Friday, October 21, 2011

Jet Stream Weather

As many of you know, the jet stream is a current of strong winds---much narrower than its length--that is often found in the upper troposphere--typically between 25,000 and 35,000 ft--in the midlatitudes (generally 35-50N).  The jet stream (a.k.a, the jet) is associated with major weather systems-- the wind and rain storms that strike the region-- particularly in the winter.  Jet stream winds can reach over 200 mph, and can greatly accelerate or retard the motion of jet aircraft, depending whether they are heading east (fast trip) or west (long trip).   Midlatitude jet streams can meander north and south in a wavelike manner. 

The jet stream was first described by German meteorologists in the 1930s and this feature become very obvious to pilots on bombing missions during World War II.   The jet stream is generally strongest in the mid-Pacific and some missions directed at Japan found themselves making little headway to the west at times.

During a typical year the jet stream core is north of the Northwest in late summer and then slides down into our area in late October and November.   Think of the jet stream as an atmospheric hose and we are on the receiving end during November through January. When is the hose most predictably and intensely over us?  The last week of November.  The wettest and stormiest week of the year climatologically.  In December the jet actually heads south of us more frequently and the weather actually improves in our area...believe it or not.

Today and Saturday the jet stream will dip down close to us... let me show you.  The following charts present the wind speed (in meters per second, roughly double for knots or mph, by colors) at a pressure level of 300 mb (roughly 30,000 ft).  Here is a forecast for  today at 2 PM.   Strong jet over the Pacific that is approaching northern Washington, with values well over 100 kts.

Tomorrow the jet pushes in just to our north

This weekend we will be hit by some disturbances riding on the jet, particularly on Saturday.
Here is the 24-h predicted rainfall charts end Saturday, Sunday, and Monday at 5 AM.
Ending 5 AM, we see modest amounts over western Washington reaching 1-2 inches on the western slopes of the northern part of the State.
 A second system gets here Saturday morning and as you will see, the rainfall, particularly in the mountains is far more serious (1-2 inches being widespread)--see map below for 24-h rainfall ending Sunday at 5 AM.  Bad for hiking!  But you can escape the rain on Saturday by heading to eastern Washington or by positioning yourself in the rainshadow to the NE of the Olympics.

Finally the 24-h rain ending 5 AM on Monday is unimpressive...some light showers, with much of the western lowlands and eastern WA being dry.  Sunday is the better day for outdoor activities.


Dawg Dash Forecast

And talking about outdoor activities, the annual Dawg Dash will take place on Sunday at the University of Washington at 9:30 AM (http://www.promotionevents.com/dawgdash/).  It will probably be cloudy with a few sprinkles, but nothing compared to the deluge of last year!

Seattle School Board Race
My next blog will discuss in detail, but I believe that supporting the challengers is critical for turning around this district. 

6 comments:

Restless_one said...

It's a bit funny that the Seattle Marathon has been scheduled for the last weekend of November. It seems like not a lot research went it this decision.

seattle annie said...

Thank you for taking the time to write this blog. We love reading all the cool info. We bought a house in Westport recently and we appreciate all the new weather forecast information from the new doppler. Thank you for all the support in helping to get it for our region...John and Anne Murray

Cliff Mass Weather Blog said...

well restless..makes sense in one way...what a great way to burn off all the calories ingested over that weekend!

Paula said...

Ah! That perfectly explains my observation that the weather is so much nastier in November in the autumn than it is in January in the dead of winter, which has always been a bit of a curiosity to me. Now it makes sense!

PATRICIA said...

Excellent post, Cliff. Watching the jet is always a key to understanding our Nov-Dec pattern.

Unknown said...

wow, the forecasts for today were way off. It has been a downpour here in north Seattle for the last 6 hours. we are drenched.