Tuesday, September 18, 2018

Why is autumn air so wonderful?

I can't tell you how many people have commented to me the last few days about the wonderful quality of the air they are breathing.   Some are almost exultant, describing their pleasure in inhaling air that is brisk, clean, and cool---just perfect.    It is true--their is something about autumn air that is wonderful. 

So let's check on some of the characteristics of the air we are enjoying today.

First, it is clean---really clean--with the smoke of summer a distant memory.

For example, the particulate level (PM2.5 small particles that are bad for your lungs) is way down, as low as it goes around here (see plot since June 1 below).  Other pollutants, like low-level ozone, are also down.

With a lack of particles, the sky has been very blue, when the clouds were not there.  The image from the Seattle Space Needle Panocam shows the azure skies.  Blue skies and sun are a psychological plus, even for cloud-accustomed Northwesteners.

What about relative humidity?  According to a number of studies, the optimal relative humidity range for human health is around 40-60%, with low relative humidity promoting flu and breathing problems.  No problem.  Ou relative humidities the past few days have rarely dropped below 50% and have been around 50-60% every afternoon! (see plot below for the past 12 weeks)

Cool, crisp air is a pleasure for the lungs, but not too cold.  The plot of air temperatures at the UW for the last 12 weeks shows our perfect situation the last few days.  High temperatures in the mid-60s, but with temperatures only dropping into the mid-50s.  Ideal.

And there is something else, some essence of fall air that I can not describe with available numbers--some aroma from the falling leaves, a smell of rain on the previously dry ground---you know what I mean.

Clean, cool, unpolluted air of the proper humidity.   Fall is here...time to take a deep breath.  And time to appreciate living in one of the rare locations where it all comes together in such a wonderful way.

Sunday, September 16, 2018

Catastrophic Overuse

Is it possible that the media is tending to excessively use the terms catastrophe and catastrophic in described major weather events such as Hurricane Florence?

For example, watching my colleagues at the Weather Channel, the adjective catastrophic was used nearly continuously:  Florence would bring catastrophic winds, catastrophic storm surge, catastrophic rains, and catastrophic flooding.

Many environmental advocacy websites, such as Grist, went full into catastrophe mode with Florence.

Now when is the use of catastrophe and catastrophic suitable?   Let's check the venerable Merriam Webster dictionary for guidance (see below).   According to the dictionary, a catastrophe is a momentous tragic event, with effects ranging from extreme misfortune to utter overthrow or ruin.

The origin of the  English word catastrophe is  from the Greek word katastrophÄ“ ‘overturning, sudden turn,’ from kata- ‘down’ + strophÄ“‘turning’ 

Is a category 1 hurricane that was rapidly downgraded to a tropical storm, which produced a very modest storm surge and few reports of hurricane-forced winds over land a catastrophe?  Yes, there has been heavy rain and flooding in some areas, but such flooding is not unusual in an area periodically hit by tropical storms (e.g., Hurricane Matthew struck the region with similar impacts in 2016).

Catastrophe is far more appropriate for major, life-upending events, such was what Hurricane Katrina did to New Orleans or Hurricane Maria's effect on  Puerto Rico.  Those were true catastrophes.
New Orleans after Katrina. This is what catastrophe looks like
The National Weather Service is generally far more sober and responsible in their use of adjectives.  Words like dangerous, highly dangerous, life-threatening are more often used by National Weather Service forecasters.   They tend not to use catastrophe for more modest events.

As an aside, here is a graphic from GoogleTrends that shows the frequency of search including catastrophic.  Big increase with Florence, which is declining now as the storm fades away.

The use of over-the-top adjectives for major, but fairly regular, events undermines our ability to communicate the potential for truly disruptive storms.  Crying wolf will desensitize people to our messaging, endangering them when the really big events are predicted.   

Using screaming words like "catastrophic" may garner more clicks and viewership for a while...but in the end it will turn folks off.

Friday, September 14, 2018

Hurricane Florence: Stunningly Good Track Forecasts, Problems with Intensity

Hurricane Florence is has recently made landfall near Wilmington, North Carolina as a category one storm with sustained winds of approximately 85 mph (see latest radar image below).  It is now weakening rapidly.

Very few locations over land have experienced hurricane force winds (SUSTAINED winds of 74 mph or more) so far, but several coastal locations have received gusts of 70-90 mph (see max gust map below)

This storm is a "classic" on how well our forecast models have done:  superb track prediction and mediocre intensity forecasts.  And the U.S. models did very well on track--better than the European Center model.

To illustrate, here are the forecast tracks from several models initialized on September 9th (last Sunday) at 11 AM PDT.  Stunningly good prediction....aiming the hurricane landfall on the southern N. Carolina coast.  All the models were doing the same thing...giving us some confidence in the track.
Absolutely marvelous. 
 Looking at the position errors of the storm (which includes the position along the track as well), shows increasing position errors in time for the various models, with the U.S. GFS model being the best.  Nice to see.  (US GFS is AVNO and the European Model is ECMF, plot produced by Professor Brian Tang, U. of Albany)

Clearly, track forecasts are very, very important.   Anyone with a bit of sense will prepare or evacuate (in vulnerable areas like the coast) if a hurricane is heading towards them.

But it would be nice to get intensity right as well.    For the same 120 hr period shown in the track figure above, here is the intensity (central pressure of Florence) that was observed (black line) and forecast (colored lines).  The observed storm strengthened rapidly during the early period, leveled off, and then weakened .  The operational global model (GFS, shown by the blue AVNO line) was initially too weak and was too strong at the end.  The new operational model (FV-3) was not deep enough for most of the storm.  The high resolution US hurricane model (HWRF, purple line) was way too deep more of the time, including at landfall.

As I have noted before, getting track right demands getting the large scale flow correct--which we can pretty much do today.  But getting intensity right demands we simulate the internal dynamics of the storm at high resolution...and do it correctly days ahead.  Very difficult to do...and may be impossible according to some research (including research done at the University of Washington by my colleague Greg Hakim and students.)

One final piece of interesting hurricane information.  Do you think the number of landfalling storms on the U.S. continental (mainly the SE US) is increasing, decreasing, or staying the same?   I suspect many would say increasing, based on the considerable hype in the media.

The truth?  A slow decline during the past few decades as shown by several papers in the peer-reviewed literature and government statistics.   Here is a plot (a) from a 2018 paper by Klotzbach et al. in the Bulletin of the American Meteorological Society for hurricane landfalls on the continental US (1900-2017).  Slight downward trend.  Similar trend for major (categories 3 and above) hurricanes.

The big question is what will global warming do to the frequency of US landfalling hurricanes. Some research suggests that the most intense hurricanes will get more frequent, but the total number will decline.   But what will happen to landfalling U.S. storms is something that is actively being studied.

Wednesday, September 12, 2018

Changes in Uncertainty for Hurricane Florence: Are We Communicating this Well Enough?

Hurricane Florence has been a study in contrasts.   For the last several days its track has been one of the most predictable on record.  Most major models have been spot on, the various ensembles have been tightly clustered, and the uncertainty in the forecast tracks is low.

Then as the storm approaches the Carolina coast everything changes:  the predictability of the storm is greatly reduced with all kinds of possible forecasts, including stagnation along the coast.

Why did this happen and how can my profession communicate such changes in predictability better?

To see the issue, here is an ensemble forecast from the European Center system showing the 51 forecasts started at 5 AM PDT on Monday.  Remember, that in an ensemble system a forecast model is run many times varying the initial conditions and model physics--showing the range of potential events.  This figure shows the probabilities of the storm being at any location, based on the ensembles.  Offshore there was little uncertainty in track but the the tracks really diverge near the coast.

A similar situation is true for the NWS global model ensemble (GEFS)

In contrast, the tradition way of showing uncertainty by the National Hurricane Center shows a progressive increase of uncertainty (see below).  This is because they simply use historical errors over time based on many forecasts.  The current figure communicates a loss of skill over time regarding the track forecasts, but there is a huge loss of information found in the ensembles...and such uncertainty information is very valuable.  Ensemble-based diagrams are far better.

Why were the forecast tracks reliable and tightly clustered out over the Atlantic, but all over the place near land?   It has to do with large-scale (or synoptic) steering flows.   To first order, you can consider hurricanes as huge tops that are pushed around by the large scale flow.  During the past few days, Florence has been steered westward by high pressure to the north of the storm, as illustrated by the sea level pressure map for Monday at 5 PM PDT.  The "L" due east of southern Florida is Florence.

The upper level (500 hPa) map for  11 AM on Monday shows high pressure north of the storm that was helping steer Florence to the northeast.

Similarly for 11 AM PDT on Tuesday.  A high pressure area to the north was effectively steering the hurricane towards the coast.

 But the situation on Thursday at 11 AM PDT is  very different.  A large ridge of high pressure is over the eastern US, without much flow over the SE U.S.  And weak high pressure surrounds the storm--thus there is little steering flow to push the storm in any direction. 

The dangers of a stagnating storm are substantial, particularly the potential for heavy precipitation, with some models going for 20-30 inches in some locations. 

Our models are now capable of providing useful information of how hurricane track uncertainty will change in time...the challenge is to find ways of better communicating the information.

Monday, September 10, 2018

Hurricane Florence: The Old Versus the New U.S. Global Weather Forecasting Model

Early next year, the U.S. will move to new global model.   The new model (FV-3) was developed at the NOAA Geophysical Fluid Dynamical Lab (GFDL) in Princeton and will replace the old Global Forecast System (GFS) model that has been around forever.

The new model is being run in parallel right now and fascinatingly, it is providing very different forecasts than the current system for Hurricane Florence, which is now approaching the SE U.S..

 Which will be right?  You will know in a week.   But first, let me show you the amazing differences.  Here is the forecast for 5 AM PDT September 14th of sea level pressure.  Both have Hurricane Florence on the Carolina coast, by the differences in central pressure is HUGE!:   913 hPa for the old GFS, 979 hPa for the new FV-3.

And  the forecast two days later are stunningly difference.  The GFS has a very deep low (913 hPa) hanging around on the coast, but the FV-3 has weakened and moved inland.

The track forecasts (see below--AVNO is the GFS, FV3G is the new model)) are initially similar, but near the coast, FV-3 swings farther south (bottom left) and the intensity forecasts are hugely different, with FV3 having weaker winds.

Considering more general verification, here is a representative upper level (500 hPa, about 18,000 ft) for the five-day forecast (see below).  The field below is anomaly correlation (1 is a perfect forecast) and you are looking at the verification of 500 hPa heights for the five day forecast.  The European Center (ECM) is the best, but new model is tied for second place  with the UKMET office.   This is improved performance.
FV3  (red line) appeared to perform better that the GFS (blue line) for the recent Hurricane Lane near Hawaii, with much lower track errors (see below)

Hurricanes are a big concern and the National Weather Service decided to replace the GFS after problems for Hurricane Sandy in 2012.  Thus, a lot of eyes will be watching the performance for Florence. The implications of the differing forecasts for Florence are huge---the next few days should be interesting.

Saturday, September 8, 2018

The Fall Transition

Summer is over west of the Cascade crest.  Heat waves are done.  Dense smoke in western Washington is finished. 

Take out your sweaters and rain gear.   You will need them.   A reason to be thankful we are living in the Northwest rather than central and southern California, which the threats of fires and smoke will continue for several months more.

Let's take a look at the latest ensemble prediction of the NOAA/NWS GFS model for surface air temperature at Sea-Tac Airport (remember an ensemble is when we run the model many times using different initial conditions or model physics).  The black line is the mean of the ensemble (generally a very good forecast) and you can see the prediction of the various ensemble members. The forecasts go through September 16th.

No heat waves..in fact, we expect cooler temperatures, with Monday and Tuesday barely getting into the 60s!    Then a very modest warm up with highs in the 60s.

And with the cooling temperatures, there will be multiple opportunities for light rain (see ensemble predictions for 3-h precipitation).  
Why all the changes?  Because of a major reconfiguration of the atmosphere with the development of a persistent upper level trough over the northeast Pacific.  

To illustrate, here is the upper-level map (500 hPa, about 18,000 ft ASL), showing a major trough right off our shore.

Wednesday at 8 AM...wow... BIG trough over the northwest.  A very cool pattern for us.

 The problem for heat wave lovers among you is the rapid weakening of the sun, which has a big impact no matter what the meteorology is doing.  

We are running out of sun (and time) for warm weather.   To give you an idea, here is the climatological temperatures for Sea Tac Airport, with the yellow line showing the daily max temps.  The vertical green line shows today.  Temperatures have NEVER gotten above the low 90sF for the rest of the month and soon never above 90F. 

The super-extended forecasts (e.g., the NAEFS US/Canada ensemble) show no hint of getting back into the 80s.  The cooler weather should be putting a damper on fire growth west of the Cascade crest, and increasing westerly flow will keep east WA and Canadian smoke away from us.  For me, I suspect the calls about "smokestorms" will be over.   

Thursday, September 6, 2018

Light Smoke Reaches Western Washington But Won't Last

The sunrise this morning had a reddish twinge to it and the Seattle Space Needle cam clearly showed a haze layer on the eastern horizon (see below)

A thin veil of smoke has extended over western Washington, with far higher values east of the Cascade crest.   The excellent http://wasmoke.blogspot.com/ web site shows the air quality situation at 7 AM (see below), indicating moderate air quality in most of eastern Washington, extending into Puget Sound and Northwest Washington.    Fine on the coast.

As shown by the small particle concentrations at the Seattle Duwamish site (see below), values have increased to around 30 micrograms per cubic meter--enough to produce a light haze that obscures the Cascades and Mount Rainier.

But some perspective is needed....this is spare change compared to what we experienced a few weeks ago (see plot since August 19th below).    Based on forecast models and the latest wind observations, I don't expect the values to get much higher in Seattle.

Why did we get some smoke?    Because the winds turned easterly (from the east) aloft and some of the smoke from the fires over NE Washington headed our way.  This is evident from the time-height cross section at Sea-Tac, which shows winds and temperatures with height over time (time increases to the left). The y-axis is height in pressure--850 is about 5000 ft, and the wind barbs are in blue.
The latest visible satellite image shows some light smoke over western WA (you can see some from the Maple Fire on the Olympic Peninsula), but huge plumes of smoke are heading northeastward over Oregon, heading towards southeast WA.

And the 10:45 AM image from GOES-16 shows smoke coming from the Maple Fire on the Olympic Peninsula and a new fire north of Mt. Rainier.  You can see the low-level haze around Puget Sound if you look closely.

Let's check out the NOAA HRRRsmoke model to see what is happening and will happen.  Below I will show integrated smoke...the total smoke in the vertical column....not smoke at the surface.

At 6 AM today , low levels of smoke are found over Puget Sound (blue), with serious smoke near the NE WA fires and in a dense plume originating out of the fires of SW Oregon and northern CA.

Similar situation at 2 PM today.

But tonight (11 PM) the onshore flow over the region will increase as an upper-level disturbance approaches and the smoke will be pushed out of western Oregon and Washington.

 And by tomorrow at 10 AM, most of WA state will be able to breathe freely, except for folks immediately downstream of the NE WA fires and the far SE corner of the state.

Clean air is expect for the weekend.

Tuesday, September 4, 2018

What kind of weather pattern produces lots of smoke in Puget Sound?

Even with all the talk about smoke here in Puget Sound, little real research has clarified the weather patterns associated with our smoky days.

OK....let's address this deficiency!

I will do this by using a technique called compositing--- finding a series of times of big, recent smoke events and then averaging weather maps for these times.  Common features should reinforce themselves, showing us the essential circulation patterns accompanying smoke.

The times I will composite are the most smoky times during the past three years:

8/21/2018    1500 UTC , 8/15/2018   0300 UTC , 8/4/2017 1200 UTC

8/9/2017 1200 UTC, 9/7/2017 0000 UTC, 8/23/2015 2100 UTC

First, let's composite a representative mid-level parameter-- 500 hPa heights (think of it like pressure around 18,000 ft)--see below.

Mama mia, there is quite a signal!  Big ridge of high pressure over the West Coast, with troughs (low pressure or height) over the eastern Pacific and the eastern U.S.

If you really want to see the wave-like anomalous upper circulation, here is the same map with the mean values for the period taken out (these are called anomalies from climatology).

Amazing. When we have smoky periods, anomalous high pressure is found over southeastern BC. Such high pressure is associated with warm, sinking air aloft. In contrast, the eastern U.S. has a cool trough when we get smoky.

What about at the surface?
There we tend to get lower heights (or pressure) over western Washington due to the warm air aloft (see below). We call that feature a thermal trough. In such a pattern, the normal east Pacific high pressure area is pushed offshore, other high pressure builds inland, and the onshore flow of clean ocean air is interrupted. In fact, with high pressure inland, we tend to get offshore (easterly flow), which pushes inland smoke over Puget Sound.

This pattern--high pressure aloft centered over southeastern BC, thermal trough over western WA, lack of onshore flow, and modest offshore flow-- is very good for pushing  smoke over western WA. And it brings warm temperatures and dry conditions across the region...good for fires.

But the question you are asking is whether this pattern has become more frequent in time and whether global warming could be the cause.  A very good question.

To help answer your question, I have plotted for over many years the values of the 500 hPa heights (think pressure at roughly 18,000 ft) over exactly the region associated with our heat waves-- the area centered on SE British Columbia-- for July and August (see below).  Higher heights (or pressure) is associated with our smokiest periods. (see below).  This figures does not provide insight into what happened during height of the smoke, but the general environment of the surrounding months.

No wonder this was the big year for smoke!  This year had the highest heights/pressures aloft since the late 1940s.  Heights appear to have risen over the past decades, which is expected as the region (and the planet) have slowly warmed.  Warming causes air to expand, which increases the heights aloft.  This year's spike up seem anomalous and unprecedented, and this is why is would be unexpected to have a repeat next year.

 Why do I say that?  Because I have done extensive research (with graduate Matt Brewer) on exactly this issue.   Heights aloft are controlled by the mean conditions (which are slowly rising with the warming earth) and transient high pressure areas, like the one over southeast BC that was associated with the smoke.    Our research (based on past trends and climate models for the next century) suggests that transient high pressure areas should be attenuated under global warming--which would work against the fires and smoke.   On the other hand, global warming would  warm temperatures and thus help reduce relative humidities,  which thus encourage fire and smoke.

So the bottom line is this.  The anomalous patterns (big ridge of high pressure along our coast), which was the proximate cause of our warmth/dry conditions/fires/smoke this year, were probably not the result of global warming.  Thus, they are probably due to the chaotic, random nature of the atmosphere.   The small background warming we have experienced in our region (1-2F) helped the fires/smoke slightly but weren't dominant factors.

But by the end of the century, as global warming really hits, the potential for Northwest fires will substantially increase, everything else being equal.