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.