Joaquin Close to Category 5 Strength; Rains Inundate Carolinas

There is plenty of life left in Hurricane Joaquin as it moves away from the Bahamas. An Air Force Hurricane Hunter aircraft detected winds around noon EDT Saturday of 144 knots at the 700-millibar level, with stepped-frequency microwave radiometer (SFMR) data showing estimated surface winds of 138 knots (159 mph). The National Hurricane Center upgraded Joaquin’s strength to top sustained winds of 155 mph in a special advisory at noon EDT Saturday, up from 130 mph in the advisory issued just an hour earlier. This immediately pushed Joaquin from the bottom to the top end of the Category 4 scale. A central pressure of 933 millibars was reported, although a radiosonde deployed in the eye of Joaquin failed, so there is some uncertainty around this estimate. Another Hurricane Hunter aircraft was en route to Joaquin as of early Saturday afternoon. Joaquin’s eye has warmed and cleared over the last few hours, reflecting the rapid restrengthening, although infrared satellite imagery shows that its core of strongest thunderstorms has become smaller and less intense.


Figure 1. MODIS image of Hurricane Joaquin heading away from the Bahamas as seen from NASA's Terra satellite on Saturday, October 3, 2015, at approximately 1:30 pm EDT. At the time, Joaquin had top winds of 155 mph. Image credit: NASA.

Joaquin’s burst of strength is especially remarkable given that a strong El Niño is under way (El Niño tends to suppress Atlantic hurricane activity by enhancing wind shear). The last Atlantic storm with sustained winds this strong was Hurricane Igor, in 2010, which peaked at 155 mph. The Atlantic’s last Category 5 was Hurricane Felix, in 2007, with winds topping out at 160 mph. The last El Niño season that managed to produce a Category 5 was 2004, when Ivan formed. However, the El Niño event of 2004-05 was relatively weak, with autumn Niño3.4 anomalies of only around +0.7°C compared to the current value of more than +2.0°C.

Joaquin is also in an area where very few Category 5 track segments have been reported since reliable records began in 1950 (see Figure 2). Record-warm waters in this part of the Northwest Atlantic are likely playing a major role in Joaquin’s unusual strength. Joaquin was designated as a tropical depression on Sunday night, September 27, at latitude 27.5°N. This makes Joaquin one of the strongest Atlantic hurricanes on record to have begun its life as a tropical cyclone at such a high latitude. In fact, Joaquin’s latest location (26.4°N. 70.9°W) is still south of its origin point.

Fortunately for the United States, Joaquin is hustling into the open Atlantic, now moving northeast at 16 mph. Track models are fairly consistent in keeping Joaquin west of Bermuda, but with only a small margin for error. Bermuda is now under a hurricane watch and tropical storm warning; at a minimum, the island can expect high surf, strong winds, and a few squalls from outer-edge rainbands, especially as Joaquin makes its closest approach on Monday.


Figure 2. In this map of all Category 5 hurricanes reported in the Atlantic since 1950, bright purple indicates the segments where Category 5 strength was analyzed. Image credit: The Weather Channel, courtesy Jon Erdman.


Figure 3. Satellite image Hurricane Joaquin taken at noon EDT October 3, 2015. At the time, the hurricane was just below Category 5 strength with top winds of 155 mph. A band of very heavy rain can also been seen feeding into South Carolina, to the northwest of the hurricane. Image credit: NASA/GSFC.


Figure 4. Flooding from heavy rain swamps the intersection of Huger Street and King Street in Charleston, S.C. on Saturday, October 3, 2015. Image credit: Matthew Fortner/The Post And Courier, via AP.

Severe flooding likely in South Carolina Saturday and Sunday
As expected, a band of torrential rain has materialized over South Carolina, paving the way for an especially dangerous situation from Saturday afternoon into Sunday. As of midday Saturday, the heaviest rain extended from the south half of the South Carolina coastline northwest across the state to the hilly Uplands region. The swath of intense rain will pivot very slowly in a counterclockwise direction, gradually translating southward over the higher terrain but moving very little near the coastline. This will put the area around Charleston at particular risk of severe flash flooding from Saturday afternoon into Sunday. CoCoRaHS maps show widespread rain totals of 4” - 8” in the Charleston area from 7:00 am EDT Friday to 7:00 am Saturday.


Figure 5. Predicted 15-hour rainfall totals from the HRRR model for the period from 10:00 am Saturday, October 3, to 1:00 am Sunday, October 4. Image credit: NWS/NCEP.


The Charleston area has a reasonable chance of beating the all-time three-day rainfall records below, possibly in just a 24-hour period!

North Charleston, SC (CHS)
11.95”, 6/9/1973-6/11/1973
11.62”, 6/10/1973-6/12/1973
11.40”, 9/19/1998-9/21/1998
10.64”, 9/4/1987-9/6/1987
10.52”, 9/21/1998-9/23/1998
Records begin in 1938

Downtown Charleston, SC (CXM)
12.39”, 6/9/1973-6/11/1973
11.92”, 6/10/1973-6/12/1973
11.73”, 9/5/1933-9/7/1933
11.72”, 9/4/1933-9/6/1933
11.31”, 9/4/1987-9/6/1987
Records begin in 1870

According to the Charleston NWS office, the record 24-hour rainfall for the state of South Carolina is 14.80", observed at Myrtle Beach during Hurricane Floyd on September 16, 1999.

Forecasters are particularly concerned that high-tide cycles in Charleston may coincide with periods of torrential rain, which could produce extreme flash flooding in the city in short order. The Saturday afternoon high tide of 8.2 feet was the highest to occur since Hurricane Hugo in 1989. The next tides will occur in Charleston at 1:34 am and 2:03 pm on Sunday.

Surrounding states are also experiencing heavy rain and flood threats. Mudslides and landslides are possible in the higher terrain of Georgia, South Carolina, and North Carolina. A strengthening of the onshore flow that has persisted for several days over the mid-Atlantic will again raise the risk of significant tidal flooding from Virginia to New Jersey, especially in the Hampton Roads area of Virginia.


Figure 6. GOES-West infrared satellite image covering the Northeast and Central Pacific, taken at 1545Z (11:45 am EDT) Saturday, October 3, 2015. Image credit: CIMMS/SSEC/University of Wisconsin-Madison.

Tropical Storm Oho may threaten Hawaii
The hyperactive Central Pacific broke its record--again--for the most number of named storms in a single season with the christening of Tropical Storm Oho on Saturday. According to NHC’s Eric Blake, Oho is the eighth tropical storm to form in the Central Pacific this year, doubling the previous record of just four. Oho is now located roughly 500 miles south-southeast of Honolulu. The steering patterns that will drive Oho are ill-defined and still evolving, which complicates the track forecast. The Central Pacific Hurricane Center currently projects Oho to arc northwest over the next couple of days, then move more briskly toward the east and northeast on a path that would keep it a couple hundred miles south of Hawaii’s Big Island early next week. There is plenty of room for this forecast to evolve, though. Oho has the chance to become a powerful hurricane, thanks to the weak upper-level flow as well as record-warm waters that have fueled so many other tropical cyclones in the Central Pacific this year. The SHIPS rapid intensification index gives Oho a good chance of rapidly strengthening from Saturday into Sunday. Oho now has top sustained winds of just 40 mph, but most dynamical and statistical models are making Oho a hurricane by Monday, and several bring it to Category 2 status by Thursday.

Elsewhere in the tropics
An array of other systems peppered the Northern Hemisphere tropics on Saturday. In the Central Atlantic, Invest 90L is looking less robust, with NHC now giving it only a 40% chance of development in the next 2 to 5 days. A late-blooming Cape Verde wave between 30°W and 35°W poses little threat over at least the next several days, and strong wind shear at low latitudes will probably cap any later development.


Figure 7. WU’s latest tracking map for tropical cyclones around the globe.

In the Northeast Pacific, Invest 94E is slowly organizing more than 1000 miles southwest of Baja California. NHC gives 94E a 30% chance of developing into a tropical cyclone by Monday and a 50% chance by Thursday. Closer to Mexico, the remants of Tropical Storm Marty could produce heavy rainfall as they move inland on Sunday into Monday. Some moisture from ex-Marty may get entrained into an upper-level storm taking shape early next week in the Southwest U.S., possibly delivering strong thunderstorms to the Arizona deserts on Monday.

In the Central Pacific, still another system--Tropical Depression 8C, the 13th tropical cyclone to develop in or pass through the Central Pacific this year--formed on Saturday morning about 1100 miles southwest of Honolulu. Moderate southerly shear should keep 8C from developing beyond minimal tropical-storm strength for at least the next couple of days as it pushes westward.

In the Northwest Pacific, Typhoon Mujigae may strengthen slightly over the next 24 hours before it moves into the coast of extreme southern China, southwest of Hong Kong. To the east, Tropical Storm Choi-Wan will slowly gather steam and may become a minimal typhoon early next week before an expected recurvature just east of Japan by midweek.

We’ll have our next update on Sunday afternoon.

Bob Henson