30 October 2020

Eta, 28th named storm, expected to form soon

All signs point to 2020 getting its 28th named storm very soon.  Invest 96L is located in the eastern Caribbean and is a late-season wave that left the African coast back around October 24th.  But only now is it encountering an environment that's allowing it to develop.  If it becomes a tropical storm and earns a name, it would be Eta, and would tie 2005's record of 28 named storms. 

As of Friday morning, the wave is centered about 140 miles west of Grenada, or 300 miles east of Bonaire.  It is expected to track westward through the Caribbean and reach Nicaragua/Honduras sometime around Tuesday. The National Hurricane Center is giving it a 70% probability of development within the next couple of days, and models unanimously favor its development too. The red shaded area on the map below indicates the potential area in which the NHC is expecting Invest 96L to develop into a tropical cyclone (don't confuse it with the "cone of uncertainty").

While the majority of model guidance favors a tropical storm or minimal hurricane heading west, the ensembles have some notable outliers, so we shouldn't get too confident in the outcome just yet.

If it should reach hurricane intensity, it would have plenty of company, historically. November hurricanes are not that rare, especially in the western Caribbean.  For some context, this map shows the 23 known November hurricanes over the past 50 years. The list includes some infamous storms such as Paloma '08, Michelle '01, and Lenny '99. The strongest U.S. hurricane landfall during November was Kate in 1985.  The official hurricane season ends on November 30th, but storms can and have formed after that artificial bound.

28 October 2020

Zeta will be Louisiana's fifth landfalling storm this season

Hurricane Zeta is poised to make landfall in Louisiana on Wednesday afternoon.  This will be the state's fifth landfalling storm this season, its third landfalling hurricane, and the sixth along the northern Gulf coast. And there's still another month left of the official hurricane season.

As of Wednesday morning, Zeta is centered 300 miles south of New Orleans and is accelerating as well as intensifying.  It's almost a Category 2 hurricane with 90 mph peak sustained winds in the eyewall and is moving north at a brisk 17 mph.  

That forward motion will increase throughout the day, resulting in an amplified and dramatic storm surge to the right of where the center crosses the coast.  The coast of Mississippi could see a 6-9-foot storm surge, while Lake Pontchartrain by New Orleans could see a 3-5-foot storm surge.  The storm's rapid forward motion means heavy rain will be less of a problem, but still not anything to ignore (heavy rain will extend far inland across the southeast and mid-Atlantic states).

These Hurricane Threats and Impacts maps highlight the areas at risk from the four hurricane hazards: wind, storm surge, flooding rain, and tornadoes. The "cone of uncertainty" is overlaid for reference, and to show that impacts always extend far beyond the cone (it is not intended to show impacts at all).

To help illustrate the statement in the opening paragraph, here is a map of the previous five landfalling storms in the area, with Zeta's forecast track added as the thick pink line.

26 October 2020

Hurricane Zeta and déjà vu for Louisiana

On Monday afternoon, Zeta was upgraded to the season's 11th hurricane.  Looking back to 1851 when official records begin, only two previous seasons had 11+ hurricanes by this date: 2005 (13) and 1950 (11), so this is a truly remarkable level of activity. 

But, as if having 27 named storms and 11 hurricanes this season were not enough, Zeta looks like it will make landfall in roughly the same place where Tropical Storm Cristobal did in early June, Hurricane Laura in late August, Tropical Storm Marco in late August, Hurricane Sally in mid September, and Hurricane Delta in early October. It's only 340 miles between Sally's landfall point in eastern Alabama and Laura's landfall point in western Louisiana.

Zeta is forecast to clip the northern Yucatan peninsula on Monday night into Tuesday morning as a hurricane, then enter the Gulf of Mexico. While it should maintain hurricane intensity through its U.S. landfall on Wednesday, model guidance is consistently showing it weakening a bit as it approaches land, so it *could* be a tropical storm by that time. Regardless, heavy rain, strong wind, and storm surge will be hazards across a wide area. Keep in mind that a hurricane's impacts are far larger than a track line or the "cone of uncertainty".

Regarding track guidance, two of the global ensembles are packed pretty tightly now, with solutions ranging from western LA to the western tip of the FL panhandle, clustered most tightly around central-eastern LA.

The official hurricane season ends in 35 days, but nature does not have to stick to that artificial bound, so there's plenty of time for more storms this season. The next couple of names on the list are Eta and Theta. There's a slight hint among long-range models of another development in the western Caribbean in 10-14 days, but that is too far out to be reliable... think of it as the next potential time and region of favorable conditions.

25 October 2020

Zeta becomes 27th named storm, threatens northern Gulf coast

After a seemingly-endless wait, the disturbance in the western Caribbean was upgraded to Tropical Depression 28 on Saturday night, and then to Tropical Storm Zeta on Sunday morning.  This is the season's 27th named storm, and ties the record 2005 season, at least by that metric.

Zeta is expected to continue to organize and intensify, possibly hitting hurricane status by the time it reaches the northern Yucatan peninsula on Monday night.  Then, the environment is forecast to become increasingly hostile and Zeta should begin to deteriorate as it nears the U.S. coast.  However, that doesn't eliminate impacts from it, so stay tuned as we get closer to the middle of the week (landfall should be some time on Wednesday).

As if stuck in a rut, Zeta is also forecast to make landfall along the northern Gulf coast, just as Cristobal, Fay, Laura, Marco, Sally, Beta, and Delta did before it this season!

Rainfall, as usual, is a concern with this storm.  As of Sunday morning, the only part of the U.S. that will be impacted by it in the near future is extreme south Florida, where flood watches are in effect through Monday. But the Yucatan and western Cuba will also see heavy rain, and eventually, the north-central Gulf coast region will in a few days.

Catching up on the ACE (Accumulated Cyclone Energy) metric, 2020 is at about 145% of average for the date, but we can see where it lies in relation to historical mega-seasons. The top 5 through October 25th are 1933, 1926, 2004, 2017, and 1893. 2005 and 1995 just fall short of that list at 6th and 7th places. 

Elsewhere across the basin, things look quiet in the foreseeable future. But if and when the time comes, the next couple of names on the list are Eta and Theta.

23 October 2020

As Epsilon passes by Bermuda, Zeta could form in western Caribbean

Hurricane Epsilon is now a Category 1 hurricane with peak winds of 85 mph... it's centered about 200 miles east-northeast of Bermuda.  It should remain a strong tropical cyclone through the weekend, but will gradually transition to an extratropical cyclone as it passes east of Newfoundland on Sunday. Then, it could actually be a pretty powerful storm for the British Isles by the middle of next week.

The area of disturbed weather in the western Caribbean is now labeled Invest 95L.  Although still not a tropical depression, its organization has improved drastically in the past day, and NHC is giving it a 60% probability of formation.  It's presently centered near the Cayman Islands, but is broad and messy.  

Any activity in the western Caribbean in October is cause for concern; some very notorious hurricanes have explosively developed there.  In this map of ocean heat content, high values over the entire western Caribbean mean that there is a nearly endless supply of very warm water at its disposal.  If this develops, it would be Tropical Depression 28 and then Tropical Storm Zeta. You may recall that the record 2005 season finally ended with Zeta, which formed on December 30th.

Model guidance is generally not aggressive with it (yet). We also need to interpret the guidance with caution, because models are not great with these broad and disorganized low pressure systems.  The overall consensus is for it to track toward the northwest, bringing it into the Gulf of Mexico early next week, perhaps as a tropical depression or tropical storm.  

But there are some notable outliers in both the global model ensembles and in regional hurricane models -- those outliers bring it north over Cuba and into south Florida or the western Bahamas on Monday-Tuesday. Those also happen to be the quicker and stronger solutions (this particular HMON run shown below ("HMNI") has a Category 1-2 hurricane just east of south Florida on Tuesday morning, for example). Given that this outcome would bring at least tropical storm conditions to urban southeast Florida starting Monday afternoon, it's worth keeping a very close eye on it.

With such a track spread, it's basically impossible to come up with a meaningful rainfall forecast. But Jamaica, Cuba, the Yucatan peninsula, the western Bahamas, and south Florida could easily see multiple days of heavy rain from today through Monday. Check https://www.nhc.noaa.gov/ for the latest forecasts.

22 October 2020

Epsilon becomes season's 4th major hurricane

Since my previous update on Monday, Tropical Depression 27 quickly strengthened into Tropical Storm Epsilon, the 26th named storm of the 2020 season.  Then it reached Category 1 hurricane intensity on Tuesday night (the 10th hurricane), and Category 3 hurricane intensity on Wednesday night (the 4th major hurricane). 

In this spectacular satellite image showing mid-level water vapor, Epsilon is the green thing in the middle in its envelope of moisture (blues) but is embedded within a huge trough flanked by dry air.  Not something you see every day, or year. Bermuda is the tiny red dot just west of Epsilon.

Fortunately for Bermuda, it is staying further east than originally predicted, so impacts will only be long-distance such as possibly getting periods of tropical storm force winds and large surf as the large and powerful hurricane passes by. Tropical storm force winds extend as much as 310 miles from the center.

Elsewhere in the basin, the pesky area of disturbed weather in the western Caribbean is still festering. 

NHC is giving it a 20% probability of development during the weekend as it drifts by south Florida and the Bahamas.  Regardless of development, it will bring elevated chances of heavy rain with it. Should it happen to reach tropical storm status, the next name on the list is Zeta. 

19 October 2020

New Depression could threaten Bermuda as Hurricane Epsilon

After a 1.5-week lull in activity, Tropical Depression 27 has just formed southeast of Bermuda. This area of interest first appeared on the National Hurricane Center's five-day outlook for potential formation five days ago... spot on!  Although not yet a named storm, it is fully expected to become a tropical storm later today and take the next name in the Greek alphabet: Epsilon. This would be the season's 26th named storm.

The current forecast from NHC brings the storm to a Category 1 hurricane by Thursday, and then maintaining that intensity into the weekend as it approaches Bermuda.

Looking at the model guidance, there is not much to suggest that it will be much stronger than that, but the models should improve once it matures a little more. This plot has forecasts from regional and global dynamical models (deterministic and ensemble means), statistical-dynamical models, and consensus. 

This has already been a rough year for the tiny island of Bermuda: Tropical Storm Edouard passed by on July 5, Hurricane Paulette passed directly over on September 14, and then Hurricane Teddy passed to the east on September 21.

Through October 19, this year's ACE (Accumulated Cyclone Energy) is at 142% of average for the date. That amount of ACE is half of what the 1933 season had accrued by this date and 54% of the 2005 season by this date.

Elsewhere across the basin, there is a slowly-evolving large-scale circulation in the western Caribbean -- this is rather typical for this time of year. If something should eventually form from it, it would head generally north toward Cuba and the Yucatan.  If and when the time comes, the name after Epsilon will be Zeta.

08 October 2020

Hurricane Delta following in Hurricane Laura's footsteps

In the 2+ weeks since my previous post (the first half of that was due to the Atlantic taking a break, and the second half was due to me taking a break), there was Tropical Storm Gamma and Hurricane Delta. Tropical Storm Gamma lasted for just under three days and was located near the Yucatan peninsula.  Then right behind it was Delta, the season's 25th named storm and 9th hurricane. Delta experienced a period of remarkable rapid intensification on October 6th, jumping from a tropical storm to a Category 4 hurricane in one day as it approached the northern Yucatan peninsula... reminiscent of Wilma in 2005.

As of Thursday afternoon, Delta is a powerful Category 3 hurricane and is headed north toward Louisiana. A clear eye is becoming evident, signaling some additional strengthening. As the title of the post mentions, Delta's landfall location will be almost identical to Laura's back in late August.  Much of that area has still not cleaned up or started to recover before Delta's arrival. The last time two hurricanes impacted western Louisiana in the same year was 1886.

As it tracks north, it will encounter slightly cooler water temperatures, lower ocean heat content, and somewhat drier air; these factors should limit significant intensification prior to landfall (meaning a Category 4 or 5 is very unlikely).

Landfall is expected in western Louisiana on Friday evening, though conditions are already going downhill with rising water levels and outer rainbands scraping the coast. Tropical storm force winds should begin in coastal areas of eastern Texas and Louisiana by Friday morning.

No major changes in intensity are expected, so it should make landfall as a Category 2-3 hurricane, and produce a 7-11-foot storm surge to the right of its center. Rainfall totals could reach one foot in parts of western Louisiana.

The Accumulated Cyclone Energy (ACE) for the season is about 138% of the average value through October 8. Other years that had higher values by this date were 2019, 2017, 2010, 2008, etc -- in other words, this season is not that notable by this metric. Sure, we have cruised through named storms at a record pace, but 14 of the 25 named storms were around for three days or less!

Elsewhere across the basin, no new activity is expected in the coming week. But if and when the time comes, the next couple of names on the list are Epsilon and Zeta.