February 22, 2024

How unusual is it for aircraft to exceed the speed of sound?

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The media this week was full of stories talking about aircraft going faster than the speed of sound.

Freak, supersonic wind speeds, some exceeding 800 miles per hour.


For reference, the speed of sound around cruise level (roughly 35,000 ft) is around 664 mph.  Some eastbound, north Atlantic flights arrived 30 to 45 minutes early, one over an hour early!

So what happened this week over the North Atlantic and how unusual was it?  All will be revealed here.

First, a bit about planes going "supersonic".   To be truly supersonic, the aircraft has to move faster than the speed of sound (roughly 664 mph a cruise altitude) relative to the air.   

They never do.  

When the media talks about "supersonic" they are talking about the speed with respect to the ground, which is NOT the same thing.  Aircraft can go very fast relative to the ground if the aircraft has a large tailwind.....as was experienced earlier this week.


Since the average jet aircraft flies around 560 mph at altitude, a tailwind of over 110 mph would result in it moving faster than the speed of sound relative to the surface.  

The winds aloft were truly screaming along the U.S. East Coast on Sunday and Monday of this week.  Here is a short-term forecast of the winds at a pressure of 200 hPa (about 38,000 ft) for Sunday morning.  Wow.   

Over the mid-Atlantic states and offshore, some winds were forecast to reach 215 knots (247 mph).  That is cranking.  And since these winds were from the west, aircraft heading to Europe were going over 800 mph to the east.


So how unusual were the winds, say over Washington, DC during this event?    

There is a radiosonde (balloon-launched weather station) at Sterling, Virginia (Dulles Airport), just to the west of Washington, DC.     On Sunday morning, the ballow measured a speed of  218 knots from the west.

Impressive.  But how unusual?  Below is a plot  (red line) of the daily 250-hPa (about 33,000 ft) strongest winds at that location for a period from 1950 to today.  The star indicates the Dulles winds on Sunday.  

That morning, there were the second strongest winds at that level in over 70 years! That is very, very impressive.

Monday evening the strong winds moved offshore, still giving European-bound aircraft a mighty boost eastward (see wind speeds below)


Although these winds were exceptional over Dulles and clearly stronger than normal, they are not as exceptional when considered on a hemispheric or global scale.

To illustrate, below is a plot of the winds aloft (250 hPa level, about 33,000 ft) at the same time for the entire hemisphere.  Several strong-wind hot spots are evident, with several showing winds exceeding 160 mph.


A forecast of the winds for next Wednesday at the same level shows very strong winds over the SE US, with plenty of "supersonic" potential around the world.


The southeast U.S. has done very well for strong winds during this period,  but typically the strongest jet stream is found over southern Japan and out into the Pacific.  

The strongest jet stream ever observed by a radiosonde?  259 mph near Yanongo Japan in 2004.


February 20, 2024

Substantial Snow Returning to the Pacific Northwest

During even the strongest El Nino years there is often a period at the tail end of winter when the atmospheric circulation associated with El Nino collapses.

When the atmosphere adjusts to considerably more solar radiation over the northern latitudes the "lock" of the tropical Pacific is lost.

This re-adjustment will likely occur next week, leaving the door open to cool wet flow over our region.

This means lots of mountain snow and even the chance of flakes near sea level.

Let me show you!

The forecast for total snowfall through 1 PM Friday is hardly impressive, with a few inches in the mountains.  Yawn.


By 1 PM Sunday, the totals in the north Cascades and southern BC are healthier, reaching nearly a foot in favored locations.


Fast forward to 1 PM Tuesday, and the Washington Cascades and Olympics have gained several feet.  Considerable lowland snow is evident.



And by 10 PM Wednesday, February 29th. some mountain areas have gained 4 feet and parts of the western Washington lowlands have a thin veneer of snow.


So what is going on? 

 Examining the situation at around 18,000 ft (500 hPa) this morning, a strong trough of low pressure is found off the California coast. This is the kind of pattern that has been dominant over the last few months.


Fast forward to the end of the week (Sunday evening) and a very strong trough of low pressure is moving in.   Classic snow producer for the NW.


But the shocker is the prediction for March 1 (below).  Very cold, powerful trough off our coast.  Very wet, very cold, and very snowy.


All of this is a bit far out in time to be sure, but most of the solutions of ensemble forecast systems, in which the model is run many times with slight changes, produces similar predictions.  You may not want to put away your winter gear quite yet.

Keep tuned.




February 18, 2024

Dry Air Storm Hits Western Washington

 How do I put this tactfully?

Have any of you noticed any dry or cracking skin?

Or perhaps a parched mouth at night, requiring a sip of water?

I have and I know exactly why it is happening:

Desiccating, easterly downslope flow moving from eastern Washington into the coastal regions has occurred repeatedly during the past week.

A dry storm if you will.  The Northwest version of the Sahara.

Consider the outdoor relative humidity at Seattle Tacoma Airport during the past week.  Under 50% for nearly a day--dropping as low as 30%!  A day before it got down to similar levels.

Portland (below) was even worse,  with additional low-humidity episodes earlier in the week. There is a reason Portland is so bad...and it has to do with the Columbia Gorge (more later).

But these are just point values, let me now show you the lowest relative humidities that occurred in Washington and British Columbia on Friday (below).  

Amazing.  A large portion of northwest Washington and British Columbia had relative humidities dropping into the 20s!      No wonder skin my need some ointment!


As you might expect, the uber-skillful UW WRF modeling system predicted the dry storm well in advance, as shown by the relative humidity prediction for 10 PM Thursday (below)


So why such dry air?

The map below shows sea level pressure, near-surface winds, and lower-atmosphere temperatures at around 2500 ft ASL(color shading, blue and purple are cold) at 7 AM last Friday morning.  Cold, dense air to the east produced high pressure, while a Pacific low-pressure center was offshore.  There was an intense pressure difference over the Cascades due to cold air banking up on the eastern slopes of the barrier.


Over terrain, air near the surface tends to move nearly directly from high to low pressure, and thus the pressure difference forces strong easterly (from the east) winds.    Some of the winds moved directly over the mountains, while in places with gaps (like the Fraser River Valley and Columbia Gorge), cold air moved westward within sea level in the gaps.
 
The cold air has low amounts of moisture since cold air holds far less moisture than warm air.  As that air from eastern Washington crossed the Cascades and then descended the western slopes of the barrier, the air was warmed by compression, which caused the relative humidity to plummet.

Why? 

 Because relative humidity is the ratio of the amount of water vapor in a sample of air divided by the maximum amount it can hold.   Warm air can hold more water vapor than cold air.  Thus,  relative humidity plummets for warming, sinking air.


The effects of downslope flow is the reason why the lowest relative humidities were downstream (west) of the terrain crest in the figure above.

The future outlook for relative humidity?   Good news....no more dry storms on the horizon.



February 16, 2024

California Enjoys the Northwest's Water

 Two years ago, media outlets were headlining strident messages that California had moved into a "megadrought" and that there was little hope for relief.

Story after story claimed that global warming had permanently changed the situation and that California's reservoirs would remain unfilled.


Fast forward two years and these apocalyptic warnings appear more like a rendition of chicken-little than reliable climate science.

Massive rainfall has hit the Golden State, while we in the Northwest are enjoying a warmer-than-normal winter with less snowpack than normal.

Below is the percent of average precipitation for the last month over the western U.S.  Some parts of California have received over 400% of normal!


To illustrate the soggy California situation, below is the accumulated precipitation at Los Angeles since October 1 (the current water year). Brown indicates normal values.   LA is now running about 40% above normal


Compare this to Seattle (below).  We are slightly above normal.   Precipitation-wise we running nearly exactly on climatology.   That may surprise some.


As I have described in several previous blogs, we are now experiencing El Nino conditions, which tend to make California very wet, while our area generally experiences near-normal precipitation.   

The latest forecast of accumulated precipitation through Tuesday shows a continuation of the pattern with heavy precipitation expected in California.  And yes....only modest precipitation over Washington.  Oregon enjoys a piece of this wet bounty.


The origin of this wet situation is a strong El Nino low center off of Califonia, illustrated by the upper level (500-hPa pressure, about 18,000 ft) at 10 PM on Monday.  The purple shading indicates an unusually strong low.  I am getting tired of this pattern.


The current extended prediction of accumulated precipitation through April 1 by the European Center model suggests this pattern is not going away, with very wet conditions over California, but drier than normal in the Washington Cascades (see below).


But what about the Northwest snowpack during the next few months?    I will leave that to an upcoming blog.




February 13, 2024

Some Lowland Snow is Coming Western Washington Late Wednesday and Thursday

Historically,  the window for lowland snow over western Washington closes the last week of February.

It appears that we are going to get some flakes before it is too late..... but not that many.



Let me start by showing you the latest forecast by the UW WRF prediction system.    Below is the forecast sea level pressure and low-level temperature map for 10 PM tomorrow (Wednesday) night.  

Cold air (blue color) is found in British Columbia, Alberta, and Montana.  Some pushes into eastern Washington.

A low-pressure center moves on to the northern Oregon coast, drawing cool air into western Washington.  The trouble for western Washington snow is that the low is a bit too far south and temperatures are marginal.  This pattern would push cool northeasterly flow through the Fraser River Valley into Whatcom County and the San Juans.




The latest UW high-resolution forecast for snow accumulation through 1 PM Thursday,  shows some lowland snow from roughly Olympia to Chehalis, and plenty over the south Cascades.  The southeast side of the Olympics gets a few flakes as well.

The Columbia Basin gets some light snow.


Nothing exciting perhaps, but snowflakes are always of interest.   There is, of course, some uncertainty in the forecast, and the way to get at that is to examine an ensemble of many forecasts.

The City of Seattle SNOWWATCH website shows the ensemble forecasts for Seattle (see below).  The gray lines are individual forecasts, and the ensemble average is in black.  Many forecasts are going zero, but the average is about .5 inches and snow depth would be much less than that.  Perhaps a dusting.  Quite possibly nothing.


In short, I would not be getting the sled out, but you might spot a few flakes.



February 11, 2024

Why does El Nino Influence West Coast Weather?

As I have noted in a recent blog, it is no surprise that the low snowpack and warm temperatures have occurred over the Northwest during the past month or so.

This is a very typical pattern during El Nino years.

In particular, during El Nino years, when the tropical Pacific is warmer than normal, we often experience low pressure off the West Coast, with storms shunted into California.

To illustrate the current situation, here is the difference from normal of sea level pressure for the past two months.  You can see the anomalous low pressure to our southwest.

This pattern pushes the jet stream south and leaves California wetter than normal.  But their boom is our bust.

But why does El Nino set up this pattern?

Why do warm sea surface temperatures in the tropical Pacific associated with El Nino alter the pressure and wind patterns off the West Coast.

The answer to this question was found in the late 1970s and early 1980s by two researchers at the UW:  graduate student John Horel and Professor Mike Wallace.

First, the warm waters of El Nino (illustrated by the shaded area near the equator) perturb the atmosphere above.  

How?  By creating lots of thunderstorms over the warm water.  Thunderstorms that inject huge amounts of energy into the atmosphere (see below).


 A series of waves....called Rossby Waves...then propagate into the midlatitudes, causing a series of low and high-pressure pressure areas (note the L's and H's in the figure in the figure below). 

 The jet stream....the current of strong winds in the upper troposphere... is distorted by this wave pattern (see the arrows in the figure).  This pattern is generally associated with low pressure off the northern CA coast.

This situation is analogous to throwing a rock into a stream, with waves propagating away from where the rock hits the water.  

In the atmospheric case,  big thunderstorms above the warm water act as the "rock,"


During the next few months as El Nino makes way for La Nina, the central Pacific water will cool, the thunderstorms will shift westward, and the wave pattern change substantially.




February 09, 2024

Precipitation Returns to the Pacific Northwest

There is some nervousness about the water situation in some quarters because of the current El Nino situation.

But although El Ninos tend to produce lower snowpack and warmer temperatures, the total precipitation is much less impacted.  Consistent with this, several wet systems are expected during the next week.

On Sunday, a wet system will move into the region,  providing welcome precipitation over the region, including heavy stuff over southwest BC.

This will be snow over higher elevations (above 4000 ft).

Another moist system will come in on Wednesday associated with a Pacific warm front (see below), with bountiful snow in the mountains.


The forecast over the next ten days, based on the European Center ensemble model, suggests copious precipitation over southwest BC and substantial amounts along the entire coast. 

 This is not a drought situation by any means.  In fact, California will be in an excellent situation regarding water supplies.


The US Army Corps of Engineers and others controlling our local dams/reservoirs will hopefully store as much of this wet bounty as possible.   They can do this because the weather models do not suggest any dangerous flooding situations for which they would need to draw down water levels.

And for those worried about water and climate change, here is an interesting factoid.   

There has been no decline in spring precipitation over the past several decades.  In fact, just the opposite:  the trends have been positive.

For example, here are the spring precipitation totals back to 1947 at SeaTec.   Upward trend (brown line).

Or consider Yakima, amid prime agricultural land in eastern Wa (below).  Also getting more moist.


So perhaps the talk of "languishing"  and  "little hope" should be put on the shelf somewhere.


February 07, 2024

A Dominating El Nino

Although the current El Nino may be entering its declining stage, the sea surface temperatures in the tropical Pacific are still quite war, and thus the impacts of the El Nino continue.

The evidence is very strong that this El Nino is the key driver of the low mountain snowpack this winter.    Let me show you.

Below is the latest snowpack situation from the SNOTEL network.  The western U.S. snowpack water content is below normal over the coastal states, ranging from about 80% of normal in California and Oregon to around 60% over Washington State.


So why this snowfall pattern?

The precipitation difference from normal for the past 60 days shows a relatively wet California and Oregon, but drier than normal conditions in western Washington and the Cascades.


And most of the west has been warmer than normal.  It all fits.  The temperature and precipitation pattern explains the snow distribution. 


But can we blame this on El Nino?  

NOAA has a nice webpage in which they show the typical winter conditions associated with El Ninos based on many events.

Precipitation? Wet California,  dry western Washington.  Looks like this winter!


The temperature during El Nino winters?  Warmer than normal over the West Coast.  Just like this year.

So the precipitation and temperature anomalies from normal fit the El Nino expectations like a glove.

Do you want even more evidence that the low snowpack has the fingerprints of El Nino all over it?

The warm western U.S. and wet California situation this year has been caused by persistent low pressure off the West Coast.    Here is a map of the difference from normal at the height of the 500 hPa pressure level (about 18, 000 ft) for December 1-February 5.   

An anomalous low is found offshore.  Such lows have warm southwesterly flow on their southern and eastern flanks.   This explains the warmth of this winter and wet conditions over California.  Note the higher-than-normal heights (pressures) over the northern Plains.

This is exactly the kind of pattern typical of El Nino years.   Want proof?  Another NOAA website has a composite of El Nino years for the same level.  Same pattern.  Strong low offshore.


Some local media are blaming the lack of Northwest snow on global warming, but they are wrong.  The cause is the naturally varying El Nino/La Nina oscillation.

A strong El Nino is the enemy of Northwest mountain snows.  

But this enemy has only months to live.  Wise reservoir managers understand that in such El Nino years, they must store as much water as possible during the rainy season.  The managers of the City of Seattle reservoir system are some of the wise ones--  Seattle's reservoirs are now well above normal! (see below).  I suspect there will be no lack of water for the Puget Sound region this summer.






How unusual is it for aircraft to exceed the speed of sound?

Announcement:  Doing an online zoom session with Patreon supporters at 10 AM tomorrow (Saturday) ______________________________________ The ...