17 September 2014

Edouard becomes first major hurricane since 2012

On Tuesday morning at 11am EDT, Edouard was upgraded to a Category 3 hurricane (115mph).  A major hurricane is defined to be any Category 3-4-5 hurricane, and the last one was Sandy when it made landfall on eastern Cuba on October 25, 2012.

However, Edouard only held that intensity for 6 hours.  The last two major hurricanes (Sandy 2012 and Michael 2012) also each held that intensity for just 6 hours.  In one day, Hurricane Rina in October of 2011 racked up as much time as a major hurricane as all other storms combined did in the subsequent 1,064 days!

Visible satellite image over Edouard from 8:45am EDT. (NASA)
As of today at 5am EDT, Edouard's intensity is back down to 90mph.  It is centered about 600 miles northeast of Bermuda and heading northeast at 20mph.
It will continue to weaken over decreasing water temperatures and increasing wind shear as it accelerates to the northeast toward the Azores.

Edouard's activity brings the seasonal ACE (Accumulated Cyclone Energy) up to about 52% of average for this date.  So although 4 out of the 5 storms became hurricanes so far, the overall numbers and intensities are falling well short of average.

Only 1 of the 4 hurricanes made landfall anywhere (Arthur), but NONE of them have existed in the tropics!!  All four hurricanes formed north of 24N, a sign that the weak El Nino is suppressing activity in the tropical Atlantic, as expected.  The figure below shows the typical influence of El Nino on hurricane activity in the East Pacific and in the Atlantic... 2014 couldn't fit this any more perfectly!

Typical influence of El NiƱo on Pacific and Atlantic seasonal hurricane activity. (NOAA)
Elsewhere across the basin, there's an easterly wave about to exit the African coast today, and models generally develop it over the next few days, but not very aggressively.

15 September 2014

Edouard becomes strongest Atlantic hurricane in nearly 700 days

Since my last post on Friday morning, Edouard was upgraded to a Category 1 hurricane on Sunday afternoon, then again to a Category 2 hurricane early Monday morning.  Eduoard is the fifth named storm and the forth hurricane... the last time 4 out of the first 5 storms became hurricanes was 1996 (which coincidentally used the same name list as 2014)!

The intensity is currently estimated at 105mph, making it slightly stronger than Arthur back in July of this year, stronger than any storm in all of 2013, and tied with Sandy on the morning of October 29, 2012: 686 days ago.

As of 5am EDT today, Edouard is located about 700 miles east-southeast of Bermuda in the middle of the Atlantic.  The forecast track is shown below - recurving by 60W and staying very far away from any land.

Additional strengthening is likely, and within a day or two, Edouard could become a Category 3 (major) hurricane.  It would be the first major hurricane in the Atlantic since Sandy BRIEFLY reached that status when making landfall on eastern Cuba on October 25, 2012.

Visible satellite image of Hurricane Edouard from 8:15am EDT.  (NOAA)
The disturbance that was passing over southern Florida on Friday entered the Gulf and dissipated.  Aside from Edouard, there are no other areas of interest today.

12 September 2014

Edouard forms, and possible Gulf storm next week?

At 11pm EDT on Thursday (03Z on Friday), TD6 was upgraded to Tropical Storm Edouard.  This is latest date for the fifth named storm since Ernesto formed on September 22, 1994.  But, climatologically, it's actually right on schedule... the average date for the fifth named storm is September 11!  And if it becomes a hurricane soon (which it should), it would be well ahead of the average date for the forth hurricane, September 28.  (these dates utilize the full 1851-2013 period of record... they change if a different period is chosen)

As of 5am EDT on Friday, Eduoard's intensity was estimated at 40mph and it is moving west-northwest at 15mph.  It is battling moderately strong wind shear and dry air, but over the next 3-5 days the shear should relax and Edouard is forecast to intensify to a minimal hurricane out in the open central Atlantic.

Visible satellite image from 8:30am EDT over the tiny and sheared Tropical Storm Edouard. (NASA)
The track forecast with cone of uncertainty can be found here; the recuravature around 55W is supported by all models.

Next... the disturbance that was over the northwest Bahamas yesterday is now inland over the southern Florida peninsula.  However, dry air and shear (sound familiar?) have taken their toll on the small embryo circulation.  But what happens when it enters the Gulf of Mexico?

Visible satellite image from 8:45am EDT.  (NOAA)
First, a look at the past 24 hours of rainfall over Florida (8am-8am).  Parts of Palm Beach County got nearly 5", and 1-3" fell across portions of Broward and Miami-Dade Counties.  The remainder of southern Florida will likely be similar as the weak system drifts slowly west today.

Estimated rainfall totals from 8am Thu through 8am Fri. (NOAA)
The disturbance will enter the Gulf of Mexico by early Saturday morning, then  models (the colored lines on this map show track forecasts from 9 skillful models: the first 4 are global and last 5 are regional) diverge on how quickly it curves back to the north.  As of now, conditions in the Gulf this weekend and next week appear to be only slightly conducive for development.  SSTs are very warm, there is less dry air, but the wind shear should remain strong.  Through the next 5 days, models generally forecast this to become a tropical storm, but none bring it up to a hurricane.  It certainly can't be dismissed though... if the longer-range shear outlook is wrong, the intensity forecast could be very wrong.  The next name on the list is Fay.

11 September 2014

Update on two tropical disturbances

11am EDIT: The eastern disturbance upgraded to Tropical Depression 6.

The quick version: neither one is a depression yet, but the eastern one is very close and the western one should remain weak but bring wet weather to FL in the coming days.

First, a look at the one that is over the Bahamas and headed for Florida.  It has not gotten much better organized, and time is running short for it to do anything.  It's now centered over the far northwestern Bahamas, and thunderstorm activity remains sparse.  Although the water temperature under it is plenty warm, the vertical wind shear is picking up and should restrict any rapid organization.

Visible satellite image from 9:30am... the approx center of the surface circulation is marked with a red X. (NASA)
So, it will continue drifting west and just bring enhanced rainfall and some gusty winds to parts of central and south Florida over the next 2-3 days.  A three-day rainfall forecast valid between Thursday morning and Sunday morning is shown below.  After crossing the FL peninsula, whatever remains will enter the Gulf, but there's too much uncertainty to speculate that far out... we'll have to wait to see if there's anything intact after crossing Florida.

72hr rainfall forecast, valid Thu 8am - Sun 8am. (NOAA/WPC)
I have a long and updating radar loop from Melbourne FL at http://andrew.rsmas.miami.edu/bmcnoldy/tropics/al92/AL92_11-12Sep14_MLBlong.gif.

Now onto the eastern disturbance, which is close to becoming a tropical depression or tropical storm.  It's centered about 800 miles west of the Cape Verde islands this morning... and moving west-northwest at about 15mph.

Visible satellite image from 9:15am.  (NRLMRY)
It is forecast the gradually strengthen, possibly reaching hurricane intensity early next week, but also recurve by 50W, never coming close to any land.  If named, the next name on the list is Edouard.

Trivia for today: the last time the fifth named storm occurred so late in the season was 20 years ago!  Ernesto was named on Sept 22, 1994.

10 September 2014

Two weak disturbances to watch

Tropical tidbit 1: The last time there was not a named storm in the Atlantic on September 10 was in 2000! And even then, a subtropical depression formed (not named).  So, the last time there was not any tropical or subtropical cyclone at all on this date was 1992.

Tropical tidbit 2: You may recall a previous bit of trivia I shared: the last time the A, B, and C storms all became hurricanes was also in 1992.  AND, the A storms in 1992 and 2014 both made landfall on the U.S. as hurricanes.

As I mentioned in my update yesterday, there are two areas of interest right now: one over the Bahamas and one just west of the Cape Verde islands.  Neither are very impressive, but are worth watching over the coming days.

First, the system over the Bahamas... it's actually being generous to even consider it for discussion, but because of its proximity to Florida, I'll give some highlights.  It's not from an easterly wave, but rather an upper-level Low that interacted with a weak surface trough over the past few days.  It occasionally spawns some widespread thunderstorms, but this morning isn't one of those times, as you can see in the benign-looking satellite image here:

However, global models do show the disturbance persisting (not developing) and heading west into Florida.  It should result in nothing more than some welcome rain for the area in the coming 1-3 days.

Total rainfall forecast in south Florida valid from today through Monday. (NOAA/WPC) 
The second system of interest is an easterly wave that exited the African coast this past Saturday.  It is now centered about 450 miles west of the Cape Verde islands, or about 1900 miles east of the Windward islands, and is extremely disorganized.

As has been the story for the entire season so far in the deep tropics, a copious amount of low-mid level dry air is choking it off and limiting any development... the SST and wind shear are both favorable for development. Global and regional models alike forecast it to continue moving WNW and eventually get better organized once it escapes the Saharan Air Layer. According to a consensus of leading models, getting a hurricane out of this one is actually not out of the question by Sunday-Monday.

Infrared satellite image (grayscale background) with a depiction of the Saharan Air Layer overlaid (yellow-red).  The center of the disturbance is marked with a red I. (CIMSS)

09 September 2014

Peak of hurricane season comes quietly this year

Although the Atlantic is fairly calm today, if you average activity over the whole 163 years of records, September 9 is actually the climatological peak!

There are numerous ways to define "activity" of course, and there are numerous time periods one could use to create the average.  But for this claim, I'm using a standard metric called Accumulated Cyclone Energy (ACE), and the full 1851-2013 period.

 It is not a sharp peak, but rather a broad hump that spans about two weeks (roughly the first half of September).  On average, Sep 2-16 generates 1/4 of the entire season's ACE!

[ACE is calculated for each named storm every six hours.  The higher the peak wind and the longer it lasts, the more ACE a given storm will accrue.  It's the sum of the squares of the six-hourly peak winds,... for example, if a storm's intensity is estimated to be 65 kt in an advisory, the ACE is 4,225 kt^2, but it's commonly shown in units of 10,000 kt^2, so that value would become 0.4225.]

Now let's look at just the average counts of tropical cyclones through the season (tropical storms, hurricanes, and major hurricanes).  The pattern is similar, but additional interesting details come through.  Major hurricanes (Category 3+) are almost exclusively found in Aug-Sep-Oct.

Rather than breaking it down into daily counts, the next figure shows the average cumulative counts.  This is handy if you wish to see that the third hurricane typically occurs by September 12, for example.

These figures and dates do change if the period used for the climatology changes.  One could look at more recent times, like 1950-2010, 1981-2010, or whatever is desired!  However, the fewer years that go into the averages, the more noisy the plots will become and the less robust the results will be.

Now to wrap up with current activity.  There are no named storms, but there are a couple areas of interest that could form in the coming 2-5 days.  The 5-day formation outlook from NHC highlights where those areas are now (X) and where formation might occur within the next five days (shaded blobs).  Note that these are NOT track forecasts!  The yellow area is given a 20% chance of development, and the red area a 70% chance.  I will probably send out a more complete update on these disturbances tomorrow if they continue to develop.

Visible satellite images of the two disturbances are shown here for reference:

02 September 2014

Dolly forms in Bay of Campeche, landfall tonight

Over the past week, an easterly wave has been making its way across the Caribbean.  As it neared the Yucatan peninsula on Sunday, it began to get better organized.  Shortly after it crossed the peninsula and entered the Bay of Campeche, it was upgraded to Tropical Depression 5.  Then, at 5am EDT on Tuesday, it was upgraded to Tropical Storm Dolly.

Today, as of 8am EDT, Dolly is centered 145 miles east-southeast of La Pesca, Mexico and is moving west-northwest at 13mph.  At this rate, landfall will occur late Tuesday evening (local time).  Maximum sustained winds are 50mph and additional intensification is possible up until it makes landfall. The greatest threat from this storm will be the heavy rain and resulting mudslides and flash floods.

Dolly is the fourth named storm of the season, and ushers in the climatologically most active couple of weeks of the Atlantic hurricane season.  In the figure below, a timeline of the average tropical storm and hurricane counts are shown, and today's date is highlighted by a green line.  This season, the overall activity as measured by ACE (Accumulated Cyclone Energy) is just 54% of average as of today.

Dolly's origin and track is interesting because past storms in this "lineage" have been similar.  Prior to Dolly, Diana was the female name in this list (it was retired in 1990).  But, going back to 1990, Diana 1990, Dolly 1996, and Dolly 2008 all crossed the Yucatan peninsula and strengthened as they moved west... also note that all three of them were hurricanes at landfall.

Elsewhere across the basin, global models have been bullish on developing an easterly wave that's still over Africa, but is expected to enter the Atlantic on Thursday.  This wave had its origins over eastern Africa back on August 28th.  The next name on the list is Edouard.

Today is a very significant day in hurricane history.  On the evening of September 2, 1935, the infamous Labor Day Hurricane made landfall on Florida's Upper Keys (near Islamorada) as a Category 5 hurricane... and it remains the most intense U.S. landfall on record.  Maximum sustained winds were 185mph (stronger than Andrew and Camille); it generated a 18-foot storm surge that totally inundated the low-lying islands, and the incredible winds leveled everything in its path.  It was responsible for over 400 deaths.