Dangerous flash flooding possible Monday night

Flooding Likely…But Where?

The remains of Hurricane Florence are on their way, bringing an elevated to high risk of flash flooding to the southern Finger Lakes, Southern Tier, and southern Central New York.

This has been one of the most difficult forecasts I’ve encountered in years, with very little consistency among the models, a very sharp cut off between extremely heavy and no rain, and a possibly high impact, dangerous event resulting.

As the remains of Florence stream north through the Mid-Atlantic and Pennsylvania, the atmosphere over the southern Finger Lakes will become primed for significant precipitation. The atmospheric processes in play will be capable of localized torrential downpours of two-plus inches per hour. Extreme rainfall amounts are likely to result, but locating the areas most at risk is proving to be an extremely difficult task.

There is still a wide range of possibilities in how far north the precipitation associated with Florence comes. Complicating matters is that the very heaviest rain is likely right along the northern edge of rain shield. This is a scenario where Location A could see over five inches of rain, while Location B twenty miles north sees almost nothing.

This band of particularly extreme rain could set up anywhere over the Southern Tier or Southern Finger Lakes with little to no rain just to the north. To the south of this band, flooding will still be a significant concern with a widespread 2 to 3 inch rain event possible.

I am very concerned that significant, dangerous flash flooding on the scale of what happened in southern Seneca County in August will be possible tonight. Widespread minor flooding is also possible south of the hardest hit areas.

Quick-Look Details

Here are the quick-to-the-point details you need to know.

• Timing: The heaviest rain will fall Monday evening and Monday night, between 8 pm and 5 am. Lighter rain may begin as early 3 pm and showers could linger into Tuesday morning.
• Amounts: Areas in the orange shading on the map have a shot at widespread amounts of 2-3 inches, provided the rain makes it that far north as expected. Localized amounts of 4-8 inches are not out of the question. Areas in the grey shading may see under one-tenth of an inch of rain or may even stay dry.
• Worst Case Scenario: Dangerous, destructive flooding hits several communities in the southern Finger Lakes. Widespread minor flooding across the southern Finger Lakes.
• Best Case Scenario: The rain fails to make it much further than the NY/PA state line and the region stays mostly dry.
• Confidence: Extremely low given the significance of the event and how close it is (12 hours away)

Important Take-Away Messages

First, I want to stress once more the difficulty in making this forecast and the range of possible outcomes. The line between nothing and extreme impacts is so fine and so uncertain that challenge of communicating the forecast is as difficult as the forecast itself.

Our region could be completely missed. Hope and pray it is.

If it is not missed, however, a very dangerous situation will unfold. Nighttime flooding is even more dangerous than flooding in the daytime. The reduction in visibility makes it more difficult to accurately gauge the dangerous of flooding. Nighttime flooding can also catch people unaware as they sleep, taking away time necessary for escape when every moment counts.

The best thing to do now is prepare. Have a plan in place before you go to bed tonight, especially if you live in a flood prone area, in an urbanized location, or near a small stream, drainage ditch, or culvert.

Remember though, with extreme flooding, even areas not typically at risk from flooding can sometimes be impacted.

Never, ever drive through flood waters. Most flooding deaths occur in vehicles. If a road is flooded, alert the authorities and turn around.

I will be closely monitoring this scenario throughout the day and into the evening. A live blog and/or live video session is likely unless something changes in our forecast.

Please stay tuned, stay safe, and let your friends and family know about their local no-hype weather source.