The Most Valuable Precipitation of the Year

Precipitation at some times of the year is more valuable than at others.

Here in the Pacific Northwest, March and April precipitation is by far the most prized and valued.

Fortunately, the latest forecasts suggest we will enjoy bountiful amounts during this period, which should be a relief to those worried about the water supply.

Why is spring precipitation so important?

It can moisten the soil and help fill the rivers before our dry summers.

It can build up water behind our dams and in our reservoirs, providing water and power during the summer.

For years without good snowpack (e.g., 2024), it can provide needed water.   

It doesn't make much difference if we fill our reservoirs with melted snow or spring precipitation.

As you can see from the average cumulative precipitation at Olympia (below), quite a bit of the annual precipitation at this station occurs in March and April (between the red bars).  

Bottom line:  it is not too late to get substantial precipitation, even in a normal year.

Although there has been a lot of gnashing of teeth about this year's precipitation, the water year totals (from October 1 to now) are actually near normal, something shown by the cumulative precipitation at Seattle and Yakima below (red is normal and green is this year).

The issue is that the snowpack is about 50% of normal as of today.  However, reservoirs are in very good shape, with some well above normal (e.g., the Yakima reservoir system).

As I have noted before, the prediction models have been emphatic about our progression into a far wetter pattern.

To illustrate, below is the total precipitation forecast from the European model through Friday, March 20th, over the Northwest.   Quite a lot over the Cascades!

Much of that will be snow, as shown by the totals through March 19th

The latest extended forecast from the NOAA Climate Prediction Center is for wetter than normal conditions for the second half of the month.

With La Niña rapidly weakening, there is no reason to expect that we will get a persistent dry pattern, as in January.  

I suspect our summer water situation will be fine.

A "Dirty Ridge" is Coming to the Northwest

No, this is not some kind of meteorological pornography.

A "Dirty Ridge" is when moisture, often accompanied by clouds and precipitation, makes its way around the northern portion of a high-pressure area ( ridge is another name for high pressure).    

Wet conditions with high pressure, not some type of obscenity.

Before I start, some review. Low-level winds typically rotate clockwise around higher pressure areas (see below). Thus, winds are southerly on the western side of the high and westerly (from the west) on the northern side.

You can see this windfield around the forecast high at 7 PM Friday night (below, for the 850 hPa level, about 5000 ft above the surface).

Consider a forecast map for 7 PM on Friday of sea level pressure, surface wind (arrows), and the transport of water vapor by the wind (shading, called integrated water vapor transport).

Strong winds from the southwest and west on the western and northern sides of the high draw moist air northward from the subtropics around the western flank of the high and moves it eastward over the northern flank of the storm.   The clouds and rain are the "dirt" of the dirty ridge.

Fast forward to Tuesday morning (March 10), and the high is still offshore, and moisture continues to move around its western and northern flanks  (see below).

This circulating moisture will be forced upward by local mountains, resulting in precipitation (the 72-h precipitation ending 5PM Sunday is shown below).  

Considering the relatively low snowpack over the region (about half of normal right now), this precipitation will be welcome, even if it is associated with a dirty ridge