28 September 2023

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.

21 September 2023

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.

15 September 2023

Lee heading for Canada, Margot weakening, Nigel on the horizon

Since my update on Tuesday, Lee and Margot have "behaved" as forecast: Lee turned north, weakened, passed west of Bermuda, and is headed for Nova Scotia.  Margot maintained Category 1 hurricane intensity but just dropped back down to a tropical storm early Friday morning. Finally, Invest 97L became Tropical Depression 15 on Friday morning and will be named Nigel once it reaches tropical storm intensity.  On the satellite loop below, Lee is in the upper left, Margot is in the upper right, and TD15 is in the bottom middle.

Lee is now a Category 1 hurricane and a very large one at that.  Tropical storm winds extend as far as 320 miles from the center, and hurricane winds extend as far as 105 miles from the center. A storm surge of 1-3 feet is possible along much of the New England coast.

It is forecast to make landfall in western Nova Scotia midday Saturday though strong winds will arrive before that... it could be an upper-end tropical storm or Category 1 hurricane, and possibly no longer technically be classified as a tropical cyclone, but that does not change the expected impacts.  This process is called "extratropical transition", and it involves changes in the dynamic and thermodynamic structure and properties of the cyclone.  Extratropical cyclones can also be very intense and impactful (remember Sandy 2012?).

Margot could be around for a while longer, generally maintaining tropical storm intensity.  It found itself in an area of weak steering flow and is just circling around west of the Azores.  Even if it eventually reaches the Azores sometime in the middle of next week, it should not be too much of a concern.

Elsewhere, the Invest 97L and 98L merger happened and has been carried as just 97L... at 11am Eastern on Friday it was upgraded to Tropical Depression 15 and it's quite close to reaching tropical storm status. The next name is Nigel which is a new name on the list, replacing Nate from the 2017 season. 

TD15 is located about 1200 miles east of the Lesser Antilles in the deep tropical Atlantic.  There is strong agreement among the models that TD15/Nigel will continue tracking to the northwest for the next five days or so, then turn north by the time it reaches about 60°W.  The models also agree on it becoming a strong storm -- very likely a hurricane by Monday.

The Accumulated Cyclone Energy (ACE) is at about 148% of average for the date and is the highest value for the date since 2017, then 2008 before that.  In other words, pretty exceptional.