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.

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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.

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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!

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