Finally! Near Normal Highs in Western Washington, While Eastern WA Remains in the Freezer

In football, there is some discussion of a two-level defense.   Well, western Washington has a two-level defense against cold air, and this protection was fully in place this morning.

Here are the minimum temperatures during the past 12 hours (click to expand).   Upper 20s and 30s in western WA.  Single digits in eastern WA, and -30 to -40 F in central and eastern Montana.  You read that right...many locations east of the Rockies were below minus 30F this morning.

Perhaps a better way to see this (and certainly more colorful) is a plot of the temperatures forecast at 4 AM around 2500 ft above sea level this morning by the UW WRF model.  Purple and white colors in Montana (-27 C and cold), light purple and dark blue over the Columbia Basin (-9C and colder), while light blue (-1 to -4C) over western WA.    As Mark Twain said:  Cold! If the thermometer had been an inch longer we'd all have frozen to death.

So why are we so favored with more comfortable temperatures compared to our frozen brethren in Montana?  It all has to do with mountains and pressure.

The latest National Weather Service surface analysis shows high pressure dominating Alberta and upper Plains States (the lines are isobars, lines of constant sea level pressure).  This high pressure is associated with cold air in the lower atmosphere.   Such cold air originates in the Canadian arctic and them pushes southward in the interior of the continent.  With high pressure over the interior, the flow is generally offshore (easterly) over the Northwest.

Fortunately for us in western WA, we have double protection from the cold air in the interior:  the Rockies and the Cascades (see map below).  The high Rockies block the coldest/densest low-level air.  Any air that makes it across is in fact warmer, and its temperature increases due to compression as it slides down the western slopes of the Rockies into eastern WA.  Thus eastern WA is not as cold as eastern Montana.

The coldest air is blocked by the Cascades (except for the narrow Columbia River Gorge) and when it sinks down the western slopes of the Cascades it warms by compression again. So the mountains are blocking the coldest air and warming air by compression

Looking at the last four weeks at Seattle's Boeing Field, we see that the last three days have gotten close to the normal highs (around 50F, shown by the purple line), although the minimum temperatures were still on the cold side.

But let's check out Pasco in central WA (see below). OMG!  The high temperatures aren't even reaching the normal LOWS.    We are talking about temperatures 20F or more BELOW normal.   Extraordinary cold.   Mark Twain would have a field day.  Something about dogs and hair I believe.

A map of the differences of daily average temperature from normal for the 7 days ending March 1 over our state is stunning.  Eastern WA is not just cold, but it is crazy cold, with large areas being more than 20F colder than normal.

You don't even want to know about Montana, where temperatures have been more reminiscent of Mars than Earth.    Strangely enough, the origin of this cold weather may originate in an area of warm water in the tropics near the International Dateline....but discussion of that will have to wait for another blog.