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Disturbance 92L getting more organized; storm surge basics explained

By: Dr. Jeff Masters, 2:06 PM GMT on August 25, 2009

A tropical wave (92L) with an increasing amount of heavy thunderstorm activity is located a few hundred miles northeast of Puerto Rico, and is tracking west-northwest at 20 mph. Recent visible satellite imagery shows some increased organization of the storm, with upper-level outflow on the north side and a hint of a surface circulation trying to form near 22N 62W. However, the disturbance is moving underneath an upper-level cold-cored low pressure system, and this upper-level low is generating 20 - 30 knots of wind shear due the strong upper-level winds from the west. The upper low is also dumping cold, dry air into 92L, and this is retarding development of the storm. Dry air is getting ingested into 92L's thunderstorms and creating strong downdrafts that are robbing 92L of heat and moisture. These downdrafts are creating surface arc clouds that spread out from where the downdraft hits the ocean surface (Figure 1). Nevertheless, 92L appears determined to become a tropical depression over the next day or two, and NHC is giving 92L a high (greater than 50% chance) of developing into a tropical depression by Thursday morning.

Figure 1. Visible satellite image for 9:15 am EDT 8/25/09. Dry air from an upper-level low is getting sucked into 92L's thunderstorms, creating strong downdrafts that appear as arc clouds that spread away from where the downdraft hits the surface.

As 92L moves underneath the center of the upper low, wind shear may decline below 20 knots, and this is expected to occur by tonight. The lower shear may allow more development, and the Hurricane Hunters are on call to fly the storm this afternoon, if needed. The upper-level low is expected to move west-southwest over the next two days, and will continue to interfere with the development of 92L through Thursday. By Friday, when 92L should be several hundred miles off the coast of South Carolina, The upper-level low may be far enough south of 92L that the disturbance will find itself in a region with light upper level anticyclonic winds, which would favor more rapid development. A strong trough of low pressure will be approaching the U.S. East Coast at that time, and all of the models predict that this trough will be strong enough to turn 92L to the north. The models disagree substantially on how close 92L will be to the coast at that time, with several models bringing the storm ashore over North Carolina on Friday night and over New England on Saturday night, and other models keeping the storm out to sea. The intensity of 92L at that time is also problematic, with the HWRF model calling for a weak 40 mph tropical storm, and other models predicting a stronger tropical storm. Keep tuned.

Elsewhere in the Atlantic
The GFS model predicts the development of a tropical wave coming off the coast of Africa late this week.

Storm surge basics
I've developed an extensive set of web pages dealing with storm surge, which will include detailed storm surge inundation maps for the entire U.S. coast for each Saffir-Simpson Category hurricane. I'll be running portions of this material in my blog this week, and plan to release the full set of storm surge pages next week.

If you live near the ocean, the storm surge is the most dangerous part of a hurricane's hazards. The high death tolls of the ten deadliest U.S. hurricane disasters, including the Galveston Hurricane of 1900 (over 8000 killed), the Lake Okeechobee Hurricane of 1928 (2500 killed), and Hurricane Katrina of 2005 (1833 killed), were primarily due to the storm surge. The storm surge is water that is pushed onto shore by a hurricane. It is rarely a "wall of water" as often claimed, but rather a rise of water that can be as rapid as several feet in just a few minutes. The storm surge moves with the forward speed of the hurricane--typically 10 - 15 mph. This wind-driven water moving at 10 - 15 mph has tremendous power. A cubic yard of sea water weighs 1,728 pounds--almost a ton. A one-foot deep storm surge can sweep your car off the road, and it is difficult to stand in a six-inch surge. Compounding the destructive power of the rushing water is the large amount of floating debris that typically accompanies the surge. Trees, pieces of buildings, and other debris float on top of the storm surge and act as battering rams that can cave in any buildings unfortunate enough to stand in the way. If you receive an evacuation order for a hurricane storm surge, it is a very good idea to get out sooner rather than later. The storm surge can begin to rise a day before the storm hits, cutting off escape routes when low-lying highways are flooded. This is particularly true along the Gulf of Mexico shore.

Figure 1. Hurricane Katrina's storm surge pours over the 8-foot high north levee of the MRGO/Intra-Coastal Canal, directly under the Paris Road Bridge in eastern New Orleans. The photo was taken by Dan McClosky, the manager of Entergy's Michoud Power Plant. According to an interview at wwltv.com, "there were waves up on top of that, that were probably 15 to 18 foot on top of what you saw from the hurricane protection levee that was out there," McClosky said. Mike Collins of Austin, Texas has put together a nice in-depth description of this photo, which was judged as being authentic on the hoax debunking web site snopes.com.

The storm surge is created by wind, waves, and low pressure
There are three mechanisms that contribute to the storm surge:

1) The action of the winds piling up water (typically more than 85% of the surge).
2) Waves pushing water inland faster than it can drain off. This is called wave set-up, and is not forecast by the SLOSH model as of 2009. Wave set-up is typically 5 - 10% of the surge.
3)The low pressure of a hurricane sucking water higher into the air near the eye (typically 5 - 10% of the surge).

The storm surge depends greatly upon the size and intensity of a hurricane, the angle that it approaches the shore at, how deep the water is close to shore (the slope of the seabed at the coastline), and how fast the hurricane is moving.

Figure 2. Depiction of a fifteen foot hurricane storm surge occurring at high tide of two feet about mean sea level, creating a seventeen foot storm tide. Note that there are 10-foot waves on top of the 17-foot storm tide, so the external high water mark (HWM) left on the outside of structures by this hurricane could be 27 feet or higher. Image credit: NOAA SLOSH Display Training Manual (PDF File).

Definitions: storm tide, storm surge, high water mark
The storm surge is how high above current sea level the ocean water gets. The number we are most interested in regarding storm surge is how many feet above mean sea level (MSL) inundation will occur. This number is the storm tide, not the storm surge. The storm tide is the height of the storm surge above mean sea level (MSL), corrected for the tide. For example, in a location where high tide is two feet higher than mean sea level, and low tide is two feet lower than mean sea level, a 15-foot storm surge would cause a 17-foot storm tide if the hurricane hit at high tide or a 13-foot storm tide at low tide. Keep in mind that on top of the storm surge will be large waves capable of causing severe flooding and battering damage, and these waves are not included in storm surge forecasts. The waves on top of the storm tide break when they reach shallow water, and create an external high water mark (HWM) on structures. The high water mark can be much higher than the storm surge or storm tide. For example, the maximum storm surge of Hurricane Katrina was 27.8 feet in Pass Christian, MS (measured inside a building where waves couldn't reach). However, the highest high water mark was much higher--34.1 feet on the outside of a building in Biloxi, MS, where a high tide of about 1 foot combined with 11-foot high waves on top of the 22-foot storm surge to create the 34.1 foot high water mark.

Figure 3. High water marks on East Ship Island, Mississippi, after Hurricane Katrina in 2005. Left: Bark stripped off a tree with salt-burned pine trees in the background (note the 25 ft (7.65 m) long survey rod for scale). Right: Massive beach and over wash erosion illustrated by damaged and snapped pine trees along the beach. Image credit: Fritz et al., 2007, "Hurricane Katrina storm surge distribution and field observations on the Mississippi Barrier Islands" (PDF File), Estuarine, Coastal, and Shelf Science (2007), doi:10.1016/j.ecss.2007.03.015.

I'll have an update this afternoon.

Jeff Masters


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