Minimum Temperature Heat Wave

September has really been remarkable over western Washington in one way:  the consistently high minimum temperatures.   I have mentioned this before, but it just doesn't end.  Here are the September temperatures at Sea-Tac Airport (yellow line) and the average max (red) and min (blue) based on climatology.  For every day but one, the minimum temperatures were above normal.  Many days in the last two weeks had minimum temps of 2-5F above normal.

Now this pattern is not limited to western Washington...take a look at a map of the average difference between climatology and the observed minimum temperature over the west during the past month.   Nearly the entire region had very high minima, with extensive regions being 2-6 F above normal.

The effects of these high minima have been clear in my vegetable garden.   Of great importance to me, my tomato plants are healthy and still producing.

So why have the minimum temps been so high?

First, we have had a very persistent area of low pressure offshore, with enhanced southerly flow on the western side of the low (see the map of the difference of sea level pressure from normal).   Remember that air goes counterclockwise around lows in the northern hemisphere.

In addition, the water over the eastern Pacific has been much warmer than normal-- the warm blob we have discussed a lot (see graphic).  In this map (SST differences from climatology over the past week), we see warm water (1.5-4C above normal) off the West Coast.   So southerly flow moving over warm water keeps us toasty at night.

We had a few rainy days, but that is over now and the daytime temperatures will surge again, but with a different pattern:  a major ridge of high pressure will form over the eastern Pacific.  Here is the forecast for Sunday at 5 AM PDT from the European Center.  Big ridge.  But it will get wet along the BC coast and there is a chance of some light shower on Saturday before the ridge amplifies on Sunday.   Want to be sure of the weekend weather...go south of Olympia.

And the 6-10 day forecast from Climate Prediction Center? Above normal over the western U.S. and colder than normal over the east.  This is looking a lot like the persistent pattern last winter.  Getting scary.