Why Do Strong Southerly Winds and Western Washington Rainshadows Come Together?

There were two singular aspects of the weather on Saturday.  First, there were extreme variations in rainfall across the region, with an amazing rain shadow over Puget Sound.

As shown below, for the 24 hours ending 10 PM yesterday, there was nearly 5 inches of rain over the western slopes of the Olympics, but only 0.01 inches near north Kitsap County in the Olympic rain shadow.

You read that right, a factor of FIVE HUNDRED in rainfall.

If you want to be further impressed, consider southeastern Vancouver Island, where a peak rainfall of 11.17 inches was observed over the coastal side while only 0.57 inches fell in the lee.   

And then there were the winds.

Strong, gusty winds from the south and southeast were found across western Washington, with Paine Field winds gusting to 62 mph, with 50-55 mph observed at several sites (below, max gust yesterday).

As shown by the wind observations at Paine Field in Everett, the winds really revved up between 2 and 5 AM on Saturday and then stayed strong for hours.

Paine Field sustained winds (cyan) and gusts (purple)

And now the interesting part.  The rain shadow and the strong winds were directly related.  

The flow that produces a strong rain shadow also results in powerful southerly winds.

Let me explain.

The heavy precipitation on the western side of the local terrain and the rain shadow to the east of the mountains are associated with strong moist westerly flow (winds from the west) reaching our region.   

To show this, the winds around 5000 ft (850 hPa pressure) yesterday afternoon are found below.  The lines are like pressure (called heights), and the winds aloft tend to be parallel to the height lines.  The closer the lines are together, the stronger the winds.  Lower heights are to the north and higher heights to the south, which is associated with higher pressure to the south and lower pressure to the north.  Remember this.

Such strong incoming flow rises on the western side of local causing precipitation enhancement and then sinks on the eastern slopes producing a rain shadow (as shown in the schematic below).

The windward enhancement was obvious in the weather radar yesterday afternoon (see below)

The accumulated precipitation during the event simulated by the UW WRF model, is below.  We have an amazing ability to simulate these precipitation contrasts.

But this wind field is also associated with strong winds from the south over Puget Sound and the Georgia Strait.

As noted above, to get strong winds from the west or southwest approaching the mountains aloft, there needs to be lower pressure to the north and higher pressure to the south.  In meteorological classes, we explain this fact, which is associated with geostrophic wind balance.   

Now, let me show you the predicted sea level pressure pattern yesterday afternoon.  

You will note low pressure over British Columbia and higher pressure over Oregon.  There is a large north-south pressure difference over western Washington State  At low levels in the presence of terrain, air moves directly from high to low pressure, which explains the strong southerly winds.   Offshore, away from terrain, the low-level winds are more parallel to the sea-level pressure lines, which are called isobars.

One final thing, why were winds particularly strong over the north Puget Sound area, such as Paine Field?

Because the pressure differences (a.k.a. pressure gradients) are particularly strong there, something shown by a high-resolution simulation for nearly the same time (see below).    

Why is the pressure difference large there?   Because of the tendency to produce a low-pressure area in the lee (downstream) of the Olympics.   Analogous to an eddy in the lee of big rock in a stream.

Strong southerly winds also bring up warm air.....and yesterday's high hit 65F at SeaTac, 6 degrees above normal.

Today should be warm and dry for most of western Washington until late afternoon when a moist Pacific front will cross the region.