By Jeff Masters & Bob Henson , from Weather Underground

Dangerous Hurricane Joaquin has intensified to a Category 4 storm with 130 mph winds and a 936 mb pressure, making it the strongest Atlantic hurricane in five years. The last stronger storm was Hurricane Igor of 2010, which bottomed out at 924 mb on September 15, 2010. Joaquin is now the second major hurricane of 2015 in the Atlantic, joining Hurricane Danny, which peaked as a Category 3 storm with 115 mph winds on August 21. Joaquin’s motion has slowed to a 5 mph westward crawl over the Central Bahamas, which are receiving an epic pounding from the mighty hurricane. David Adams of Reuters told me this afternoon that he has been calling down to the Bahamas, and no phones are being answered on Aklins Island–but Reuters’ Nassau correspondent informed him that flooding was bad on Aklins Island and Long Island. The last hurricane hunter aircraft departed the storm after a 12:47 pm EDT eye fix, and new plane will be in the storm early this evening. The Hurricane Hunters found that Joaquin’s eye had shrunk from 41 miles in diameter early this morning to 27 miles in diameter this afternoon. Shrinkage of the eye is common in intensifying major hurricanes, and eyewall replacement cycles that lead to temporary weakening of the storms typically occur when the eye diameter gets down to about 10 miles. Wind shear continued to be in the moderate range, 10 – 20 knots, on Thursday afternoon, and visible and infrared satellite loops showed that Joaquin was a moderate-sized hurricane with impressive organization, with a solid core of intense eyewall thunderstorms surrounding a clear eye. Upper level wind analyses from the University of Wisconsin show that the hurricane has maintained an impressive upper-level outflow channel to the southeast, and it appeared a new outflow channel was ready to open up to the northwest, which would support continued intensification. Ocean temperatures in the region remain a record-warm 30°C (86°F), but may start to cool due to Joaquin’s slow motion. This cooling of the waters could well put the brakes on further intensification by Friday morning.

Figure 1. GOES-13 visible image of Hurricane Joaquin taken at 3pm EDT October 1, 2015. At the time, Joaquin was a Category 4 storm with 130 mph winds. Image credit: NOAA Visualization Lab.

Two major Atlantic hurricanes during a strong El Niño: a first
Strong El Niño conditions currently exist in the Eastern Pacific, where ocean temperatures have warmed to 2.3°C above average in the region 5°N-5°S, 120°W-180°W (called the Niño 3.4 region). Major hurricanes are uncommon when ocean temperatures get this warm in the Eastern Pacific, and 2015 is the first strong El Niño year to experience two major Atlantic hurricanes since El Niño records began in 1950. The other four years that had strong El Niño conditions during the peak August-September-October portion of hurricane season have had only one major hurricane each. Those years were 1972, 1982, 1987, and 1997. Joaquin is now the second strongest Atlantic hurricane observed during strong El Niño conditions, behind only Hurricane Debby of 1982, which had 135 mph winds.

Joaquin is an uncommon beast: a major hurricane that did not develop from an African tropical wave. These waves serve as the origin of about 85% of all Atlantic major hurricanes. According to TWC’s Stu Ostro, Joaquin’s origin can be traced, in part all the way back to a upper-level trough that came off the coast of the Carolinas on September 15 (animation here). This trough became a cold upper low northeast of the Leeward Islands, then warmed and transformed into a warm-cored tropical cyclone. These sorts of systems are usually too far north to have warm enough water temperatures to make it to major hurricane status, but with the waters of Joaquin’s birth a record warm 30°C (86°F), this was not a problem for the storm.

Impact of Joaquin on the Bahamas
Joaquin’s main threat to the Bahamas is likely to be wind damage. The 5 pm Thursday Wind Probability Forecast from NHC gave the highest chances of hurricane-force winds of 69% to San Salvador Island (population 930). Hurricane-force winds are slightly less likely on Cat Island (population 1,500), to the northwest of San Salvador Island. Heavy rains of 10 – 15 inches in the Central Bahamas may also cause considerable flooding damage, as well as the large waves of the storm riding up on top of the expected 5 – 10′ storm surge.

Figure 2. This Maximum Water Depth storm surge image for the Bahamas shows the worst-case inundation scenarios for a Category 4 hurricane with 145 mph winds, as predicted using dozens of runs of NOAA’s SLOSH model. For example, if you are inland at an elevation of ten feet above mean sea level, and the combined storm surge and tide (the “storm tide”) is fifteen feet at your location, the water depth image will show five feet of inundation. No single storm will be able to cause the level of flooding depicted in this image. The regions of the Bahamas most vulnerable to storm surge tend to lie on the southwest sides of the islands. Since Joaquin is approaching from the northeast, the storm’s peak on-shore winds will be affecting the northeast sides of the islands, where deeper offshore waters tend not to allow larger storm surges to build. NHC is forecasting peak water levels (the depth of water above the high tide mark) of 5 – 10 feet from Joaquin in the Bahamas. See wunderground’s storm surge pages for more storm surge info.

Joaquin likely to miss the U.S.

Confidence is growing that Joaquin will move out to sea this weekend, although a U.S. landfall still cannot be ruled out. Among the 12Z (8 am EDT) Thursday operational runs, three models–the Canadian GEM and the U.S. GFDL and NAVGEM models—continue to call for a landfall on the U.S. East Coast. Other models, including the four that performed the best for three-day outlooks during the 2014 Atlantic hurricane season (GFS, ECMWF, HWRF, and UKMET), now agree that Joaquin will turn sharply toward the north or northeast and begin to accelerate by late Friday or Saturday, feeling the influence of an upper-level low drifting well to the northeast. The less-likely possibility of a U.S. landfall hinges on the idea that a strong upper-level trough would produce a cut-off low in the Southeast that would hook Joaquin around it, much as happened with Hurricane/Superstorm Sandy in 2012. That cut-off low is still expected to form, but the balance between the eastern U.S. low and the Atlantic low in terms of influence on Joaquin now appears to favor the latter.

NHC nudged the forecast track for Joaquin further east in its 5:00 pm EDT outlook. In the associated discussion, they stressed: “We are becoming optimistic that the Carolinas and the mid-Atlantic states will avoid the direct effects from Joaquin. However, we cannot yet completely rule out direct impacts along on the east coast, and residents there should continue to follow the progress of Joaquin over the next couple of days.”

Figure 3. The European model ensemble run at 8 pm EDT Wednesday September 30, 2015 (00Z Thursday, October 1) had four of its 50 members (grey lines) that tracked the movement of Joaquin exceptionally well during the period 00Z – 18Z October 1. All of these four members had tracks for Joaquin that missed the U.S., with one of them hitting Canada. The operational (high-resolution) version of the European model is shown in red. Image taken from a custom software package used by TWC.

Historic rain/flood/surge still in the cards for U.S. East Coast
Joaquin’s presence, even at sea, is one of many factors now lining up to help produce a weather event that may rival many U.S. Category 1 hurricane landfalls in terms of impact. Along the immediate coast from New Jersey to the Carolinas, several days of high seas, beach erosion, and coastal flooding can be expected from a long fetch of easterly wind setting up between Joaquin and a strong, sprawling ridge of high pressure far to its north. The concern is not so much the intensity of the onshore flow (assuming Joaquin does not approach) but its sheer persistence. “The duration of this wind event is absolutely mind-boggling,” said storm-surge expert Hal Needham in a blog post on Thursday morning. Onshore winds of more than 20 mph could be affecting the mid-Atlantic coast for more than 96 solid hours, regardless of Joaquin’s track. High water will be present for as many as 10 high tides over several days, increasing the risk of erosion and flooding along the coast as well as up to a few miles inland. “This developing situation is truly historic and has not been observed in the modern history of the Mid-Atlantic Coast,” says Needham.

Figure 4. The combination of Hurricane Joaquin and high pressure well to its north will put a squeeze play on the U.S. East Coast, with several days of onshore flow expected to produce high surf and coastal flooding. This map shows surface winds in knots (multiply by 1.15 for mph) predicted by the 12Z Thursday run of the GFS model for 06Z (2:00 am EDT) Sunday, October 4. Image generated by wundermap using “model data” option.

Another exceptional part of the story from this weekend into Monday will be the mammoth rainfall amounts expected to fall over parts of the Southeast and mid-Atlantic, particularly South Carolina. These areas are not unfamiliar with torrential rain, especially from landfalling tropical cyclones, but the amounts this weekend will be amazing even by southeastern standards—and this is assuming Joaquin does not make landfall. Strong and extremely moist flow pushing from the Atlantic against the Appalachians may boost rainfall totals to record or near-record totals from the mountains and foothills of northeast Georgia to western Virginia. The Greenville/Spartanburg NWS office is already warning that “all the ingredients are in place for a rainfall event of historic proportions across the area.”

At the same time, a frontal boundary may develop between relatively cool air associated with the Southeast upper low and warmer, more humid air being funneled across the Carolinas from Joaquin’s circulation. This boundary, which should extend east or southeast from the Appalachians to the Atlantic, will help the extremely heavy rain to extend into the coastal plains of the Carolinas. According to Peter Neilley (WSI), the boundary may end up bearing some resemblance to the “norlun trough” feature that can boost snow amounts in nor’easter winter storms.

With onshore flow pushing a storm surge of 1 – 2′ along much of the East Coast over the next five days, the rain water will not be able to drain effectively into the ocean, causing rivers to back up and flood more severely than they otherwise would. A mandatory evacuation is now in effect for Ocrakoke Island in North Carolina’s Outer Banks, out of concern about rising water levels in the Pamlico Sound that separates the Outer Banks from the NC mainland. Transportation could be hindered for days in coastal regions by this complex setup, and there is the potential for major flash flooding and river flooding throughout much of the Carolinas depending on the timing and placement of the heaviest rains.

Figure 5. 5-day rainfall amounts for the period from 8:00 pm EDT Thursday, October 1, to Tuesday, October 6. Almost all of South Carolina is projected to get 10” – 20” of rain. This map assumes that Hurricane Joaquin will remain offshore from the U.S. East Coast. Image credit: NWS Weather Prediction Center.

Value of the Hurricane Hunters
The Hurricane Hunters have been in Joaquin almost continuously the past two days, sending back invaluable information on the storm’s position and intensity. Without their data, we might well have classified Joaquin as a much weaker storm, since satellite estimates of Joaquin’s strength were too low during a large portion of the hurricane’s intensification period. Satellite estimates of Joaquin’s strength using the Dvorak technique from NOAA/NESDIS gave the hurricane a constant rating of T5.0 between Wednesday evening at 7:45 pm EDT and Thursday morning at 7:45 am EDT. The Tropical Storm Current Intensity Chart for this corresponds to a Category 2 hurricane with 105 mph winds and a 970 mb pressure. During this period, the Hurricane Hunters documented that Joaquin actually underwent a period of rapid intensification, with a pressure drop from 954 mb to 942 mb, and winds increasing from 105 mph to 125 mph. However, by 2 pm EDT Thursday, the satellite estimates of Joaquin’s strength finally matched what the Hurricane Hunters were seeing, with both saying top winds of 130 mph.

Bob Henson was on WUTV on the Weather Channel at 6:20 pm EDT Wednesday, and Jeff Masters at 8:20 pm.

For more on this story go to: http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3136