Cry me a river

Last night’s epic rain storm that lashed the Savannah Georgia area dropped enough water on the city to make six billion cups of coffee!

How’s that for a headline? OK, I’m not trying to minimize the impact of the storm system hitting the US West Coast right now, people are getting hurt by this thing. But I wish we still had actual journalists who could just report facts and not dramatize things. Headlines like “Cali pounded by eight trillion gallons of water!” and “Up to 37 million people at risk for life threatening floods” aren’t helpful. Journalism isn’t’ supposed to be an exercise in creative writing or manipulating the reader, it is supposed to be about conveying information to create an informed citizenry. </rant>

OK, so what’s the no-drama version? Depending on the patterns of circulation around the earth, from time to time long narrow regions of consistent flow can develop. Over water, they pick up moisture as they move along and can collect large amounts of water. When they hit land, especially land with mountains, the moisture is forced out of the air in the form of rain and snow. That, in brief, is what an “atmospheric river” is – a band of very moist air transporting water through the atmosphere. NOAA has a nice page on the subject if you would like to learn more, and for teachers here’s another NOAA science page geared to grades 6-12.

These atmospheric rivers often develop over the Pacific and head towards the US Southwest – in fact, they are a critical factor in the hydrology of the southwest, reinforcing the snowpack and providing a large part of the rain and water storage the region receives. Most of the time that’s normal and expected. But every now and then, especially during El Niño years when the atmosphere over the Pacific is wetter than usual, these atmospheric rivers can collect a lot of water. A normal atmospheric river can transport about the same amount of water as the Mississippi River; an El Niño “Pineapple Express” – a river starting near Hawai’i and dumping on California – can transport 10 to 20 times as much! That is what is happening now … here’s an image from yesterday evening (it’s still dark over the Pacific as I write this on Monday morning) …

Once the “river” hits the Sierra Nevada mountains there is significant orographic enhancement (a fancy term that means the air gets pushed up and the water wrung out in the form of rain and, at higher elevations, snow). Here is the GFS model excess rain estimate for today – you can see the “river” running into the mountains and dumping a bunch of rain/snow:

So that’s the basics. In terms of impacts this will likely have several billion dollars of damage and disruption. Unfortunately there will likely be deaths from this, mostly from people doing things that they probably shouldn’t like driving through standing water (much less moving water – small streams can be a lot more powerful than you think). Turn around, don’t drown (link to Cal OES article).