Indian Summer

In much of the U.S. it is called Indian Summer.  In France the term is St. Martin's Summer, while in eastern Europe Old Ladies Summer and Gypsy Summer are often used.  All of these names describe a warm period in the autumn that follows a period of cool weather-- in fact, some demand a killing frost before the warmth.   Well, we don't get too many killing frosts west of the Cascades in autumn, so the term Indian Summer here is usually reserved for periods in mid-fall when the temperature rise to summer-like levels*.  And we have real shot of this on Tuesday of next week.

Well, let me first document that we have had some cool weather--in fact the last two weeks have generally been cooler than normal, and last night there was a real chill in the air (I know about this personally, I was out searching for my lost dog in Mountlake Terrace).  Here are the temps at Sea-Tac airport compared to the normal highs and lows:

 During the last two weeks only one day had max temps above normal and most were far below.  Today will be another cool one.   But this will change in a big way.

Today (Friday) there will be considerable clouds and some light sprinkles, particularly in the central Cascades.
Saturday will be sunny and warmer...very nice...but you won't mistake it for summer--Indian or otherwise.
Sunday a weak trough will bring cooling and lots of clouds (maybe a bit of sun)

But Monday starts the warm-up and Tuesday it will be in full bloom. Want to see some weather maps? (of course you do!)  Here is the upper level (500 mb) chart for Tuesday morning at 5 AM.  Wow...big ridge over us...and a trough over CA that is working its way north.

Here is the sea level pressure, surface winds, and lower atmosphere temperatures at the same time. Good offshore flow with higher pressure east of the Cascades and lower pressure west of the mountains.  You will notice a low pressure area near the Oregon/CA border.

With offshore flow, there will be virtually no clouds and air will be compressed as it sinks from higher elevations.  Here are the predicted surface (really 2-meter) air temperatures at 2 PM on Tuesday.  Temperatures above 70F over land, particularly on the eastside of western Washington and along the coast, downstream of the Olympics. Expect the warmest temperatures in the foothills...like North Bend and Issaquah. 

Western Washington is warmer than eastern Washington in such situations.

So get your bathing suit ready, Tuesday will be your day.  I suspect this will be our last excursion into the 70s this year...better enjoy it.

* Sequim dropped below freezing on Wednesday night.

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Tale of Two Radars: Rainshadow and Windward Enhancement

 Dog Alert!  See Message at the Bottom---Particularly If You Live in Mountlake Terrace.

With weather radars on both sides of the Olympics now, we can do something we have never done before:  examine simultaneously both the enhancement of precipitation on the windward side of the mountains and rain shadowing on the other.

Here is the storm-total precipitation from the new Langley Hill radar for the past two days.

I know it is hard to see the geography well.... but if you examine this closely you see values of over 2.5 inches on the southwest side of the Olympic, with lighter amounts offshore.  This is a good example of orographic enhancement, with the showers coming off the Pacific being enhanced from uplift on the western and southwestern slopes of the mountains as the air is forced to rise by the terrain.  The sharp cut-off of precipitation over the mountains is not real...it is due to mountain blockage effects.

What about the other side?   Here is the view from the Camano Island radar:

A complex pattern, but with a big hole of light precipitation to the northeast of the Olympics--this is the rainshadow, which extended from the north Sound to Bellingham. Rainshadows are caused by air descending and warming down mountain slopes on the lee side of a barrier.  The rainshadow was shifted away from Sequim and Victoria because the coastal winds have been more west-southwest than southwest or south-southwest.  And you will notice the precipitation really picked up over the western slopes of the Cascades as the air was forced to rise again--some amounts exceeded four inches.   Now these values are not well-calibrated, but the qualitative picture is certainly realistic.

For comparison, take a look at the visible satellite image this morning at 10:45 AM.  You can see the convective showers approaching the coast, the enhancement on the windward (western) side of the mountains, and if you look carefully you can spot some rainshadowing (really cloudshadowing here!) over northeastern Puget Sound.  Of course, the big kahuna rainshadow is to the east of the Cascades, where you could have escaped all the showers today.   And then air rises over the western side of the Rockies and clouds return from Spokane westward. 
 

At the UW we have software that combines the National Weather Service and Canadian radars--here is a view at 4:20 PM:

Really nice.  You can see the showers offshore, how they become more intense over land, how they die just east of the Cascade crest, are absent over eastern Washington, and start again...in a wimpy way... over Idaho.  And I haven't even mentioned the blustery winds of 20-35 mph all around the region today.

You got to love the weather around here...lots of variability!  Don't you feel sorry for people back east where the weather is so uniform and boring?

My Dog (see picture to the right) was seen today (10/12) near the intersection of 236th St SW and Cedar Way.  If you see her please call immediately (206 719 5603) 

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Its All in the Timing

How many times does this happen?  The National Weather Service forecasts rain coming in at a certain time and the forecast is off by a few hours, either early or late.  The truth is that this happens all the time and such timing error increase with the length of a forecast.  And variations in timing of consecutive forecasts provide valuable information on forecast uncertainty.  Consider the rain over Saturday night.

Here is a radar image from the new Langley radar at 7 PM on Saturday night. 

A frontal band is moving in, crossing the north-central WA Coast. Now here is the 74 forecast valid this time (1-h precipitation at ending 7 PM).  Got the right idea of frontal system...but too far south.

 Here is a forecast for the same time from the next cycle (62h forecast).  Whoa!  The frontal band has shifted way north...too north.

Next forecast cycle and the 50 hr forecast...the band is shifted south...not too bad.

The 38 hr forecast?  Pretty similar but a tad stronger and farther south.

The 26hr forecast? Similar, but slightly weakened.

And finally the 14h forecast. Very much the same, but perhaps a bit more diffuse.

These are very typical forecast sequences.  Go far enough out in time and the forecasts are shifting in a big way, particularly in the time that features pass a location.  As you get closer in time, the solutions tend to stabilize.  On occasions when the solution stabilizes late or not at all, forecast confidence is low, and vice versa.    But just because a solution stabilizes does not mean it HAS to be right, and sometimes the forecast models equilibrate on  a solution that is not correct.

Forecasters have a special name for the variation in forecast for the same time (e.g., 10 PM August 5) but for different forecast intervals (12, 24, 36 hr forecast, etc):  dmodel/dt.  If any of you have calculus you know why.  For those who haven't, this translates as the change in model solution over a specific time interval... a.k.a. the time  derivative.   Large dmodel/dt--low confidence in the forecast, small dmodel/dt--more confidence.

Ten years I did a paper with Brian Colle and David Ovens that showed that models  were typically fast by an hour or so.  I haven't repeated this study, but I suspect that although the errors are probably less now, errors in timing are not usual.  That is why the coastal radar is so important.  When our models are off by a few hours--too fast or slow, the radar will give us a heads up when weather systems are 6-9 hrs out.

Dog Alert
There have been repeated sightings of our dog in and near Terrace Creek Park in Mountlake Terrace.  If you are in that area, please let us know if you see her.

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Is West Coast Weather Getting More Extreme?

A question that is often asked is whether Northwest or West Coast weather is getting more extreme.  More heavy rain and flooding events?  More windstorms?  Extreme losses of snowpack in the mountains?   Some groups have answered these questions in the affirmative and have stated, suggested or hinted that such extremes are connected with anthropogenic global warming.  They often provide hand-waving explanations of such extreme behavior--for example, it is accepted that the atmosphere will gain water vapor as the planet warms (by roughly 7% per degree C).  Won't that cause extreme precipitation to increase?

My students and I have looked into this issue, as have other weather/climate scientists.  As I shall discuss, this is not a simple issue and uncertainties abound, but the bottom line is that there is little evidence for area-wide increases in extreme weather due to human-induced global warming during the past several decades over our region.  That is not to say there won't be any in the future, but the impacts are anything but clear.

Lets take extreme precipitation. With UW graduate students Mike Warner and Adam Skalenakis, we looked at the trends of heavy precipitation over the past 60 years at coastal stations from southern CA to British Columbia. Specifically we looked at the trends for the top 20, 40, and 60 events of two-day precipitation.  Some of the results are shown in the figure below. Southern/central CA is a mixed bag, but northern CA and southern Oregon show a DECLINE in heavy events. On the other hand, northern Oregon and WA have seen an increase in the big rainstorms.  BC show weak upward trend.

Now heavy precipitation, of course, has a big influence on rivers...what do big river discharge events look like?  Well, we looked at the trends over the same period of maximum annual discharge of unregulated rivers (see figure below).  Importantly, we got the same pattern! Decline over southern Oregon and northern CA, increase to the north.

Up arrows indicate increasing trend, down arrows, declining trend for max annual river discharge.

So there is no uniform change in extreme precipitation along the West Coast:  some places are up and some are down.  Some nice work by members of the Climate Impact Group at the UW, looking at the output of a variety of global climate models, suggests that this pattern is the result of natural variability and not the result of any kind of global warming signal.
    What about big windstorms?  Storms such as the Inauguration Day Storm and Chanukah Eve storm?   Fascinatingly, the pattern is the same over the past half-century--- upward trend over Washington and northern Oregon and decreasing frequency of such storms to the south.  We are trying to understand exactly what is causing this configuration.

The Chanukah Eve Storm of 2006

   What about the next fifty years or so?  Will global warming change extreme weather events over our region?   The honest answer---we don't know.   The computer models do not agree.   And one can easily think of scenarios where there will be a decline of extreme precipitation and storms.   Extreme weather events are closely associated with the jet stream.  Most climate models indicate the jet stream will move northward under global warming.   Will the most extreme events gets displaced northward?

   In short, the changes in extreme weather along the West Coast has been non-uniform and don't suggest a human-induced global warming signal.  Global warming due to our greenhouse gas emissions will be very significant, but the local implications regarding extreme weather are quite uncertain.

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Western Snow Season Begins

 In a real sense, the winter season began today along the entire West Coast.   Nearly simultaneously, snow struck higher elevations in the Cascades (including Paradise on Mt. Rainier) and in the Sierra Mountains (Donner Pass and Squaw Valley Resort), while rain wind hit southern California (see pics below).

Paradise at Mt. Rainier

Donner Pass

Hollywood

Why such widespread winter-like conditions so early in the season?  The answer is a very high amplitude meridional (north-south) trough of low pressure along the West Coast.

Take a look at the upper-level (500 mb) chart this morning.  Pretty amazing...low heights over the Northwest and a strong trough and strong winds (indicated by the closeness of the height lines) over central and southern California.

The latest Climate Prediction Center forecast for the next two weeks is for colder than normal and wetter than normal conditions over the Northwest....and the pattern looks very similar to last year's La Nina situation.  Forget about watering your garden.

And now the bad news--the computer models are more emphatic than ever that we will have to deal with a second La Nina year...yes, a double dip.   Here is an example of a collection of various models:

Most of the them are calling for negative sea surface temperature anomalies (differences from climatology) and the Climate Prediction Center's coupled atmosphere-ocean simulation system is even more emphatic.  Although this year's La Nina is a bit weaker than last year's, examination of previous double dips indicate that the second year can have a similar impact even if weaker.  The good news....skiing should be good this year and we will have lots of water next year.

Finally, some major radar news.  The Camano Island radar is back on line after being updated to dual-polarization.   So far there is no place for you to see the new dual-pol radar products on the web.   I am hoping that we can soon put some of the them on the department web site within a few weeks.  Several of you have also asked is there a place where are the radars are combined for your viewing pleasure...there is...my department is combining the U.S. and Canadian radars and you can access these graphics here.  This interface uses google maps technology, allowing you to zoom into the area of interest.

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Visibility Loss at Daybreak

A month ago, Greg Johnson of Skunk Bay Weather asked me why visibility often gets much worse after sunrise and then improves at sunset.  And several other of you have asked the same question.

Here is what Greg was talking about.  First the view at 5:32 AM on August 24th.  Great visibility and the mountain peaks are clearly evident.

A little over an hour later (6:53 AM) and the it is all murk.  No mountains!

You can watch his video of this "event" by clicking here.
Or another good example in July is found here.

Did some strange cloud come in with sunrise?  A rising of obscuring vapors forced by the sun? Camera problems?   None of these I suspect!

Let me tell you what is happening--and I have confirmed my interpretation with a department radiation expert.

Before sunrise, there are particles and droplets in the air, particularly near the surface, but they are not being illuminated by the sun.   The mountain peaks are either getting illuminated or are silhouetted by the rising sun.  Then the sun comes up and some of the light get intercepted by atmospheric particles and a portion of this light is scattered back to your eye--that produces the hazy look.   You get a lot more light from the scattered particles than distance objects, causing the objects to fade out.

This effect is worsened by being in a location with lots of moisture and particles...like the coastal location above.  And the reduction of visibility is worst near sunset since most of the scattering is in the forward direction (forward scattering).

Interestingly, away from the water, visibility often improves mid-day as drier, cleaner air gets mixed down from aloft, as the surface heats and the lower atmosphere becomes convectively mixed.  But that is for another blog.

Pledge Breaks.    Yes it is pledge break time at local public radio stations.  If you do support public radio, I recommend my new station KPLU.  I am not only impressed with their programming, but they have proven to be a real class act in all my interactions with them.

Dog Update:  My little dog is still out there...probably in the Montlake Terrace to Brier area...keep your eyes out for her if you live up there!  Thanks so much..cliff

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Sometimes a forecast goes wrong..

One often learns a great deal from failures, and certainly Friday was not a great success for my profession....and it serves as a good illustration of how useful weather radar information is when the computer forecast models have problems.

The weather predictions for Friday afternoon was for relatively warm conditions  (70F for Seattle, 72 Tacoma) with only a slight chance of rain (roughly 10%). 

 The computer models did not show any rain over western Washington, such as this prediction by the NWS NAM model for the three hours ending at 2 PM:

 Or the UW WRF for the same period:

 Similar to solution to the NWS model.  And yes, my forecast for KPLU--made around 7:30 AM, and before the precipitation was obvious in the radar, was too dry and warm as well.

It became evident in the morning hours that the models were missing a major ingredient as precpitation was evident offshore and then moved in over us--thanks to the new Langley Hill radar (images at 10:30 AM and 12:03PM):

Most of this precipitation was due to mid-level instability (and convection) and very little reached the surface.  Thus in some sense the model verifications really weren't that bad...but the cloudiness resulted in the max temperature being well below the predicted and the edge taken off a supposedly nice day.  Sea Tac for example only reached 65F...5F below the forecast.

Here is a nice video showing the development of the mid-level convection:

http://www.atmos.washington.edu/~ovens/loops/flash2.cgi?/images/webcam2/movies/20110930.swf

So if one watched the radar carefully, you could have seen the forecast going wrongand alter it....we call this nowcasting.

So what went wrong?   There appear that there was a weak offshore disturbance that was poorly described in the initial state of our numerical models.  Here is the 9:30 AM infrared satellite photo that suggests such a feature:

During the next few months the NWS and the UW will be using the information from the Langley Hill radar to initialize our models, perhaps this will help improve the short-term forecasts of such events, but it will do little to help predictions more that 6 hours out.  To improve these will require better use of satellite data and more satellite data.

Note:  The Camano Island radar is down now for upgrading to dual-polarizaton...should be up by the end of the week....cliff


DOG Alert

We had a solid spotter report of my lost cockapoo in Mountlake Terrace near Terrace Creek Park...if any of you live or work up there can you keep an eye out?  For a picture of her, check the link on the right.  Thank you so much. 
http://misscockapoo.blogspot.com/2011/09/missing-black-female-cockapoo.html
Here is the area she was seen last in:

 

This dog was lost on 9/10 and has moved 10 miles in 19 days.  

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Virgin Pacific Front

The fronts we experience in the interior of western Washington and Oregon, or east of the Cascades are generally pretty wimpy affairs, having been highly modified and weakened by the coastal mountains (including the Olympics) and then the higher Cascades.   Wind shifts are generally weak, precipitation intensity moderate, and the structures torn apart by the terrain.

But out over the Pacific, men are men, women are women, and fronts can be well-defined and far stronger.  Until the Langley Hill radar was installed, our knowledge of Pacific frontal features were mainly based on research work done by aircraft and coastal radars completed over relatively short periods during field experiments--Peter Hobb's group at the UW did the classic work on such systems.

Well now we have a wonderful operational radar looking offshore and we had a view of a beauty of a front.

Now we start with a visible satellite picture at 12:45 PM on Monday as the front approached the coast:

Nice looking frontal band, but exactly where is the front? How strong is it?   Well, we don't have to worry anymore folks...we have the Langley Hill radar.  You can see it offshore (below) --a band extending SSW to NNE crossing the NW tip of the Olympic Peninsula.  Notice the corrugations in the front?   Classic...there are cores of heavy precipitation and gaps between them.

Take a look at the observations at Destruction Island (just off the central WA coast) for the last few days--and take a special look on the 26th (times are in UTC-- 12 is 5 AM, 0 is 5 PM).  You can see the trough of low pressure with the front and winds gusting to 60 knots.

Impressive....and stronger than any of the winds that hit Long Island during Irene. 

The strength of such Pacific fronts are not uniform...they are much stronger in the core areas of heavy precipitation than the gaps in between.  Sometimes the frontal characteristics are INTENSE in the cores.  During the 90s we have field experiment called COAST in which we flew the NOAA P3 Hurricane Hunter aircraft into a Pacific front.  The scientists wanted to go out and cross one of the cores to see what it was like.  The NOAA P3 pilots agreed...they had been through many hurricanes, so they were not concerned.   Well, the passage through the core was very, very intense--with several g's up and down and even the coffee pot broke off in the rear of the aircraft.   Several scientist thought they were going to die. The pilots were shaken...no more flights through cores at low levels--this was a lot worse than going through hurricane eyewalls!

DOG Alert

We had a solid spotter report of my lost cockapoo in Mountlake Terrace near Terrace Creek Park...if any of you live or work up there can you keep an eye out?  For a picture of her, check the link on the right.  Thank you so much. 
http://misscockapoo.blogspot.com/2011/09/missing-black-female-cockapoo.html
Here is where she was seen:

Reminder:
Friday, September 30th 6-8pm
West Seattle Meet & Greet Happy Hour with Candidates Marty McLaren & Sharon Peaslee (and I will be there too!).   Refreshments, of course.
TOPIC: Improving Math, Science and STEM in SPS
7020 18th SW (north of SW Myrtle)
Donations appreciated. - all are welcome

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The New Radar Documents A Frontal Passage

Monday Morning Update:

Here is the storm total precipitation ending 10:30 AM on Monday from the new radar....notice the enhanced precipitation on the windward (SW) slopes of the Olympics:

In science, major advances in understanding often come with a new instrument, one that casts light on a previously unobserved phenomena.  Certainly, that is true of meteorology; through new understanding and additional data from new sensors, the skill of prediction increases.

The frontal system that passed through during last night and today was observed from beginning to end by the new Langley Hill radar---lets take a look at only one viewpoint:  the reflectivity at the lowest elevation angle (.5 degrees).   As we have talked about many times in this blog, such reflectivity is closely related to precipitation intensity.

First, consider the image at 5:34 PM yesterday.  Note the precipitation intensity (reflectivity) is given in dbz and yellow is very heavy stuff, green a lot lighter, etc.  Looking offshore we could see the approach of the Pacific frontal system (something we could not do before).   During the day yesterday (Saturday) the skies over Puget Sound clouded up and some light showers formed as the air over the western interior destabilized at midlevels--producing lots of altocumulus castelanus.   You can see the light showers in the radar, indicated by the north-south linear features.   Not forecast well.   This was not frontal precipitation, but rather the instability was initiated by the approach of the upper level trough associated with the front.

Four hours later around 8 PM the offshore precipitation was approaching rapidly and some light showers remained over the western slopes of the Cascades,

As the evening progressed the band reached the coast, as shown by this image at 1:44 AM.  Look how nicely the radar shows the band from Vancouver Island to south of Tillamook, Oregon.  The location of this radar was chosen very, very carefully to allow such a comprehensive view.  Behind the band the precipitation is spottier and more intense, indicative of convective precipitation.

And now the great disappearing trick!  The Camano Island radar had a blind spot west and south of the Olympics.  Of course, our new radar has a blind spot too--NW of the Olympics!   Here is a view of few hours later that shows this.  Believe me, the band extended north of Everett.  And look offshore...you can see instability showers over the Pacific, with some with red colors---very, very heavy rain.  Interestingly the surface front (with the lowest pressure and windshift) was NOT with the continuous precipitation band over Puget Sound, but was still offshore at this time.

In fact, there was another band of precipitation, fairly convective looking, that come in with the front around 9 AM Sunday (see below).

 Around 12:38 PM there were lots of showers over the western slopes of the Olympics and coastal mountains.   You see the regular streakiness in the convection?  That is associated with convective roll circulations.  And see the bluish colors just off of Gray's Harbor?  That is the radar seeing the surface of the ocean.

?  

And finally the situation after the front around 5 PM on Sunday...a few convective showers over the Pacific.  Showers and sunbreaks.

This radar is going to teach us a great deal about Northwest meteorology.  Knowledge that will both aid my colleagues in the National Weather Service and help us gain a better understanding of the interactions of incoming weather systems with our coastal terrain.

 Seattle School Board Candidate Events

There are two important gatherings to listen and talk to Seattle School Board candidates:

Wednesday, Sept 28th-7:30pm
The Stranger's School Board Candidate Debate
Town Hall - 1119 8th Avenue, Seattle
NOTE:  The event is FREE but tickets are required.
Click here for ticket information.

Friday, September 30th 6-8pm
West Seattle Meet & Greet Happy Hour with Candidates Marty McLaren & Sharon Peaslee (and I will be there too!).   Refreshments, of course.
TOPIC: Improving Math, Science and STEM in SPS
7020 18th SW (north of SW Myrtle)
Donations appreciated. - all are welcome

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Sweating in the Pacific Northwest

 Note:  I made some changes to the html code that should make the blog work far better for IE9 browsers.  Let me know if anyone is having problems with performance of this blog on any browser...cliff

Bicycling to work today, I found myself wiping the sweat from my brow as I entered my building.  It really felt like the East Coast during summer out there. In fact, the last few days we have had some unusually humid air over the region, with nighttime minimum temperatures far higher than we are accustomed to.    The dewpoints were quite high this morning...rising into the mid-60s in some locations.   In fact, during the past day we have had the most humid air of the summer invading our region.  What is going on?

During the past several days we have had a pattern with unusually high pressure to our southeast and low pressure to our northwest (see graphic at roughly 18,000 ft--500 hPA pressure).   The result is a strong current in the lower to middle atmosphere from the southwest extended well back into the subtropics.  A flow that effectively transports moisture into our region.

 Here is a graphic showing the total water vapor in a column of air over the NE Pacific this morning--you see the current of high water vapor values heading our way from north of Hawaii?   This is often called an atmospheric river.

This river of moisture has been mainly directed into Vancouver Island and central BC and they have had very large amounts of rain this week.   You can also see the current of moisture--at least the part associated with clouds--in the visible satellite image this afternoon:

We have been just on the southern side of most of the clouds and rain, but within the current of higher than normal moisture.

With a current of warm, moist air coming towards us, our temps have been above normal and with the humidity our low temps have been WAY above normal.   Water vapor is very active in the infrared--in fact water vapor is the most important greenhouse gas in the atmosphere.  Such gases tend to warm us up at night--- they intercept the infrared radiation leaving the earth and reemit some back down to the surface, thus lessening cooling.  There is a reason deserts are cool at night--not much moisture.

Take a look at the temperature plot for the last week:

 See how high our minima have been the last three days? 10-15F above normal!

In short, we have been in the sweet spot--on the southern edge of a plume of warm, moist air.

Tomorrow will be the last day of the semi-tropical warmth, with temps getting into the lower 80s in places.  And the pattern shifts and we will have rain on Sunday and clouds/rain for much of the next five days.....

well...autumn did begin today.

Dog Update
   Still looking for my little cockapoo Leah.   My family is convinced that someone has picked her up in the area near the intersection of  105 N St., Holman Rd, and Greenwood Ave N.  Here is the area:

If any of you live in this area, please keep your eyes out for her (see link on right for pics). Let me know if you see anything.

 Seattle School Board Candidate Events

There are two important gatherings to listen and talk to Seattle School Board candidates:

Wednesday, Sept 28th-7:30pm
The Stranger's School Board Candidate Debate
Town Hall - 1119 8th Avenue, Seattle
NOTE:  The event is FREE but tickets are required.
Click here for ticket information.

Friday, September 30th 6-8pm
West Seattle Meet & Greet Happy Hour with Candidates Marty McLaren & Sharon Peaslee (and I will be there too!).   Refreshments, of course.
TOPIC: Improving Math, Science and STEM in SPS
7020 18th SW (north of SW Myrtle)
Donations appreciated. - all are welcome

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