WeatherWatcher: Atmospheric Rivers -- we have one right now

 

UW WRF-GFS 4km 72 hour rainfall forecast ending at Saturday 4pm

An atmospheric river has arrived, one in a series of many that have been coming in off the Pacific ocean and mainly impacting California and Southern Oregon. There's a lot of hype implied with this type of weather system, but they are in fact very common and very normal in West Coast winters.

In the 1990's the term "Pineapple Express" was typically used to describe an atmospheric river. They are basically a plume of subtropical or tropical moisture that is pulled in by a low pressure or trough in the northeast Pacific and directed into the west coast by the jet stream. 

Probably at least half of our rain events through the fall, winter, and spring seasons are the result of an atmospheric river to some degree. Some atmospheric rivers bring a half inch of rain, others can bring 2-3 inches of rain during a 2-5-day single event. Some have lasted over 7 days, bringing continuous moderate rainfall to the area at the rate of a half inch to one inch per day.

The mountains often get much heavier rain in most rain events. Atmospheric rivers can cause river flooding in the lowlands due to the typically higher snow levels often associated with them. The big snowstorm we had December 26-30, 1996, ended in a long-lived atmospheric river that started as snowfall. 

Several snowstorms that week brought up to about 20 inches of accumulated snow on the ground. I recorded a low temperature of 10°F one morning that week with an analogue thermometer under the trees in the front yard. Then the atmospheric river hit bringing another 4 inches of snow. We had a total of 24 inches of accumulated snow, before that event changed everything over to rain. 

Then it warmed up to the mid 50's with several inches of rain falling over the course of the next several days. All the snow was melted within a 24-hour period. Weight of the rain-soaked snow caused a lot of roof collapses in the area.

Forecast:

For our current atmospheric river, I consider it a moderate one. It's expected to last about 3 days. During the 72-hour period ending at 4pm Saturday we are expected to get around 1.5 to 1.75 inches of rain. This is enough to get standing water in places and maybe some minor urban flooding where storm drains are blocked with debris.

Starting Wednesday evening, as of this writing, rain is expected to pick up if it hasn't already. Rain is expected to be steady and moderate through Sunday afternoon. Temperatures will range in the mid 50's for highs and lower to mid 40's for lows.

We get a bit of a break from the rain Sunday evening through Tuesday with only a chance of rain showers during that period. Temperatures cool down a bit with highs ranging in the mid to upper 40's and lows near 40°F. Tuesday night more rain is expected to return and last through Wednesday.

Longer range shows a drier pattern setting up for the last week of January. This could be a good time to get some yard work clean up done. Current model ideas show potential for colder air first week of February. That's too far off to say what might happen but it's a pattern that suggests potential for more wintery weather.

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