REX Will Dominate Northwest Weather This Week

During the past two weeks, the Northwest has enjoyed absolutely normal weather.   Typical temperatures (see plot below, purple and cyan are normal highs and lows) and nearly normal precipitation (see cumulative precipitation for the same period).

 Normal has been good for us, giving relief to our plants, restoring water to the surface soil layers, and radically reducing the water usage in urban areas such as Seattle (see below).

The occasional precipitation has been associated with transient upper-level troughs that have moved through the region, such as one moving across the Northwest this morning (see upper level, 500-hPa, ap for 5 AM this AM)

But this week something interesting is going to happen:  a very stable REX BLOCK will develop over the eastern Pacific, shutting off precipitation, leaving us with lots of sun, dry conditions, and moderate temperatures. 

It will be wonderful.

So what is a Rex Block?   It is a configuration of the atmosphere where the upper level circulation has a ridge of high pressure (or heights) north of a trough of lower pressure/heights.  Here is a schematic of such a situation.

A Rex Block is a very stable atmospheric configuration, with the ridge and trough reinforcing each other, keeping the flow pattern in place  (for reasons I talk about in my graduate synoptic/dynamics class, but won't go into here).

The ridge/high part produces descending motion and fine weather, particularly on its eastern side.  Storms are sent far northward--in the example shown, well into Alaska where they belong.

The Rex Block has nothing to do with kings or dinosaurs, but with a meteorologist named D. F. Rex, who wrote a seminal paper on this features way back in 1950 (see below).

Rex, D. F. (1950). "Blocking Action in the Middle Troposphere and its Effect upon Regional Climate". Tellus. 2 (4): 275–301.

Now that you are Rex-trained, lets look at the upper-level forecasts for this week.

11 PM on Sunday?  Rex is here!  Huge ridge of high pressure/heights over the northeast Pacific, with a low underneath.

 2 PM on Tuesday....classic Rex!

 5 AM on Thursday.   A Rex fiesta.

The REX block shifts westward on Friday (see below), allowing a trough to move southward over us late Friday night.   Some showers and cooling if it happens.

Will the REX block reestablish itself?   Stay tuned.

from Cliff Mass Weather and Climate Blog https://ift.tt/2znu8hL