16 October 2019

Odds increasing of Gulf storm this weekend

Surface wind streamlines as of Wednesday morning. Two circulation centers are apparent: one on each side of the Isthmus of Tehuantepec. (earth.nullschool.net)
Since yesterday's update, the disturbance over central America has drifted into the southern Bay of Campeche.  It's still disorganized, and is really close to another disturbance on the Pacific side of the low-lying 130-mile-wide Isthmus of Tehuantepec!  This has been a feature of interest going back to last *Thursday* when it was a disorganized blob off the east coast of Nicaragua.  These things can take a long time to cook (remember watching Michael for about a week last year before it became a Depression?).  There are zero indications that this will have much else in common with Michael though.

The most recent European model ensemble indicates a pretty decent chance of this developing in the coming days as it heads north to northeast. Some of the ensemble members have a trackable low pressure center near the northern Gulf coast on Saturday, but none are strong (shown here).  If this becomes a tropical storm, the next name on the list is Nestor.  Nestor was introduced to this list in 2013 after Noel '07 was retired.

As I shared yesterday, there's a short list of seven named storms that developed in the western Gulf of Mexico after October 1 since 1960.  The one that took off toward the northeast into Florida's Big Bend region was Josephine in 1996.

Tropical Depression 15, the one that was way out by Cabo Verde, has dissipated and never reached tropical storm status and so was never named.

As of today, the Atlantic has had 13 named storms, 5 of which became hurricanes, and 3 of those became major hurricanes (Category 3+).  I whipped up a seasonal tracking map through today to refresh your memories on when and where things happened.

15 October 2019

TD 15 forms by African coast, also closely watching Gulf of Mexico

Enhanced satellite image of Tropical Depression 15. (EUMETSAT)
On Monday afternoon, a very strong and late African wave developed into Tropical Depression 15, way out at 20.2°W -- just 200 miles offshore! The Cabo Verde season, in which tropical waves exit the African coast near the Cabo Verde islands, typically spans mid-August through mid-October... but by mid-October it's pretty meager. As Phil Klotzbach at CSU pointed out, this is the easternmost tropical cyclogenesis this late in the year on record.

The forecast is not very threatening to anyone though. It may never reach tropical storm status as it passes by the Cabo Verde islands then off toward the northwest where it is forecast to dissipate.

Elsewhere, a disturbance over central America has some potential for development in the Gulf of Mexico later this week.  The National Hurricane Center is giving it a 40% chance of development into at least a depression by the end of the weekend.

Global model ensembles are divided on the system developing on the west side of Mexico or the east, but the trend has been shifting eastward, favoring the Gulf.  At this point, model guidance does not suggest that it becomes anything too strong, but that can change at this long lead time. So we will just keep a very close eye on it and model trends... especially interests in the western Gulf this weekend.  The next name on the list is Nestor.

Looking back to the start of the satellite era in 1960, I could find just seven named storms that developed in the western Gulf after October 1.  The two most recent were Marco (2008) and Matthew (2004). Jerry (1989) is the one that made landfall near Galveston as a Category 1 hurricane on October 16.

Tracks of seven named storms that formed in the western Gulf after October 1 (1960-2018).
Today is also the 14th anniversary of Hurricane Wilma's formation (Tropical Depression 24 at the time).  If you missed it on Twitter, there's a thread with new and interesting graphics at https://twitter.com/BMcNoldy/status/1184106020299104261

12 October 2019

Melissa forms off northeast U.S. coast

Tropical Storm Melissa, the season's 13th named storm, began its journey over North Carolina on Monday, then left the coast on Tuesday and continued to slowly organize... it was finally upgraded to a subtropical storm on Friday morning.  However, it may only be around for another day or so. Melissa will be yet another short-lived and messy storm this year, similar to Andrea, Chantal, Erin, Fernand, and Imelda -- those five storms combined were named for less than four days!

Peak sustained winds are at 50 mph and further weakening is expected as it accelerates toward the east into higher wind shear and colder water.  There are no associated tropical storm or storm surge watches/warnings. The storm is very tiny; the tropical storm force winds extend an average of just 45 miles from the center!

The recent transition from a subtropical cyclone to a tropical cyclone is nicely illustrated in this cyclone phase space diagram. It's a bit technical, but the low starts at location A on Wednesday, it's presently at location C on Saturday, and the forecast is at location Z on Friday. In this model-based analysis of the storm's structure, we see that it began as an asymmetric warm-core system (subtropical), it migrated into the symmetric warm core realm (tropical), is currently in a near-neutral zone, and is headed for the asymmetric cold core realm (extratropical).  So, while these classifications have a bit of subjectivity to them, there is also an objective foundation on which the transition is based.

Cyclone phase space diagram for Tropical Storm Melissa. (FSU)

Elsewhere, the Atlantic basin should be quiet for the foreseeable future. But don't tune out just yet... even late October has a history of producing some infamous storms!

02 October 2019

Lorenzo's trip to Ireland

Large and powerful Lorenzo has just passed the Azores and is headed toward Ireland.  It still has peak sustained winds of 80 mph and is racing toward the northeast at 43 mph. Tropical storm force winds extend as far as 390 miles from the center, while hurricane force winds extend as far as 150 miles from the center.

It will undoubtedly be a major problem in Ireland, with destructive winds and significant storm surge.  Port cities such as Galway will experience severe flooding on Thursday. Tropical storm force winds should arrive in Ireland on Thursday morning, followed quickly by hurricane force winds and a rapidly rising storm surge.  I have a long radar loop from Ireland at http://bmcnoldy.rsmas.miami.edu/tropics/radar/.

I found 18 tropical or post-tropical cyclones that passed within 100 miles of the Irish coast since 1851, and the most recent was Ophelia in October 2017, which made landfall as a post-tropical cyclone with sustained hurricane-force winds (80mph).  Debbie was the same intensity and was debateably still a hurricane when it made landfall in northwest Ireland in September 1961.  Edna was a little stronger than Ophelia and Debbie (85mph), and made landfall in northwest Ireland in September 1953 as a post-tropical cyclone.  Then the strongest on record was a post-tropical cyclone with nearly Category 2 sustained winds (90-95mph) that made landfall in southwest Ireland in September 1883.  We will see on Thursday where Lorenzo will fit into history.

Tracks of tropical and post-tropical cyclones that passed within 100 miles of Ireland since 1851. The gray lines indicate the storm was post-tropical.
Lorenzo formed back on September 23, and it alone has generated 27% of the season's total ACE (Accumulated Cyclone Energy) so far.  With this boost, the ACE is at 145% of average for this date, using the past 50 years as a climatology.

Elsewhere, the Atlantic basin remains relatively quiet for the foreseeable future.  There is a disturbance south of Cuba that has a low chance of development as it heads west toward the Yucatan Peninsula, and there's a wave still over Africa that models indicate could develop early next week in the far eastern Atlantic.

29 September 2019

Amazing Lorenzo becomes season's 2nd Category 5 hurricane

Late Saturday night, Lorenzo beat the odds and was upgraded to a Category 5 hurricane... the second of the season. The last season with two Category 5s was 2017 (Irma and Maria), and then 2007 before that (Dean and Felix).  In my previous blog post on Thursday when Lorenzo was a Category 3 hurricane, I mentioned this possibility and included the historical odds: "It will almost certainly keep going into Category 4 status, and could even reach Category 5 intensity (160+ mph) at some point.  Only about 2% of Atlantic named storms ever achieve Category 5 status".

Enhanced infrared satellite image of Hurricane Lorzeno from around the time it was upgraded to Category 5 intensity. (NOAA)
Not only did it beat the odds, it did so extremely far east; in fact, as Phil Klotzbach at Colorado State University nicely illustrated on a map, it reached Category 5 status about 600 miles further east than the previous easternmost: Hugo (1989).  I added labels to point out the three frontrunners.

Locations where all known Category 5 hurricanes first reached that intensity. (Phil Klotzbach, CSU: https://twitter.com/philklotzbach/status/1178137123687284737)
While this seems quite far north to have such an intense hurricane, the ocean temperatures in that area are normally quite warm this time of the season, and this year they're even warmer.  The water under Lorenzo when it was a Category 5 hurricane is about 28°C (82.4), which is about 1°C (1.8°F) warmer than average... just enough to give it that extra jolt.

Sea surface temperatures (°C), with Lorenzo's Category 5 position marked by a black X. (NASA)
As of 5am EDT on Sunday, it has weakened just slightly to an upper-end Category 4 hurricane with 155 mph sustained winds. The large hurricane (tropical storm force winds extend an average of 205 miles from the center), is forecast to continue tracking to the north then turn northeast, which will bring it near the Azores early Wednesday.  Beyond that, it is expected to begin the transition to an extratropical cyclone, but it will remain quite strong as it heads toward the British Isles on Friday. Both areas could easily experience hurricane conditions including destructive winds and significant storm surge.

Looking back at the history of NHC intensity forecasts for Lorenzo and comparing them to the observed intensity, we can see that the first peak was fairly well-anticipated, but the second and even stronger peak was certainly not.

Elsewhere across the basin, things will remain quiet for the foreseeable future.

26 September 2019

Karen battling dry air as Lorenzo becomes season's 3rd major hurricane

In the far eastern Atlantic, Lorenzo has rapidly intensified to become the season's 3rd major hurricane (Category 3+), with peak sustained winds of 125 mph as of 6am EDT -- just yesterday morning it was at 80 mph.  It will almost certainly keep going into Category 4 status, and could even reach Category 5 intensity (160+ mph) at some point.  Only about 2% of Atlantic named storms ever achieve Category 5 status, so don't hold your breath.

By the way, curious about other percentages? Of all 1637 known named storms (subtropical storms, tropical storms, and hurricanes) in the Atlantic from 1851 to 2019 so far:
  - 58% become Category 1 hurricanes
  - 20% become Category 3 hurricanes
  - 2% become Category 5 hurricanes
The forecast continues to confidently include a turn to the north beginning tonight which will keep this large and very intense hurricane far from land.  The exception to that could be the Azores... Hurricane Lorenzo has a chance to pass near or even over the Azores next Wednesday-Thursday.

Karen is now a minimal tropical storm.  Dry air has taken its toll on the tiny storm, and even after a slight resurgence in thunderstorm activity overnight, its days appear to be numbered.

The forecast from NHC still shows a northward motion through Friday morning, followed by a turn and/or loop toward the west, but continuing to weaken the whole time.  It could dissipate by the end of the weekend.  There is no model guidance that shows any threat to the Bahamas or the U.S. now.

Elsewhere, the basin is quiet, and no new development is expected in the foreseeable future.

25 September 2019

Karen moving away from Puerto Rico, Lorenzo upgraded to 5th hurricane

Karen made landfall on the eastern side of Puerto Rico on Tuesday afternoon as a tropical storm and is now centered about 240 miles north of the island.  As of 11am EDT, the peak sustained winds are 45 mph and it is moving toward the north at 15 mph. The northward motion is expected to continue roughly through Friday, at which point things get messy.

The NHC forecast is in line with most of the model guidance, and shows a stall sometime around Friday, followed by a sharp left turn (there could easily be a little loop in the process) toward the west. That stall-and-turn or loop-and-turn depends on a subtropical ridge building and strengthening to its north, but it also depends on how strong the storm itself is since storms of different intensities are generally steered by different layers of the atmosphere.

Models all agree on a stall and/or loop of some sort, and the large majority show some degree of westward track after that.  But what's not shown on the map above are the intensities.  In some cases, there's barely a trackable system, and there's really no support among dynamical models for anything of hurricane intensity. Clearly, given the potential for impacts in the Bahamas and the southeast US, we'll be watching model trends very closely, but as of now, it's not a cause for concern.

Lorenzo became the season's 5th hurricane on Wednesday morning, and is fully expected to become the season's 3rd major hurricane in a few days.  It is far from land in the deep tropics west of Cabo Verde, and models all agree on a north turn to begin later this week which will keep it out in the middle of the ocean. As of Wednesday morning, the peak winds are 85 mph and the official forecast brings it up to 125 mph (Category 3) by Friday evening.

Tropical Storm Jerry has been transitioned to a post-tropical cyclone, but is still headed toward Bermuda.  It is nearly devoid of rain and thunderstorms, but does have tropical storm force winds associated with it.  It will make its closest approach to Bermuda late Wednesday night. If any rain showers or thunderstorms develop in it, there are radar loops at http://bmcnoldy.rsmas.miami.edu/tropics/radar/. Recall that Jerry formed last Tuesday and peaked as a Category 2 hurricane northeast of the Leeward Islands.

24 September 2019

Three tropical storms keeping the Atlantic busy

Tropical Storm Karen is now closing in on Puerto Rico, Tropical Storm Jerry is a little bit closer to Bermuda, and Tropical Storm Lorenzo is nearly a hurricane out by Cabo Verde.

Karen was briefly downgraded to a depression on Monday night, but has regained tropical storm status and is now centered just 80 miles south of Puerto Rico and moving to the north at 7 mph.  As of this post, heavy rain has not reached the island, but will later today, as will tropical storm force winds. There are long, updating radar loops at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Since the winds won't be too strong, the biggest concern by far is the rain, and the resulting flash flooding and mudslides. Areas to the right of the storm track could see some minor storm surge, as well as the gustiest winds from the circulation and an elevated tornado threat.  These maps show the outlook for the four hurricane hazards (heavy rain, wind, storm surge, and tornadoes) as of Tuesday morning:

Beyond today's encounter with Puerto Rico, the forecast gets abnormally complicated.  Although the NHC is bound to making a single track forecast and drawing the same "cone" around it that gets drawn around every other storm and forecast all year, the uncertainty is definitely larger than average historical track errors would indicate.

A long list of unknowns is the reason for this, such as its proximity to Jerry, its interaction with Puerto Rico, its sensitivity to wind shear, its sensitivity to dry air, and the strength and position of a building subtropical ridge.  A stronger storm is deeper and is steered by different layers of the atmosphere than a weak storm.  Although the latest NHC forecast shows a northward motion through Friday followed by a sharp left turn and westward motion during the weekend, model guidance is all over the place. This map shows forecast tracks from NHC (thick red line with red dot) as well as three hurricane models, three global models, two global model ensembles, three global model ensemble means, and a couple that are a consensus of some of the others.  It's the dreaded "squashed spider".

So for now, consider Puerto Rico and the Virgin Islands as the places at risk from Karen... everyone else just relax and wait a bit longer to see what's going on.

Jerry is a highly-asymmetric tropical cyclone with barely any thunderstorm activity near the center, and deteriorating quickly -- but it is bringing periodic strong rainbands to Bermuda.  As of 8am EDT today, the peak sustained winds are 60 mph and tropical storm force winds should reach Bermuda by early Wednesday morning.  It's headed north at 8 mph and is expected to make a turn to the northeast later today, bringing the center near or over Bermuda on Wednesday afternoon.  There are also radar loops from Bermuda at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

And much further east, Tropical Storm Lorenzo looks like it's rapidly on its way to becoming the season's 5th hurricane and most likely the 3rd major hurricane in a few days.  It passed well south of Cabo Verde on Monday, and model guidance is in agreement on a turn toward the north this weekend that will keep it very far away from any land.

23 September 2019

Jerry heading for Bermuda, Karen heading for Puerto Rico, soon-to-be Lorenzo near Cabo Verde

The tropical Atlantic continues to bubble with activity, and there could be three named storms by the end of the day.
Tropical Storm Jerry is centered about 350 miles southwest of Bermuda and is forecast to pass just north of (or over) the island on Wednesday as a tropical storm -- about 6.5 days after Category 3 Hurricane Humberto passed north of the island. It is forecast to continue to weaken in the face of wind shear, dry air, and cooler ocean temperatures, but tropical storm conditions should reach Bermuda on Tuesday evening. Beyond the Bermuda encounter, Jerry will dissipate over the cold north-central Atlantic later in the week.
Next on the list is Tropical Storm Karen, which formed on Sunday from a tropical wave near Trinidad and Tobago. It has since entered the far eastern Caribbean and is battling moderate vertical wind shear (it looks like it's losing that battle). The forecast brings it north, and tropical storm warnings are in effect for Puerto Rico and the Virgin Islands.  It's not even a given that it remains intact that long, as some models dissipate it before then.
But, assuming that it holds together, models are in good agreement on a northward track for another 1-2 days after crossing Puerto Rico on Tuesday.  At that point, a strong ridge of high pressure is forecast to build to its north, which would force it to make a sharp left turn to the west.  The ridge is pretty robust in the models, so a continued westward track would bring it toward the Bahamas, Florida, and/or the southeast US coast in about one week.

Track density from the 23Sep 00Z ECMWF 50-member ensemble. The 00Z deterministic run is shown by the cyan dashed line, and the 5-day NHC forecast is the black solid line. (UAlbany)
But that all comes with the uncertainty built into the survival of the tropical cyclone in the next couple of days, the exact strength and position of the ridge, and a list of environmental factors that Karen will need to contend with beyond two days.  At this long lead time, it's just worth being aware of this "left turn" possibility, and knowing that the global models and their ensembles have supported this scenario for a couple days now.

Finally, Tropical Depression 13 is located south of the Cabo Verde islands and is very close to becoming the 12th named storm of the season: Lorenzo.  Models are in excellent agreement on this becoming a strong hurricane in a few days, and on a turn to the north before reaching 60°W.  It appears at this time that it will never be a threat to land.  The NHC forecast brings this close to major hurricane status on Saturday as it begins the northward turn.

20 September 2019

Jerry's turn to take aim at Bermuda, next storm brewing over Africa

As of 8am EDT on Friday, Jerry has strengthened to a Category 2 hurricane with 100 mph peak sustained winds and will pass north of the Leeward Islands later today.  Since the wind field is quite small, impacts on the islands will be minimal, but tropical storm watches are up.  The hurricane also looks very ragged today, hardly the mental image we conjure up when thinking of a Category 2 hurricane.
While confidence in a turn to the north this weekend is high, there's a slim chance (as indicated by about 15% of the ECMWF ensemble members) it stays south and tracks along or near the Greater Antilles. It's just too soon to completely rule that out.  And regarding the NHC "cone of uncertainty", remember that it's designed to enclose the track of the center of the storm just 2/3 of the time, using historical errors. There's historically a 1/3 chance of it tracking outside the cone.

There is a much greater chance that Jerry will pass near Bermuda next Tuesday, not even six days after Category 3 Hurricane Humberto's visit.  This one-two punch is brutal, and actually happened five years ago when Hurricanes Fay and Gonzalo hit the island 5.5 days apart.  When the atmosphere gets stuck in a rut, watch out.

The remnants of Tropical Depression Imelda have finally loosened their wet grip from southeast Texas, and the rainfall totals are staggering.  Local amounts over 42 inches have been reported, and there is significant flooding in the Houston to Beaumont region.

Storm total rainfall estimates as of Thursday evening. https://twitter.com/JZTessler/status/1174821051550883840
This storm *barely* got a name -- it was Tropical Storm Imelda for six hours, just one single advisory on Tuesday afternoon.  That's it.  Clearly, it does not take a hurricane or even a tropical storm to cause major impacts.  But by sneaking into the "named storm" category, the potential exists for the name to be retired.  That decision would not be made until well after the season ends, and a special committee of the World Meteorological Organization convenes (think Jedi High Council).  In the following charts, I am confidently assuming that Dorian will be retired, but leaving Imelda out for now.  What stands out is that "I" storms and September storms are by far the most retired, so Imelda would certainly fit that history! This year was Imelda's first time on a list, having replaced Ingrid when it was retired in 2013.  Other "I" storms that got retired on their first use were Ike, Igor, and Irma.

Shifting our attention way east, a strong tropical wave still over Africa has a lot of support for rapid development in the model guidance. NHC is giving it a 70% chance of becoming at least a tropical depression by early next week, possibly even before reaching Cabo Verde's longitude. Specific examples from the most recent ECMWF and GFS runs are shown below (the shading is low-level cyclonic vorticity, a measure of the curvature of the wind).  The next name on the list is Karen.