Cabo Verde season isn't over yet... watching Tropical Storm Sean and a second disturbance

"Cabo Verde season" is the portion of Atlantic hurricane season that refers to when easterly waves trek across Africa and emerge over the Atlantic Ocean near Cabo Verde... and a small percentage of them go on to become long-lived hurricanes.  The vast majority of the most infamous hurricanes are of Cabo Verde pedigree.  This season doesn't have exact bounds, but is generally mid-August through early October.  The large-scale environment tends to be too hostile for those easterly waves to develop before and after that timeframe.  So, the point of this introduction is that it's getting to be rather late to be watching one let alone two systems out there!

I suspect some of the reason for that is the anomalously warm water still present out there. On this map below, the sea surface temperature anomaly is the background image (in °C), Tropical Storm Sean's forecast track and track forecast cone are overlaid in the center, and the easterly wave's current position and potential formation zone in the coming week are shown by the yellow X and corresponding shaded area.

Sean is the season's 19th named storm, but probably won't be around much longer.  Conditions were only marginally favorable for development in the first place, and increasing vertical wind shear should limit the storm's lifetime to just 3-5 days.  But, a couple hurricane models show it turning to the north a little sooner, and missing some of the interaction with the stronger shear... allowing it to intensify in 4-5 days.  So we can't tune this out just yet.

The easterly wave that just left Africa is given a 30% probability of development in the coming week by the National Hurricane Center.  It's not expected to move very quickly, but models generally indicate that it will gradually develop and stay in the deep tropics... potentially reaching the Lesser Antilles in about 10 days.  However, at this long lead time, it's premature to venture a guess if it will be anything of concern to the Antilles... just something to watch and be aware of. Should it become a tropical storm, the next name on the list is Tammy.  

By the end of the day, the Accumulated Cyclone Energy (ACE) will be at about 124% of the climatological average for the date, and 105% of an average full season.

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Tropical Storms Philippe and Rina lurking in the deep tropics

Tropical Depression 17 formed on Saturday morning and was upgraded to Tropical Storm Philippe shortly afterward... and Tropical Storm Rina just formed on Thursday morning.  Both are located east of the Lesser Antilles and most likely will not affect land.  They are rather close together and in this satellite loop, it's even hard to tell them apart!  (Philippe is west of Rina)

Philippe has been hovering as a mid-range tropical storm all week, and its future is very interesting and uncertain... much more than normal.  This example is from the American global model (GFS) ensemble and is representative of the spread we're seeing in the other models.  One cluster dissipates the storm or at least keeps it weak as it heads west toward the Caribbean, while another cluster stalls, turns north, and becomes a strong hurricane.  In a few of the scenarios, this could be a close call for the extreme northeast Caribbean islands, so certainly something to pay close attention to there.  The NHC forecast is a hybrid of these outcomes: their forecast takes it west, then turning north well before reaching the islands, but keeping the intensity steady as a weak tropical storm. 

Tropical Storm Rina is located just 650 miles east of Philippe and is not expected to strengthen much.  It is the 18th named storm of the season... keep in mind the climatological average number of named storms in an entire season is 14.  There are a couple reasons for its modest intensity outlook: the proximity to Philippe and increasing vertical wind shear.

The Accumulated Cyclone Energy, or ACE, is at about 132% of average for the date, and 99% of an average full-season's total. Impressively, today is the 40th consecutive day of ACE accrual... the activity has been nonstop from Emily through Rina!

Although there are no other features of interest out there to monitor yet, the next couple of names on the list are Sean and Tammy.  As we head into October, activity from Africa begins to dwindle, and we start looking to the western Caribbean and Gulf of Mexico for formation areas.  The ocean temperature in these areas is also very warm compared to normal for this time of year, so not only does that give incipient storms a nudge, the area of "favorable" warm water is larger than normal too.

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Monitoring two new disturbances as Nigel exits the scene

Activity in the Atlantic has been relatively uneventful for the middle of September, so since my previous post on Friday, Lee did of course make landfall in Nova Scotia, Margot dissipated, and Nigel became a hurricane.  Today, Nigel is heading for colder water and won't be around much longer, but there are two areas of interest to keep an eye on.  The next two names are Ophelia and Philippe.

I'll start with Nigel.  It was upgraded to a hurricane on Monday morning and tracked north through the middle of the Atlantic.  It reached Category 2 intensity for a while late Tuesday into Wednesday, but is now a Category 1 hurricane again, and expected to become an extratropical cyclone by Friday morning as it heads toward Iceland... two weeks after leaving the African coast.

The first area of interest to watch for development is a low pressure that is beginning to form along an old stalled-out cold front.  It's centered east of the Florida peninsula and has been designated as Potential Tropical Cyclone 16 by NHC on Thursday morning.  That means it still hasn't formed, but is expected to and warrants some tropical storm warnings.

It already has characteristics that are more subtropical than tropical, but either way, it would get named and NHC will issue advisories and forecast products for it.  The forecast brings it north to a landfall in North Carolina on Saturday morning as a (sub)tropical storm. Tropical storm force winds will likely arrive in eastern NC on Friday evening.  You can find the full suite of the latest forecast products at https://www.nhc.noaa.gov/graphics_at1.shtml?start#contents

Parts of North Carolina, Virginia, and Maryland could see 2-4 feet of the storm surge from this over the weekend, and the threat of flooding rain is increasing from North Carolina up into New England.

Much further east is an easterly wave that left the African coast on Tuesday.  It's located just southwest of Cabo Verde and is expected to track generally west-northwest for the next week or so, then its future path becomes harder to predict.

This is not an Invest yet, so we are limited to global models and their ensembles for guidance.  The two skillful ones we typically look at are the American (GFS) and European (ECMWF).  These two images below show the tracks from the GFS (top) and ECMWF (bottom) ensembles, ending next Friday evening. Although both of them are pretty consistent in bringing it up to hurricane intensity by Tuesday-ish, and both of them have a general tendency to turn it north by the time gets to 60°W-ish, the GFS ensemble does have more members that show a hurricane clipping the northeast Caribbean islands.  This is something we need to watch closely, because if that route starts looking more likely, it makes the northward turn less likely which has implications in the following days.

Again, the next two names are Ophelia and Philippe and it's not clear yet which of these two systems will become a tropical (or subtropical) cyclone first.

The Accumulated Cyclone Energy, ACE, is about 142% of average for the date.

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