“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.
A widespread storm surge event will impact the U.S. Atlantic Coast later this week, as the complex interaction between Hurricane Joaquin, a pre-existing cold front, and a building dome of high pressure impact much of the Eastern Seaboard.
The National Hurricane Center upgraded Joaquin to a hurricane as of the 8 a.m. Eastern time advisory this morning. They forecast Joaquin to slowly drift westward towards the Bahamas through early Friday morning, before picking up forward speed and tracking to the north from Friday through Monday.
Although many hurricanes that track up the Eastern Seaboard eventually curve towards the northeast and remain offshore, building high pressure over Eastern Canada may block Joaquin from making this turn. This scenario may force Joaquin to either track due north, or even take a turn towards the west, on a path that could potentially resemble that of Hurricane Sandy three years ago.
Strong Pressure Gradient Will Generate Widespread Onshore Winds
Regardless of Joaquin’s exact track, the strong dome of high pressure over eastern Canada will set up a widespread pressure gradient, which will produce prolonged onshore winds along much of the Eastern Seaboard. The GFS model output depicted in the image above, initialized at 06Z (2:00 a.m Eastern time) this morning, maps this strong gradient for a forecast 48 hours into the future (06Z or 2:00 a.m. on Friday). The lines on this map, called isobars, depict lines of equal pressure. Tightly packed isobars indicate large pressure gradients and strong winds.
The National Weather Service (NWS) hourly weather forecast graphs depict this prolonged wind event visually. The graph for the Atlantic City, N.J., (Point Forecast at Lat: 39.38N, Lon: 74.4W) depicts winds greater than or equal to 20 mph to begin by 7:00AM Eastern time on Thursday and continue until 12:00 a.m. on Sunday. The direction of these winds will range from north-northeast to east. If this forecast holds, it means that an onshore wind, capable of generating substantial storm surge, would exceed 20 mph for 65 consecutive hours. This graph also depicts winds greater than or equal to 30 mph impacting this area for seven consecutive hours on Saturday.
Such forecasts are subject to change, particularly in a complex situation like this one, as Joaquin’s exact track is unknown. Wind speeds should increase dramatically closer to Joaquin’s track. However, such a large pressure gradient between Joaquin and the massive high pressure to the north will mean that a widespread, prolonged onshore wind event is likely, regardless of Joaquin’s exact track. This means that hundreds of miles of coastline will likely experience storm surge inundation, even if Joaquin stays offshore or makes landfall somewhere else.
Heavy Precipitation Will Exacerbate Flooding
Heavy precipitation is another factor that should increase Joaquin’s inland and coastal flooding potential. A low-pressure system tracked along a front on Tuesday, producing substantial rains over northern Virginia, the Mid-Atlantic States and New England. As of Wednesday morning, more than 1.5 inches of rain fell across much of this region over the past 24 hours, with many areas observing more than 3 inches. Unfortunately, the NWS Weather Prediction Center forecasts more than 7 inches of rain to fall during the next five days over a broad area from North Carolina through Southern New Jersey, with amounts exceeding 3 inches across northern New Jersey, southern New York and eastern New England.
Such heavy rains increase the risk of multiple hazards. Obviously, more heavy rain from Joaquin falling on soil that is already waterlogged will produce freshwater flooding, as the soil cannot store any more water, increasing runoff into streams and rivers. Waterlogged soil also provides less support for trees, making it more likely that they will fall when struck by strong winds.
From a coastal flooding perspective, waterlogged soil and excessive freshwater runoff exacerbates the impacts of storm surge events for several reasons. It is easier for a storm surge to push inland if the surge finds water levels already elevated in coastal wetlands, estuaries, tidal inlets and rivers. This past Friday evening while I was on a bayou in south Louisiana looking for alligators (it’s quite fun actually), two locals told me that Hurricane Ike’s surge in 2008 was particularly bad because it arrived only 12 days after Hurricane Gustav, and the water levels from Gustav had not yet fallen. New water accumulated on pre-existing water compounds flood potential.
Another complex problem related to heavy rain and storm surge is that rainfall drainage is usually gravity fed, depending on a slope between the land and the nearest water body. When a storm surge pushes inland and elevates the level of a bay, river or tidal inlet, fresh water drainage slows down considerably. In other cases, a storm surge can actually “dam” freshwater discharge near the mouth of a river, causing the river to actually back up and inundate inland communities.
These interactions are complex and difficult to forecast. However, we do know that storm surge and heavy rainfall work together to compound flooding, and Wahl et al. (2015) recently found that this phenomenon is becoming more frequent, particularly along the U.S. Atlantic and Gulf Coasts.
Long story short, the combination of prolonged onshore winds and heavy rainfall preceding and accompanying the approach of Hurricane Joaquin should generate substantial coastal flooding impacts along the Eastern Seaboard. Stay tuned to updates from your local National Weather Service and the National Hurricane Center for forecast updates on this developing event. I will attempt to update my blog frequently over the next week, as this complex situation unfolds.