“Hurricane” Hal Needham is a storm surge scientist who specializes in data-driven storm surge analysis. He is the founder and president of Marine Weather and Climate.
An extraordinary onshore wind event will develop along the Eastern Seaboard over the next several days. Although Hurricane Joaquin is grabbing the headlines, and may directly produce the greatest wind, rain and surge impacts, it is just one of three players taking part in a complex drama that will unfold over the next several days.
The first player is a cold front and associated low-pressure system that dumped 2-4 inches of rain across a wide area from Virginia through Maine from Tuesday through Wednesday. This heavy rainfall saturated soils and elevated rivers, which will compound impacts from approaching Joaquin. Saturated soils will cause more of Joaquin's rainfall to runoff and cause flooding, while increasing the risk of downed trees, as wet soil provides less support than dry soil.
Player No. 2 is a large area of high pressure building over eastern Canada behind this cold front. The strong pressure gradient between this massive area of high pressure and Hurricane Joaquin, Player No. 3, will generate prolonged onshore winds from North Carolina through New York. These winds will begin to blow hard long before Joaquin's arrival.
The duration of this wind event is absolutely mind-boggling. Data from the National Weather Service point forecast for Atlantic City International Airport (ACY), New Jersey, suggest a strong onshore wind event will occur over coastal New Jersey for more than 100 consecutive hours (Yes, four days and then some)! Let me explain in more detail.
I've defined an "prolonged onshore coastal wind event" as a scenario in which winds along the Mid-Atlantic (New Jersey in this case) coast blow from between the N and E (compass bearings 0 to 90 degrees), with maximum sustained winds ≥ 20 mph for at least 75% of the observed hours. I chose these compass directions because Ekman Transport tends to deflect water to the right of the wind direction in the northern hemisphere, making a NE wind very efficient at piling up water along the East Coast.
Using this metric, coastal New Jersey will observe a strong onshore wind event lasting 112 hours, based on point forecast data for ACY. This forecast predicts that winds will exceed 20 mph by 12 p.m. Eastern time on Thursday and remain above this threshold for 88 out of 90 hours, until 5 a.m. Eastern time on Monday. Mind you that this extraordinary wind event occurs before Joaquin even makes its closest approach.
Winds are then forecast to drop slightly below the 20 mph threshold, before quickly increasing with the closest approach of Joaquin. Adding the winds from Joaquin's closest approach increases our coastal wind event to 112 hours, with onshore winds ≥ 20 mph forecast at 102 of those 112 hours (91.1% of observations).
This developing situation is truly historic and has not been observed in the modern history of the Mid-Atlantic Coast. I worked with Josh Gilliland, a PhD student at Louisiana State University, and the only Cleveland Brown's fan I know, to analyze the wind history for ACY. We were interested to find previous events that may have resembled the current forecast.
Hourly wind data were from 1950-2015 were provided by the National Climate Data Center's Integrated Surface Database (NCDC-ISD). Gilliland adjusted these observations to 10m height, in accordance with wind data standards.
We found only six previous "prolonged wind events" that lasted ≥ 48 hours at ACY. None of these events were produced from onshore wind events, as they all were related to frontal passages, with strong winds blowing from the NW. All of these events occurred during winter or spring:
The longest duration onshore wind event we could find since 1950 was related with a winter storm, from January 9-11, 1956. Within a 42-hour period, onshore winds > 20 mph were observed on 39 hourly observations (92.9%). During a 6-hour period, sustained winds ranged from 28-29.9 mph, but never topped 30 mph.
The tide gauge record along the Mid-Atlantic Coast depicts a prolonged surge from this January 1956, onshore wind event. Unfortunately, this event fell within an 8-year period with no NOS tide gauge records for Atlantic City. However, tide gauges at Sandy Hook, N.J., and The Battery, N.Y. (south end of Manhattan), depict this surge event.
Storm surge levels at The Battery reached approximately 2 ft, with a storm tide above MLLW peaking at 6.71 ft. See the graph below.
The January 1956, onshore wind event produced even higher storm surges/storm tides along the New Jersey Coast. The graph below depicts a storm surge of approximately 2.5 ft and a storm tide of 7.09 ft above MLLW at Sandy Hook, New Jersey.
Our current forecast predicts onshore winds for a longer time period (nearly 3x as long as 1956), with generally stronger wind speeds and potentially the approach of a hurricane or strong tropical storm. All of these factors would lead us to believe that the upcoming surge will be higher and last longer than the event in 1956.
So what does all this mean?
First of all, we are moving into new territory. It is looking more and more likely that the Eastern Seaboard, from the Carolinas through the New York City area will experience prolonged coastal winds. Long-duration winds may also set up across New England, but in many locations they may blow offshore for long periods. Nonetheless, coastal flooding will be likely as far north as New England early next week.
For now, one of the take home points is that this widespread wind event will create a long-duration storm surge even if Hurricane Joaquin does not make a direct landfall on the U.S. Atlantic Coast.
Also, even if Joaquin's storm surge levels do not come close to those produced by Hurricane Sandy (2012) along the Mid-Atlantic Coast, or Isabel (2003) along the NC/VA coast and Chesapeake Bay, the combination of long-duration surge and heavy rainfall on previously saturated soils may flood some areas that were not inundated from Isabel or Sandy.
We should also consider these factors:
1. This surge event will last for many high tide cycles. The impacted region observes two high tides per day, so a 5-day surge event may stick around for 10 high tides. This will increase the risk of flooding in many locations.
2. Joaquin may flood areas not flooded by Sandy or Isabel, even if those storms produced higher surges at the coast. I am particularly concerned about areas "slightly" inland, such as 5-15 miles from the coast. How can rainfall drain into bays and rivers when they are elevated for multiple days from storm surge?
3. Even though our analysis focused on coastal New Jersey, wind, rainfall and surge impacts will likely extend from the Carolinas to New England.
We're facing multiple-hazards here and the interactions between rain and surge are complex to say the least.
If you've made it this far I would like to extend a hearty congratulations for making it through one of my longest blog posts ever! Well done, my friend!
Stay tuned to the National Hurricane Center, your local National Weather Service Weather Forecast Office, and your local media for future developments related to this complex forecast. I will continue to update this blog daily through the passage of Hurricane Joaquin.