30 Sep 2015

Flooding rains threaten eastern US

Eastern coastal states of the US look set for some heavy and prolonged rainfall during the next five days, which will bring the risk of major flooding in places.

Tropical Storm Joaquin, soon so be a hurricane, is currently sitting just north of the Bahamas, but its influence will extend hundreds of miles ahead of it.

At the moment, there is a big, strong area of high pressure across south east Canada, giving the eastern side of the US an easterly wind – feeding in moist air off the Atlantic Ocean.


As soon-to-be Hurricane Joaquin moves a bit further northwards, the large contrast between high and low pressure will increase the strength of the onshore wind.

Subsequently, this will feed in even more moist air off the Atlantic Ocean, which will have an increasingly tropical origin – meaning that the amount of moisture it can hold is much greater.

A foot of rain possible

This flow of increasingly tropical air will inject an abundance of warmth and moisture into the atmosphere, producing clouds that will give copious amounts of rain.

The latest forecast from the US National Weather Service suggests that there could widely be 2-5 inches of rain from the Carolinas up to Maine.

Within this zone, North Carolina to southern New Jersey could have as much as 8-12 inches of rain in the worst affect areas.

US_rainfall_NOAA_wpFlooding concerns

Some of these states have already had a few inches of rain in the past few days, so the ground is saturated in places.

This means that with such great amounts of rain still to come during the next five days, there is the likelihood of some major flooding.

However, the forecast is not an easy one for US forecasters, because the exact strength and path of soon-to-be Hurricane Joaquin is very uncertain.

Some computer models have the storm make landfall in the Mid-Atlantic states, whereas others keep the storm offshore and push it eastwards across the Atlantic Ocean, as it gets swept up by the jet stream.


Whichever way the storm ends up going, the theme for heavy rainfall and flooding seems certain for the eastern US coastal states.

The uncertainty lies in the path that Joaquin will take, which could potentially increase rainfall amounts even more if it makes landfall.

I’ll be keeping a close eye on the situation in the coming days and posting updates on Twitter – @liamdutton

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