Friday, June 25, 2010

Low Clouds over the Pacific

Perhaps the most important characteristic of June weather over the west side of the Northwest is the persistent low-level clouds--the cause of the annual June gloom or Junuary, as some wits like to call it.

There is a reason for the low clouds: -the NE Pacific is full of them this time of the year. Look at the visible satellite picture above (from Friday at 12:30 PM PDT). Most of the Pacific is covered in clouds. In fact, more of the Pacific has cloud cover now than during the middle of the winter! Nearly all of these clouds are stratus and stratocumulus, associated with some drizzle at best.

Even coastal California gets hit by this stuff--in fact it is worse for them than for us. Never plan a summer vacation in Monterrey if you want lots of sun.

Why so many clouds? Ironically, it has to do with high pressure. During the spring and early summer the east Pacific anticyclone (high pressure area) strengthens and moves northward and the Aleutian low attenuates. Below is the sea level pressure for yesterday at 5 PM....you can get a feel for the situation. High pressure is associated with weaker winds and sinking air.

Sinking is strongest aloft and weakens towards the surface (the air can't move through the surface). Sinking causes warming, so there is more warming aloft...this produces a stable situation with little mixing (inversion or near inversion conditions). Air near the surface thus gets moister and moisture without the mixing down of dry air aloft. Another factor is warm air moving off Asia and Alaska....as the air cools down over the warm it eventually can cool to saturation.

The result of both mechanisms: lots of low marine clouds. With higher pressure offshore (associated with the high) and lower pressure inland, the low clouds are pushed into the western lowlands. Generally you can escape them by traveling east of the Cascades, since they are quite shallow.

High pressure and cooler air on the western side and lower pressure east of the Cascades produce a bounty for some of us--strong winds and lots of wind energy downstream of gaps in the Cascades, like downstream of the Columbia Gorge or in the Ellensburg region (I have a section in my book on this).

Anyway, knowing why we have the low clouds won't get rid of them, but perhaps understanding their cause and possible benefits will make them easier to tolerate.




6 comments:

patrick said...

I love how you explain the weather. As a sailor becoming more knowledge about the weather and forecasting is important to my safety and my enjoyment of the boat.

Still, can't you make it sunny with 15 knot winds alternating directions on Saturday and Sunday and then rotating each weekend so we can sail downwind to and from our destinations but still get to N one weekend and S the next. ;-)

Cliff Mass Weather Blog said...

I can for the right price...

Dan said...

Hey Cliff-

Can you explain why I couldn't find any trace of this morning's rain on the radar (I use National Weather Service and KIRO). I live over in Sandpoint and try to use your trick to time my bike commute with the radar, but I couldn't do it this morning (I got soaked!).

One of my colleagues at the University said he had the same problem this morning when trying to time his walk to the bus stop. What gives with the lousy radar signature of this morning's showers?!

Thanks.

Kevin Purcell said...

@Dan:

I actually reported this to Phil Regulski at the Rainwatch site as that site's decluttered images didn't show any rain either but on CapHill at 9AM Friday but we had blowing drizzle/light rain. And the rain gauge at Volunteer park showed a measurable rain rate of a tenth per hour (10 times more than a trace -- enough to get wet).

http://www.wunderground.com/weatherstation/WXDailyHistory.asp?ID=KWASEATT110&day=25&year=2010&month=6

He said he wasn't seeing rain gauge tips at that time (9am) in his network but did see them in the following hour (as the drizzle moved?).

He also pointed out (because I hadn't noticed) that KATX was in clear air mode. So with higher sensitivity you could see a large patch of drizzle (but below 30dBz but orange in clear air mode) on the wxdisc radar plots at the UW.

I suspect there was a fair amount of perhaps local drizzle between 9am and 10am around the city but it was so diffuse and relatively low rate it got decluttered out of the radar. For the Rainwatch site it wasn't a significant amount for them so it's not a problem for them.

One comment I had to Phil was it's interesting how the low apparently objective measurements deviated from the perception at ground level. If you stayed outside on CapHill for the half hour around 9am Friday you were going to get wet but the radar said it was fine and the rain gauges weren't tipping. So that method isn't perfect!

Oh, and to use the comment my Irish Mom used to use in England about fine rain making you wetter. I used to laugh but now I think perhaps the Mythbusters need to take a look :-)

JP said...

These low clouds are common. Lots of Seattle-ites are totally unaware of this. This is why you can leave Seattle in dreary weather and have a great hike in the Cascades. On Friday I tried to get some friends of mine to go but they all backed off based on the weather in Seattle saying it would be worse in the mountains. This is true in the winter but not this time of year. I went up Defiance Mountain off I-90 just west of Snoqualmie Pass. Once you get above 4000' feet it's beautiful sunshine and you can see clouds rolling through the valleys below you. I was able to see Rainier and Adams quite well. Bonus only one other car in the otherwise overcrowded parking lot!

cloudman said...

Isn't the low cloud pattern in southern California (aka the May June Gloom) also due to the temperature difference between the surface high pressure offshore and the thermal low developing inland causing a stronger temperature gradient?

Thanks,
Jeff
Venice, Ca