Monday, September 18, 2017

Normal Weather Returns to the Northwest (Hint: Mountain Snow, Heavy Rain, Wind, Clean Air)

Just when folks thought that heat and smoke would continue forever, normal weather returned to the Northwest.

Let's see....we had light snow above 5-6 thousand feet, providing a dusting at Timberline Lodge.

Air quality improved over nearly the entire region (green circles) as strong onshore flow brought in clean air and the cool/wet weather put down the current fires.


The infrared satellite imagery shows one system after the other, with an upper level trough and very unstable air moving through the region, bringing heavy showers and even some lightning, as I write this.


The radar image for around 8 PM (Monday) shows bands of moderate to heavy (yellow-orange) precipitation moving through.


And the precipitation totals for the 24h ending 7 PM Monday show rain all over the region, with some places (western Olympic Peninsula and the southern WA Cascades) getting over a inch.  Some locations east of Portland got two inches.  The weather gods want to stop the fires in the Columbia Gorge.


The cause of all this frisky weather?  An upper-level trough that replaced the unending upper level ridge of the western U.S. (see below)


 But there is still a possibility we could achieve an amazing record:  the driest calendar summer (June 22-Sept 21) in the history of Seattle.  

Yes, it is still possible.  The record is .58 inches.  So far we have had about .30 inches.  More rain is coming...   Keep your fingers crossed.





Saturday, September 16, 2017

La Nina and This Winter's Weather

I am always nervous about predicting the character of the upcoming winter's weather for a number of reasons.  Seasonal forecasting skill is not good, with our long-range numerical models having very little skill past three weeks. Furthermore, our main seasonal forecasting tool with any skill, the relationship between El Nino/La Nina and local weather, only explains some of the interannual (between years) variation.  In addition, the state of the tropical Pacific (which determines whether we are in El Nino, La Nina or La Nada) often changes during the spring/summer.

Earlier in the year it appeared that we would have a neutral (or La Nada) winter, but recently the waters of the tropical Pacific have cooled and the National Weather Service has released a La Nina Watch (see below).

La Nina is associated with cooler than normal waters in the central and eastern tropical Pacific Ocean, with enhanced easterly trade winds near the equator and the shifting of convection (thunderstorms) westward in the Pacific.


Moving the convection to the west has huge impact on the rest of the atmosphere, even outside of the tropics.  This is the source of long-range forecast skill with El Nino/La Nina.

Let's look at the change in tropical Pacific sea surface temperatures for the Nino 3.4 region (see map below)


Examining a map of anomalies (differences from normal) reveals that during the past few months, the Nino 3.4 area ocean temps have dropped below normal (blue color).

Just as important, the temperatures BELOW the surface have also cooled. Here are a series of views below the surface at the equator (the x axis in longitude across the Pacific, and the y axis is depth below the surface) for July through now.  Cooler than normal temperatures (blue) have developed.


 Atmosphere/ocean coupled models, such as the National Weather Service CFS, are predicting the tropical Pacific will cool further this winter (see graphic below of several of their predictions in time).
 And with all this input, the official NOAA/NWS Climate Prediction Center forecast for this winter is that La Nina will be observed (see below).


 OK, what does all this imply for the Northwest winter?  Generally cooler and wetter than normal.  More snow than average in the mountains.

Here are some statistics from the National Weather Service for the region encompassing western Washington and the western slopes of the Cascades. The eastern side of the State is similar.

For precipitation, two plots are shown, one for fall (OND) and the other for mid-winter (JFM).  The red line is the mean and 50% of the years are within the blue boxes. The extremes are shown by the "whiskers".    La Nina years tend to be wetter than normal (neutral) and El Nino years.


 What about temperatures?  A bit cooler in the autumn, but much cooler during mid-winter.


 The implications for snow is clear, especially after January 1.... a higher probability of the white stuff, particularly in the mountains.  Yes... a reasonable year to get an annual pass at your favorite ski area.

The strength of this relationship depends on the amplitude of La Nina, and at this point the models are only going for a modest one.  And the La Nina/El Nino connection is not dominant, with natural variability being larger.  Finally, one should NOT expect more precipitation than last winter, which was the wettest on record by several measures.

But after the smoke and heat of last summer, I suspect many Northwesteners are breathing a sigh of relief.  And the upcoming week promises plenty of clouds and rain to get us in the mood.

Wednesday, September 13, 2017

The Hurricane Irma Forecast: Triumph or Disappointment?

The quality of the National Weather Service forecast model projections for Hurricane Irma have received a lot of comments in the mainstream and social media.  Some have been positive, but others have been critical, suggesting that the NWS models have performed poorly, lagging behind the well-known European Center model.



Others have criticized the changing hurricane track, particularly the uncertainty over which side of Florida that hurricane would traverse.


Other media outlets noted apparent errors in the storm surge forecasts.

So how good were the American and other weather forecast models when it comes to Irma?
Have we made progress? 
Are there still problem with U.S. forecast models, something that became well-known during Hurricane Sandy in 2012? 

This blog will take on these questions.

My bottom line:  the Irma forecast was a triumph for the weather prediction community, but it also revealed continued problems with U.S. numerical weather prediction and our ability to communicate the uncertainty in model forecasts.

The Triumph

In many ways, the extended forecast of Hurricane Irma was an extraordinary triumph for weather prediction technology, with major global modeling systems (e.g., US GFS, European Center, Canadian, UKMET) suggesting a major threat to Florida a week or more out.   Even a decade ago, we could not have done this well.


Let us begin with a review of Irma's track (see below with legend).  Starting offshore of Africa as weak tropical disturbance, it headed westward, revving into a hurricane east of the Cape Verde Island, and by the time it approached Puerto Rico, Irma had exploded into a category 5 storm.   Subsequently, it moved WNW until it paralleled the northern Cuban coast before taking a sharp right turn that sent it across the Florida Keys and then northward over the western side of the Peninsula.  Irma made landfall on Florida on Sept. 10th.


Below are the ensemble track forecasts for Irma from the U.S. GEFS system (21 members or individual forecasts) and the European Center (ECMWF, 51 members) for the ten-day forecasts initialized on August 30th at 1200 UTC.  We use ensemble forecasts to get an idea of forecast uncertainties and to produce probabilities.

Starting with the European Center ensemble, although uncertainty increases in time, most of the ensemble members are taking a strong storm towards Florida.  A big warning MORE THAN TEN DAYS AHEAD of U.S. landfall.  Amazing.


The smaller U.S. ensemble (GEFS) initialized at the same time is also bringing Irma towards the U.S., but has a greater tendency to bring to storm up the Atlantic coast.


Now, let's examine the ensemble predictions for the two systems for the forecasts initialized two days before landfall (Sept 8th at 0000 UTC).  I will use the wonderful graphics produced by Professor Brian Tang of University of Albany (as an aside I just finished a wonderful visit there, meeting with Brian and his colleagues/students).  The individual ensemble are shown by the thin white lines and probabilities based on these tracks are shown by the shading.  The official (National Hurricane Center) forecast is shown by the black line and high-resolution version of the modeling system by the red-dashed line.

Starting with the European Center forecast, the model predicted the sharp right turn and the high probability for the storm to pass along the western half of Florida. Quite good.


The U.S. GEFS forecast also had a right turn, but it was taking the storm more along the eastern side of Florida, which was not correct.


Although the European Center solution was clearly superior, both U.S. and EC forecasts are very good....showing the threat to the U.S. more than a week ahead of time and predicting a sharp right turn days before. Other major modeling systems, such as the United Kingdom and Canadian models, did the same thing.  Predicting the exact location of the right turn days ahead is simply beyond the science at this time and may always be, but the models were all excellent in predicting that such a turn would occur in the vicinity of Florida.

A triumph for the technology of numerical weather prediction, with substantial credit going to those who have built the complex observing and modeling systems that made this possible.

Whispering warnings

During ancient Roman triumphs, a slave would stand behind the victorious general whispering in his ear "remember you are mortal" and I will act in this role now, at least for the American conquerors.

The U.S. global model was clearly inferior to the European Center model for this hurricane, as it was for Hurricane Sandy, Hurricane Harvey last month, and for many others.

To show this, here are the forecast errors for the hurricane locations (track errors)for the high-resolution forecasts of various global models and some of the U.S. hurricane models for 12 to 120 hours (the graphic produced by Brian Tang).  Track errors increase with time, as would be expected.  The U.S. global model (GFS, shown as AVNO--dark red) track errors are MUCH larger than the European Center (orange, ECWF), particularly for the longer forecasts (370 km error for the U.S. and 185 km error for the EC at 120 hours).  For many hours, the U.S. track error is TWICE the EC error.

Just as concerning, the high-resolution U.S. hurricane models had track errors that were substantially worse than those of ECMWF, including the newest U.S. hurricane model (HMON, light green) and the model that was developed (at a cost of tens of millions of dollars) over the past five years, HWRF (aquamarine).

Hurricane Harvey?   Simliar story,  with HMON going wacky at some hours.

Many of the HMON forecasts were completely out to lunch, producing unphysical results.  For example, here are the pressure forecasts for HMON (green), HWRF (purple),  and observed (black).  HMON took the storm down to a completely unrealistic 850 hPa central pressure (observed was around 930 hPa).  With crazy pressures and often large track errors, HMON clearly has very serious problems and should not be shown publicly.  Why it is even necessary is another major question.

 The general superiority of the European Center model is also suggested by other statistics, including the 48h track errors for tropical storms and hurricanes produced by the National Hurricane Center for the last several decades.   Clearly, there has been great progress since 1984, with track errors going from around 200 km to less than 100 km.   That is the triumph stuff.  But this sample of many storms shows that the European Center is consistentlythe best (light blue dots).  The EC would be even more dominant at longer projections.


So, if was whispering in the ears of an American weather general, I was note the following:

  • The ability of the U.S. to forecast hurricanes has clearly improved. Congratulations.
  • Five years after Hurricane Sandy, the U.S. has not caught up to the European Center, with both improving at roughly the same rate.   We need to do better.
  • The European Center does a better job at assimilating a wide variety of observations and their model has better physics (e.g., descriptions of moist processes) than the U.S. models--we need to up our game.  The proposed new U.S. modeling system (FV-3) is not going to fix these areas.
  • The U.S. has spent large amounts of money on hurricane models (e.g., HWRF and now HMON), but in many ways they are inferior to a coarser global model (EC), particularly for track forecasts.  
  • A clearly deficient hurricane model has recently been developed (for reasons that are not clear)...HMON.  It is not ready for prime time, so why show it?
  • There was a lot of confusion about which side of Florida would be hit by Irma.  Much of this confusion can be traced to inadequate communication by the National Weather Service and the media, with substantial misunderstanding of ensemble prediction by the lay community.  We need to do much better in this domain (more in a future blog).
  • National Weather Service verification of model quality and hurricane forecast skill is very poor.  Why do we have to depend on an innovative professor (Brian Tang), and unofficial web sites (e.g., weathernerds) to supply such information?
A triumph for sure but many problems remain for U.S. hurricane and numerical weather prediction.



Tuesday, September 12, 2017

The Northwest's Summer Drought Will End on Sunday

It has been an amazingly dry late summer, with very little rain during a period when showers typically return.

If you want to be impressed, here is the cumulative precipitation at Sea-Tac Airport for the last 6 months that actually occurred (purple) and which is normal (blue).   You might be startled to note that the precipitation for the past 6 months has actually been ABOVE normal, but that was due to an unusually wet spring.  For most of the warm season (since approximately 15 June), Seattle's precipitation has been essentially flatlined.  Earlier in the summer (before late August) this is not that unusual, but having nothing for the first half of Sept. is not a common occurrence.

Dry conditions will continue through Saturday, so enjoy the nice weather while you can.  Here is the cumulative precipitation for the entire country from the NWS GFS model through 11 AM Sunday.  Washington is pretty much dry.


But later on the day on Sunday, the first substantial frontal system will come through...one that will provide a real wetting-down of the region.  The 24-h precipitation ending 5 AM Monday provides the story, with 1-2 inches in the mountains and light rain east of the Cascade crest.  This event will provide substantial aid for reducing the wildfires that are still burning in our region.


The cooler weather of late has restrained the regional fires-- as shown by the MODIS satellite imagery yesterday, many of the fires are still burning, with local smoke.  But the huge plumes of the past few weeks are not apparent.

 The excellent weather.com forecasts for Seattle (partially based on National Weather Service models),  show that the really warm temperatures  (85F and more) are history for us, with 70s and then 60sF in our future.  The sun's radiation is rapidly weakening now as the day's shorten---and the atmosphere is finally responding.




Saturday, September 9, 2017

Hurricane Irma Takes a Western Route Northward

It is looking increasingly likely that Hurricane Irma will take a more western route than initially predicted, with negative implications for the west coast of Florida and more benign conditions for Miami and the populous eastern side of the peninsula.

The NWS Key West weather radar at 9:34 PM EDT Saturday was clearly picking up the storm's eye and spiral rain bands.

Wind gusts so far have gotten to 55-65 mph at some coastal locations, with the worst yet to come.


The European Center Model has been the most skillful weather prediction model for this storm, and its latest forecasts (initialized Saturday morning) has most forecast tracks (from its ensemble system) moving up the Florida West coast (see graphic courtesy of WeatherBell Analytics).

As I noted in my last blog, this is a VERY hard forecast, with the storm predicted to make a sharp right turn near a relatively narrow peninsula.   Small errors in position and time of the turn have HUGE implications for the forecasts.  The storm has also weakened substantially (now a category 3), with the latest model runs suggesting some intensification (perhaps to a cat 4, but NOT a cat 5).

Let me show you the pressure and surface (sustained) wind predictions from the European model, courtesy of WeatherBell.  At 8 AM EDT Sunday, Irma is nearly over the Florida Keys,

 By 8 PM Sunday (EDT), the eye is near the SW Florida coast,
 And near Tampa at 8 AM Monday.  The strongest winds stay on the west side of the Peninsula, where substantial damage would occur.
Similarly, the largest precipitation totals (5-15 inches) remain over the west side the peninsula (see graphic).  A huge storm surge is probable along the western shores.

In a later blog, I will analyze the predictions of the various modeling systems for this event.

Thursday, September 7, 2017

American Versus European Models and Irma's Big Turn

Hurricane Irma, which passed north of Puerto Rico last night, is a remarkable storm, with estimated sustained winds of 180 mph.  A category 5 hurricane and one of the strongest on record.  The radar image last night from the NWS radar on Puerto Rico showed a double eye structure, which is not uncommon during eyewall replacement cycles.


But what is equally remarkable is the predicted track of the storm, in which most model forecasts suggest will include a sharp right turn as Irma reaches Florida.   Such a turn has many implications for forecast uncertainty and the impacts of this hurricane on the U.S. southeast.

Let's look at the forecast tracks of Irma, first using the vaunted European Center ensemble in which many forecasts are made, each slightly different in terms of initial state and model physics.   Virtually all forecasts direct Irma WNW initially before turning right near Florida.

  But the tracks tends to spread out in time, with some just offshore, some over Florida, and some even over the Gulf.  The impacts, both in terms of wind, storm surge and precipitation would be very different depending on exactly which track the storm actually follows.



The U.S. GFS model, with less members in its ensemble and coarser resolution, is generally similar, but with more members offshore.


The general agreement between the U.S. and European models is considerable, and other national modeling systems are doing similar things.  So I think we have great confidence in the turn.

But why the sudden turn?

Because of the passage of an upper-level trough to the north, with Irma starting to feel the steering effects of the westerly flow of the midlatitudes.  The forecast 500 hPa chart (around 18,000 ft) with heights and winds illustrate the predicted environment as the hurricane approaches Florida.


The trouble with such tropical-midlatitude interactions as hurricanes move north is that such situations can produce a lot of uncertainty, something suggested by the ensembles.   It is sort of like jumping on to a rapidly moving merry-go-round.  You know generally where you are going once you get on, but you are not sure which horse you will end up on.  A very challenging forecast.   But with a storm this large and powerful, anywhere in southern Florida will have serious impacts.


Last night's high resolution European Model run was scary, with the storm making landfall near Miami and very strong onshore flow to its north, which would produce a large storm surge, with water being pushed up on the coast north of the low center (see below).  Storm surge prediction is something the National Weather Service is lagging behind in and no such guidance is available yet for Irma, based on the ensembles shown above.  When Irma is closer to Florida, surge forecasts will become available.

Accumulated precipitation from this European Center run?

5-15 inches.  Heavy but not like Harvey.  Why?  Because the storm is not predicted to stall.