How atmospheric rivers are helping forecasters predict the upcoming storm

Tuesday, February 11, 2014, 5:03 PM -

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.

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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

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

NEXT PAGE: WHAT ARE ATMOSPHERIC RIVERS?

For the most part, forecasters do a relatively good job in predicting these events given some of the forecast products shown above. Relatively new research shows some new techniques and concepts that help explain and show where exactly these higher impact events occur. Atmospheric rivers are a relatively newly-discovered feature that is said to be responsible for large amounts of moisture transport within these large systems (Dettinger et al. 2011). Zhu and Newell, 1998 describe these atmospheric rivers as long narrow bands of enhanced water vapour flux (about 2000+ km long and up to about 1000 km wide or less) which is carried with the main moisture plume as shown in Figure 3 above. They also indicate that these atmospheric rivers develop within the warm sector of extratropical cyclones ahead of the polar front. According to Dettinger et al., these relatively narrow atmospheric rivers are confined to within the lower 2.5 km of the atmosphere. Zhu and Newell, 1998 suggest that the standard physical model of atmospheric moisture transport, which tends to ignore the dynamic and energetic effects of moisture, needs to take into account atmospheric rivers which are said to be responsible for nearly all moisture transport within mid-latitudes.

Neiman et al., 2008 used the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) mission to obtain atmospheric profiles showing temperature and moisture over the data sparse Pacific. The data was used to successfully analyze and show the atmospheric river that was responsible for the flooding in the U.S. northwest in early November of 2006.

The Earth System Research Laboratory is now producing water vapour imagery to help meteorologists track atmospheric rivers and their impacts on coastal rainfall events, cyclone development, and flooding potential. Figure 4 below shows the current integrated water vapour from this morning. Currently we can see an atmospheric river associated with the low to hit British Columbia this afternoon and evening.

Figure 4. Water vapour imagery in units of g/cm^2. Shows location of atmospheric river with low forecast to push into BC this afternoon and evening. Source: http://www.esrl.noaa.gov/psd/psd2/coastal/satres/data/html/wx_cl.html

Figure 5 shows the GFS projection of where this feature will be by 5:00 p.m. MST and then by 4:00 a.m. MST on February 11 (Wednesday). On Tuesday evening, the main plume will affect the western side of Vancouver Island and the northern tip of Washington. By morning as the arctic front dips south into Washington the arctic river will become more of an issue for Washington, Oregon, and northern California. This doesn’t mean British Columbia won’t be getting significant rain but there could be some more significant numbers for the northwestern U.S. as this plume remains in place.

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Figure 5. GFS forecast of where the atmospheric river will be located this evening around 4:00 pm MST (the left image) and tomorrow morning at 4:00 am MST (the right image). Source: http://www.esrl.noaa.gov/psd/psd2/coastal/satres/data/html/ar_detect_gfs.php

What does this mean then exactly? These newer techniques could certainly help improve forecasts in terms of more extreme events. Foremast models do a good job overall with these high impacts events. Since the Pacific has very little observation network tracking atmospheric river could help narrow down higher impact regions leading up to the events. Since this main plume will not linger very long in BC I suspect current model projections will be doing a pretty good job in resolving rainfall amounts. The presence of these plumes or atmospheric rivers could certainly enhance a systems development which could lead to under forecasted rainfall amounts at times.

References

Zhu, Y., and R. E. Newell, 1998: A proposed algorithm for moisture fluxes from atmospheric rivers.

Neiman, P. J., F.M. Ralph, G.A. Wick, Y.-H. Kuo, T.-K. Wee, Z. Ma, G.H. Taylor, and M.D. Dettinger, 2008b: Diagnosis of an intense atmospheric river impacting the Pacific Northwest: Storm summary and offshore vertical structure observed with COSMIC satellite retrievals. 

Dettinger, M.D., Ralph, F.M., Das, T., Neiman, P.J., and Cayan, D., 2011: Atmospheric rivers, floods, and the water resources of California. Water, 3 (Special Issue on Managing Water Resources and Development in a Changing Climate), 455-478.