March 2, 2021

Weather News – Road Conditions – weather forecast

Heavy Precipitation and the Mid-Winter Water Update for Our Region

3 min read

Note: My next blog will be on the sudden stratospheric warming over the Arctic and the implications for the long-term weather forecast. Major changes in the large scale weather circulation in the offering!

As promised, a potent atmospheric river has hit our region, with very heavy precipitation on the upwind (windward) sides of regional terrain.

Let be begin by showing you a satellite-based view of total atmospheric moisture from late yesterday.  Purple and red are the highest amounts.   Satellites can sense water vapor content based on the emission of radiation by water vapor.  

You can see the plume of moisture heading into us from the southwest.  Amazingly,  our moisture plume can be traced back thousands of miles to the western Pacific near the Philippines and Indonesia: the region of the warmest water on the planet, known as the warm pool. 

The map below shows the precipitation for the past 24 h (ending 8 AM).  Some locations on the south and southwest side of the Olympics have gotten 5-6 inches so far, as have some rain gauges in the mountains of SW Washington.  Around Seattle about 1.5 inches have fallen, while only a few tenths of an inch have wet the ground over northern Whidbey Island in the rain shadow.  Seattle already has had 5.12 inches this month and by the end of the day we will reach the typical monthly total (5.57 inches).  A whole month’s rain in less than half a month!

Weather radars can be used as remotely sensing rain gauges by making some assumptions.  Here is the “storm total” for the past two days from the Camano Island radar.  You can see the profound rainshadow to the northeast of the Olympics, with about a tenth of an inch at its core.   But 3-5 inches not far away west of the Hood Canal.

Several of you have emailed me, asking about the current level of our reservoirs.  As of yesterday, Seattle’s water supply was in good shape (see below), with above-normal cumulative precipitation, above normal snowpack, and much above normal water storage.   Today will provide a substantial water addition.

But what about the drought-sensitive Yakima River Reservoir System, so critical for agriculture in the east? (see below).  Currently above normal and WAY above last year.

And Washington State’s snowpack is in good shape, as shown below and noted in previous blogs, and Oregon is just slightly below normal.

The bottom line:  water resources and snowpack are in good shape for our region at this point, something not surprising for a La Nina Year.

Many of you are asking about cold and snow in the lowlands.   I will talk about this in future blogs, but the latest forecasts are projecting a turn to radically cooler conditions by the end of the month.  As a teaser, here is the predicted 7-day snowfall ending January 29th.  Yes, lowland snow.  But don’t get too excited yet…. forecast skill is marginal that far out.   Seattle and other municipalities should make sure the plows and salt supplies are in good order.

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