This post will break from the usual format and focus mostly on the heavy rain and flooding threat posed by the remains of Debby as they move into the area on Friday.
It is my goal with this post to give you the clear-cut, no-hype information you need to properly understand the large and small-scale details of this situation. Armed with accurate information, you will not only be able to more accurately assess your own risk, but will also be able to help inform others and combat the spread of weather hype.
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Large Scale Overview
Rainfall will come in two waves, starting today (Thursday) but focusing on Friday.
First will be scattered showers today, a few of which are already in the area this morning. Showers will gradually increase this afternoon, including the potential for a few heavier downpours. While most areas will see low rain amounts, an isolated amount of an inch or so cannot be ruled out.
The main batch of rain will arrive as widespread, moderate to heavy rain starting Friday morning and ending late Friday afternoon or early in the evening. The heaviest rain will be during the afternoon hours.
As the rain moves north into our area, it will narrow as it interacts with an incoming frontal boundary from the west. The heaviest rainfall will be directly west of the path of the low pressure center that was once Hurricane Debby.
Along the path of the low and to the east, rainfall amounts will be less, but winds will be stronger, and the threat for isolated tornadoes cannot be ruled out.
The low pressure center is expected to pass, south to north, directly over the central or eastern Finger Lakes. However, the exact location still has some wiggle room, making the location-by-location small scale details difficult to nail down.
Small Scale Nuances
With the precipitation area narrowing as it moves north, the track of the low becomes even more important.
The width of the two-inch or greater rainfall area may only end up 50-100 miles wide. Therefore, it is highly likely that not all the Finger Lakes will see rainfall exceeding two inches. Still, with a perfectly aligned track, that width may encompass most of the area.
The greatest concern will be in an even narrower band, possibly just 10-20 miles wide. Within this small south-southwest to north-northeast band, rainfall amounts of 3-5 inches with locally up to 6 inches are possible.
Recent rainfall has not been uniform either, so soils are unevenly saturated, making some areas less capable of holding onto the incoming heavy rain and thus more prone to flooding.
And, of course, complex terrain and urbanization locally enhance the flooding risk as runoff rushes down steep slopes or pools on pavement.
Flash flooding is a highly localized event dependent on a myriad of factors. It is very likely that there will be flash flooding in our area. The potential for locally dangerous flooding remains.
The area of greatest risk will be small and cannot be accurately pinpointed at this time, but is likely to be somewhere over the Finger Lakes region.
However, if I had to take a stab at it, I would narrow the most likely area to be along or east of Keuka Lake, but west of I-81 up into the Syracuse area.
Positive Factors
There are a couple of factors working in our favor, however.
First, the aforementioned narrowing of the heaviest rain will reduce the areas that will see over 2 inches and over 4 inches of rain, respectively.
The speed of this system continues to be projected to be faster and faster, with rain now expected to end by Friday evening, if not during the late afternoon. While the rain will come down hard and excessively, the shortening time span is also leading to lower rainfall projections.
Lastly, this has been a well-advertised event. Unlike many flash flood events which are sudden and unexpected, there is plenty of time to make preparations, something I urge especially for those areas that have proven susceptible to flash flooding events in the past, those near small streams and areas of complex terrain, and in urban areas with poor or even slightly poor drainage.
Other Threats
On the east side of inland tropical systems and their remains, wind and severe weather can become a concern. We saw this in early July when the remains of Beryl moved through, spawning microbursts and tornadoes in our area.
There are discrepancies on the models for how strong the winds will be along and east of the low pressure center. Wind gusts should at least be in the 40-45 mph range, but gusts over 50 mph are not out of the question.
Also, late in the afternoon, a band of thunderstorms will develop near and south of the low pressure center and move east. Locally damaging winds and brief tornadoes are possible with any of these storms.
The threats for wind and severe weather are higher to our east but will ultimately depend on the exact path of the low. Eastern reaches of the Finger Lakes could start to see some of these impacts if the low tracks along its western possibilities through the central FLX.
The labels of eastern and central are, in this case, deliberately vague, given the uncertainty of the exact track of the low.
Extended Weather Forecast
With rain ending no later than early Friday night, the weather will be much improved by Saturday. Expect a mix of sun and clouds, maybe a stray spit of rain, and much less humid conditions. Dewpoints will drop from the 70s on Friday to the 50s Saturday.
Sunday will be similar, just with a bit more in the way of clouds and a better chance for a few scattered showers. Both days will have highs in the low and mid 70s.
Blustery winds will also develop both days from the west-southwest. Gusts as high as 30-35 mph are expected.
A few showers will also be possible Monday, with just low chances for a stray spit of rain here or there the rest of next week. The dry weather will be welcome news and should help soils recover after Friday’s rain.
Temperatures will remain in the 70s next week with low humidity and coolish nights in the 50s. Highs may creep back up to around 80 by next weekend.
More Information:
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This graphic represents an average over the entire Finger Lakes region. Localized variations should be expected.
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Dan
Appreciate the write up. Would you say travel from Ithaca to the NYC area is inadvisable on Friday afternoon?
Meteorologist Drew Montreuil
Yeah, if it is avoidable, I’d make other plans.
Jim Kennedy
PS Drew, note, Geneva is in – barely – Ontario County, Ontario not being listed on counties of concern. No biggie, and good luck you you both in the storm and “managing” it.
Meteorologist Drew Montreuil
Thanks for your note, Jim! Yes, the list of counties in the ALT text of the map didn’t include Ontario, but Geneva is kinda in the shading a bit on the actual map. Bottom line… the weather doesn’t care about our made up political boundaries, haha! Best of luck to you as well.