What is spit flow, what does it do to West Coast weather, and why do we think our weather will have split flow personality for the rest of the winter?
All of this will be revealed in this blog.
So what does a split flow look like? Here is the forecast 500 hPa weather map for 1 PM Saturday (at roughly 18,000 ft, black lines are height lines, winds shown by the barbs). Winds are parallel to the lines and strongest when the lines are closest together.
There is a trough of low pressure out in the Pacific with a single current of strong winds north of Hawaii. But east of the trough the winds spit into two currents, the strongest going into Alaska and the weaker into California. Winds over the Northwest are light. This is split flow.
Weather systems tend to follow the stronger winds and they get torn up by the split. The result is that most of the real weather is going north and south of the Northwest: into Alaska and California.
Here is an example from last Sunday, January 3 at around 2 AM. Take a look at the infrared satellite image below. You can see a weather system moving into California and a large system out over the Pacific, south of the Aleutians. Clouds over SE Alaska. Nothing over us.
And guess what the upper atmosphere (500 hPa) flow pattern shows? You guessed it...a split flow!
You better get used to split lows. Strong El Nino's tend to favor them after January 1 and the flow pattern the last few weeks has been like this very often.
How do we know this? Because if we average (or composite) the upper level flows for other strong El Nino years, that is the pattern we see. Here is the deviation of the strong El Nino average from normal (called the anomaly) for 500 hPa. The purple shows a large negative anomaly (must deeper than normal low pressure offshore). Such a trough offshore results in the split flow pattern.
The latest 10-day precipitation forecast from the NWS GFS model suggests a continuation of this persistent El Nino split flow, with lots of precipitation in California. (see below). Lucky devils.