Oct 4, 2012

Atmospheric Anomalies and Northeast U.S. Hurricanes

First let me define an atmospheric anomaly: it is a deviation or departure from normal. For example in my blog  Hurricanes: Forecasting-motion-and intensification  I talked about the subtropical highs across the oceans. During the summer months this subtropical high is usually at its strongest and greatest extent. During the some summer and fall months this high could be stronger (weaker) than "normal". The difference between normal from a stronger (or weaker) than normal subtropical high is the anomaly.

Anomalies occur with atmospheric features: jet stream, surface pressure, heights (pressures) aloft, precipitable water, temperatures, etc.

For a hurricane to make a landfall over the Northeast the atmospheric pattern has to feature a wind pattern that permits the hurricane to move very quickly to the north, especially once the system moves north of the Gulf Stream.  Since tropical systems thrive on warm water the cooler water temperatures to the north of this current would "sap" the storm of its strength. The faster the storm moves when it is north of this ocean current the less time it is over these "cooler" waters and thus the rate of weakening would be less. There is actually a critical speed for the storm once it moves north of the Gulf Stream - 30 mph; a forward motion less than 30 mph and significant weakening occurs. The most intense hurricanes (Category 3 ) to hit Long Island and New England were moving at speeds of 40 mph or more. (In the Long Island Express Hurricane of 1938 the forward motion of the storm accelerated to 60 mph for a brief time!)

In order for this fast forward motion to occur the east west pressure  must be anomalously great from the surface to great heights above it.

Here is a list hurricanes that have made landfall across the Northeast U.S (NY or New England) along with the maximum intensity (using the Safir-Simpson intensity scale) of the storm during its life span as well as the intensity of the storm when it made its Northeast U.S. landfall.
 
Hurricane
Maximum Intensity
Intensity at Landfall






New England Hurricane of 1938
5
3
Great Atlantic Hurricane (1944) 
4
3
Carol (1954) 
3
3
Edna (1954) 
3
1
Donna (1960)
5
2
Esther (1961) 
4
1
Belle   (1976)
3
1
Gloria  (1985)  
4
1
Bob  (1991)  
3
2
                               
A couple of notes: Hurricane Donna is the only hurricane to cause hurricane force conditions along the entire U.S. East coast - from the Florida Keys to Maine.  Also note that "Irene" is not on the list as it was NOT a hurricane when it made its third landfall near NYC in August of 2011. Even though it was not a hurricane it still was a devastating storm.

From Ryan Hanrahan's blog Way Too Much Weather here are anomaly maps for Northeast U.S. landfalling hurricanes for 72 hours, 48 hours, 24 hours prior to and for the day of the landfall.


500 millibar anomaly 72 hours prior to landfall
The map to the right is a 500 millibar anomaly map 72 hours prior to a landfalling hurricane the blues to purple colors indicate areas of below normal heights (negative anomalies)  at 500 millibars;  the yellows and oranges indicate heights above normal (positive anomalies). Note the the two areas positive anomalies over the North Pacific Ocean and the other over the Northeast States and southeast Canada.  The "blue" spot is an area of below normal heights that extends from the Dakotas to Louisiana





(500 millibar anomalies 48 hours prior landfall)
The map to the left is the same anomaly data but for 48 hours before land fall note the positive anomaly over the Northeast Pacific remains in place but now has expanded southward along the west coast. This in turn causes the negative anomaly over the middle of the U.S. to strengthen both in terms of intensity and amplitude. The anomaly over the Bahamas is associated with the hurricane. The other positive anomaly over the Atlantic also persists but is now situated over the Canadian Maritimes and the waters to the east and south.



(500 millibar anomalies 24 hrs before landfall)
Twenty four hours prior to landfall (map to the right) the negative anomaly over the middle part of the U.S, has intensified greatly with the greatest or lowest heights centered over the Great Lakes and a full latitude trough south the the Gulf Coast and the Southeast States. The positive anomaly over the Atlantic has also increased in strength and is now centered over the Canadian Maritimes; once again the positive anomaly over the Pacific has not moved either.




The day of the landfalling hurricane has the greatest anomalies persisting  across the Great Lakes States; the secondary negative anomaly associated with the hurricane is over the East Coast and the positive anomaly continues strong with little movement across the Maritimes.

For the very fast moving storms the anomalies located over the Central U.S. and the Maritimes would exhibit very large departures from normal. The greater the difference between the two anomalies the strong the winds aloft would be.



References:
Atlantic Hurricanes - Dunn and Miller
Secular Fluctuations in Vulnerability to Tropical Cyclones in and off New England MWR Aug 1955 Namias
Relation of Long-Period Circulation Anomalies to Tropical Storm Formation and Motion JOM April 1959 Ballenzweig