The Rain Cometh

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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.