Monday, October 20, 2014

Heavy Rain Coming and Windstorm Threat

An atmospheric river, a warm current of large amounts of water vapor, will be approaching our region starting later tomorrow (Tuesday), but it will be coming from a less frequent direction:  the west.  And the result will be the heaviest rainfall to strike our region since last winter.

Time to replenish our renowned water resources!  And a good reason to be glad you don't live in California.   And speaking of parched California, they are going to get a piece of the action.

Let's start by looking at the latest view from space, showing the amount of moisture in the atmosphere.  Reds and purples are large amounts, and not surprisingly such enhanced values tend to be in the tropics.   But a huge northward push of moisture is occurring in the central Pacific and a tendril of it is moving westward towards us.    It has our name on it.


Let me show you the forecast atmospheric moisture for 2 AM on Wednesday morning.  You can see the heavier moisture (blue and white) heading towards us.   And when that moisture is forced to rise by our regional terrain, there will be lots of precipitation.


How much?  Here is the forecast from the UW WRF model for the 72 hours ending 5 AM on Thursday.  Big values (more than 5 inches in the Olympics, North Cascades, and northern Oregon coastal mountains. An inch or two in the lowlands.   Rake the leaves from the drains near your house or apartment!


The next 72 hours?  Still wet, but less so.  1-5 inches over the region west of the Cascade crest.  One good thing:  northern California will get enough rain to begin to fill their thirsty reservoirs.


As with most atmospheric rivers, there will warm temperatures and strong, but not damaging, winds.  To illustrate here are the forecast temperatures, heights and winds near crest level (850 hPa, about 5500 ft) on Tuesday at 11 PM.  You see how packed those height lines are approaching the Oregon/WA coast....those are associated with strong SW flow.  Reds are warm air.



Let me make clear...this is NOT going to be a record event.  But it will be the wettest period since last winter.

Finally, a number of the modeling systems are predicting that a deep low pressure center will approach our coast on Saturday morning.  Here is the solution from the U.S. GFS model (the solid lines are isobars, lines of constant pressure) for 8 AM Saturday.   Huge pressure difference (gradient) along the southern Oregon coast...with big winds (50-75 mph).  The European Center model has the same low, but fuarther north and offshore.   It will take a few days for the solutions to stabilize...but this is worth watching.


The latest run is in...and it is getting more threatening for western Washington!   This is valid 5 AM Saturday.  Only a 108 hr forecast...close enough that you got to be concerned.  985 hPa low.  This would be a major windstorm folks.


Here is pressure and sustained winds (kts) for the same time.   Wow...serious winds along the coast.


And by 10 AM Saturday, the low moves north of Puget Sound after crossing the Olympics...the most threatening windstorm path.  I am going to COSTCO tomorrow to buy batteries.




Saturday, October 18, 2014

The Extraordinary Minimum Temperature Heat Wave in the Northwest

I have been watching Northwest weather for a long time and I have never seen anything like what we have experienced during the last month:  

An extended and intense minimum temperature heat wave.

Something amazing has been going on this fall, and for some reason the Ebola-crazed media hasn't picked up on it.   But that is why we have blogs.  Gardeners know something weird is happening. Vegetable plants are not dying.   Tomatoes are still ripening.

There are movies about this issue.

Here are the temperatures at Seattle-Tacoma Airport during the past 4 weeks, with the average high (red) and lows (blue) shown.   Only ONE day in that entire period has seen the temperature dropping to the average low.  For most days, our minimum temperatures have been 5-10 degrees above normal. Our minimum temperatures last night were close to the average maximum for the date!
UPDATE MONDAY MORNING:  Here is the latest 4 weeks.  Our low temperatures the last few days have been around the NORMAL HIGHS.  And yesterday broke the record daily high at Sea-Tac Airport.

And this is not Seattle alone, here is the same trace for Bellingham.  Same thing.  Bellingham cooled to 59F last night!
Or Quillayute on the coast.   Mega-warm.
A plot of the minimum temperature anomaly (difference from climatology) for the western U.S. over the past month shows that our regional is RED HOT, with minimum temperatures 6-8F above normal on average.

A close-up over Washington State shows some areas are 8-10F above normal.


And the latest NOAA Climate Prediction Center extended forecasts show no end in sight
to the warmth:


Now why is this happening?   This is an important  question because one can expect some folks in the media and advocacy groups to start saying this is a "sign" or "consistent with" global warming due to mankind's emissions of greenhouse gases.  There is no reason to think that is true.

There are two main reasons for the warmth and they are both associated with the anomalous atmospheric circulations we are having.

Reason #1:  a persistent area of low pressure over the eastern Pacific.  The figure below shows the sea level pressure anomaly (difference from normal) for the past month.   There is an area west of us with pressures well below normal.   Such anomalous low pressure is associated with stronger than normal southerly and southwesterly winds over us that blow in warmer than normal air.
Here are the wind anomalies near the surface for the same period...look closely you will see they are southerly over us. It all fits.

This is probably the major cause.   Then there is something else, something I have talked about in previous blog:  the warm water BLOB off the coast.

Below is the sea surface temperature anomaly map for the past week.  You see the orange and red colors off the coast that indicate temperatures 2-4F above normal?  The BLOB still lives.  So air passing over the eastern Pacific  is exposed to warmer than normal water.  Me like BLOB, BLOB is good.

As I noted earlier, the BLOB has little to do with global warming but was produced by anomalous high pressure over the Pacific last winter and year.

So our ridiculously warm temperatures this fall are being produced by an unusual combination of high pressure a year ago that produced the blob and low pressure this fall that is bringing up warm air from the south.

There is no reason to think that these circulation anomalies are caused by human greenhouse gas emissions.  And remember that the eastern U.S. has been colder than normal.

Well, time for me to go out to my garden to harvest some more red tomatoes.


Wednesday, October 15, 2014

The Western Puget Sound Rain Shadow

In Seattle, it was quite a wet, blustery day, but if you had taken a short ferry ride to Bainbridge Island you would have experienced hardly a drop.  To illustrate this, here is the 24hr precipitation ending 8 PM Wednesday over Puget Sound and nearby environs.

.01 inch in Bainbridge...barely enough to wet the pavement and .02 inches in Belfair on the Kitsap Peninsula.  Cross the Sound and there was over .40 inches.  In general, it was much drier on the western side of the Sound.  Yes, Sequim to Port Townsend were fairly dry too.


As you might imagine, this contrast was due to rain shadowing in the lee of the Olympics and upslope enhancement on the western side of the Cascades.   We could watch it happening in today's weather radars (showing you the radar-based 1-hr precipitation)

11 AM.  Rain shadow NW of the Olympics, consistent with SW flow.

 At 12:59 PM, the flow was in transition (becoming more westerly) and you notice an increase in precipitation on the western side of the Cascades)

This transition was even more obvious at 3:46 PM,  A broad rain shadow downstream of the Olympics was obvious, as was the substantial enhancement of rain on the western side of the Cascades.


Even more so at 5:02 PM


We did have radiosonde launch at Qullayute at 4 PM and here is the sounding of winds (barbs), temperature and dew point (the dark lines slanting to the left).  Height is in pressure units (hPa), with 700 being 10,000 ft, 850 about 5500 ft., etc.).  Winds approaching the Olympics were from the west-southwest at that time, which fits the radar pattern.


My point is that Olympic rain shadows don't only hit Sequim and environs, they can extend down the eastern slopes of the Olympics under the right conditions.  And it also illustrates that if you are weather-wise and are ready to travel an hour or so, you can often find much better weather.  Or stormier weather if you prefer.


Monday, October 13, 2014

Waterspout Hits Southern Puget Sound: First Tornado Warning Here in 17 Years

On Saturday around noon, several of you were startled to get a tornado warning on your smartphones.

The cause: a waterspout that developed near Anderson Island in the southern Sound and which remained intact for about a half-hour.   Here are some pics I found on the KOMO and KING-5 web sites.   An extremely well-formed funnel and you can see from the first that the winds reached the surface, kicking up lots of spray.  Beautiful pictures.


Waterspouts are the weaker cousins of the strong tornadoes one finds over the Midwest U.S.
According to the official Storm Prediction Center definition:

 A waterspout is a tornado over water--usually meaning non-supercell tornadoes over water. Waterspouts are common along the southeast U. S. coast--especially off southern Florida and the Keys--and can happen over seas, bays and lakes worldwide. Although waterspouts are always tornadoes by definition; they don't officially count in tornado records unless they hit land. They are smaller and weaker than the most intense Great Plains tornadoes, but still can be quite dangerous. Waterspouts can overturn boats, damage larger ships, do significant damage when hitting land, and kill people. 

This waterspot, and virtually all of our waterspouts/tornadoes around here, are associated with non-supercell thunderstorms.  Supercells are the big Kahunas of the thunderstorm world with very high tops (reaching 40-60K ft), intense rain, hail, and most importantly rotation.

This waterspout came out of a relatively wimply NW thunderstorm.  Let me show you.  We start with the composite reflectivity--a measure of the highest precipitation rate in the storm. Got some red...that is pouring rain, probably with some small hail in the south Sound. Impressive for around here.  But equally strong thunderstorms were hitting in the north Sound with no waterspouts. No sign of any hooked echoes...which indicate  supercell storms.


How high were the thunderstorm tops?....we have a radar-based diagnosis of this.  Only 20,000 ft!   Folks in the Midwest would laugh at such tops.



And since the National Weather Service radar is Dopplerized, it can show the radial winds toward or away from a radar.  Neither this shot from the Camano Island radar nor one from the Langley radar showed any hit of thunderstorm rotation as we would see in a supercell storm.  Rotation would be indicated by a pairing of warm (red,orange) and cool (green, blue) colors.  Nothing there.


Nearly all of the waterspouts and tornadoes in our area are from the non-superstorm type.   That usually involves a fairly strong thunderstorm passing/developing over a region where the wind changes rapidly with horizontal distance.   We call that horizontal wind shear.  Such wind shear is associated with some inherent rotation around a vertical axis that get spun up by the vertical motion of the thunderstorm.   A schematic of this effect is shown below and I have a whole section on this process in my book on Northwest weather (which is a good gift for the upcoming holidays by the way).


Looking at the surface map at noon on Saturday (see below), I wasn't impressed by the amount of shear, but clearly there was enough.  Perhaps we had 15 knots over southern Puget Sound and much weaker wind over land (Kitsap, Anderson Is.).


Most Puget Sound tornadoes seems to develop on the shear associated with a Puget Sound convergence zone or the strong wind shear near terrain.  This one appears to be an exception.

The last time a Tornado Warning was issued for the greater Seattle Metro was on  Dec. 12, 1969, so the younger folks reading this blog will never have experienced such excitement.


Dawg Dash:  Weather Looking Decent!

 One of my favorite events will occur Sunday morning at 9AM:  the UW Dawg Dash fun run.  For more information, check out their site: http://depts.washington.edu/alumni/blogs/dawgdash/  The latest forecast suggest no major storm at that time.  Little or no rain.

Saturday, October 11, 2014

Wimpy El Nino

We are now close enough to the winter season to have a fairly clear idea of the El Nino situation ahead.  Remember that El Ninos are associated with warmer than normal water in the tropical Pacific and that such anomalies can influence Northwest weather (less storms, warmer, less snow).

Originally there was a lot of talk (last spring) of the potential for a Super El Nino, with some of the global warming "advocate" sites talking about its effects on the global temperature record (global temperatures can warm substantially with strong El Ninos).



However, the sea surface temperatures in the critical  central Pacific is only modestly warmer than normal and the atmospheric circulation has not reacted in a way to reinforce the warming and push us towards a moderate or stronger El Nino.

Let's start with one of the key measures:  the sea surface temperature in the Nino 3.4 area of the tropical Pacific.  The official definition of El Nino is an anomaly  there greater than .5C.  We are not there now and in fact the SST has declined recently.


A number of groups run statistical and full-physics models to simulate El Nino.  They suggest we will have an El Nino, but a weak one.  However, verifying these models for the forecasts started earlier in the year, suggest they have been pushing warming in an unrealistic way.


The National Weather Service Climate Forecast System (CFS) model is now going for an entirely marginal event, barely reaching the .5C criterion


And the official probabilities for El Nino are now only about 65%.


So we should not expect much more than a marginal El Nino during the upcoming fall and early winter months.  And amplitude matters.   Weak El Ninos have lesser impacts.

The correlation of our weather with El Nino is not perfect to start with.  And for weak El Nino years the relationship weakens further.

Let me illustrate this for you.

Here are the precipitation patterns across the U.S. for some STRONG El Nino years.  The most consistent implication is  the wetter southeast U.S.   There is a tendency for southern/central California to be wetter.  The Northwest is less consistent.



But for weak El Nino years, the precipitation patterns are all over the place.  I would not place bets on anything.  This is the story of our upcoming winter unless El Nino revs up unexpectedly.


So based on the correlation with El Nino, we have very little guidance for the upcoming winter. Sorry.

The latest NWS Climate Forecast System forecasts for December-January-February is for warmer than average over much of the U.S.  No hint of the feared "polar vortex."
What about precipitation?  Very little signal over the Northwest.  But wow...California is much wetter than normal.  This would really help the drought.  Fingers crossed.

And this week should be a shock to some.  Showers today.  Generally dry on Sunday and Monday day.  But they the celestial spigots will turn on.

Hopefully, the rain will be over by next Saturday.  Why?   Because one of my favorite events will occur Sunday morning at 9AM:  the UW Dawg Dash fun run.  For more information, check out their site: http://depts.washington.edu/alumni/blogs/dawgdash/