Halloween 2019, unfortunately, will not go as hoped for much of the Ohio Valley and Northeast. A rapidly strengthening low pressure system will make its way northeast from the Mississippi Valley, the Ohio Valley and Southern Ontario. It will bring a significant wind and rain threat right around the time trick or treating will begin in the northeast. In addition to the wind and rain threat, the reach of the system will be felt along the east coast as well. A severe thunderstorm risk is a concern for eastern Pennsylvania down to North Carolina. It will be an active Halloween day.
Earlier in the week, there was significant disagreement among the GFS and Euro (as well as ensembles) as to both the formation and intensity of the low pressure system. The Euro had a prominent low forming, while the GFS was not in agreement; keeping flow more zonal in nature. This is no longer the case.
Stratiform rain will likely begin around 12z (8am) for Windsor and push east, with the rest of Southern Ontario being hit by 15z (11am). It will likely be sustained for much of the day. The heaviest precipitation will be before the cold front coming through between 21z and 00z (5-8pm). Current estimates range from 30-50 mm with potentially more west of Hamilton due to rainfall currently ongoing (6:30pm Wednesday). There is a chance that parts of Southern Ontario get into the low's "dry slot", which gives an opening around trick or treat time, but there will still likely be the wind threat to deal with (more on that in a minute). This may lead to localized flooding of areas with the plentiful amount of leaves in eaves troughs and sewers, so monitor those throughout the day tomorrow.
The greater concern is the wind threat with this system.
The sounding to the left (subject to timing changes) shows the strongest winds above an inversion. This is pre-cold front. You can see the southeasterly winds still ongoing at the surface. The strongest winds are not likely to be mixed down to the surface at this point despite the low continuing to deepen.
The second sounding, about 3 hours later, shows a well mixed layer up to about 875 mb. At this height, there are 50 kt winds ongoing, which may get mixed down with precipitation in addition to the mixed layer itself. This is the most significant threat the system poses. Gusts could end up exceeding 100 km/hr; especially with winds coming off Lake Erie with a lack of friction to slow them down.
The entire system will have winds ranging from 70 to 90 km/hr, but north shore Lake Erie will be the most at risk area. Shoreline areas were hit by another strong low on Sunday, so water levels are already above normal. Long Point, Turkey Point, Port Colborne, and Crystal Beach will likely be hit quite hard by strong winds and waves potentially exceeding 10 feet at some points. Coastal properties should be quite vigilant of this threat especially because it happens overnight and into the morning.
The wind threat will begin on Thursday night and continue into Friday afternoon, with less of a threat to the coast lines beyond Friday morning due to the low pushing to the NE out of the Great Lakes area (this means the wind direction will shift from SE, to E/NE (parallel to shoreline) and finally as the pressure gradient becomes weaker, stronger winds will begin to subside by Friday night. Coastal areas around Buffalo, as the winds turn more easterly in the morning, look to be hit significantly by a surge of lake water.
Special weather statements have already been issued by Environment Canada and I think an upgrade to a watch/warning is highly likely by morning. If you have any loose items in the yard, or even Halloween decorations outside, make sure they are well fastened down, or just take them inside unless you want to unintentionally decorate the neighbor's yard.
This isn't the best case for the kids looking to trick or treat, but the high winds may just be spooky enough throughout the night.
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