It’s shaping up to be an extraordinary hurricane season. Either Harvey or Irma by themselves would qualify 2017 as exceptional. Both of them together in one year gets 2017 in the conversation with the really big years like 2005. Unfortunately, it’s far from over.
I will briefly mention Hurricane Jose, but frankly he is at worst a moderate threat to the northeast states. Jose has lingered in the western Atlantic for most of a week, making a good sized cyclonic loop, and is set to turn north on his way to a close brush with the northeast US coast.
Fortunately, while Jose has regained hurricane strength, conditions are not favorable for him to retain that strength as he nears the coast. A combination of NW mid-level shear, upper-level SW shear in the region immediately to his north and colder waters as he approaches the coast will almost certainly reduce Jose to a tropical storm as he makes the turn out to sea.
I will make even briefer mention of Tropical Storm Lee in the eastern Atlantic. Lee will be a statistic, almost certainly no threat to any land.
But the system that has my stomach in knots right now is currently designated “Potential Tropical Cyclone 15.” This designation of potential tropical cyclones is a new thing for the National Hurricane Center this year. The idea is that the old system of waiting until one had a developed tropical depression (closed circulation, sustained convection) meant that there were times when the warning given to threatened locations was inadequate in the case of a system which rapidly develops close offshore. Thus this year they have decided to commence advisories on such systems if they have a high probability of developing near a populated area.
As you can see in the image at the top of this diary, PTC15 is located in the same general area where Irma and Jose rapidly developed into major hurricanes. Unfortunately, the environmental conditions which enabled those rapid developments are still present. Thus there is every reason to have high confidence that PTC15 will develop into hurricane Maria before reaching the islands. It could very well reach major hurricane status as it approaches the general area of Puerto Rico.
This system poses a significant threat to the northernmost windward islands, the leeward islands, Puerto Rico and Hispaniola over the next five to six days. Beyond that things are less certain but interests in the Turks and Caicos and the southeastern Bahamas need to pay close attention.
In the long term, this system could pose a threat to the Bahamas and some portion of the US east coast.
It goes without saying that the leeward islands in particular are in no condition to take another blow like this. Please keep these folks in your thoughts as they are in mine.
I’m planning to post additional diaries (probably one per day) as this situation develops further, with updates as appropriate.
Saturday, Sep 16, 2017 · 5:51:28 PM +00:00 · jrooth
NHC just issued an intermediate advisory upgrading PTC15 to Tropical Depression 15.
Tropical Depression Fifteen Intermediate Advisory Number 1A
NWS National Hurricane Center Miami FL AL152017
200 PM AST Sat Sep 16 2017
...DISTURBANCE BECOMES A TROPICAL DEPRESSION...
SUMMARY OF 200 PM AST...1800 UTC...INFORMATION
----------------------------------------------
LOCATION...11.9N 51.6W
ABOUT 695 MI...1120 KM ESE OF THE LESSER ANTILLES
MAXIMUM SUSTAINED WINDS...35 MPH...55 KM/H
PRESENT MOVEMENT...W OR 280 DEGREES AT 20 MPH...30 KM/H
MINIMUM CENTRAL PRESSURE...1006 MB...29.71 INCHES
WATCHES AND WARNINGS
--------------------
CHANGES WITH THIS ADVISORY:
The government of Barbados has issued a Tropical Storm Watch for
Barbados and St. Vincent and the Grenadines.
SUMMARY OF WATCHES AND WARNINGS IN EFFECT:
A Tropical Storm Watch is in effect for...
* St. Lucia
* Martinique
* Guadeloupe
* Dominica
* Barbados
* St. Vincent and the Grenadines
A Tropical Storm Watch means that tropical storm conditions are
possible within the watch area, generally within 48 hours.
Interests elsewhere in the Lesser Antilles should monitor the
progress of this system. Additional Tropical Storm or Hurricane
Watches will likely be issued later today.
For storm information specific to your area, please monitor
products issued by your national meteorological service.
DISCUSSION AND 48-HOUR OUTLOOK
------------------------------
Satellite images indicate that the disturbance has become better
organized and is now classified as a tropical depression.
At 200 PM AST (1800 UTC), the center of Tropical Depression Fifteen
was centered near latitude 11.9 North, longitude 51.6 West. The
depression is moving toward the west near 20 mph (30 km/h). A
slower west-northwest motion is expected during the next couple of
days. On the forecast track, the system is expected to approach the
Leeward Islands on Monday.
Maximum sustained winds are near 35 mph (55 km/h) with higher gusts.
Strengthening is expected during the next 48 hours. The depression
is forecast to become a tropical storm later today and could be near
hurricane strength when it approaches the Leeward Islands.
The estimated minimum central pressure is 1006 mb (29.71 inches).
HAZARDS AFFECTING LAND
----------------------
WIND: Hurricane conditions are possible within portions of Leeward
Islands by Monday night or Tuesday, with tropical storm conditions
possible on Monday.
RAINFALL: The depression is expected to produce total rain
accumulations of 5 to 10 inches with isolated maximum amounts of 15
inches across portions of the central and southern Leeward Islands
through Tuesday night. These rains could cause life-threatening
flash floods and mudslides.
SURF: Swells generated by the depression are expected to begin
affecting the Lesser Antilles by Sunday night. These swells are
likely to cause life-threatening surf and rip current conditions.
Please consult products from your local weather office.
Saturday, Sep 16, 2017 · 8:04:35 PM +00:00 · jrooth
Here are the GFS ensembles from the mid-day run:
And here are the ECMWF ensembles from the same time:
As you can see, the members of both models are pretty tightly clustered for the first five days, putting “Maria” in the vicinity of the eastern end of Hispaniola after having crossed or passed very near Puerto Rico. After that, the members of both models diverge quickly.
Saturday, Sep 16, 2017 · 8:43:33 PM +00:00 · jrooth
And it’s official: Tropical Storm Maria
Satellite images indicate that the system located several hundred miles east of the Lesser Antilles has become much better organized throughout the day. The low-level center of circulation is now well defined, and banding features have become better established in all quadrants. The initial wind speed is increased to 45 kt, in agreement with a Dvorak classification from TAFB. This makes the system a tropical storm, Maria becomes the thirteenth named storm in the Atlantic basin this season.
...
The tropical storm is located within conducive environmental conditions of low wind shear, high amounts of moisture, and over warm 29 deg C SSTs. Since these conditions are not expected to change much, steady or even rapid strengthening is likely during the next 3 to 4 days. Slight weakening is predicted by the end of the forecast period due to some land interaction and a slight increase in wind shear. The NHC intensity forecast is raised significantly from the previous one to come into better agreement with the latest guidance.