Major Cool Down

For those of you unhappy with the unseasonal, but wonderfully warm and dry, weather of late,  a smile will soon be on your face.  An extended period of cool, cloudy, and sometimes rainy weather is in store.

The key transition is a profound change in the upper level flow pattern.   For the past week, the circulation aloft  has been dominated by a ridge of high pressure over the western U.S., something documented in the upper level (500 hPa) height anomaly map (difference from normal) shown below.  A large positive anomaly (yellow/orange colors) is evient over the western states.  The means high pressure (or heights)

But the situation is rapidly changing right now.   Let me show you the 500 hPa height anomalies predicted by the U.S. GEFS ensemble system for Friday afternoon (5 PM) and for the following Thursday:  substantial negative anomalies over the western U.S. coastal zone, which means troughing or low pressure.

Reflecting this upper level pattern, the latest NOAA Climate Prediction Center 6-10 day forecast is for cooler than normal and wetter than normal conditions over the Northwest (see below).  

As I will describe in my next blog,  the global circulation pattern has experienced a profound shift during the last few weeks.   El Nino has collapsed and high pressure over western North American has shifted.  None of our global models indicate a return of ridging (high pressure) over our region during the next several weeks.

Tonight (Tuesday), an upper level trough is approaching the West Coast, with the ridge retreating to the east (see graphic)

An approaching trough produces upward motion, which helps force thunderstorms, and many were observed over eastern Oregon during the afternoon and early evening (see radar for 5 PM).  This trough will also force more marine air inland.

Tomorrow will be a transition day and after that we will be in the upper 60s for at least a week. This cool/wet period will reduce wildfire potential east of the Cascade crest and reduce evaporative loss for agriculture.