26 September 2021

Sam rapidly intensifies to Category 4 hurricane

Not surprisingly, Sam did indeed strengthen significantly over the past couple of days.  As of Sunday morning, it's a Category 4 hurricane... three days ago it was a tropical depression!  It is also the season's 4th major hurricane (Category 3+) now; Grace, Ida, and Larry were the other three.

There is a whole spectrum of intensification rates of course, from very slow to very fast.  Conventionally, the top 5% of rates is given the designation "rapid intensification" which corresponds to an intensity increase of at least 35mph in a 24-hour period.  The National Hurricane Center provides advisories with intensity estimates every six hours, so a new 24-hour period begins every six hours.  Sam experienced a total of seven RI periods, five of which were consecutive!  (in this table, the intensity is given in knots (nautical miles per hour)... 1kt=1.15mph... and RI periods are highlighted in red)

While that peak rate of 45 kt in 24 hr is impressive, it is very far from a record.  The record-holder in the Atlantic is Hurricane Wilma in 2005: 95 kt in 24 hr.  The record anywhere in the world belongs to Hurricane Patricia (East Pacific, 2015): 105 kt in 24 hr.

Of course, the ACE (Accumulated Cyclone Energy) is exploding due to Sam, and it will surpass the value that a full average season accrues on Tuesday... before October even starts!  By the end of the day, it will be at about 124% of average for the date.

Now on to the forecast.  Model guidance continues to show Sam turning north before reaching the Caribbean islands, then continuing northward.  It's not in any rush, so we'll still be dealing with  Hurricane Sam for at least another week.  In this new product from OU grad student Tomer Burg, a super-ensemble plot shows the track spread valid next Sunday morning.  By "super-ensemble", I mean this includes members from three different global model ensembles as well as their deterministic runs: GFS, ECMWF, and UKMET... a total of 118 members. (aside: this is a very nice product... I remember it being talked about 15 years ago as a way to replace the static "cone of uncertainty" with a dynamic one, the issues are timeliness and reliability)

Sam is about to leave an area of low ocean heat content and pass over much higher ocean heat content in the coming five days.  All of the rapid intensification that just occurred was over low OHC!  A slow-moving very strong hurricane requires high OHC to sustain itself.  OHC is integrated energy, so it's far more useful than just the sea surface temperature.  Big hurricanes really churn up the ocean, upwelling water from below.  If that water is cool, the hurricane will weaken, but if the warm water is deep, very little weakening will occur.  I overlaid the current NHC forecast "cone" on the OHC map below.

In short, Sam is not a threat to land any time soon, but is forecast to remain a major hurricane for probably another week or so.  It's definitely one to watch in Bermuda.

Elsewhere across the basin, there are some areas of interest. One is former-Peter near Bermuda which has a small chance of re-development, another is a wave in the deep tropics that doesn't look so healthy, and the most likely area to watch is an easterly wave that's still over western Africa.

That wave over Africa is strongly favored to develop by long-range model guidance. It's a really long way from anything, but it's worth introducing. The next name on the list is Victor.

By the way, if you missed yesterday's post, please check it out: I highlight this blog's 25th anniversary and the 20th anniversary of my "Tropical Cyclone Radar Loops" webpage!


  1. Thank you Brian ! I really enjoy your blogs, been following you since Irma. I especially found the Saros Cycle piece to be super interesting. We here in S. Fl are fortunate to have you watching out for us !
    Thanks again!

  2. Thank You for the factual details.

    I love science, from age 4 and still at age 60, but I dislike when some just say things that though are part of the known proven science as the TS "engine" run on warmer waters. But at times (as with 2021SAM) these statements are not the full picture.
    As to a well respected website, several respected member posted that SAM was over the hottest waters (which are warmer in part due to aGW) but in fact SAM was not over those areas. Instead more closer to 28 to 29C temperatures.

    Wish I could share my theories that explain this action by SAM 12+ hrs. before but this is too professional of a site for unproven "science".

    Thank You for your good professional investigative science work.