Warm West and Cold East

 It has certainly been warmer than normal over the Northwest during late summer, but what you might not know is that the eastern U.S. has been far cooler than normal.

Consider the plot of the temperature anomaly (difference from normal) for the past two weeks over the entire U.S. from the NOAA/NWS Climate Prediction Center.  WAY colder than normal from the Rockies to the East Coast.

Here is a high-resolution view of the temperature anomaly at 5 PM PDT on Sunday. 

Amazing.  Some portions of the eastern U.S. were 10-20°F below normal!  Very warm over SW Canada.

Why this anomaly pattern?   A persistent, perturbed upper-level  (500 hPa pressure, about 18,000 ft)  pattern with a ridge (higher pressure) centered over SW Canada and a trough (lower pressure) over the eastern US. (see below, and anomaly from normal for the last week below).

But all good things have to end.

The large-scale pattern will be changing over the weekend, with rain and cooling temperatures over the west and moderation of the cool temperatures over the east.

Tonight, we see the previous pattern with a ridge in the west and a trough in the east.

Same story on Thursday afternoon. 

But BIG CHANGES by next Wednesday with a massive trough of low pressure over the western U.S..  It will feel like autumn.  And that pesky, West Coast ridge moved into the central part of the U.S.

Going out even farther (and, of course, with less skill) for Sunday, 14 Sept, the forecast model shows a broad trough (blue colors) over the NW Pacific and the ridge moving into NE Canada.

Consider the latest precipitation forecast total through the same time (next Sunday).  Several inches in the mountains and light rain in the lowlands.  I suspect this will essentially end the wildfire season.

 

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