Wednesday, October 24, 2012

Who Remembers Hazel? 
Comparisons between the October 1954 major hurricane 
and current observations about Hurricane Sandy 

3:50 PM EDT 10/24/12 (Forecaster Mike & The Long Range Team) On October 15, 1954 one of the most powerful October hurricanes in history slammed into the Carolinas, charged through the Chesapeake Bay with winds of 90 mph and by the time it crossed Pennsylvania into Canada, was still the equivalent of a Category 1 Hurricane. This was Hurricane Hazel, and over 50 years later it is still a benchmark against which late season tropical systems are compared. 

Believe it or not, some strong similarities exist between the historical setup of Hazel in the days prior to its landfall, and the current atmospheric pattern before us with Tropical Storm Sandy. Our team has conducted a detailed analysis of this comparison, as featured below. 

# 1 - SURFACE ANALYSIS The graphic on the left shows the weather at 1:30 PM on October 11th, 1954. The graphic on the right shows the surface analysis at about 12:30 PM on October 23rd, 2012. 

  • Remarkable similarities between the two maps. First, note the two high pressure centers, both very strong, near the SE United States. In 1954, the High pressure was slightly offshore and a little larger than in 2012, but the positioning is very similar.
  • Secondly, note the low pressure in the upper Midwest. The low pressure there in 2012 is slightly weaker, and as a result more spread out. Still, in both years, there is a stationary front stretching from that low pressure towards the Atlantic, causing unsettled weather to move along near the Great Lakes.

Another remarkable similarity is the conditions over Eastern Canada. For example, but years show a strong high pressure close to the Hudson Bay. This high is slightly further east in 1954 than in 2012, which also pushes the low pressure on the east side further north and east and closer to Greenland in 1954.

# 2 - UPPER AIR ANALYSIS: The surface maps only tell a small portion of the story, especially considering the fact that the timelines of the storms are very different. The upper atmosphere gives a better look at the overall pattern that forced Hazel on the path she took in 1954 and we can use that to try to pinpoint where Sandy may go in 2012. 

The graphic on the left shows the 500 mb analysis from October 11th, 1954. The graphic on the left shows a current analysis of the 500 mb level on October 23rd, 2012. Some strong similarities exist at this level of the atmosphere as well:
  • Notice the upper level ridge that dominates the eastern United States in both maps. Another important aspect is the strong upper level low near the Labrador Straight (between Greenland and Canada). This feature will play a significant role down the road for both storms.
  • If we head closer to the time of Sandy and Hazel’s impacts though, the upper air projections by computer model for Sandy start to differ more from the observed analysis at the time of Hazel. Let’s take a look at the upper air analysis for two different computer models. 

The graphic on the left shows the CMC model projection for the time at which Sandy is at about 30º North Latitude. The graphic on the right shows Sandy at approximately the same latitude according to the GFS model.

Right away, we can see that the low pressure I mentioned earlier is already having an impact. The important features to look for here include the blocking high south of Greenland, the upper level low out in the Atlantic, of course Sandy, and the trough moving into the Eastern United States.

The strongest differentiating feature in these two maps is the upper level low over the Atlantic. This low is stronger on the GFS model, which causes it to cut into the high pressure. On the CMC model, you can see that there is no weakness below the blocking high over Greenland, which down the road, forced the storm back westward. However, according to the GFS, the low pressure is stronger, which causes a break in the high pressure, and provides a weakness that Sandy can pass through, and head safely out to sea. 

Now let’s compare this with what Hazel did in 1954. 
Right away, we can see some important differences. The biggest thing to point out is the position of the high pressure in the Atlantic. In 1954, that low pressure system was not present in the Atlantic, instead it remained near Greenland, the opposite of the blocking high we see in 2012. However, there still was a blocking high pressure east of New England, which you can see to the NE of Hazel, located at about 30º North on October 15th, 1954. With the high pressure to its northeast, Hazel was automatically forced westward, where it made landfall as a devastating hurricane in North Carolina.

A similarity with 2012 is the trough moving into the eastern United States. In 1954, Hazel interacted with the trough as it hit the eastern seaboard, and quickly transitioned from a tropical system into a non-tropical, but still strong, system moving inland.

# 3- NAO COMPARISON  When looking at storms anywhere near the eastern seaboard, it is often helpful to look at the North Atlantic Oscillation, which is an important control of the blocking in the North Atlantic. 

The NAO indices between the two years are very different. This is actually reflected on the upper air analysis shown above. As Hazel was strengthening in the Caribbean around the 9-11th of October 1954, the NAO was strongly positive but dropping quickly. The NAO continued to plummet through Hazel’s North Carolina landfall on October 15th, 1954. After bottoming out on the 16th, the NAO index started to climb rapidly once again.

This year however, you can see that the NAO is strongly negative, and is anticipated to remain strongly negative. However, we have seen in the past that slight bumps in the NAO index can have a huge impact on storms in our area. This strongly negative NAO index today is reflected by the blocking high that we discussed above. 

It remains to be determined what Sandy will end up doing, but it is always useful to look to the past when predicting the future! 

1 comment:

ravensbbr said...

Andy! Terp! ...anyone? Anyone? Early (outside, dubious, but possible) powderhounding in the offing...anyone? :-P