Published on 28 Nov 2012

Pineapple Express set to bring torrential rain to west coast of US

You’ve probably arrived at this blog with a sense of intrigue – no doubt drawn in by a rather fruity sounding title.

Rest assured, there aren’t going to be pineapples falling from the sky, but there will be some very wet weather affecting western coastal states of the US during the next five days.

The source of this meteorological phenomenon’s name will become clearer by the end of this blog, but first of all, let me explain what’s going to happen and why.

As is the case in the UK, the weather in the US is also driven by the jet stream – a fast moving ribbon of air, high up in the atmosphere, that determines how weather systems form and where they go.

A dipping jet stream forming low pressure

When the jet stream moves from west to east, as well as travelling in a straight line, it also bends around. In cases where it dips and curves in a broad U-bend shape, something called a trough forms.

As the ribbon of air approaches the base of a trough, it slows down slightly as it goes around the bend then accelerates once it has passed through it.

It’s the acceleration of the jet stream after passing through the U-bend that is a key factor because air high up in the atmosphere is sucked upwards and removed faster that it can be replaced at the surface.

As a result of this process, air has a tendency to rise and pressure falls (forming an area of low pressure), resulting in cloud, wind and rain.

This is what’s going to happen just to the west of the US over the next five days.

Reservoir of tropical moisture from near Hawaii

In addition to the dipping jet stream, mentioned above, there’s also going to be a plume of tropical moisture surging north eastwards from near Hawaii.

This will place a reservoir of warmth, energy and moisture beneath the dipping jet stream, feeding developing areas of low pressure – effectively giving them longevity and the ability to produce a lot of rainfall.

The process is similar to what happens in the UK when we get south westerly winds from near the Azores, interacting with the jet stream over the Atlantic, bringing heavy rain and flooding.

Western coastal states of US in the firing line

Once the moisture-laden areas of low pressure have developed over the eastern Pacific Ocean, the jet stream catapults them into the west coast of the US – one after another, like a rain-train.

As a result, western coastal states of the US will see torrential rain during the next five days, with a significant risk of flooding.

The latest forecast from NOAA suggests that northern California will be worst hit, with as much as 14.5 inches (around 370mm) of rain falling over the mountains.

Five day rainfall totals for the US – courtesy of NOAA

Greatest amounts rain will fall over mountainous areas because air is forced to rise even higher in the atmosphere – squeezing even more moisture from the clouds.

Oregon and Washington State will also experience heavy rain, but it’s not likely to be as much as northern California, with 4-7 inches (100-175mm) expected.

So why Pineapple Express? Well, Hawaii is a renowned producer of pineapples on a large scale. With this being the approximate location of the phenomenon’s origin, it’s how it has gained its name.

For updates on how the Pineapple Express is affecting the western US, follow me on Twitter – @liamduttton

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