High Pressure Dominates the Region But A Radical Change Ahead

There will be no precipitation for the rest of the month over our region. None.

The reason?   A strong area of high pressure that has built in across the area, and one that will remain in place through the middle of next week.

Let's start with the upper level (500 hPa, about 18,000 ft) map for 5 AM Friday.  HUGE ridge over the U.S. West Coast. The problem with having high pressure over us during fall, is that it promotes low clouds and fog...like we had today.

The ridge will stay strong all day Friday and then weaken a bit on Saturday, as a weak disturbance passes to our north (illustrated by the upper level map at 11 AM Saturday).

The ridges strengthens, but shifts westward on Monday, shunting Pacific storms way to the north.  With the ridge to the NW of us, cooler, but still dry weather will be the rule.  Less fog.

On Tuesday, the ridge is still strong, but closer to us.

The bottom line is that we have a week of dry weather ahead, with initially warmer than normal temperatures (mid-60s Friday and Saturday), dropping into the upper 50s by early next week.  Perfect fall weather.

Have you noticed that the leaf colors have been particularly good this year? What I have read is that warm days and cool nights in late summer and early fall is good for fall color.   Taking a look at the temperatures at Sea-Tac for the past 12 weeks (below), we have had a number of warm days, with typically cool nights.  Plenty of sun, which supposedly is good for color.

 Enjoy the weekend...a beautiful one to get out.   There will be nice weather until Wednesday...but then the bottom drops out.   A strong upper-level trough will approach from the northwest (see map for 8 PM Thursday).  This has a classic structure for bringing cold, wet weather into the Northwest.  If this was a month later, I would be warning about lowland snow.

In fact, the surface chart (pressure in solid lines, colors are temperatures), shows very cold air (blue and purple) just to our north.

So put your umbrella and heavy jackets away for the the next 5-6 days, but get them ready.  By Friday you will need them, with HIGH temperatures only reaching into the mid-40s!