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Joaquin Approaching Bahamas; Possible U.S. Landfall This Weekend

By: Jeff Masters and Bob Henson 9:14 PM GMT on September 30, 2015

Hurricane Joaquin is developing into a formidable storm as it churns over record-warm waters east of the Bahamas. Top sustained winds in Joaquin were 85 mph as of the 5:00 pm EDT advisory from the National Hurricane Center (NHC), and the central pressure had dropped to 967 millibars. Joaquin's center was located roughly 175 miles east-northeast of the central Bahamas. A large, distinct eyewall was evident on Wednesday morning imagery from microwave imagery and from visual observations from an Air Force hurricane-hunter aircraft. An eye was intermittently apparent on visible satellite imagery by early Wednesday afternoon, although the eye remained largely obscured by clouds. Infrared imagery showed an enlarging shield of showers and thunderstorms (convection) around Joaquin's core. Northwesterly wind shear was still limiting the amount of outflow on Joaquin's north side, but the shear appears to be slowly relaxing.

Figure 1. Enhanced infrared image of Hurricane Joaquin, collected at 1915Z (3:15 pm EDT) Wednesday, September 30, 2015. Image credit: NOAA/NESDIS.

With its continued fairly slow motion (only 8 mph) toward the southwest, Joaquin remains on a track somewhat further south than earlier expected. Hurricane warnings are now up for the central and northwestern Bahamas, with a tropical storm warning for the southeastern Bahamas. Joaquin is expected to move very slowly over the central Bahamas over the next two days, before embarking on a much more rapid northward track on Friday that will take it safely away from the Bahamas.

Figure 2. GOES 13 image of Hurricane Joaquin approaching the Bahamas as seen on Wednesday, September 30, 2015, at 9:15 am EDT. At the time, Joaquin had top winds of 75 mph. Image credit: NOAA Visualization Lab.

Impact of Joaquin on the Bahamas
Winds were rising across the Central Bahamas on Wednesday afternoon, and were a brisk 31 mph, gusting to 37 mph, at  2:34 pm EDT at a personal weather station on Exuma Island. Shemp's Webcam from Exuma Island on Wednesday afternoon showed a steady increase in clouds, and in whitecaps on the waters, as Joaquin approached.

Joaquin's main threat to the Bahamas is likely to be wind damage. The 5 pm Wednesday Wind Probability Forecast from NHC gave the highest chances of hurricane-force winds of 62% 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 - 20 inches in the Central Bahamas may also cause damage, as well as the large waves of the storm riding up on top of the expected 2 - 4' storm surge. The latest 8 am EDT (12Z) Wednesday run of the GFS model portrays the center of Joaquin passing over San Salvador Island around 2 pm EDT Thursday. Tropical storm-force winds of 39+ mph have likely already begun on the island, so they are in for a long battering. More concerning for the Central Bahamas is the latest 12Z Wednesday forecast from the European model, which has Joaquin penetrating about 70 miles farther to the southwest, stalling out over Exuma Island on Thursday night.

Figure 3. This Maximum Water Depth storm surge image for the Bahamas shows the worst-case inundation scenarios for a Category 2 hurricane with 105 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 2 - 4 feet from Joaquin in the Bahamas. See wunderground's storm surge pages for more storm surge info.

Hurricane history for the Bahamas
The last hurricane to affect the Bahamas was Hurricane Sandy of 2012, which passed through the Central Bahamas as a Category 2 storm with winds of up to 105 mph. Sandy caused two deaths and damage estimated at $703 million, equivalent to 9% of the nation's GDP. The most severe damage was on Cat Island and Exuma, due to wind and storm surge. According to EM-DAT, the most expensive hurricane in Bahamian history was Category 4 Hurricane Frances of 2004, with damages estimated at $1 billion.

Where will Joaquin go after the Bahamas?
Joaquin is trapped to the south of a high pressure system whose clockwise flow will push the cyclone very slowly to the southwest or west-southwest at about 3 - 6 mph. As the storm progresses to the southwest, the strong upper-level winds out of the north currently bringing high wind shear of 20 knots will gradually decrease, allowing Joaquin to strengthen. The 2 pm EDT Wednesday run of the SHIPS model predicted that wind shear over Joaquin would fall to the light to moderate range, 5 - 20 knots, on Thursday and Friday. These conditions should allow Joaquin to intensify to a Category 2 hurricane, and possibly a major Category 3 hurricane, by Friday. As Joaquin progresses to the west, the storm will also increasingly "feel" the steering influence of a strong upper-level trough of low pressure situated over the Eastern United States.

Figure 4. The operational GFS and ECWFM model runs from 12Z Wednesday, September 30, offer wildly differing projections for Hurricane Joaquin for 12Z (8:00 am EDT) Sunday, October 4. Image credit:Levi Cowan, www.tropicaltidbits.com, via Phil Klotzbach.

The big trend from the 00Z Wednesday suite of computer model guidance was a marked convergence among models toward a landfall in the vicinity of North Carolina or Virginia. Among the major dynamical models, only the ECMWF remained adamant that Joaquin would head to sea well before reaching the southeast U.S. Little had changed in the 12Z Wednesday round, as the non-ECMWF models (GFS, HWRF, GFDL, UKMET) were heavily clustered around a potential landfall in or near North Carolina on Saturday or Sunday while the ECMWF kept Joaquin well out to sea at the same point.

Figure 5. The operational ECMWF (top) and GFS (bottom) model runs from 0Z Wednesday, September 30, showed similar depictions of the eastern U.S. trough steering Joaquin, but much different depictions of an intensifying upper-level trough east of Newfoundland. This may help explain why the ECMWF operational model is taking Joaquin out to sea. Image credit: Ben Papandrea, WSI.

Why the discrepancy between the ECMWF and other models? The leftward hook prominently featured in the other models is being driven by the increasingly negative tilt (NW-to-SE) to the upper trough deepening over the eastern U.S. late this week. The models are projecting that this trough would pull in Joaquin on its northeast side, in much the same way that a strong upper-level low did with Hurricane/Superstorm Sandy in 2012, although in this case the process would unfold a couple of hundred miles to the south. The ECMWF run shows a very similar upper-level pattern to the other models across the eastern U.S., but Joaquin's interaction with the trough occurs later in the ECMWF, giving the hurricane a chance to escape some of the trough's influence. Another feature of interest is an upper-level low east of Newfoundland, which is depicted as being much stronger in the GFS than in the ECMWF (see Figure 5 at right). This low is associated with increased ridging in the Northwest Pacific that would help shunt Joaquin into the U.S. East Coast, in tandem with the eastern U.S. upper trough.

Back in 2012, the ECMWF model caught on to the leftward hook of Sandy's track several days before other models. The ECMWF's high overall skill means we cannot entirely discount its out-to-sea forecast for Joaquin just yet. At the same time, the strong consistency among other leading models in projecting a landfall in or near North Carolina cannot be ignored. We can gain more perspective on the mid-Atlantic scenario by looking at the ECMWF and GFS ensemble output from 00Z Wednesday. In each ensemble, the model is run a number of times for the same situation, but with the starting conditions varied slightly to represent the uncertainty in our starting-point observations of the atmosphere. The ECMWF and GFS ensembles from 00Z Wednesday are much more similar in flavor than you might expect from looking at their single operational runs. Both models have a majority of ensemble members heading for the U.S. East Coast, with a few outliers heading to sea.

What kind of U.S. impacts could Joaquin bring?
A hurricane watch could be required for portions of the U.S. East Coast as early as Thursday night, and NHC is now citing the possibility of a major hurricane landfall in the Carolinas. If Joaquin moves along the track projected by most models, hurricane-force winds could arrive somewhere on the NC/Mid-Atlantic coast over the weekend. Very high seas and a significant storm surge could also be expected. It is relatively rare for a hurricane to make a Sandy-like left hook into the U.S. East Coast. Such a track was unprecedented for New Jersey in hurricane annals, and even in the NC/VA area, it is uncommon enough that the likely effects would be both unusual and high-impact. The closest analogue from recent years is 2003's Hurricane Isabel. Isabel brought a major storm surge into the Chesapeake Bay and nearby waterways, plus widespread impacts from high wind and heavy rain. Such an outcome would depend heavily on the exact track of Joaquin. Storm surge expert Dr. Hal Needham of LSU has a detailed look at the potential for storm surge from Joaquin along the U.S. East Coast in his Wednesday morning blog post, Widespread Storm Surge Event to Impact U.S. Atlantic Coast.

Regardless of Joaquin's precise motion, any approach toward the U.S. East Coast would exacerbate what is becoming a serious flooding threat over a large area, due to a preexisting front being overtopped by near-record amounts of water vapor streaming over the region ahead of the trough that will help steer Joaquin (see Figure 7).

Figure 6. Rainfall observed over the 24-hour period ending at 1200 GMT (8:00 am EDT) Wednesday, September 30, 2015. Image credit: NOAA Advanced Hydrologic Prediction Service.

Figure 7. Moisture from several parts of the tropics was converging on the Northeast United States between 700 and 500 mb (roughly 2 to 4 miles high) at 1530Z (11:30 am EDT) Tuesday, September 29, 2015. The depiction of moisture channels on this image was created by Sheldon Kusselson atop a map of precipitable water (the amount of water vapor above particular points) produced using data from multiple sensors by the Cooperative Institute for Research in the Atmosphere. The water-vapor analysis system was recently highlighted in a paper in the Journal of Operational Meteorology. Image credit: Sheldon Kusselson and CIRA/CSU.

Flash flooding has popped up in several spots from Virginia to Maine over the last day (see the embedded tweet below from Belfast, ME). Portland, ME, received 5.61" of rain between midnight and 3:00 pm EDT Wednesday, overwhelming drainage systems and causing widespread street flooding. This morning's 7-day outlook from the NOAA Weather Prediction Center showed a vast area of 5-10" rainfall amounts from North Carolina to southern New England, and flood watches are posted for most of the U.S. East Coast. Model output suggests that localized 7-day totals of 10-20" or more are not out of the question, depending on Joaquin's exact track. The potential for major, widespread flooding-related impacts from Joaquin, in combination with the already-unfolding heavy rain event, should be taken very seriously.

We'll have our next update on Joaquin by Thursday morning at the latest.

Jeff Masters and Bob Henson

Bob Henson will be on WUTV on the Weather Channel at 6:20 pm EDT Wednesday, and Jeff Masters will be on at 7:20 pm.


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.