May Inundation

(see update at bottom)

It is POURING outside right now.  Streets are starting to flood. This weekend is going to be an unusually wet one for May.

First, here is the 12-h precipitation ending about 9:45 PM Saturday from Seattle RainWatch.  Some parts of southern Seattle have already gotten 1 to 1.5 inches. Even heavier amounts on the Kitsap.

The wide view shows the large difference between heavy rain in the south and central Sound and the light precipitation in the rain shadow NE of the Olympics. (this figure is based on the Camano Island radar so you can't just the precipitation over the Olympics or over the coast).  This pattern is consistent with southwesterly flow aloft.

Seattle RainWatch not only provides real-time information on rainfall amounts, but it sends out warnings to Seattle personnel and its developers (like Jeff Baars and myself).  Here is what my email looks like tonight:

Take a look at the high-resolution WRF weekend total precipitation forecast (5 AM Saturday through 5 AM Tuesday).  2-5 inches over the mountains and 1-2 inches over the lowlands.  Even eastern Washington gets some real rain.

Guess I won't have to water my vegetable garden tomorrow.

Update: here is the 24h precip total from RainWatch ending 7:30 AM Sunday.  3-4 inches near the Hood Canal  and over 2 inches in parts of SW Seattle.  But only light rain over the NE Olympic Peninsula.

The 24 h precip map from observation sites (available on the Seattle NWS website) shows the serious precipitation SW of the Olympics, with a profound rain shadow over the Strait and San Juan Islands.  .04 inches at Port Angeles and .02 inches on San Juan Island, while Hood Canal had 3.65 inches.  It is nice to live in a place where you can pick your weather.

Why is the coast sometimes the warmest place in the Northwest?

It happened again on Wednesday: the warmest location in the Northwest was along the coast (see max temp map for that day below) and not inland.  At North Bend, Oregon it got to 91F!  The Washington coast was warmer than eastern Washington!

We are used to thinking about the coast being a cool place.   Pacific water temperatures (around 50F) are generally much cooler than inland temperatures virtually the entire summer and if the air is coming off the water, cool temperatures and low clouds are the rule.

But the strange thing is that during the run up to many of our heat waves, the coast is often the warmest place for several days.  But why?

It all has to do with offshore flow and sinking down mountain slopes!

With cold water offshore, the only way for the coast to warm up is to isolate it from the cold water:  that means offshore, EASTERLY flow.   And that happened on Wednesday.

Here are the surface observations and 9-h model forecast for the Oregon coast at 2 PM on Wednesday.  Yep...we have offshore flow along the coast!

This offshore flow is associated with high pressure inland and an area of low pressure (the thermal trough), moving northward from California. (see map showing sea level pressure, surface winds, and lower atmosphere temperature below).

Higher pressure inland and lower pressure along the coast helps produce easterly flow at low levels.

But there is something else:  flow down the terrain. As air sinks down terrain, it is compressed (air pressure is greater lower down) and warmed (air warms when compressed as in your bicycle pump).  As shown in the figure below, under easterly flow there are three chances for air to sink:  on the Rockies, on the Cascades, and finally on the coastal mountains.   The air progressively warms as we move past one barrier after another to the west.  The result is that the warmest air in this situation will be west of the final barrier:  the coastal mountains.

So you want to imagine being on the coast of Maui or the Cayman Islands, slathering on sunblock while you drink something tropical in the warm sun?   Just pick an offshore flow day along the Northwest coast in spring or summer.  Close your eyes and imagine.

But one thing:  don't go into water.  If you do, the tropical illusion will be replaced by hypothermic shock.