Area Forecast Discussion

WHAT HAS CHANGED

Increased chance for showers and storms Saturday afternoon and evening.

KEY MESSAGES

1) Rain chances return Sunday and continue into early next week. Longer duration rainfall totals could be high and lead to localized flooding and/or river rises

2) Much cooler temperatures arrive by mid-week

DISCUSSION

KEY MESSAGE 1... A more active pattern is on the horizon beginning later Saturday evening and continuing through next week. A breakdown of the southeastern CONUS ridging will open the door for rounds of waves embedded within quasi-zonal flow and associated rain.

Round one arrives late Saturday and continues into Sunday as an initial mid-level wave rides along a baroclinic zone through the Ohio Valley. Ongoing convection Saturday morning poses an interesting evolution into the evening hours as CAMs attempt to handle the previously poorly modeled extent. Warm starts are now capturing it better and suggest that outflow extends as far south as south-central Illinois, sparking additional development there. This is progged to track east through the afternoon and arrive into our eastern Ohio counties just around dark. This should point toward weakening, and the 12z suite of CAMs suggests that, though maybe not quite to the extent that they think as point soundings ahead of it still show some elevated instability. The severe threat should be low provided the timing is correct, but lingering showers and rumbles of thunder are certainly in the cards, especially for the southwestern part of our area where the storms should be diving toward the best instability axis during the day. Farther north, ongoing warm advection and low-level jet driven showers likely continue as the broad ascent spreads east. While light as drier lower levels on our 12z balloon need saturated, the expectation is scattered light showers within mostly cloudy and cooler air.

Another push of precip, this one likely a bit more widespread and moderate to heavy at times, arrives Sunday morning as another wave rides atop the surface boundary. This time, a more prolonged period of southerly flow on the warm side is able to bring HREF mean PWAT values up to 1.6-1.9" and near the 99th percentile of climatology. A deep, saturated column with fumes of 100-300 J/kg of MUCAPE within the warm cloud layer should be supportive of efficient rainfall rates. That said, it likely will take repeated rounds of heavy rain over the same area to raise flash flood concerns as mean cloud bearing layer wind is around 30 knots, and HREF prob of 1" in an hour in any given hour tomorrow doesn't reach much above 20-30% thus supporting the lower probability of intense rainfall rates but rather longer duration totals to monitor. There are higher pockets of 50-60% probability for >1" in 24 hours. The deep column moisture should offset a severe threat with limited instability.

Following a brief lowering of rain chances early Monday, it'll remain active into the middle part of the week. While the setup doesn't appear to favor heightened flash flooding concerns any given day, long duration accumulation has the potential to be more notable. Latest NBM 48 hour QPF from Monday morning - Wednesday morning has the highest values south of Pittsburgh where a slow moving/stalled boundary is progged to be. Location of that will be paramount for who gets the bulk of the rain, and right now, most ensembles place it south of Pittsburgh. Mean values are around 1.25- 1.5" there, and 90th percentile values up to 2-3.5". Farther north, the mean is much less at 0.5-0.75" with the high end around 1.5-2". There are even some solutions (mostly a southward displaced boundary) in which the northern part of the area sees very little rain. While this all told wouldn't pose as much a threat for flash flooding, areal flooding and river flooding chances may increase especially if the high-end potential is realized. The primary way to realize that high end potential would be to be positioned immediately north of the stationary boundary with overrunning, and/or on the northwest side of a developing coastal low. Should the stationary boundary be too far south or the coastal low to far east, these threats would not be realized. The area with highest potential for impacts will be south of Pittsburgh and will need continued monitoring into early week.

KEY MESSAGE 2... The other story of the week is going to be the much cooler air in place with several days where the highs may not get out of the 50s for some. Surface high pressure across New England will induce stout, prolonged east-northeast flow locally. NBM probs for daytime highs exceeding 65F is basically a 50/50 shot from the ridges to Pittsburgh, then a bit higher farther west, all that with a summertime-biased NBM and likely a bit too high even at that; AI guidance and some deterministic models are running a handful of degrees cooler. Normal highs are in the low 70s and lows in the low 50s throughout next week, so while lows should be right around normal, we could be looking at highs some 10 degrees below.

AVIATION /18Z SATURDAY THROUGH THURSDAY/

VFR prevails through at least this evening as a mix of mid and high clouds overspread the region along with some light rain showers from pockets of elevated convection. A cu field has developed, owing to the onset of daytime heating with cigs hovering at borderline MVFR/VFR levels. However, that cloud deck should remain mostly scattered in coverage so not much impact is expected from it.

Rain chances increase overnight into Sunday as the next weather system approaches, with a greater coverage of showers and thunderstorms spreading across the area after 00z and especially after 06z. Expect cig/vsby reductions to accompany the increasing shower and thunderstorm activity. Winds remain light and generally out of the south or southeast through the TAF period.

Outlook... Restrictions are likely to return by Sunday morning with an increased chance for showers and thunderstorms associated with an approaching disturbance.

PBZ WATCHES / WARNINGS / ADVISORIES

PA...None. OH...None. WV...None.