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Spaghetti plots help track hurricanes

CAPE CANAVERAL AIR FORCE STATION, Fla. -- A 'spaghetti plot' is a powerful tool for predicting the track of hurricanes. A spaghetti plot shows many lines on a map, each line being a hurricane track forecast from different hurricane prediction computer models. The overlapping long strands look a bit like a plate of spaghetti.

Sometimes the various track predictions will cluster together very tightly. This indicates that the computer models agree closely on where the hurricane should track and gives meteorologists increased confidence in the final prediction.

Sometimes the track predictions will spread far apart, indicating that the computer models disagree on the hurricane path, giving meteorologists less confidence in the forecast. Usually, most of the predicted paths will generally agree, except for a few paths that diverge from the consensus. The consensus of several models usually performs better than even the best single forecast model.

In August 2011, Hurricane Irene was an excellent example of the benefits of spaghetti plots. The major hurricane (Category 3) came within 180 miles of the Space Coast, yet no evacuations or even hurricane watches or warnings were issued for Brevard County. This was in large part because the spaghetti plot provided a high level of confidence for the likely track of the hurricane passing safely offshore Brevard County.

Spaghetti plots have become popular very quickly. It is a simple concept that makes it easy to interpret a lot of complex information quickly and easily on one map. It has drawbacks though. People often want to know which line is the best forecast. But looking for the best single forecast defeats the purpose of the spaghetti plot--consensus works best. The spread of the plots also indicates forecast confidence.

Why are there different forecasts? The computer models have different features and assumptions that give them different strengths and weaknesses under different weather conditions. It is okay for the models to have different forecasts. An experienced expert meteorologist is needed to know which models perform best under what situations and which have been performing better recently. That is one of the main jobs of the forecasters at the National Hurricane Center. Usually a consensus of a few of the best models gives the best prediction. The spread in the forecast tracks also allows "enveloping" to provide margins of safety. For example, if a cluster of track forecasts are approaching Florida, the National Hurricane Center might initially put the official track forecast toward the landward side of the consensus cluster to be cautious, until confidence increases enough to lower the risk to the coastline safely.

Beginning in the 2010 hurricane season, the National Hurricane Center added the spread in the track forecasts to their Wind Speed Probability Product--an objective application of spaghetti plots. This provided up to a 15 percent improvement in skill of the product under some conditions. This technique was developed in part by a collaborative project between the Naval Postgraduate School and the 45th Weather Squadron.
A spaghetti plot provides only the track of center of the tropical cyclone. The size and intensity of the wind field is not included, nor are other hazards like storm surge, local flooding, and tornadoes that can occur far from the storm center. Other tools are used for those hazards.

Weather safety training, including hurricane preparedness, is available from 45th Weather Squadron, or you can contact william.roeder@us.af.mil, at 321-853 8410.