“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.
This weekend’s winter storm threatens to slam the Mid-Atlantic with coastal flooding and high waves. While this epic storm will most be remembered for inflicting blizzard conditions near the Nation's Capital and dumping feet of snow, coastal flooding is a concern from Virginia through New Jersey.
The National Weather Service is forecasting a strong onshore wind event for Tidewater Virginia, the Delmarva Peninsula, and the Jersey Shore. The grid point forecast for Cape May, N.J., depicts sustained winds exceeding 40 mph, with gusts in the 50s, for an 10-hour period on Saturday.
The most intense winds will be from an ideal direction to cause coastal flooding. Strong northeast winds are the most efficient at generating coastal flooding along the Delmarva Peninsula and coastal New Jersey, because water tends to deflect to the "right" of winds in the Northern Hemisphere.
The strong winds will build the biggest surge through Saturday afternoon and evening, which is unfortunate timing on the monthly calendar. The moon is full on Sunday, meaning high tides this weekend will be at their highest levels for this month.
However, on the daily calendar, the timing of the most intense winds is actually quite good. This region experiences two high tides per day, and it appears the storm will be most intense around the time of the "lower high tide." This makes a difference as the "higher high tide" is approximately 1 feet (0.3m) higher than the "lower high tide" this weekend at both Atlantic City and Cape May, N.J. The "higher high tide" occurs between 7:00- 8:00 a.m. on Saturday and Sunday mornings at both of these sites. This timing is ideal, as the peak storm surge should not occur near the time of highest high tides.
We can expect maximum storm surge levels to reach approximately 3.0-3.2 feet above predicted astronomical tides on Saturday afternoon and evening. Storm surge levels should drop to around 2.3-2.4 feet by dawn on Sunday. These surge levels would create the following total water levels (surge + tide) at Atlantic City and Cape May. Homeowners should be concerned about the total water level (storm tide), not just the level of storm surge.
I have provided predictions in tabular form below. The timing and height of predicted tide levels come from NOAA Tides and Currents. Water levels are in height above Mean Lower Low Water (MLLW). The storm surge predictions are my own and added to the predicted astronomical tide. For surge prediction I used the "Hurricane Hal's Secret Rocket Ship Model," which is basically my intuition after working with surge data for eight years now. But, hey, everyone likes thinking about rocket ships, right?
These are "deterministic" predictions (not probabilistic), meaning I'm sticking my neck out there and trying to make an "exact" forecast, and not considering the likelihood or unlikelihood of my prediction. I'm not doing advanced modeling here, so please account for a margin of error.
These water levels would cause minor to moderate coastal flooding near the time of high tides.
Comparison to October's "Gradient Surge"
Coastal New Jersey experienced a storm surge in early October from persistent onshore winds caused from a strong pressure gradient between Hurricane Joaquin in the Bahamas and a strong high pressure system parked over Eastern Canada.
Storm surge levels above predicted astronomical tides reached 2.51 feet in Atlantic City and 3.08 feet in Cape May. The total water level (surge + tides) above MLLW reached 7.11 feet in Atlantic City and 8.04 feet in Cape May.
These surge levels were high enough to cause minor to moderate coastal flooding along the Jersey Shore, particularly near Ocean City, Sea Isle City and Avalon. I came across powerful waves washing across the causeway of Avalon on the afternoon of Sunday, Oct. 4.
The chart above shows that I am predicting similar total water levels this weekend. I think total water levels will be similar because it takes time to move a lot of water. Although winds this weekend should reach higher levels than in October, the duration of sustained winds greater than 30 mph should only last about 24 hours, whereas October's gradient event produced prolonged onshore winds for 3-4 days.
The water levels in October's storm and what will be observed this weekend were well below Hurricane Sandy's maximum water levels in the region.
Storm Surge Impacts
Although I'm predicting similar water total water levels to October's event, coastal flooding impacts could be worse in some locations this time for two reasons:
1. Wave heights may be higher. It takes time for wind to displace a column of water in the ocean, but surface wind waves often generate more quickly. As sustained winds could top 50 mph, with gusts in the 70s offshore, massive waves will pummel the coast. So waves riding on surge inundation could do considerable damage.
2. October's storm surge produced moderate coastal erosion in New Jersey. After the surge subsided, I personally found 7 foot (2.13m) cuts in Sand Dunes in the town of Avalon. A lot of sand and grass washed away, and there has not been time for natural or artificial beach nourishment. Therefore, similar storm tide levels this time may flood areas not flooded in October.
Want to interact on social media about this winter storm’s storm surge/ coastal flooding? Check out the forum I created at the new U-Surge Project.
Post your thoughts or questions about this surge event and let's get the party started!
Take care and stay safe everyone!