Monday, September 24, 2018

Super Convergence Zone Dumps Heavy Rain

One of the most striking peculiarities of western Washington weather is the Puget Sound convergence zone  (PSCZ), which can produce a heavy band of precipitation across northern Puget Sound and into the Cascades.

The PSCZ  results when low-level winds on the coast are westerly (form the west), move around the Olympics, and then converge together over Puget Sound, producing upward motion, clouds and precipitation.

This is exactly what occurred Saturday evening, as illustrated by the plot of surface observations at 7 PM (a large scale and close up view is shown below)

The air was relatively unstable Saturday evening and an upper level trough was producing additional the convergence zone really revved up.    

Now let me impress is a radar image around 7 PM Saturday.  Red indicates very heavy rain, and yellow is moderate to heavy. Intense convergence zone band.  Note the sharp southern edge of the rainfall, which is very typical of strong convergence zones.

There was substantial convection and thunderstorms in that heavy precipitation band, and lots of lightning was noted by the regional lightning sensors (see below-24h lightning ending 1 AM Sunday)

Seattle RainWatch combines radar and observations to provide a good estimate for rainfall.   Here is the RainWatch 24-h total ending 5 AM Sunday.  A substantial area got 1-2 inches, which even more in limited regions.  Pouring over Snohomish County but downtown Seattle was dry.   You got to love living around here.

Local rain gauges had 24-h rainfall totals that were consistent with RainWatch, with over 2 inches around Monroe.

I know someone that had a wedding celebration near Monroe...thankfully they had tents, but it was a soggy affair.

Now the really exciting part for me is how well this event was forecast....our high-resolution models was very skillful, days ahead of time.  To illustrate, here is the forecast from the UW WRF high-resolution (4/3 km grid spacing) model for the 24h precipitation ending 5 AM Sunday.  

Nice convergence zone.  It underplayed the total amounts a bit...but it was clear that Snohomish County was going to be drenched, which downtown Seattle would be dry.

Saturday, September 22, 2018

REX Will Dominate Northwest Weather This Week

During the past two weeks, the Northwest has enjoyed absolutely normal weather.   Typical temperatures (see plot below, purple and cyan are normal highs and lows) and nearly normal precipitation (see cumulative precipitation for the same period).
 Normal has been good for us, giving relief to our plants, restoring water to the surface soil layers, and radically reducing the water usage in urban areas such as Seattle (see below).

The occasional precipitation has been associated with transient upper-level troughs that have moved through the region, such as one moving across the Northwest this morning (see upper level, 500-hPa, ap for 5 AM this AM)

But this week something interesting is going to happen:  a very stable REX BLOCK will develop over the eastern Pacific, shutting off precipitation, leaving us with lots of sun, dry conditions, and moderate temperatures. 

It will be wonderful.

So what is a Rex Block?   It is a configuration of the atmosphere where the upper level circulation has a ridge of high pressure (or heights) north of a trough of lower pressure/heights.  Here is a schematic of such a situation.

A Rex Block is a very stable atmospheric configuration, with the ridge and trough reinforcing each other, keeping the flow pattern in place  (for reasons I talk about in my graduate synoptic/dynamics class, but won't go into here).

The ridge/high part produces descending motion and fine weather, particularly on its eastern side.  Storms are sent far northward--in the example shown, well into Alaska where they belong.

The Rex Block has nothing to do with kings or dinosaurs, but with a meteorologist named D. F. Rex, who wrote a seminal paper on this features way back in 1950 (see below).

Rex, D. F. (1950). "Blocking Action in the Middle Troposphere and its Effect upon Regional Climate". Tellus. 2 (4): 275–301.

Now that you are Rex-trained, lets look at the upper-level forecasts for this week.

11 PM on Sunday?  Rex is here!  Huge ridge of high pressure/heights over the northeast Pacific, with a low underneath.

 2 PM on Tuesday....classic Rex!

 5 AM on Thursday.   A Rex fiesta.

The REX block shifts westward on Friday (see below), allowing a trough to move southward over us late Friday night.   Some showers and cooling if it happens.

Will the REX block reestablish itself?   Stay tuned.

Wednesday, September 19, 2018

Time to Drop the Saffir-Simpson Hurricane Scale

One of the most familiar aspects of hurricane season is the constant talk about the strength of developing storms using the Saffir-Simpson scale, which ranges from Category 1 (a marginal hurricane) to Category 5 (a powerful monster).

Today's Saffir-Simpson scale is based only on the maximum SUSTAINED wind of a hurricane, the average wind over a few minutes and not the peak gusts.

As we shall discuss below, the S-S scale is an anachronism that poorly communicates the real threats accompanying hurricanes, and in fact can be quite deceptive, resulting in people being unaware of the real dangers that can threaten them.  It is also too hurricane centric, not highlighting major threats from "lesser" tropical storms and disturbances.

Hurricane Florence is a good example of the problem.  It reached the Carolina coast as a marginal category 1 storm, and few inland location experienced hurricane-force winds (74 mph or more).   But because the storm slowed to a crawl as it made landfall, very large rainfall amounts (as much as 30-40 inches over five days near the coast) occurred, producing very serious flooding.

So folks in vulnerable areas might have felt complacent when they heard that "only"  a category 1 storm was approaching.    And even weaker tropical storms, not even considered a hurricane, can produce similar levels of precipitation, such as Tropical Storm Allison in 2001 (41 inches in Beaumont, Texas).

Hurricanes can produce serious damage in a number of ways:

1.  From the effects of the strong sustained winds and gusts damaging buildings and other structures.
2.   From storm surge, in which the hurricane winds push water up on to coastal regions and into rivers open to the ocean.  Storm surge is generally the most damaging aspects of hurricanes.
3.  Heavy rain, which can result in flooding, both near rivers and in low areas.

Storm surge is the greatest killer in hurricanes

The problem is that the Saffir-Simpson scale only quantifies the winds.

But the deficiencies of the S-S scale are far worse than that:

1.  It does not quantify the size of hurricanes, which hugely impacts the potential to do damage.  Hurricanes vary substantially in size, with the big ones obviously able to cause far more extensive damage.
2.  It does not quantify the amount of storm surge, the real killer.
3.  It does not quantify the amount of rainfall over any period.
4.  It does not quantify the level of flooding in the interior.

And it is worse than that.  

The effects of identical hurricanes can be very different depending on their speed of motion.  Slow moving storms like Harvey or Florence, did major damage because they moved very slowly, allowing big rainfall accumulations.  If they had moved more quickly, as for most storms, their effects would have been radically less.

And worse than that.

The effects of a storm can vary by terrain and coastal bathymetry (variations of water depth offshore), allowing identical storms to have differing impacts depending on where they hit.

Replacing the Saffir-Simpson Scale

The S-S scale made some sense when atmospheric and hydrological sciences were in a primitive state; when we lacked the capabilities to model and forecast the details of tropical storms and hurricanes and their impacts.

But times have changed.  Our ability to forecast hurricane tracks out nearly a week is now stunningly good.  And closer in time, we have good skill in predicting wind, rainfall, flooding, storm surge and the like, including the geographical distribution of the threats.     That is why the WeatherChannel folks and others were pushing the "catastrophic" rainfall and flooding threats during the day before landfall.

So why not drop the problematic and often confusing measure of hurricane strength (and one that neglects often-dangerous tropical storms) expressed in the S-S scale and simply warn folks of the specific threats, such as high-wind warnings, heavy precipitation warnings, storm surge warnings, flooding warnings.  And as we develop better probabilistic and uncertainty information, that can be communicated.

Forecasters can simply say a tropical storm or hurricane is approaching ("HURRICANE WARNING")  and describe the specific forecast threats and how they vary in time and space.

Here on the west coast of the U.S., we often experience the landfall of Pacific cyclones that are the equivalent in damage potential to minor or even major hurricanes, but we do well with providing the specific threats, without any categories.    Categories for tornadoes (the Fujita scale) are probably fine, since tornadoes are only associated with one type of threat (wind damage).

Probably OK.
If we do drop the Saffir-Simpson, there is one group that will probably complain:

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 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.