04 August 2020

12-day tour ends for Isaías as it heads into Canada

As forecast, Isaías did briefly intensify to a Category 1 hurricane before making landfall near Myrtle Beach SC on Monday night. During the day on Tuesday, it accelerated northward through NC, VA, MD, PA, NY and will cross into Canada tonight as it transitions to an extratropical cyclone. It left the African coast on July 23rd -- twelve days ago -- but has spent only half that time as "Isaías".


Isaías is now the fifth named storm to make landfall on the continental US this season already... there have only been nine named storms in total, and it's only August 4th!  This radar loop from Wilmington NC shows the evolution of the storm as it made landfall... this and many other radar loops of the storm are available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/ -- including a 3-day loop than spans its trek from the Bahamas to Virginia.


As usual, rainfall is the greatest hazard posed by this landfalling tropical cyclone.  Over the past three days, I have measured nearly 5 inches of rain at my house near Miami, but higher totals have been measured in the mid-Atlantic and northeast US states... preliminary estimates are in the 8-10 inch range in parts of MD and PA. Additionally, between Monday and Tuesday, there have been 27 tornado reports from North Carolina up into New Jersey.


Wilmington NC, which was hit by the eastern eyewall of the hurricane as it made landfall, experienced its largest storm surge in recorded history, beating the previous record set just two years ago by Hurricane Florence.


As of Tuesday afternoon (5pm local), Isaias is still a tropical storm with 65 mph peak sustained winds; it's centered near Albany NY and is racing toward the north-northeast at 40 mph. The list of tropical storms that have passed within 150 miles of where Isaias is on Tuesday afternoon is slim, but certainly includes ones you've heard of. The six most recent are Irene (2011), Gloria (1985), Frederic (1979), David (1979), Belle (1976), and Agnes (1972).

Invest 94L, which I've been mentioning for several days now, is located just south of Bermuda but is looking less and less likely to develop. Elsewhere, the Atlantic basin is quiet for the near future.


03 August 2020

Isaías taking aim at mid-Atlantic and northeast US states


As of Monday morning, Isaías has not regained hurricane intensity, but is really close.  The very warm Gulf Stream water is giving it lots of fuel, but strong wind shear is constantly trying to tear it apart -- a battle of conflicting environmental influences. Hurricane warnings are in effect for northern South Carolina and southern North Carolina... tropical storm warnings extend up into Massachusetts, and storm surge watches/warnings span the coastlines of SC and NC.  As I mentioned in Saturday's post, this storm will be impactful along the entire US east coast.


Even if Isaias does briefly strengthen to a hurricane again before landfall, the greatest hazard by far will be rain, followed by storm surge, followed by wind. The following map shows the flash flood risk associated with Isaias' trek along the coast over the next couple of days.


And the map of peak storm surge values again confirms how widespread the effects of Isaias will be.


Landfall is forecast to be near Myrtle Beach SC on Monday evening, and Isaias could be a minimal hurricane by then. There are long radar loops available at http://bmcnoldy.rsmas.miami.edu/tropics/radar/

Elsewhere, there is still that tropical wave north of the Lesser Antilles (Invest 94L) with a decent chance of development according to NHC.  The support for its development is dwindling in the models though. Should it form and get named, the next name is Josephine. It will remain out over the open ocean, perhaps bring some weather to Bermuda, but even that is a pretty low chance.

02 August 2020

Isaías weakens along Florida coast, hazards continue up US east coast


During the day on Saturday, Isaías got completely overwhelmed by a combination of dry air and wind shear... suffocation and decapitation. While that doesn't immediately "kill" a tropical cyclone, it weakens it in a hurry. It was downgraded to a tropical storm on Saturday evening, and remains a tropical storm with 65 mph peak sustained winds on Sunday morning, though those winds are well offshore. It is not expected to regain hurricane intensity (but briefly could because it's so close to the 74 mph cutoff between a tropical storm and a hurricane).


Since passing over the warm Gulf Stream ocean current, vigorous thunderstorms have re-erupted and persisted over the center, breathing new life into the struggling storm. In the enhanced satellite image at the beginning of the post (from Sunday morning), the mid-level dry air is shown by the oranges to the west of the storm, and the cold cloud tops associated with the strong thunderstorms near the center are the bright blues/greens/reds.

There have been minimal impacts on the east coast of Florida because the thunderstorm activity and stronger winds have been confined to very close to the center or east of the center.  However, the northern Bahamas have received very heavy rain and fairly significant storm surge. A 1-day radar loop from Miami shows the (lack of) structure as it made its closest approach before heading north.  This and other long radar loops can be found at http://bmcnoldy.rsmas.miami.edu/tropics/radar/


Tropical Storm Isaías will continue to bring periods of rain and minor coastal flooding to the eastern Florida coast from West Palm Beach northward.  A 2-4-foot storm surge is possible up to Cape Fear, NC as it tracks along the coast over the next couple days. The greatest hazard will be heavy rain and flash flooding in the Carolinas, the mid-Atlantic states, and then the northeast U.S. through Wednesday.


The map below shows the full path that Isaías took, beginning when it left the African coast back on July 23rd. It battled dry air the entire time, but was able to briefly reach Category 1 hurricane status immediately after leaving Hispaniola.


Tropical Depression 10, which was out near Cabo Verde, has already dissipated and never became a tropical storm. The tropical wave near the northern Lesser Antilles, identified as Invest 94L, still has a good chance of forming this week, but model guidance is in agreement on it turning north into the open Atlantic.  The next name on this list is Josephine, and the record earliest formation of the "J" storm is August 22nd.