Thursday, October 1, 2015

Wind, Rain and Surge Will Not Miss the East Coast, Regardless of Joaquin's Track

The National Hurricane Center's 5PM EDT update forecasts Hurricane Joaquin to track farther east than previous forecasts. These changes reflect eastward shifts in various models, such as HWRF, GFS and UKMET, to reflect solutions closer to the ECMWF (European) model, which all along has been predicting Joaquin to take an eastward turn.

The 5PM EDT update from the NHC shifted Joaquin's track to the east. Source:
While this updated forecast provides a collective relief for many along the Eastern Seaboard, it's not time to celebrate yet. Hurricane Joaquin is just one player in a complex drama that is taking place along the U.S. East Coast, and, in a sense, other characters in this play will not allow Joaquin to "miss" the East Coast.

A large, strong area of high pressure is building over Eastern Canada, and the gradient between Joaquin and this dome of high pressure will generate strong onshore winds for more than four consecutive days from North Carolina through New England. This situation will play out regardless of Joaquin's exact track. Persistent northeast winds will cause prolonged storm surge flooding and coastal erosion, particularly from Virginia through the New York City area.

The maps below show GFS model output. These maps were initialized (run) at 1200UTC (8AM EDT) on Thu Oct 1, and predict conditions for 1200UTC (8AM EDT) on Sat Oct 3 and Sun Oct 4. Note the tightly packed isobars in boxes 1 and 2, which are predicted to occur 24 hours apart.

 The GFS Mean Sea Level Pressure forecast for 1200 UTC (8AM EDT) on Sat Oct 3 depicts Joaquin near the Northern Bahamas and a very tight pressure gradient from North Carolina to New England. Source:, edited by Hal Needham.
  The GFS Mean Sea Level Pressure forecast for 1200 UTC (8AM EDT) on Sun Oct 4 moves Joaquin to the NNE, but keeps the tight pressure gradient along the U.S. East Coast. Source:, edited by Hal Needham.

The two maps below depict ECMWF model forecasts for the same times (1200 UTC/ 0800 EDT on Sat Oct 3 and Sun Oct 4). This model also predicts Joaquin will be centered near the Bahamas on Saturday, but predicts the storm to track NE, away from the U.S. mainland on Sunday.

However, notice how the isobars, or lines of equal pressure, stay packed in the boxes on both of these maps as well. When we step back and look at the scale of this widespread wind event, we begin to understand why Joaquin won't fully "miss" the U.S. East Coast, even if it tracks out to sea.

The ECMWF forecast for 1200UTC (8AM EDT) Oct 3 forecasts Joaquin near the Bahamas and a tight pressure gradient along the U.S. East Coast, from North Carolina to New England. Source:, edited by Hal Needham.

Although the ECMWF model forecast for 1200UTC (8AM EDT) Sun Oct 4 predicts Joaquin to track east of the Bahamas, and away from the U.S. mainland, note the strong pressure gradient that remains in the box. Source:, edited by Hal Needham.

Unfortunately, these strong winds will persist four or five days, which spells trouble because the East Coast will need to endure at least 10 high tide cycles during this surge event. Although surge levels may not reach as high as Hurricane Isabel (2003) or Sandy (2012) in this region, this prolonged surge will generate substantial flooding and excessive coastal erosion.

In addition to prolonged coastal winds and storm surge, a stalled out cold front will produce torrential rains in the Carolinas and Virginia, with portions of South Carolina forecast to receive > 15 inches of rain through late Sunday.

Low pressure tracking along a stationary front off the Southeast Coast will generate torrential rains that may exceed 15 inches in South Carolina through Sunday. Image:

The combination of prolonged onshore winds, storm surge, beach erosion and torrential rains will produce substantial weather impacts along much of the Eastern Seaboard, regardless of Joaquin's exact track.

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