Breaking down the flood threat Thursday into Friday

Analyzing the Flood Threat

With a new storm system moving into the region, the flood potential picture is complicated in the wake of the early week rainfall.

The incoming rain will be associated with an area of low pressure that will lift out of the Ohio Valley and into southeastern Canada. Rain will come at the Finger Lakes in three stages starting Thursday afternoon and ending Friday evening.

Stage 1: Thursday Afternoon-Evening

As deep moisture pushes back into the Finger Lakes this afternoon, an area of showers and thunderstorms will likely develop over southwestern New York and push northeast into the Finger Lakes.

These thunderstorms seem most likely over the western and northern Finger Lakes before sunset. This is when the storms will have the most energy to work with and will be most capable of lightning and heavier downpours. Areas such as Dansville, Rochester, and Geneva should see these storms.

For these areas, it would take around two inches of rain in a three hour period to produce flash flooding. This is within the realm of possibility, but will probably not happen. Should rain amounts exceed 1.5 inches, local terrain and urbanization could produce flooding issues even if the two-inch threshold is not met.

Further south and east, the storms may hold off until after sunset. While still capable of heavier downpours, the threat of excessive rain is less. Unfortunately, it will also take less rain to produce flooding in the Southern Tier, where a little over an inch in a three hour period could set off flooding. Again, this is in the realm of possibility, but not overly likely to happen.

The stripe of yellow Level 2-LOW risk that extends from the eastern suburbs of Rochester to Geneva and down into the Southern Tier represents this potential/

Stage 2: Friday Morning

After Stage 1 departs during the overnight hours, a new batch of rain showers will move in for Friday morning.

This will again move from southwest to northeast across the region, but the core of the heaviest rain will likely be a bit further to the south and east.

This will, therefore, have more of an impact in the flood-weary areas, but thankfully, this will be the least concerning of the three stages.

While rather widespread, rainfall rates will likely remain light to moderate with less of a chance for heavier downpours.

The main concern with Stage 2 will be if Stage 1 produces flooding across the southern Finger Lakes and Southern Tier. Generally, though, the risk is minimal Friday morning.

Stage 3: Friday Late Afternoon-Evening

This is the most uncertain of the stages.

As low pressure passes over or just north of Lake Ontario, a cold front will push into eastern New York with showers and thunderstorms.

However, there are indications that a second area of showers and thunderstorms may develop off of Lake Ontario and push into mainly the north-northeastern Finger Lakes.

Should this verify, areas east of Rochester and north of Ithaca and especially near and north of Auburn would be most prone to thunderstorms.

Some of these same areas could see heavy rain from Stage 1, and multiple storms could train over the same areas with a west to east storm motion.

Between Stage 1 and Stage 3, a few localized areas could see 2-4 inches of rain with an outside chance of even more.

While the counties along the shore of Lake Ontario are the least prone to flooding, total amounts over 3 inches would be concerning.

What About that Level 1- Minimal Area in the Eastern FLX?

As I mentioned in yesterday’s analysis of my flood forecasts early this week, flash flooding is a very complicated phenomenon to predict.

Not only is their difficulty in predicting where the rain will fall, but taking previous rainfall into account is vital.

Given how the rain was distributed earlier this week, and how the three stages are expected to unfold at this point, I think the threat for flooding across Tompkins, Cortland, southern Cayuga, and southern Onondaga Counties is less than most of the rest of the Finger Lakes.

Firstly, these areas generally did not see nearly as much rain earlier this week. Even so, the amount of rain required for flash flooding is fairly similar to the rest of the region.

However, I expect the heaviest rain from Stage 1 to focus in areas further west, while Stage 3 will be further north. Stage 2 will probably impact these areas the most, but again, that is the least concerning of the stages.

When taking all these factors into account, the differences were great enough to keep these areas at a lower stage. That does not mean flooding cannot happen here, just that it is even less likely.

Overall, I expect the Finger Lakes to get through this event with little to no significant flooding. Should flooding occur, it *should* be mostly minor and localized. This is why there is no Level 3- Elevated risk areas on the map.

Nonetheless, I will be watching closely and updating as necessary.

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