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How atmospheric rivers are helping forecasters predict the upcoming storm

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By Brad Rousseau
Meteorologist
@bradrousseau
Tuesday, February 11, 2014, 5:03

The trend across Canada’s west coast this year could be summarized as feast or famine. The coast has been plagued by prolonged spells of little precipitation broken by periods of short but persistent and intense rain (or snow for the high elevations).

The current pattern setting up has some long range models are projecting nearly a months’ worth of rainfall (according to February’s monthly average using the 1981-2010 climate normal per Environment Canada) for some parts of the coast over the next week to week and a half. The pattern responsible for this trend is well known and often referred as the “pineapple express”. Simply put, the pineapple express is a branch of the polar jet which extends from the tropics near Hawaii and pushes into the western coastline. In some cases the split polar jet can combine with subtropical jet as well. The jet sets up along the polar front where cyclones develop and push into the west coast. This pattern can often persist for a few days or for a week or so, as the current forecast indicates. There is a lot more to this process than explained but for simplicity sake I’ll give the shortened version for now. Figure 1 below shows the approximate location of the polar front for this evening through the Pacific and into western coastline. Figure 2 shows where the 300 mb jet stream is forecast to setup. Note how the two setup in relation to one another.


RELATED: Defining a pineapple express


Figure 1. 850 mb wind and temperature forecast for this evening through tonight. Front positions are analyzed as well. Image courtesy of Weather Bell.

Figure 1. 850 mb wind and temperature forecast for this evening through tonight. Front positions are analyzed as well. Image courtesy of Weather Bell.

Figure 2. 300 mb jet stream location for this evening through tonight. Note its relation to the polar front. Source: http://models.weatherbell.com

Figure 2. 300 mb jet stream location for this evening through tonight. Note its relation to the polar front. Source: http://models.weatherbell.com

These systems, with the amount of rain forecasted, obviously carry with them huge amounts of moisture. The satellite image below in Figure 3 shows exactly that. We can see the large area of cloud which indicates the massive tropical moisture plume pushing northeastward along the polar front within the main core of the 300 mb jet stream.

Figure 3. Visible satellite image from earlier today showing the large plume of tropical moisture pushing northeastward. Source: http://hoot.metr.ou.edu/satellite/GOES-W/PacC/VIS

Figure 3. Visible satellite image from earlier today showing the large plume of tropical moisture pushing northeastward. Source: http://hoot.metr.ou.edu/satellite/GOES-W/PacC/VIS

NEXT PAGE: WHAT ARE ATMOSPHERIC RIVERS?

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