The Debby train begins to move north It’s taken a few days, but the atmospheric pieces are working into position to move Debby along. While the remnant effects on Maine appear to be marginal, I will remind you that tropical systems are nothing to fall asleep on. While the general idea is for a rainy affair, there is always the surprise factor. These systems have a mind of their own, and will do what they please. Staying aware and having multiple ways to receive weather alerts will be key until this event passes on Saturday. One more dry, comfortable day Very little fanfare for what is expected during daylight hours. Any areas of patchy fog over interior areas near lakes and rivers burn off by mid-morning. A southerly wind flow develops with surface high pressure exiting to the east. Clouds will be on the increase as the day unfolds. It will be another seasonably cool day by a few degrees, with comfortable humidity levels. That all changes starting Thursday night. Timing, flash flood potential, and rainfall Thursday 8 AM (12z) to Saturday 8 PM (00z Sunday)-- I'm using the precipitable water forecast idea here as it covers all of the bases on timing, conditions, and potential threats. The dry air that the region has enjoyed over the past couple of days is moving out on Thursday. Dew point temperatures and relative humidity values begin to climb. A warm front moves in Thursday night into Friday, which brings the risk of heavy showers and thunderstorms. With plenty of convective energy building aloft along with the increase of spin in the atmosphere, the risk of a severe thunderstorm is present and could feature a damaging wind or a potential tornado threat. Dew point temperatures climb into the 60s. The dew points are expected to continue rising into the 70s Friday night as a cold front approaches from the west. The main idea of the heaviest rainfall chance comes in this window into Saturday morning. Given all of the tropical energy, I can’t rule out the threat of a severe storm in the overnight hours. There is plenty of spin in the atmosphere, potential for damaging wind, and perhaps an isolated tornado. As the upper low captures the post-tropical low of Debby, a strong cold front passes through the region on Saturday. Rainfall is expected to end over southwestern areas by around 10 AM and the rest of the state by early to mid-afternoon. Dry air funnels in on the backside to bring comfortable conditions to all areas Saturday night.  With the storm's track to the northwest, the better risk for flash flooding is focused on the higher terrain. We see this happen with inside runner systems with south/southeast wind flow, which have become more common in recent years.  With the track further to the west, the higher rainfall ideas have shifted that way as well. Seeing how this is lining up, I can see bust potential on the low end over DownEast areas (less than ½”), and on the high end for the mountains (greater than 3”). No matter where you may be perched, the greatest threat for flash flooding will be where heavy rainfall and storms repeat. These are the areas where small rivers, brooks, and streams will need to be monitored. With the heaviest rainfall potential for the mountains, there is likelihood of trail and dirt road washouts. The bigger towns run the risk of urban street flooding where dumpers set up. The highways may be slowed to a crawl due to torrential rainfall reducing visibility and standing water. Low level jet moves through Friday night Friday 8 AM (12z) to Sunday 8 AM (12z) – A look at the low-level jet associated with the system shows the best chance for damaging downdraft wind gusts associated with the heaviest rainfall. The horizontal wind gusts range in the 20-30 mph range over land and perhaps 30-40 mph along the shorelines. I can’t rule out isolated power outages because, you know, but I don’t expect any widespread issues. The storm is expected to intensify as it heads into the Gulf of St. Lawrence and brings breezy conditions on Sunday. The wind speeds could be stronger on the backside than the front. High surf for the shorelines Friday 2 PM (18z) to Sunday 2 PM (18z) - As far as the ocean is concerned, I am not expecting much concern. I suspect there will be a high surf advisory posted as seas build in the 6-9’ range. If there is a chance for some isolated splash over, it would be during the afternoon high tide during the 3 PM hour. Tides are running off astronomical levels at this point. Rip currents will be high, given the situation. It would be best to watch the ocean from a safe distance and respect the danger associated with the storm.  PTW continues due to the financial support |
Mike Haggett
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