The Warm Pacific Helps Keep the Northwest Toasty

If one looks at the recent temperatures at Northwest stations for the past month,  one is struck by the fact the minimum temperatures have persistently shifted towards warmer temperatures.  Let's look at  the proof:

 Here are the observed temperature at Quillayute, Olympia, and Sea-Tac Airport for the past 12 weeks.   The average daily highs and lows for the dates are shown by the red and blue lines, respectively.  Note how the low temperatures are consistently 5-8F above normal.

Why is this the case?   One reason is that we have had very warm water off our coast; the air reaching us has to transverse this warm H2O.   Here is the  sea surface temperature anomaly for the past 3 months. A lot of water around 3C (5.5F) about normal.

For the last week the water has been even warmer! (see below)

Our low level air passes over this warm water for thousands of kilometers and warms up;  thus, the low temperatures at night are very sensitive to the offshore water temperature.   During the day when the surface is heated and air is mixed in the lower atmosphere, the upper air temperatures are much more important.

To prove this to myself, I decided to have some modeling fun....to play god with the atmosphere! Dave Ovens, who runs the WRF modeling at the UW,  took one of our real-time forecasts and replaced the warm sea surface temperatures over the Pacific with the lower climatological sea surface temperatures.  Here is the difference in the sea surface temperatures between the two runs we tried, one with the actual warm warm and the other with the normal cooler water (see below)  The time for the start of the simulations is 4 PM April 24, 2015.

All we changed was the surface surface temperatures.    Here is the difference in surface air temperature for the 168h (7 day) forecast.  The temperatures over our region warmed up by 2-3C.

We did a few other runs, with similar results.

Another contributor to the warmth has been the ridge of high pressure over the West Coast and eastern Pacific--a ridge that has been crazy persistent.  And to complete the circle, most of the warm sea surfaces over the eastern Pacific resulted from the high pressure over the West Coast.