Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters from 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather
By: Dr. Jeff Masters , 3:24 PM GMT on February 14, 2006
Did the Blizzard of 2006 have hurricane-like characteristics? Yes, it did. To explore this more fully, let's look at the two basic types of large scale (synoptic-scale) storms that meteorologists define:
1) Tropical cyclones (hurricanes), which have a warm core, and derive their energy from the latent heat of condensation. When water vapor condenses into rain, the phase change from gas to liquid liberates some extra heat energy--the latent heat--that was used to evaporate the water in the first place. Since maximum evaporation in the atmosphere occurs over the warmest ocean waters, tropical cyclones thrive in the late summer when ocean temperatures are at their peak.
2) Extratropical cyclones (mid-latitude cyclones), which have a cold core, and derive their energy from the potential energy released when cold air aloft sinks and is replaced by warmer, less dense air. Extratropical cyclones develop where two air masses of sharply different densities (and thus, temperatures) intersect. Extratropical cyclones exist only outside of the tropics (thus are "extra"-tropical), where there is some cold air to be found. The ordinary low pressure systems that bring rain and snow to residents of the mid-latitudes are examples of extratropical cyclones.
In recent years, meteorologists have begun to discover that many extratropical cyclones--including Nor'easters, which are strong wintertime extratropical cyclones that affect the Northeast U.S.--can make a partial transition to a warm-core system once they move out over the warm waters of the Gulf Stream. Like a hurricane, deep convection will appear near the center of the storm, and the hybrid system will begin to draw energy from the latent heat of condensation. These storms can "bomb" and deepen at rates of 10 mb/hour, and reach central pressures normally associated only with major hurricanes.
Figure 1. The Blizzard of 2006, visible satellite image from NOAA for Sunday February 12 2006. Note the eye-like feature south of Rhode Island. The "eye" was near the edge of the Gulf Stream, where water temperatures increased sharply from 6 degrees to 12 degrees C.
According to Louis Uccellini and Mel Shapiro, extratropical cyclone experts with NOAA, these storms may be undergoing a "seclusion" process that creates an semi-isolated tropical system in the midst of an extratropical cyclone. In the seclusion process, a strong extratropical cyclone draws in warm air from the south, and latent heat of condensation from the cyclone's intense precipitation makes this air even warmer. This extra-warm air spirals into the center of the low and wraps around to the west side, where it is pinched off. As result, one has an isolated "warm core" center where deep convection builds and spiral banding can occur. However, unlike a hurricane, there is no eyewall, and no cloud-free eye created by sinking air (subsidence) in the center. The eye-like feature in an extratropical cyclone has upward moving air, and is merely the center where the surface winds spiral into. Spiral bands of convection can develop in the warm air near the center, mimicking the spiral bands of a hurricane. If these convective bands become intense, subsiding air on the flanks of the bands may create subsidence that warms and dries out the surrounding air, creating cloud-free regions near the center that may give it a more eye-like appearance. Another difference with hurricanes is that the upper-level high pressure system (anticyclone) over the extratropical cyclone is displaced to the northeast (downwind) of the center. In a hurricane, the anticyclone is directly over the eye.
The Blizzard of 2006 developed a distinct eye-like feature when it moved offshore over the warm Gulf Stream waters. The storm was undoubtedly tapping the hurricane's source of energy--latent heat of condensation--at the time the photo in Figure 1 was taken, since we can see evidence of spiral banding occurring neat the center of the storm. As seen in Figure 2, the Sea Surface Temperatures increased sharply from 6 to 12 degrees C (43 to 54 degrees F) near where this eye-like feature developed, right along the edge of the Gulf Stream current. There was plenty of water warm water for the storm to tap into for an extra energy source.
However, the storm missed the warmest waters of the Gulf Stream to its south, and did not intensify much compared to other historic blizzards--the storm's central pressure only dropped to about 980 mb, and a pressure of 960 mb is more typical of a classic Nor'easter "bomb". Also, note that the band of heavy snow that extends from Long Island through Connecticut northeastwards is well away from the center of the cyclone. This band is what gave the Blizzard of 2006 its prodigious snow amounts, and the band developed just as the storm moved off the coast--well before the warm ocean waters had time to create a warm-core seclusion in the storm and enhance the storm's snowfall. This band of heavy snow was created by processes unrelated to the formation of a warm core in the cyclone. The band had some similarities to the intense bands of lake-effect snow one finds in the lee of the Great Lakes--drier, fluffier snow than one usually finds in a Nor'easter, and very high snowfall rates of up to four inches per hour. Lake effect snow bands, and the extreme snow band of the Blizzard of 2006, are examples of developments on the "mesoscale"--the scale of a few tens of kilometers--and are not well handled by computer forecast models. These models typically chop the atmosphere into grid cells between 20 km and 40 km square, and thus were not able to resolve features like the extreme snow band of the Blizzard of 2006, which concentrated its heavy snow into a band just 10 or 20 kilometers wide.
Figure 2. Sea Surface Temperatures at the time of the Blizzard of 2006.
From the Sea Surface Temperature plot in Figure 2, we can see that much warmer water lies further south, along the North Carolina coast. The Gulf Stream moves parallel to the coast here. The Blizzard of 2006 missed tracking over this warmer water, since the storm popped off the coast near New Jersey then tracked due east. Thus if a winter storm crossing the U.S. can take a track so that it moves offshore near the Carolinas, then move northeastwards along the axis of the Gulf Steam, it will spend a longer time over much warmer waters than the Blizzard of 2006 did, and have chance to really tap into that latent heat of condensation energy that powers hurricanes. This is what happened to a January 1989 cyclone I flew into as part of a field project the Hurricane Hunters were participating in, called the Experiment on Rapidly Intensifying Cyclones over the Atlantic (ERICA). This storm "bombed" while we flew through it, the pressure dropping an astounding 60 mb in 24 hours, bottoming out at 938 mb. I'll have the tale of the rough ride through that storm later this week.
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