Monday, July 26, 2010

Subtleties


We are now in a stretch of magnificent summer weather, with day after day warming above normal (see graphic above). In fact, way above normal (roughly 10F!). This week we will continue the sun, but with some cooling and more clouds, so we will head to more normal territory.

This time of the year a key aspect of the forecast problem is deciding how much onshore flow we will have. A lot and we are cool and cloudy all day. Modest onshore flow and we get morning clouds with highs in the mid 70s. Little flow or offshore flow, 80s and 90s.

A key parameter that meteorologists watch is the onshore pressure gradient or difference. Two big favorites here in western WA: Hoquiam minus Seattle and North Bend,Oregon minus Seattle pressure differences. If you read the National Weather Service forecast discussion they talk about them all the time!

This afternoon and this evening the Hoquiam minus Seattle pressure difference has risen to roughly 2.4 millibars (or hectopascals). Such a difference typically brings a weak onshore flow. Get to 3.5 or 4 and we get a major push. This change is associated with the passage of a weak trough aloft. So expect tomorrow to be 5 degrees cooler or so and the clouds on the coast pushing at least into SW WA.

Talking about the clouds, take a look at tonight's visible satellite picture. Plenty of low clouds offshore and these will make some progress inland overnight. In fact, the latest fog imagery (satellite pictures enhanced to show fog at night) indicates the movement of the coastal stratus inland. Hoquiam is in fog and cool SW flow has hit Shelton, which is 10F cooler tonight than yesterday at the same time.


If you look in the southern portion of the image you will see debris from some thunderstorms over Oregon.

4 comments:

smokejumper said...

I love visable pics for marine pushes. I just looked at todays, and a lot of times as it surges north, a spiral forms off the NW tip of the peninsula. You could also see it drain into the straight. Interesting.

Lindsey said...

Here in downtown Seattle, you can see the stratus off to the west this morning, and the relatively strong breeze blowing off the Sound made me wish I had brought long sleeves. Definitely a change from when I left my home in Mountlake Terrace earlier.

Jay said...

Good job a usual Cliff! Love your book!

Jonathan said...

Nice post -- it's good to see the bigger picture of what's keeping the sky so gray. Why is the marine layer so aggressive this year?