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.

19 September 2019

Humberto zips past Bermuda, Imelda drenching Texas, Jerry upgraded to hurricane

The tropical Atlantic continues to be buzzing with activity, which isn't surprising considering it's mid-September. We now have two hurricanes and a tropical depression.  Three other tropical waves have a shot at becoming tropical cyclones in the coming days as well.

Humberto made its closest approach to Bermuda on Wednesday night, and brought a peak wind gust of 116 mph to the island, knocking out power and momentarily knocking out the radar.  The final radar image before a 4-hour-long outage is shown below --  Bermuda was right in the southern eyewall of Category 3 Hurricane Humberto. It has since been downgraded to a Category 2 hurricane and is racing off toward the open northern Atlantic at 24 mph. It will become a large and strong extratropical cyclone by tomorrow.

Tropical Depression Imelda is still sitting over southeast Texas, and dumping FEET of rain in the same places that Harvey flooded in 2017.
Enhanced water vapor satellite image showing a large shield of cold cloud tops sitting over southeast Texas... very dry air is just to the west. (NASA)
Incredible rainrates of 4-6 inches per hour have set up and remained nearly stationary, resulting in rain being measured in feet rather than inches, and peak radar estimates are now over five feet. The loop below is a storm-total rainfall accumulation, where the scale updates to accommodate the maximum values.  By the end of the loop, the scale tops out at about 65 inches.

This once again serves as a reminder that it does not take a strong hurricane to cause tremendous destruction and impacts.  We saw this with Allison (2001), Harvey (2017), and Florence (2018) to name a few. Water is by far the #1 killer when it comes to hurricanes and tropical storms. Tropical cyclones and their remnants are still very wet, and when they become stationary, they're unbelievably wet. The hurricane rating scale (Saffir-Simpson Scale) is ONLY based on the storm's peak sustained wind speed... it says nothing about size of the wind field, rainfall, storm surge, or tornadoes. There's more to the story than the category!

Moving on to the newly-upgraded Jerry... it is now the season's 4th hurricane and is still headed toward the northern Leeward Islands.  Jerry should pass north of the Leewards on Friday, but could bring tropical storm force winds beginning Friday morning.  As of 11am EDT on Thursday, it has peak sustained winds of 75 mph and it's moving toward the west-northwest at 16 mph.

Model guidance is in quite good agreement on it recurving to the north prior to reaching the Bahamas later this weekend, but it is not a closed case. The skillful European model ensemble still has 10-15% of its members indicating that Jerry could remain to the south and track along/near the Greater Antilles. Assuming the most likely scenario, the only encounter with land it will have beyond the Leeward Islands is possibly Bermuda early next week -- about six days after Humberto's visit.
Of the three areas of possible formation shown on the map at the top of the post, the only one strongly supported by models is the easternmost.  A potent tropical wave inland over Africa is expected to come off the continent on Saturday and quickly get its act together.  The next name on the list is Karen.

18 September 2019

Major Hurricane Humberto passing by Bermuda, Imelda flooding Texas, Jerry forms in deep tropics

The Atlantic family portrait on Wednesday afternoon. (CIRA/RAMMB)
Humberto is now a Category 3 hurricane with 120mph peak sustained winds.  As of 2pm EDT, it's centered about 100 miles west-northwest of Bermuda, tracking quickly toward the east-northeast at 20mph, and the tropical storm force winds extend an average of 175 miles from the center.  This is the second major hurricane of the season, after Dorian.  Bermuda will experience hurricane conditions on Wednesday evening/night, including 1-3 feet of storm surge and 2-4 inches of rain.  The latest version of the radar loop shown below is available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Imelda was only a named tropical storm for a few hours, and was downgraded to a tropical depression on Tuesday evening, but the flooding threat remains very significant.  Rainfall totals over the past two days are shown below, and the forecast for the next two days is right below that.  This will be a major event (areas with over two feet) when it's all over, but the amorphous storm is crawling north at just 5mph, so it's far from over yet.

Tropical Depression 10 was upgraded to Tropical Storm Jerry early Wednesday morning, making it the 10th named storm of the season.  It is forecast to become the season's 4th hurricane on Thursday as it tracks toward the Leeward Islands.  As of Wednesday afternoon, it's centered 750 miles due east of Martinique and moving toward the west-northwest at 15mph.

The track guidance is still largely in agreement on it passing north of the Leewards on Friday as a hurricane, but tropical storm conditions could arrive in the islands by midday Friday. If the track stays on the south side of the guidance envelope, the northern Leewards could experience hurricane conditions on Friday.  Uncertainty sneaks in via a big patch of dry air to its west right now -- huge differences in forecast evolution crop up depending on it if the storm keeps itself isolated from that or if it wraps it into the circulation.
The forecast is still hazy beyond Friday or so, and that has big implications for the Greater Antilles, the Bahamas, and the southeast US next week.  As of now, about 15% of the ECMWF ensemble members do not recurve the hurricane out to sea before reaching land. And it just so happens that some of that 15% keep the storm rather strong. So, it is definitely something to keep a close eye on with each new model cycle.  The stronger Jerry stays these next few days, the more it will feel the presence of the subtropical ridge and the further south it will stay.

Elsewhere, two other waves have small chances of developing into depressions within the next five days, but considering the amount of current activity, they are not worth going into detail just yet.

17 September 2019

Humberto, Imelda, and soon-to-be-Jerry scattered across the Atlantic

The Atlantic basin now has two named storms, and likely three by day's end.  We have Humberto heading for Bermuda, Imelda inland over Texas, and Tropical Depression 10 east of the Lesser Antilles. 

Humberto is now a Category 2 hurricane and is continuing its trek toward Bermuda. It's centered just 450 miles west of the island and moving toward the east-northeast at 12mph.  They are under a hurricane warning, and dangerous tropical storm force winds should arrive by Wednesday afternoon.  While Humberto is the season's third hurricane, it's only the second Category 2+ hurricane (Dorian was the other).  The latest NHC forecast does bring it up to Category 3 intensity on Wednesday as it passes by Bermuda.  I have a long, updating radar loop from Bermuda at http://bmcnoldy.rsmas.miami.edu/tropics/radar/ which could become interesting on Wednesday.
Tropical Storm Imelda just formed on Tuesday afternoon... inland near Houston TX.  It's certainly unusual to have a tropical storm form over land, especially without so much as a tropical storm watch leading up to it (this was never a "Potential Tropical Cyclone", despite that term being implemented solely for this purpose in 2017).  The first advisory on it was as Tropical Depression 11, on Tuesday early afternoon right on the coastline. It has peak sustained winds of 40mph and it's moving toward the north at 7mph.  It is forecast to weaken, but stranger things have happened (like a few hours ago).

Even prior to this rapid genesis, the big concern was flooding, and that remains the case. Rainfall totals in the area could easily exceed a foot as the storm drifts northward over the next couple days.

Imelda is a new name this year, introduced after Ingrid was retired in 2013.  Curiously, "I" storms are the most-frequently retired, so we'll see what Imelda does in the coming days -- hopefully not enough to earn retirement.

Finally, Tropical Depression 10 is about 1100 miles east of the Lesser Antilles and is forecast to become the season's tenth named storm very soon: Jerry.  Model guidance is in good agreement on it reaching somewhere around the northern Leeward Islands on Friday and the NHC forecast is for it to reach hurricane status around that time.  Beyond that, a slight northward bend will bring it toward the eastern Bahamas later this weekend.  The map below shows the arrival time and probability of tropical storm winds based on the latest 5-day track and intensity forecast.

Beyond that, there is still way too much spread just in the European model's ensemble forecasts to say much.  The realistic scenarios range from a hurricane as far south as Puerto Rico to an early recurve out to sea by about 65°W.  Not only is this a small and weak system, the large-scale steering features will be manipulated and altered by Humberto.  For now, just stay tuned and I will be watching for changes or trends in the ensemble guidance.

16 September 2019

Hurricane Humberto eyeing Bermuda

Tropical Depression 9 formed near the northern Bahamas on Friday afternoon, and was quickly upgraded to Tropical Storm Humberto, the 8th named storm of the season.  On Monday it was upgraded further to Category 1 Hurricane Humberto, and could become the season's 2nd major hurricane on Tuesday as it heads for Bermuda.

Humberto's tropical storm force winds extend an average of 105 miles from the center, and it has a spectacular outflow channel to the north that reaches out past Newfoundland!  It's centered about 700 miles west of Bermuda and has 85mph peak sustained winds as of the 11am EDT advisory on Monday. Additional strengthening is forecast, and Bermuda could start seeing tropical storm force winds by Wednesday evening. The last few hurricanes to have passed within 100 miles of Bermuda were Joaquin (2015), Gonzalo (2014), and Fay (2014).  While the odds favor a hurricane's inner core not making a direct hit on a tiny island, it cannot be ruled out either.

It's worth pointing out that the skillful ECMWF ensemble has several members indicating that Humberto might not be too quick to race out to sea... a number of them slow it down, turn the hurricane back toward the west, and approach North Carolina in 5-6 days.  It's not a large probability, but something to be aware of.

Elsewhere, the basin is suspiciously quiet for mid-September, but there is a tropical wave centered about 1200 miles east of the Lesser Antilles that has a good shot at developing and becoming the next named storm: Imelda.  Over the next five days, both the GFS and ECMWF ensembles agree on this system moving toward the west-northwest and being located somewhere north of the Lesser Antilles this weekend.  Beyond that, the spread increases, but they still strongly favor a recurve to the north well before reaching and land -- with the exception of Bermuda, which is currently in the spread sometime next Monday-Tuesday.  Lots of time to watch and wait.

The Accumulated Cyclone Energy has remained right near average for the date, with Humberto's help.  It's at 102% of the average over the past 50 years, and should continue to keep up with climatology as long as Humberto is active.

12 September 2019

Disturbance near Bahamas slowly getting organized

The broad, disorganized tropical disturbance near the eastern Bahamas that I mentioned yesterday is still centered near the eastern Bahamas but is closer to reaching tropical depression status.  It has maintained thunderstorm activity around it, but seemingly lacks a defined surface circulation. A reconnaissance aircraft will fly through it later today to determine if it has a surface circulation -- it would get upgraded to Tropical Depression Nine or Tropical Storm Humberto if it does.

The National Hurricane Center is giving it a 70% chance of formation by Saturday, somewhere over the Bahamas or south-to-central Florida.  It is technically referred to as Invest 95L now, and could be renamed Potential Tropical Cyclone Nine very soon (if it's not upgraded to a bona fide tropical cyclone). There is actually no difference between the two as far as intensity is concerned.  Both an Invest and a PTC are pre-depression disturbances. The difference is that NHC can issue tropical storm watches and warnings for a PTC but not for an Invest.  Basically, it gives them the ability to start advisories on a disturbance because it could impact land with tropical storm conditions within 48 hours.  This ability and the PTC designation were introduced in 2017.

Model guidance has unfortunately become more cloudy since yesterday.  On Wednesday, there was wide agreement on a westward track into the Gulf of Mexico. That is still a possibility, but more models have started to indicate a northward turn before or around reaching Florida... completely avoiding the Gulf.  Not shown on the map below are the GFS and ECMWF ensembles; the entire GFS ensemble keeps 95L weak and tracking into the northern Gulf, while about 85% of the ECMWF ensemble members strengthen it and turn it north before reaching Florida. This large amount of uncertainty just 2-3 days out is frustrating and makes preparation difficult for people.  It also makes thinking about 4-5 days down the line next to impossible.

Through the weekend, we would not be looking at big impacts in the Bahamas or Florida, even if it does marginally develop. The biggest hazard would be heavy rain, and for a low-end tropical storm there would be some gusty winds. These hazards reaching into Florida are dependent on how far west it tracks.

Much further east, the tropical wave that left the African coast back on the 10th is still a feature of interest. It has a hostile environment ahead of it for the next several days, but is favored by long-range model guidance to start to develop in the vicinity of the Lesser Antilles around Monday.  The latest ECMWF ensemble shows a generally-weak system moving through the Caribbean with lows scattered from the central Caribbean to the far eastern Bahamas next Wednesday.

11 September 2019

Tropical wave heading for Bahamas, Florida, then Gulf coast

Although there are no named storms in the Atlantic today, there are a few disturbances and waves of interest.  The first is over the far eastern Bahamas and will have an impact in the Bahamas, the Florida peninsula, and then likely the northern Gulf coast in the coming days.
Map with surface wind flow denoted by white lines, moisture (precipitable water) in background shading, and an "x" marking the center of the three features of interest. (earth.nullschool.com)
Significant development is not expected at this time, especially while over the Bahamas and by the time it reaches Florida on Friday.  Those areas will experience rainy and breezy conditions -- even in the very slim chance it becomes a tropical depression or low-end tropical storm in the near future.  But as you can see from the satellite loop below, it is broad and disorganized.

Seven-day rainfall forecast (in inches), valid from today through next Wednesday. (NOAA/WPC)
But after crossing the Florida peninsula, some development is likely, as favored by the majority of models.  The most recent European model ensemble guidance is shown below, and it is fairly consistent with the past few runs.  It shows the track and intensity of any low detected in the member forecasts, from today through Monday, every six hours. In this run, all members remain well below hurricane intensity, but largely agree on a track into the northern Gulf coast.  If this does reach tropical storm status at some point, the next name on the list is Humberto.

The other two areas of interest are in the deep tropics, east of the Lesser Antilles. Neither is likely to develop in the foreseeable future, but will be watched very closely of course -- after all, it's mid-September and they're African waves.  However, they are both battling dry air and wind shear.  The western one would reach the Lesser Antilles sometime around Monday, and the one south of Cabo Verde has a long way to go to be a concern (see map at top of the post).

The ACE (Accumulated Cyclone Energy) is at 115% of the average for this date, using the past 50 years as climatology.  Unless a named storm forms soon, 2019 will slip behind the average again on Sunday. This time of year is climatologically active, so every day without something is anomalous.  Of the seven named storms so far this season, Dorian alone contributed 82% of the total ACE!

06 September 2019

Dorian finally makes U.S. landfall on Cape Hatteras, now heading to Canada

Eighteen days after exiting the African coast, and three days after its closest approach to south Florida, Hurricane Dorian finally made landfall in the U.S., barely.  The center of the eye just clipped Cape Hatteras, North Carolina around 8:30am EDT; the intensity was 90 mph and 956 mb. However, even without a technical landfall further south, the western eyewall and wind field battered and flooded the coast from central Florida up through Virginia, and it's not done yet.

As of Friday afternoon, Dorian is centered 300 miles south of New York City, or 750 miles southwest of Halifax, Nova Scotia.  Tropical storm force winds now extend an average of 175 miles from the center, and hurricane watches and warnings plaster Nova Scotia and Newfoundland where severe impacts such as strong wind and significant storm surge are expected to begin by Saturday morning and continue into Sunday.  The last few hurricanes to hit or get very close to Nova Scotia were Earl (2010), Bill (2009), Kyle (2008), Juan (2003), and Gustav (2002).  Juan hit the province as a Category 2 hurricane and the name was retired because of the impact there.
Also to wrap up Dorian, I have a long list of radar loops covering the journey since it passed over the Windward Islands on August 27 at http://bmcnoldy.rsmas.miami.edu/tropics/radar/. There's a 5-day-long loop that covers the entire southeast U.S., and brand new today is one I was able to pull together that covers Dorian's history-making trek over the northern Bahamas... be sure to check them out.
The name Dorian was added to the 2013 list when Category 5 Hurricane Dean was retired in 2007.  The 2013 version of Dorian was actually quite similar to 2019's Dorian in terms of origin and track (but definitely not intensity).  It began as an African wave on July 22 and tracked westward through the deep tropics as a tropical storm, over the Bahamas as a tropical depression, turned north right before reaching Florida, and then dissipated off the North Carolina coast on August 4.  But the 2019 version will certainly make this Hurricane Dorian's last appearance.

Elsewhere, Tropical Storm Gabrielle is maintaining its northwesterly track, and is forecast to continue strengthening for the next several days as it heads into the north central Atlantic, possibly becoming the season's third hurricane. It's centered about 1200 miles northwest of Cabo Verde, and is not a concern to land.

The tropical wave that left the African coast two days ago remains a feature of interest, and the National Hurricane Center is giving it a 70% chance of becoming at least a tropical depression in the next five days.  Long-range model guidance has been indicating that unlike Gabrielle, it could continue westward across the deep tropics. The latest 50-member European model ensemble has a majority of members that keep a southern path over the ten days, and this is representative of the past few cycles. However, keep in mind that this extended-range model guidance comes with a *LOT* of uncertainty, and is not an official forecast.  For now, we just keep an eye on their consistency and trends.

05 September 2019

Dorian regains major hurricane status near South Carolina coast

Composite radar image from 9am EDT. (CoD)
Hurricane Dorian has continued its northward trek along the southeast U.S. coast, and as of Thursday morning, is centered about 70 miles east of Charleston SC.  The maximum sustained winds are up to 115 mph (Category 3), and the tropical storm force winds extend an average of 145 miles from the center. The exact intensity/category does not matter much at this point: Dorian will be a powerful hurricane and will bring with it the full array of hurricane hazards (watch out for those fast-moving tornadoes in the rainbands).

The track forecast is shown below, along with the current wind field (green is the tropical storm force winds, yellow is the gale force winds, and orange is the hurricane force winds. As you see, the hurricane force winds will be very close to the SC and NC coastline, sometime onshore, sometimes offshore.

For arrival times of the dangerous tropical storm force winds, the map below show the most likely timing (line contours) along with the probability (colored contours).  Note that as of Thursday morning, the Delmarva peninsula is under a tropical storm warning and Cape Cod is under a tropical storm watch.  The full updated maps of hurricane, tropical storm, and storm surge watches and warnings can be found on the NHC website.

The National Weather Service's  HTI (Hurricane Threats & Impacts) graphics from Thursday morning break down the big four hurricane hazards onto maps: wind, flooding rain, storm surge, and tornadoes. When it comes to storm surge, keep in mind that the timing of the surge matters quite a bit: the flooding will be worse if peaks near high tide. Water will rise gradually for a while, then ramp up quickly as the strongest onshore winds approach.

You can keep an eye on the rainfall (and eyewall and rainbands) via long radar loops at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Dorian will pass the Outer Banks in NC by Friday afternoon, then by New England on Saturday, then continue northeast toward Nova Scotia on Saturday night.  Coastal flooding will likely be a problem all along the way as the storm's winds push water onshore ahead of it.

Since yesterday, Fernand made landfall in northeast Mexico and has since dissipated, Gabrielle is still a disorganized tropical storm far from land, and a wave that just left the African coast yesterday is showing signs of development. At this early stage, it appears likely that if this forms, it would turn north well before reaching land, but not as quickly as Gabrielle did. Long-range model guidance does not suggest that it would reach the Lesser Antilles.