Saturday, September 8, 2012

The Rain Cometh

Today is the 48th day without rain in Seattle.  So close to  the record of 51 days.  But it has become increasingly probable that we won't reach the finish line this time.  The reason:  a strong Pacific front that should bring rain to western Washington within the next 48h.

Being dry midsummer is not unusual in the Northwest, with the period mid July to mid August  drier here than almost anywhere else in the country.  During most years, the first rain-bearing Pacific front reaches us around the third week of August, perhaps a week later during a dry year.  But not this year, as high pressure over the eastern Pacific has shunted systems to our north.

As shown in the figure below, for four weeks our high temperatures have been normal or well above normal, particularly the last few days:

But on Sunday things are going to change and a relatively strong Pacific front will reach us, bringing clouds, increasing wind, and yes, rain.   And as I will notenlater,  this rain may bring dangerous road conditions as the water mixes with oil and dust on the roadways, creating a slippery emulsion.

Yesterday was extraordinary, with 80s everywhere and Seattle reaching 90F under strong offshore flow.  Blue, blue skies sullied by Asian dust or local wildfires.

But today, things are starting to change.  The offshore pressure difference (higher pressure inland, lower pressure offshore) has switch to onshore and the coast is in clouds.  And as shown by the upper level forecast for 5 PM, our ridge moved inland a bit, a weak upper trough is approaching from the southwest, and a strong frontal trough in now approaching from the NW (see image).

Today will still be a warm day for the Northwest, at least of the coast, but tonight there should be an onshore push of marine air, with tomorrow bringing far cooler temperatures and clouds to those of us west of the Cascade crest.  Go up or east on Sunday if you want sun.

But the big story is Sunday night, as the front approaches us, and Monday AM as the front crosses the region (see predicted surface chart for 5 PM on Sunday, solid lines are sea level pressure--you can see the frontal trough offshore ).  This well defined front will bring the first real rain of the


fall season if the models are correct.   Here is the 24-h precipitation forecasts valid 5 AM Monday and Tuesday.  Since the front is coming in from the NW there will be some lee rainshadowing east of the Olympics and mountains of Vancouver Island...so an outside change Sea-Tac could miss the precipitation.

The next 24h shows shows in the Cascades and a rather prominent convergence zone precipitation band...and chance of rain at Sea Tac. 

And be careful driving after the rain starts...the first fall rains can bring very slippery conditions as the water mixes with road oil and dust.  Since we have had a particularly extended dry spell, perhaps the conditions will be worse than normal.

Finally, some of you may have forgotten what rain is and how to deal with it.  How do you stop rain falling on your head?  What to do when your car windshield is covered with it?  Don't be concerned!   Scott Sistek of KOMO TV has prepared a wonderful training blog that describes the essential steps for dealing with this unusual phenomenon of water falling from the sky.  Check it out here.
 

9 comments:

Don said...

So close....

Unknown said...

Cliff,

I wonder about the effect of such a summer dry spell on the snowpack. Is it that, perhaps counterintuitively, it helps preserve the snowpack as the absence of rain in higher elevations slows the melt?

Thansk for the stream of blogs,

Pierre

Scrapycandy said...

Thanks for the warning heads up...I think I will drive my teen and let him have the car later in the week.

Hindu said...

And with the first of wet front of late summer, probably means no rain here in Tri-Cities, but big winds and a good dust storm. With the dry fields in the Horse Heaven it doesn't take much to get it up into the air and moving.

Kyle said...

Just rained at Husky Stadium. Big heavy drops for about a minute. 7:30pm Saturday.

rainycity1 said...

Just had our first sprinkle over here in W. Seattle... The Husband was very disgusted as I had told him he had until tomorrow evening to bring his tools in... ;-)

john said...

Before the rain comes, Cliff, I have a question about all the small black dust grains (way less than 1 mil in diameter) I see on outdoor surfaces, like leaves, here and elsewhere. I don't know if you've noticed this stuff in Seattle, but every year I see an accumulation of black granular material here in South Puget Sound at the end of a dry spell, like now. Obviously, it comes from the sky but from what? I remembered your blogs on fires and Asian smoke, I wondered, could this material be Siberian fire paticulates? Coal smoke from China? From local car exhaust? I suppose you'd have to analyze the grains and deposition pattern over a wider area to get a better handle on its source. But I just wanted your thoughts on this before the rain takes it away.

super hip kitty said...

Cliff -
Some friends(old timers) and I were reminiscing over previous beautiful dry winters and noted a correlation to have extremely good snows the ensuing winter. Any possible causation? For example, I remember my first summer here in 98 was spectacular - weeks and weeks of sunshine. That following winter I recall that Mt Baker Ski Area had to close the lifts because they had so much snow. I believe that was a/their record breaking year for snowfall?
Perhaps we should be looking forward to a similar snow season??!!

super hip kitty said...

Cliff -
Some friends(old timers) and I were reminiscing over previous beautiful dry winters and noted a correlation to have extremely good snows the ensuing winter. Any possible causation? For example, I remember my first summer here in 98 was spectacular - weeks and weeks of sunshine. That following winter I recall that Mt Baker Ski Area had to close the lifts because they had so much snow. I believe that was a/their record breaking year for snowfall?
Perhaps we should be looking forward to a similar snow season??!!