Previsão do tempo, weather forecast: Heavy Rain and the Beginning of Autumn

Wednesday, September 23, 2020

Heavy Rain and the Beginning of Autumn

Autumn began at 6:30 AM this morning and the atmosphere is clearly watching the calendar.

Tomorrow will bring the first of a series of wet weather systems, each associated with a potent atmospheric river, that will bring as much as TEN inches of precipitation to the northern Cascades and Olympics.

As a result, wildfire season in Washington State will be coming to an abrupt close.

Precipitation will be heaviest during two periods, one tomorrow and the other on Saturday, when atmospheric rivers--narrow currents of large water vapor values--will make landfall on our coast.

Here is the forecast atmospheric moisture content at 8 AM tomorrow (Wednesday), with the blue and white colors indicating the largest water vapor values. When the water vapor current rises on our local terrain it is forced to condense, resulting in bountiful precipitation.

Even more impressive is the atmospheric river on Saturday morning.....wow...this is serious! Fascinating how moisture is concentrated into such narrow "atmospheric rivers". I will explain why in a future blog.

Get out your umbrella and rain jacket--you will need it. To demonstrate that, I will show you the precipitation over the week in 24-h chunks.

For the 24h hours ending 5 AM Thursday, you can see the results of the first atmospheric river, with up to 5 inches in the Olympics and north Cascades. Even eastern Washington gets substantial rain...which is very good.

The next 24 (through Friday morning), still substantial amounts!

The next day continues the rain.

And the 24h ending Sunday at 5 AM is even wetter.

Over the past six months, Seattle has had nearly normal precipitation (see below, blue is normal, purple is observed)

While the last three months has been drier than normal, by about 2 inches. I suspect we will make up that deficit during the next week. The good news is that watering season is over, with substantial savings in expensive-water Seattle.

And there is someone else that will be very happy: