Tuesday, June 7, 2016

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.


John Marshall said...

Wonderful to see good weather returning.

Thankfully that hot, sunny stuff only hangs around long enough this year (so far!) to remind us why we love the cool, cloudy weather that many of us moved to western WA to enjoy.

I instinctively cringe and then laugh when the TV weather folks get all excited about hot, dry, sunny weather. If that's what I wanted, I would have continued to live in southern CO and souther CA.

It would be interesting to hear Cliff's thoughts on future climate modeling for western WA, especially regarding winter versus summer temperatures and moisture. Do we get wetter winters and drier, hotter summers as we saw last year? Or could we have more moisture in summer than historical due to more troughs flowing through?

Yearly averages only tell part of the story. The winter versus summer story is even more interesting to many of us.

andy gladish said...

It's interesting that the predictions for the rest of the summer are still for higher than normal temperatures.
I'd love to hear Cliff's thoughts on what's coming up for July and August- given that things have profoundly changed, what conditions contribute to the remarkable temporary ridging that we've seen this spring, and how will it change for summer?

Mark said...

During the summer months the general circulation is oceanic highs and continental lows. The oceanic high is presently centered at 160 longitude and 35 latitude. This is about normal. Clockwise circulation around the high drags cool air down to the PNW (June gloom) until the high grows larger typically around the 3rd week of June or by the first week of July. Then our skies will clear and temps will warm. It's good to see a reasonably normal pattern developing.

Melvis said...

Western WA no longer has summer
We go directly from fall to winter back to fall
Terrible for crops
We need dry warm & sun at least 4 months a year for food crops such a apples & berries
If you want consistent cool move to Canada!

tracksdc89 said...

Melvis, have you been around for the past two months, which have been exceptionally warm, sunny, and dry? And, I don't see how anyone could possibly make such a statement after the historic and persistent inordinately extreme heat and dryness of last summer.

Westen WA most certainly does get its heat and sun. And, why suggest moving to Canada? That country experiences either very hot and humid or very hot and dry summers, depending on the region.

Or are you just complaining for the sake of complaining?