June 19, 2010

The Great Weather Divide

Today, some of the media are forecasting general showers over western Washington, but the reality will be very different between north and south. A tight upper level low is moving SE into Oregon (see graphic above) and a band of clouds and precipitation are wrapping around it (see image).

Try this animation of the visible satellite imagery:

The current radar picture shows that showers are limited to the south Sound area...so north of Seattle it might be completely dry. This kind of pattern brings precipitation into eastern Washington and of course the Cascades. In fact, with easterly flow approaching the Cascades, normally dry locations could get hit by heavy showers. The air will be somewhat unstable after the surface heats, so thunderstorms are possible over eastern WA and over the mountains. As the low moves SE the showers should follow.

If you dig down on the Seattle National Weather Service web site you can see their graphical forecasts (http://www.weather.gov/forecasts/wfo/sectors/sew.php)...here is their prediction today for 12-h precipitation ending 5 PM--the probability of rain varies from 83% in Olympia to 32% in Bellingham.

So advice for today....want dry condition...go north.


  1. Its gorgeous out in Whatcom right now, planning on hiking Heliotrope Ridge by Mt. Baker this afternoon so hopefully the rain clouds don't push too far north.

  2. Cliff, Here's a question that has perplexed me all Spring; Why when the flow goes West to East like it does most of the time, downslope off the Cascades dries the Eastslopes. However, whenever precip. moves East to West, like it is today, it seems there is no drying and the Cascades do very little in limiting the rainfall over Puget Sound? BTW, as a Little League Dad of 7 years, this has to be the worst season in recent memory!!! Do you see any positive change coming in the global pattern??? These upper lows are killing us!

  3. It rained HARD early this morning down here in Olympia. I looked at the closest station--Evergreen State College--and it recorded slightly over an inch of rain after midnight. It was really dumping at one point; one of those times you wake up in the middle of the night and go "Holy Cow!" when you hear it pounding on the roof. lol

  4. Spectacular towering cumulus clouds here east skopes. Lots of heavy rain showers with thunder around. Noticed there movement is to WNW, so suburbs east of Olympia and Tacoma might be in for a wet evening

  5. agree with livingston. awesome day up here in bellingham, about to go mountain biking

  6. its interesting on how the rain is moving from the east to the west down from olympis toward centralia and west.

  7. Wish I saw this early this morning. I got rain ( and hail)on the Blue Creek trail to Red Top over onear Blewett Pass. It was not as drenching as the downpour there two weekends ago.

  8. How does the start of this summer compare to the all-time misery of the summer of 1993?

  9. I don't know if Cliff has answered this yet, but here's my 2 cents:
    East to west (more like SE to NW) flow of precip usually indicates a low which drifts to the SE of Seattle near the central and eastern Gorge area or even S of there. This causes showers to rotate in from SE as the energy from the OR coast is spun in around the circulation of the upper low. Thus, it is not drier air as it originates off the coast, and it is being forced into a more moisture laden/unstable area. The opposite is true for the east slope with the normal west to east flow, where the mountains wring out any moisture, and the air then enters an already dry stable region east of the crest. When you see drying in the Puget Sound from an easterly or northeasterly flow, it occurs due to the presence of a high east of the region. As the drier continental air is forced into the Puget Sound (sinking air), it is compressed, thus becoming hotter and drier, resulting in above normal temps and clear skies.


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