Area Forecast Discussion
Issued by NWS Wilmington, OH

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974 FXUS61 KILN 200820 AFDILN Area Forecast Discussion National Weather Service Wilmington OH 420 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/... The initial MCS that was moving across northern Indiana earlier this morning has pretty much dissipated but it does appear to have left an outflow boundary that is currently pushing southeast across central Indiana. The last several runs of the HRRR have suggested that the next MCS currently over southern Wisconsin will also drop southeast toward our area and weaken through late morning. This too could leave some sort of boundary that may push down toward our area through the afternoon hours. Meanwhile, the debris clouds from these two MCSs will also have a possible impact on temperatures across our area as we head into the early afternoon hours. As a result, there is quite a bit of uncertainty with the forecast for today. The current thinking is that the pcpn associated with these systems will generally remain north and west of our area through the early afternoon hours. However, we will likely see some amount of cloud cover from them, especially across our north through the morning hours before possibly dissipating through early afternoon. Low level thermal fields are warmer than what we saw on Wednesday with 850 mb temperatures generally forecast to be about 2 to 3 degree celsius warmer. The possible cloud cover could counter this somewhat so will only go about 2 degrees fahrenheit warmer for highs today compared to Wednesday, with the greatest increase across our south where any affect of the cloud cover should be less. Further complicating the forecast will be the possible development of thunderstorms along any remnant outflow boundary later this afternoon as we destabilize. Would think the best chance for this would be across the northern portions of our area so will generally carry a 20-30 pop across our north to account for this threat. Meanwhile, with dewpoints forecast to rise into the low to mid 70s, and if we are able to warm into the lower 90s, heat indices of up to around 100 degrees and possibly even a tad higher will be possible later this afternoon. Since it looks like it will only marginally meet heat advisory criteria and there could be a lot of competing factors (outflow boundaries, possible thunderstorm development, cloud cover), confidence just is not there to go with a heat advisory so think the best path at this time is to hold off on an advisory and issue an SPS to address the heat potential. && .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 THURSDAY/... 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 /07Z THURSDAY THROUGH MONDAY/... A weakening MCS is moving southeast across northern Indiana early this morning. This is moving into a drier airmass so expect the weakening/dissipating trend to continue so have kept the TAFs dry through this morning. Debris clouds associated with it though will still likely affect especially our northern areas through mid morning. this will likely help limit br development, especially at the northern TAF sites. There is a lot of uncertainty heading into this afternoon as the HRRR is suggesting another weakening MCS riding down along a left over outflow boundary from the current MCS while several of the other hi res models are suggesting some thunderstorm development mainly to the north of I-70 late this afternoon and into this evening. For now, have kept the TAFs dry through the afternoon and then just introduced a VCSH at KCMH/KLCK this evening to account for the possible activity along and north of I-70. 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...JGL SHORT TERM...JGL LONG TERM...Haines AVIATION...JGL

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