Over the past 24-36 hours, trends with the remnants of Hurricane Debby have been in Vermont's favor. The main trend during this time was a westward shift in Debby's central track. This westward shift put the highest rain totals across upstate New York as well as into Canada. These trends haven't changed much over the course of today. Debby's remnants will interact with a trough dropping out of the Great Lakes, which will be the focus for the more widespread rain.
It was generally expected that if these trends continued today, the Weather Prediction Center would trim back their excessive rainfall outlook for Friday to Saturday. This did happen earlier this afternoon, with the entirety of Vermont getting removed from the "moderate" category (level 3 of 4). The state is now in the "slight" category (level 2 of 4). This means that flash floods are still expected, but not to the extent that could happen in upstate New York, where the moderate category remains in place.
The first flood risk map is the CURRENT one as of Thursday evening. The first one was the one from this morning before the afternoon update:
Another big reason for a potential reduction in rainfall totals and flooding has been the trend toward a quick exit. At first, the storm looked like it would be a longer duration one, but the system looks to get picked up by the jet stream and flung away by early Saturday afternoon. The duration of heaviest rain will be from Friday evening through early Saturday morning. An initial burst of rain is likely in the morning as a warm front strengthens and lifts through the region. This front will drop the most rain once it enters Canada.
HRRR showing potential weather Friday morning (1st image) and Friday evening (2nd image):
With that said, Vermont is not completely in the clear. A flood watch has been posted for northeast Vermont and northern New Hampshire. 1-2.5 inches of rain remains likely in these areas. The White Mountains may see some upslope enhancement. This rain will come down hard at times in a span of a few hours, which will be the main threat for any flash flooding. The areas covered in the flood watch are currently over-saturated from recent rainfall events. Vermont is in a "prepare for the worst; hope for the best" type of scenario.
While the west side of post-tropical storms in this kind of setup see the highest rainfall, the east side has a greater risk for severe thunderstorms. These tropical-like setups do favor brief, spin-up tornadoes, so this will be something to watch as the line of storms moves through southern and central New England in the evening. It should be noted that these tropical-like tornadoes tend to be very brief and weak. This is not much of a threat, but the potential is there and worth bringing up.
The greatest chance for a couple isolated severe storms will be across western New England. Any severe storm that forms will likely weaken below the severe threshold as it pushes east. Again, this is a lower chance and heavy rainfall remains by and large the greatest impact. The greater severe thunderstorm threat will be to the west of New England where storms will likely fire up earlier in the day.
A full forecast update will come in an updated "impacts; timing" article Friday morning.
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