outlier compared to previous models runs so have kept more with the GFS and better upper level amplification. Surface high pressure will then move in behind the cold front allowing for pleasant weather Tuesday into Wednesday. By Thursday the mid level ridge will again set up over the midwest allowing the area to return to northwest flow and a return of rain chances. && .AVIATION /14Z THURSDAY THROUGH MONDAY/... An MCS currently over southern Wisconsin and northern Illinois should push southeast and weaken through late morning as it moves into a drier/more stable airmass. However, the last few runs of the HRRR are indicating some possible development out ahead of this through mid to late morning. Think this is probably overdone so have left the TAFs dry through this morning, but will have to continue to monitor radar trends over the next few hours. There is some uncertainty through the rest of this afternoon as to what affect this MCS and the associated debris clouds will have on our area through this afternoon. Would expect to see the debris cloudiness dissipate through the afternoon, allowing us to still destabilize nicely. If this occurs, we should see some thunderstorm development across northern Ohio late this afternoon into this evening, then working south through the overnight hours. Since there is still some uncertainty with this, have opted to cover the threat with a VCSH at the northern TAF sites from late afternoon through this evening. OUTLOOK...Thunderstorms will be possible at times between Friday and Monday. && .ILN WATCHES/WARNINGS/ADVISORIES... OH...None. KY...None. IN...None. && $$ SYNOPSIS...JGL NEAR TERM...BPP SHORT TERM...JGL LONG TERM...Haines AVIATION...JGL

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