A Junior Blob Off the West Coast--But Still Cool for a Hurricane

The latest plot of the difference of the current sea surface temperature from normal (the SST anomaly) available from NOAA shows that some of the most unusually warm water across the planet is now offshore of the West Coast.   Some of the water is 2.5-4 C degrees warmer than normal (4.5-7.2F warmer than normal).  Since it has only been there a few months, let's call it a junior BLOB.

One reason for the BLOB development has been a persistent areas of high pressure over the eastern Pacific.

Some folks have asked me whether we might get a hurricane like Dorian over the West Coast, particularly since our offshore waters are warming.  A good question.

The answer is no.  Even with a major BLOB off our coast, the sea surface temperatures over the eastern Pacific are still too cold.   Hurricane development requires ocean temperatures of at least 80F (or 26.5F). 

Below is the latest plot of sea surface temperatures around North America provided by NOAA.  Blue/Green is cold.   Red is warm (you could guess that!).  26.5C or 80F is where the color turns from yellow to orange.    The Gulf of Mexico is great for hurricanes, as is the south Atlantic.

But yikes!  The water temperatures off the West Coast even with Ms. Junior BLOB is WAY too cold (around 50F).  No hurricanes for us.   Ever.

If you look carefully at the first picture above, you will notice some blue color off of southern Florida....that is the result of the churning of the upper ocean by Hurricane Dorian, bringing up cold water from below.

To really see this effect, let me show you the sea surface temperatures measured at a buoy that was run over by the storm (buoy 41010, just north of the Bahamas).  Pretty amazing... a cooling from around 85.5 F to 81.3 F.

This cooling of the ocean's surface was enhanced by the halting of the storm's motion over the Bahamas (so the storm was stuck over cooling water), and led to the storm weakening rapidly from a category 5 to a category 2 storm.

Announcement:  I will be giving a talk "The Great Storms of the Pacific Coast" in Ocean Shores at 6:30 PM on September 7th at the Shilo Inn as part of the Coastal Interpretative Center's summer lecture series.  More information is found here: https://www.interpretivecenter.org/.   Shilo Inn is offering special room rates for those wishing to stay overnight, as well as a special buffet.   

Hurricane Dorian is a VERY Small Storm That is Moving Very Slowly

There has been a  lot of coverage in the media about Hurricane Dorian, but most have not been stressed how SMALL the storm is.   And size matters, limiting the damage to the immediate vicinity of the center of the storm.  A recent image from the Miami radar shows the storm about 120 miles offshore, with a small eye and circling eyewall rain bands.

According to the National Hurricane Center, hurricane-force winds (64 knots or more) extended only 45 miles from the center--the diameter of hurricane winds was only 90 miles.

Compare that to Hurricane Irma, which hit Florida in 2017--it has a diameter of 420 miles ifor the hurricane winds!  Hurricane Sandy in 2012 had a hurricane-force wind diameter of 1000 miles at one point.

The small size of Dorian is shown by the wind forecast for 5 AM PDT Tuesday morning-- the hurricane force winds (purple colors) stay way offshore and don't reach the Florida mainland.

The maximum gusts today (see figure) show gusty, but modest, winds of 30-40 mph along the eastern coast of Florida.  Makes sense considering the small diameter of the storm.

Dorian has essentially not moved during the past 24-hours, keeping the strong winds over the Bahamas, where terrible damage is occurring.  But the small size has another benefit... it limits the growth of large waves, which increase with fetch--the distance over which strong winds work on the water.  This is illustrated by the wave prediction for 3 PM PDT Monday afternoon from the NWS WaveWatchIII system.  Small area of large waves, with none larger than 10 meters.

Staying in one place is bad for a hurricane.   Hurricanes depend on warm water and they churn up the ocean, bringing cooler water from below.  If the hurricane doesn't move it will weaken...and Dorian has already declined from a category 5 to 3 storm.   A NOAA buoy in front of the storm illustrates the cooling.

The latest forecasts are for the storm to slowly move north-northwestward on Tuesday, with the storm gradually increasing in size.  To illustrate,  here is the NOAA/NWS HWRF forecasts (their high-resolution hurricane model), showing some increase in size, with hurricane strength winds just offshore of the coastline.  Really close.  

 Florida will probably escape serious damage from Dorian, but the show is not over, as the storm expands in size, speeds up, and then makes a grazing landfall on North Carolina.