Hurricane Ernesto to impact Bermuda on Saturday

Since my previous post on Monday morning, Tropical Storm Ernesto did indeed form later that day, becoming the season's fifth named storm.  The center passed over Guadeloupe then the Virgin Islands, dumping hefty rainfall totals over the Leeward Islands and Puerto Rico.  It was upgraded to the season's third hurricane on Wednesday morning, centered just northwest of Puerto Rico.

As I wrote on Monday, the intensity model guidance continued to indicate it would reach Category 3 intensity as it neared Bermuda, and the official NHC forecast now includes that as well.  Once it gets closer, you'll find a long, updating radar loop from Bermuda at https://bmcnoldy.earth.miami.edu/tropics/radar/

There's very little spread in the track guidance.  It will pass over or near Bermuda on Saturday morning, gradually weakening in the face of increasing wind shear by then, then continue north toward Newfoundland.  A turn toward the east is likely at some point near Newfoundland, so any impacts there would depend greatly on how soon it starts heading east.

Although it won't be a tropical cyclone anymore, a potent extratropical cyclone could then reach the UK next Thursday-Friday.

One long-distance impact that these powerful hurricanes over the open ocean create is substantial wave and swell action... the east coasts of the U.S. and Canada will experience elevated threats of coastal flooding over the next six days due to Ernesto, which also aligns with amplified tides due to a full moon coming up on the 19th.

The climatological date of the third hurricane formation is September 7, and the climatological date of the second major hurricane formation is September 19.  Not suprisingly, this season is wayyyy ahead of schedule by those metrics, as well as by ACE, which is ~304% of average for the date.

The basin continues to look quiet for the foreseeable future after Ernesto, but now that it's mid-August, we should enjoy this relative calm while we can.

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Soon-to-be Ernesto prompts tropical storm warnings for Leewards and Puerto Rico

The easterly wave I mentioned in Friday's post has not produced any surprises yet: it has been gradually getting organized and as of Monday morning, it is close to becoming a tropical cyclone.  It also still looks like it will pass over the Leeward Islands on Tuesday, and over/near Puerto Rico on Wednesday.  The next name on the list is Ernesto.

The feature was transitioned from Invest 98L to Potential Tropical Cyclone Five on Sunday afternoon.  Essentially, it's still an Invest (not yet a tropical depression or tropical storm), but the "PTC" designation allows the National Hurricane Center to issue watches and warnings for it because it's expected to form and impact land with at least tropical storm conditions, but it has not formed yet.  

The map below shows the current extent of tropical storm warnings from the northern Leewards over through Puerto Rico.

Directly from Friday's post, the forecast is following what looked like the most likely scenario then:

The most likely scenario presented by the models now is gradual development and intensification, a tropical storm in the vicinity of Puerto Rico on Wednesday, then a turn toward the north when it reaches 70-75°W (the longitudes of Hispaniola to eastern Cuba), followed by much more significant intensification north of the Bahamas. 

The model guidance has nudged slightly east since then, which is good news for the Dominican Republic, Haiti, and eastern Cuba, but it puts Bermuda on alert in five days or so.  The NHC forecast brings it up to just shy of Category 3 intensity by Saturday, but several models suggest it will still be intensifying by the time it gets there so NHC may bump up its intensity forecast if that persists.

Pointless trivia: Ernesto is still a name from the original six lists introduced in 1979.  It was first used in 1982 and every six years since then, so this will be its 8th time on the map.  For the most part, storms bearing this name have been benign, but two of them reached hurricane intensity and both made landfall: Ernesto 2006 made landfall in eastern Cuba and in south Florida as a tropical storm, then Ernesto 2012 made landfall in the Yucatan peninsula as a Category 2 hurricane.

Aside from PTC5, the basin looks quiet for the coming week.

 

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Closely watching wave east of Lesser Antilles for development next week

As of today, there are no active tropical cyclones in the Atlantic, and it will probably stay that way through the weekend.  But by early next week that could change.  An easterly wave that just left Africa on Wednesday is presently centered  about 1800 miles east of the Lesser Antilles and is given a 60% probability of development by NHC within the coming week.  The next name on the list is Ernesto.

Visible satellite image from Friday morning. The current center of the disturbance is marked by the orange X and the area of potential formation within the next 7 days is the orange oval.

For the next 2-3 days, this disturbance will be battling dry Saharan air ahead of it and to its north.  But by Monday, most models start perking up when it's about 700 miles east of the Lesser Antilles.  This time of year there are no concerns about the ocean temperature, so dry air, wind shear, and land interaction are the three factors that could put brakes on development. The first two don't look particularly hostile next week, and it won't encounter any substantial landmass until Wednesday at the earliest (Hispaniola). The two maps below show the ensemble-based tracks, color-coded by intensity, from the European model (top) and the American model (bottom) out through next Friday afternoon.

From this early look, this feature will almost certainly cross the Leeward Islands on Tuesday, though there's a lot of spread in the intensity.  Clearly, the European ensemble keeps it weaker for longer, finally reaching upper tropical storm intensity near the latitude of the Bahamas on Thursday-Friday, while the American model's ensemble is far more aggressive with several members bringing it to hurricane intensity by Wednesday.  Keep in mind that global model ensembles are not really great at intensity; they will generally be biased low.

For mainland U.S. interests, the earliest something would reach south Florida currently looks like Friday, if at all.  As of now, there's too much spread to say much beyond that, but it doesn't appear likely.

The most likely scenario presented by the models now is gradual development and intensification, a tropical storm in the vicinity of Puerto Rico on Wednesday, then a turn toward the north when it reaches 70-75°W (the longitudes of Hispaniola to eastern Cuba), followed by much more significant intensification north of the Bahamas.  We'll see if that outlook persists.

Through today's date, here's a look at the total Accumulated Cyclone Energy (ACE) by year.  The value in 2024 is the 6th highest on record, behind 2005, 1933, 1980, 1926, and 1916.  Certainly an impressive start to the season.

Climatologically, the steep uptick in activity is just around the corner though.  We're just wrapping up the typically-quiet first 10 or so weeks of the season; the next 10 weeks are typically not so quiet.

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Debby makes landfall, significant flooding problems coming this week

Since my previous post on Friday morning, there have not been a lot of surprises.  The disturbance of interest did indeed become Tropical Depression 4 on Friday night, then Tropical Storm Debby on Saturday afternoon when its center was west of the Florida Keys.  It tracked northward and briefly attained hurricane status for twelve hours as it made landfall in the Big Bend area of Florida early Monday morning.  From my post on Friday: 

"Climatologically, the fourth named storm forms on August 15, and the second hurricane forms on August 26... so if Invest 97L does become Tropical Storm Debby OR Hurricane Debby, it will be well ahead of the normal pace."

18-hour enhanced infrared satellite loop; landfall was at approximately 11:00 UTC.

I didn't include it in this post, but I have a very long radar animation of Debby available at https://bmcnoldy.earth.miami.edu/tropics/radar/

As of Monday afternoon, winds have decreased to tropical storm strength, but the forecast keeps it around for a few more days as it stalls near the coast of the Carolinas.  That stall will be responsible for a very significant flooding threat this week.  This map below shows the five-day rainfall forecast from Debby -- those high totals over northern Florida, Georgia, South Carolina, and North Carolina will undoubtedly be devastating in many locations.

Debby is another perfect example of two important lessons with tropical cyclones:

1) The storm doesn't stop at landfall.  Quite often, there are days of flash flooding and/or tornado threats after landfall and far from the landfall location, so don't tune it out.

2) There's more to the story than the category.  The only thing the category rating of a hurricane tells you is what the peak sustained wind is somewhere in the storm.  However, the large majority of fatalities and damage comes from rainfall and storm surge (i.e., water).  Even a tropical depression can easily produce catastrophic flooding.

Tropical storm warnings are still up for Florida's Big Bend region and then over in eastern GA and SC as Debby is expected to meander back out over the ocean to recharge.

Elsewhere, there's an easterly wave that just crossed the Windward Islands that has a small chance of development in the Caribbean this week... it should reach the Yucatan peninsula area on Friday-Saturday, but as of now it does not appear to be a threat.  The next name on this year's list is Ernesto.

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Debby likely to form this weekend near Florida

An easterly wave has been embedded in dry Saharan air for the past ten days since it left the African coast on July 24.  On Thursday it finally began to show signs of invigorated thunderstorm activity which indicates the dry air is now less of an influence on it.  As of Friday morning, this wave, tagged as Invest 97L, is centered over the eastern tip of Cuba and is forecast to enter the eastern Gulf of Mexico on Saturday.

NHC is giving this a 60% chance of becoming at least a tropical depression through Sunday morning, and 90% through the next seven days.  There's now very good agreement among the models that the center of this will track along the northern Cuba coast through Saturday morning, make a turn toward the northwest and enter the Gulf of Mexico midday Saturday (crossing over or near the Florida Keys), then turn north and track off the west coast of the Florida peninsula from Saturday evening through midday Monday.  Beyond that, some models suggest a bit of a stall over northern Florida, while the majority track it northeastward and intensify it over the Gulf Stream.

As far as intensity forecasts go, there's good agreement on it becoming a tropical storm (its name would be Debby) on Saturday in the Gulf, with a slight chance of reaching hurricane status.  Due to extremely warm water temperatures in the northeast Gulf, the possibility of rapid intensification can't be ignored.

Sea surface temperature (left) and the anomaly (right).
https://bmcnoldy.earth.miami.edu/tropics/sectors/

The most significant impact from this storm, whether it becomes a tropical depression, tropical storm, or even Category 1 hurricane will be the rainfall.  Hefty totals are forecast over the next five days, with the bulk coming Saturday-Monday in Florida then Monday-Wednesday in the Carolinas.  There will undoubtedly be major flooding concerns all along the peninsula and then northward along the coast from there.

Five-day rainfall forecast, ending Wednesday morning.
https://www.wpc.ncep.noaa.gov/#page=qpf

Climatologically, the fourth named storm forms on August 15, and the second hurricane forms on August 26... so if Invest 97L does become Tropical Storm Debby OR Hurricane Debby, it will be well ahead of the normal pace.

Finally, here's a look at how this season's Accumulated Cyclone Energy (ACE) stacks up against the average: roughly 360% of average for the date.  If there's no activity through August 31, this year's total would meet up with the climatological value -- but don't hold your breath.

I will most likely not be available to write posts this weekend, so for the latest reliable information as it develops, keep an eye on the NHC website.

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