THE RECORD SNOWSTORM AT NEW YORK CITY 02-12-2006
AN ALREADY FORGOTTEN STORM TO REMEMBER
This snowstorm, or nor'easter, was an unusual event, in that, although it set the all time record for a 24-hour snowfall in New York City and Hartford, CT, it was not technically a blizzard in those areas, nor in most of the areas where it dumped the heaviest snow. It was also much smaller in coverage, unlike most of the previous storms that it supplanted in the record books. Last, but certainly not least, virtually no one forecast the snowfall amounts well. This makes it
the especially heavy snow, from 22 to 28 inches, from southeast Connecticut (West Redding) southwestward into northwest Queens (LaGuardia Airport), Manhattan (Central Park), and northeastern New Jersey (Rahway). Although not shown on this map, the highest official amount recorded in the metro area was 30.2 inches at Fairfield, Connecticut, on the coast south of West Redding.
Figure 2. Chart of snowfall depths over the NYC metro area from 2006-02-12. Courtesy The New York Times and Penn State 02-13-2006.
For a complete tabular list of snowfall amounts, here is the final snowfall chart from the NWS local office at Upton, NY.
BIRTH OF THE STORM - CYCLOGENESIS
The process that caused this storm is known as cyclogenesis, where a low pressure area (cyclone), is born (genesis) in the mid-latitudes of the earth due to certain atmospheric conditions at the surface and aloft. Cyclogenesis is fairly common along the eastern seaboard during the cool season, from roughly October through April, and is usually most intense in the winter, when temperature differences over the land and the ocean are at their highest. In this case we had explosive cyclogenesis, where the low forms and then strengthens (deepens) rapidly, which meteorologists call "bombing out". These coastal lows intensify rapidly because of the strong differences in temperatures along a cold or stationary front at the surface, supported by powerful atmospheric dynamics aloft.
AT THE SURFACE: As cold fronts approach the east coast, they can bring in very cold continental polar (cP) or continental Arctic (cA) air from Canada, which moves south and eastward, and thus pushes into much milder maritime tropical (mT) air over the southeast states, the Gulf of Mexico and/or the Atlantic Ocean. The surface charts for 7AM of February 11th and 12th, shown below in figures 3 and 4, show how this collision of air masses set up and progressed at the surface. Notice in figure 3 the cold air advection (advancement) on the southwestern side of the low pressure, forming a cold front, and the warm air advection on the northeastern side of the storm, forming a warm front. Bear in mind that this process is going on in the lower few thousand feet of the atmosphere, as well as at the surface. This is the early stage of the low. The advancement of the cold air eastward around the bottom of the low will continue, until eventually it raps all the way around the low center, pushing the warm air away to the east, as in figure 4. This is the "occluding stage" of the low, where it has "maxed out" and will soon start to weaken, since the warm air that is part of the fuel for the storm is being shunted out of the system. While these diagrams clearly illustrate the clash of different air masses, which is the fuel for cyclogenesis, they only illustrate a part of the process that unfolded. It was the air in the layers higher up in the atmosphere that caused cyclogenesis to take place, effectively putting a match to the fuel below it.
THE 500mb LEVEL: Higher up in the atmosphere, at roughly 18,000 feet above sea level, is where meteorologists look for all levels where the pressure is equal to 500 millibars (or mb). When the lines of 500mb pressure are drawn on a map, showing the "500mb heights", we see areas of high and low pressure, just as we do on surface pressure maps. These highs and lows also move along from west to east, but more rapidly, primarily due to a lack of friction from land. Troughs (low pressure areas) appear as a "U" or "V", and ridges (high pressure) as inverted "U" or "V" shapes, clearly visible in the 500mb charts in figures 5 and 6 below. If a trough or ridge closes off to a circle, it is called a closed low or high. It is important to remember that the troughs are areas of cold air and the ridges warmer air, relative to each other. Note the large 500mb trough (cold pool) approaching the east coast and the large 500mb ridge over the west. Within the large, (longwave) troughs there are embedded smaller ones called shortwave troughs moving through them. These 500mb shortwave troughs have an area of spin near their base, called a vorticity max, noted by the "X" in the diagrams. A vorticity max (also called vort max) is an area of diverging air which removes air from the air columns underneath it. This net loss of air causes the surface air at the bottom of these air columns to start to rise up to replace the loss. This causes surface pressure to drop, and the surface low deepens. As the surface low deepens, the counterclockwise flow around it strengthens the cold air advection to the west of the low and warm air advection to its east. With warming air columns east of the low and cooling columns west of the low, the 500mb trough also starts to amplify (its curvature increases), which strengthens the vort max, which strengthens the divergence, and now each keeps strengthening the other. This process is called self-development, and is a positive feedback loop. It is only limited by the amount of cold and warm air advection in the lowest layers, and the strength, or depth, of the initiating 500mb trough and vorticity max . Note in these 500mb charts below how deep the trough is - it extends from central Canada all the way to the Gulf of Mexico, a sure clue that the developing low will be strong.
One other important aspect of 500mb troughs, such as the one examined here in figures 5 and 6, is that when the bottom of the trough leans towards the west (leftward) as it does in figure 5, the trough is called a positively tilted trough, and is usually the configuration we see as cyclogenesis begins on the frontal area below it. As cyclogenesis occurs and the surface low deepens, the main upper air trough also deepens as explained previously, and the base of the deepening trough starts to slant more and more towards the east, as cold air rushes southeast and warm air pushes north and west. When the trough base leans more east or southeastward (rightward) as it does in figure 6, it is called a negatively tilted trough, and signifies the period during which the intensification process is maximizing. Soon after this, the best divergence shifts closer to the trough axis (closer to the core of cold air), which draws the surface low back toward the coldest air. This eventually separates the surface low from the main baroclinic zone (the front dividing the cold and warm air), causing the self-development positive feedback loop process to fall apart. The surface low then drifts under the upper-level low, where it begins to weaken or "fill", and die out.
AT THE JET STREAM LEVEL: Even higher up in the atmosphere, at what is called the 300mb level (the cruising altitude of many commercial jets, or roughly 30,000 feet) is one more ingredient that will determine the strength of a developing low pressure area in the cool season. This is the position and strength of the jet stream, and any embedded stronger disturbances that move along through it, which are called "jet streaks". When the left exit or right entrance region of a jet streak moves overhead of a developing surface low pressure area, it will further enhance the upward motion of air over the surface low and cause the in-rushing air at the surface to rise even more rapidly. The following chart in figure 7 shows the upper air conditions at 300mb for the morning of February 12, 2006, as the low was reaching maximum intensity. Notice how the northern jet stream is diving far to the south over the central U.S. and then making a sharp "u-turn" over the northern Gulf of Mexico and back up along the eastern seaboard. Also notice how the southern jet stream, or subtropical jet, is converging with the northern jet over the northern Gulf of Mexico, adding additional energy to the upper-air dynamics. When these two jet streams converge, they can produce a powerful jet streak, clearly visible over the southeast U.S. coast. Together with figures 4 and 6, notice how the left exit region of the jet streak stacks up over the 500mb vorticity max and the surface low, adding considerable energy to the storm. Although it didn't occur in this storm, it should be noted that the right entrance region of a jet streak has a similar energizing effect on a developing or intensifying low.
THE STRENGTHENING STORM
As you would expect, knowing the history of this storm, all of the ingredients were potent, and all came together quite well. At the surface, the temperature difference along the fronts within the low was quite large. Note in figure 8 below, the surface chart from 11:25PM on Saturday evening, the 65 degree reading in the warm sector (southeast side) of the low and the 31 degree reading in the cold sector (north side) at New York City. Also note the strong 1024mb very cold high pressure over northern New England with temperatures in the single digits and below. This polar ridge of air provided plenty of cold air to the cyclone; the cold air advanced east and southeastward towards and then around the center of the potent low. This flow is called the cold conveyor belt. The very warm air on the east side of the low is pushing northward and then upward, in a counterclockwise direction (north and northwestward) over the very cold air on the cold side of the low. This is the warm conveyor belt. Remember that the warm air is moving northward at the surface up to the warm front line. North of there it is forced upward over the very cold air being brought in on the cold conveyor belt. Along with the strong upper-air dynamics just discussed, this was a classic setup for a rapidly strengthening heavy snowstorm in the northeast. This stage of the low's development is called the open stage.
THE OCCLUDING STORM
As the storm matures, there are actually three currents of air feeding into the low pressure system. These currents are called "conveyor belts". The basic motion and setup of the cold and warm conveyor belts have already been discussed. But there is also a river of dry air that begins to feed into the low as it reaches maturity. This flow is called the dry conveyor belt. Figure 9 below is a colorized infrared satellite photo of the storm at this occluding stage, showing the classic comma-shape of a mature low, with the 3 conveyor belts annotated. It was taken not long after the surface conditions shown in figure 4. Cold air is pulled all of the way around and into the center of the low (blue arrows), forming the classic "comma-head" easily visible on the satellite picture. At the same time the warm conveyor belt (red arrows) is pulling very warm, moist air all the way up from the tropics into the low's circulation. Sinking, drying air is also now feeding into the storm from farther west and gets pulled all the way up into its center. This dry conveyor belt (yellow arrows), also called the "dry slot", is a classic part of the satellite signature of a mature low. We are now at the occluding stage of the low, where it has fully matured. But at this point the center of the surface low starts sliding backward under the cold, relatively calm center of the 500mb trough, where there is little or no divergence aloft. And note in figure 4 how the warm air has been forced far eastward and no longer makes it into the center of the low. With the warm conveyor belt no longer feeding into the low's center and dry air coming into the center of the storm circulation, the primary source of energy, warm moist air, is being gradually cut off. With all of these factors coming together, the low will no longer be able to sustain itself. Shortly after this point, the low began to "fill" or weaken, and then dissipated.
WHY THUNDER AND LIGHTNING?
One atmospheric event normally associated with the rising of moist air are clouds and precipitation (convection). If this rising of moist air is strong enough, it can produce thunderstorms. Although we usually associate thunderstorms with the summer season, we can see them in any season under the right conditions. When we have an influx of very warm, moist air at the lower levels, along with high positive lapse rates and some mechanism to force the air to rise, thunderstorms become likely. Although relatively rare in winter storms of average intensity, it is actually not that unusual to get thunderstorms in a winter storm of very high intensity such as this one. As we have noted, we had quite warm, moist air getting pulled into the right side of the storm at the surface and lower levels of the atmosphere. In addition, here is a chart of mid-level lapse rates for this storm at its height; the high numbers over the northeast show just how unstable the air was, i.e. how easily it would be able to rise if lifted. And we have also seen that this low was of unusual intensity, due to the strong 500mb vort max and 300mb jet streak, giving it the ability to strongly lift the air. Remembering how rare thunderstorms are in February, see the lightning map from 2006-02-12 at 7AM EST from Penn State's weather pages and the 2006-02-12 7AM Hourly Report from the NWS. Note the conditions of "TSTRM" with "BLWGSNOW" (blowing snow) observed at Central Park. Meteorologists call this "thundersnow". Figure 10 below, the 6AM weather discussion from the local weather office for New York City, shows just how awed the local NWS meteorologists were by the conditions unfolding in front of them.
WHY WERE THE FORECAST AMOUNTS TOO LOW?
When we examine precipitation, convection and thunderstorms within a low, we naturally turn to the valuable tool of Doppler Radar, which shows us where, and how heavily, the precipitation is falling. This will help us to understand why the band of heaviest snow was so narrow, and why these storms are so hard to predict. Below left, in figure 11, is a Doppler radar image from the morning of February 12, 2006. I have also reproduced, in figure 12, below right, the chart from figure 2, annotated to show the areas that had roughly 22 or more inches of snow, the heaviest recorded amounts. Note how well this corresponds to the heavy snow band in the radar image shown in figure 11.
We can see from this radar exactly where the heaviest band of snow was falling at the time. This narrow band of very heavy precipitation with embedded thunderstorms was dropping snow at the prodigious rate of 3-4 inches per hour, with some reports of 5 inches per hour. Because it did not move much for hours (see EXTENDED RADAR PAGE), extremely high snowfall rates were experienced under that band, as opposed to the surrounding areas. Convective bands such as this are associated with what meteorologists call a "deformation zone", an area that often sets up on the northwest and west sides of a low pressure system. While the storm itself is a synoptic scale event, the deformation zone and band of thundersnow are mesoscale events, for which the NWS has specialized coverage. The SPC often issues Mesoscale Discussions (MCDs) for winter storms such as this, so I have included, for the technically ambitious, one pertinent MCD here that relates to the deformation zone and its heavy snow band, in the MCD 139 REVIEW PAGE. The bottom line is that this deformation zone and heavy snow band were not forecast well enough in advance, and are the primary reason the forecast amounts were way off. The closest snowfall forecast made the evening before was by a local New York City meteorologist who predicted 14-18 inches, although he hedged by saying that this would occur under "optimum conditions". Most other forecasters were still calling for 6-12 inches at that time. The next morning, many early risers in Manhattan who woke up to 7 inches of snow at 7AM and thought the worst was over, saw 22 inches on the ground just 3 hours later at 10AM!
Most locations outside of this area had considerably less snow, with 10-15 inches only 25 miles to the east or west of it. It's important to remember that 10-15 inches of snow, while quite a lot, is not uncommon in this region and will not set all time records. See NOAA's record for the biggest snowstorms in NYC history. The story is the same for the other major cities of the northeast coast. Because of this, the storm had much less impact on most residents outside of the banded area in figure 11.
It should be noted at this point that the forecasting models used by the NWS (and also used by most forecasters in the nation) actually came pretty close on the timing and position of the low pressure system. Where they didn't do so well was on the strength of the low and snowfall amounts. This FORECAST HISTORY PAGE shows how well 3 of our best computer models forecast the position of the storm and its precipitation areas, but not so well on the low's strength and actual snowfall amounts. But at this stage in the history of forecasting, our current computer models cannot yet forecast confidently for weather events on a scale as small as a deformation zone. In addition, it is actually still not uncommon for the forecast models to underestimate the strength and snowfall amounts in a low that is "bombing out". This rapid deepening of the low can happen faster than the models can process all of the data. Since this limits their ability to foresee unusual events such as this until they are already unfolding, meteorologists must still rely on their expertise and on local readings as they become available. It is also why we should be very skeptical of any forecast of a big blizzard more than 1 or 2 days ahead of the storm. The technology to accurately forecast a storm like this many days in advance is simply not yet available.
ONE MORE ASPECT - THE OROGRAPHIC EFFECTS
One other interesting side of this storm is the orographic effect, where mountainous terrain influences the motion of the air. Looking at this topographic map of New York, and comparing it to the area affected by this storm, it's hard to miss the coincidence of the configuration of the band of heavy snowfall with the local terrain of southeast New York State and the surrounding areas. Most other heavy coastal snowstorms in the area have shown similar patterns. This feature is well known to meteorologists who cover these storms, and while the most obvious reason for a storm to intensify in this area is the difference in temperatures of the warmer ocean water and the much colder land areas, there are also influences from the mountains on the way air is lifted as it's swept inland in a nor'easter. Since the position and elevation of the deformation zone is critical to the storm's ability to drop excessive amounts of snow, and both are always going to be affected by the topography of the region, it is a legitimate question to ask how much influence the mountains have on storms such as these. Unfortunately it is way beyond the scope of this discussion and my level of studies, but I felt I would be remiss without mentioning it. I hope to be able to return to this in a future report.
NOT A "STORM OF THE CENTURY"...
In addition to the limited area of record snows, there are other reasons why this storm didn't match up to other major nor'easters that have roared along the northeast coastline in past winters. As powerful as it was, it did not even meet the criteria for a blizzard in the areas where it dumped the most snow. Why? The NWS definition of a blizzard requires that the storm have, for three consecutive hours:
1) Sustained wind speeds of 35 mph or more, or frequent gusts to 35 mph or greater
2) Considerable falling and/or blowing snow that reduces visibility frequently to 1/4 mile or less
Looking at local conditions from the height of the storm in this 2006-02-12 7AM Hourly Report, we can see that the observed conditions were not meeting blizzard criteria at any of the locations outside of Martha's Vineyard. The primary reason was that the storm's most intense winds remained just offshore at most of the cities in its path. Remember that it is the high winds in a blizzard that pile up snow drifts, dramatically enhancing the appearance as well as the reality of just how severe the storm is. There were no "drifts up the second stories" as there were in the "blizzard of '88". Also, as covered in the "Forecast History Page", this was light, fluffy snow. Besides boosting the snowfall totals, this type of snowfall makes it much easier to deal with for anyone who has to shovel, drive or walk through it, also affecting the public's opinion of the storm's severity. And of course it makes lousy snowballs.
...BUT A VERY POWERFUL STORM
For the experts who follow and analyze these systems, this was still a quite powerful and memorable storm. The lowest central pressure in the storm was roughly 975mb or 28.79 inches of mercury estimated (I am still researching this number), a pressure seen in category 2 hurricanes. In addition, it was in a class that few winter storms attain, given the fact that it formed an eye and was accompanied by prodigious snowfall rates of 3-4 inches per hour along with convection strong enough to cause thunder and lightning at its height. This STORM REPORTS PAGE, including reports from buoys off the coast and in the path of the storm, documents just how strong this storm was, and just how close this storm came to producing blizzard conditions at the major metropolitan areas. Conditions were near-blizzard on Long Island and reached blizzard status at coastal areas of southeast New England. This STORM COMPARISON PAGE has a brief comparison of this storm to the very powerful blizzards of 1983 and 1996, both of which had widespread thundersnow and record snowfall accumulations.
This storm left behind at least one snowfall record, for one of the largest metropolitan areas in the world, that may stand for many years to come. But the coverage of its heavy "thundersnow" was relatively narrow in respect to other winter storms of record. The developers of a new scale for rating the impact of snowstorms on the northeast section of the United States, NESIS, gave it a rating of 3 (major) on a scale of 1-5, mainly because it did not impact a wider area. They placed it at number 20 out of 33 strong northeast snow storms in modern history (1956-2006). Washington, D.C. which picked up 8.8 inches of snow, and Philadelphia, with 12 inches, have both had much heavier snowfalls along with true blizzard conditions in recent memory. For that reason, and the others previously mentioned, it will probably not be long remembered by most residents of the Northeast outside of the narrow area of high impact. Even in Manhattan, where the all time snowfall record was set, many people quickly forgot about it, since most of it fell on a quiet Sunday morning, interrupting very few routines, and the city's streamlined snow collection system scooped it up quickly and dumped it into the rivers and oceans before most even went to bed on Sunday night. (It costs New York City $1 million an inch to remove a snowfall.) By Monday morning, as school kids grumbled about not getting a snow day (and their parents along with them), the sun was out along with above freezing temperatures melting the neatly arranged piles of snow at the curbs. Two days later came 3 straight days of temperatures in the high 50s, so that by the following Saturday, the 18th, the official snow depth at Central Park, and the rest of the New York City metro area, was "0 inches"! 27 inches of snow isn't what it used to be.
OF PUBLIC INTEREST:
T1. Discussion of Frontogenesis from NWS Louisville, KY: http://www.crh.noaa.gov/lmk/soo/docu/forcing2.php
T2. Regarding Frontogenesis in Snowstorms: http://www.srh.noaa.gov/mrx/research/mar26snow/mar26snow.htm
T3. NESIS Overview from NOAA: http://www.ncdc.noaa.gov/oa/climate/research/snow-nesis/
T5. NWS at State College, PA, 2006 storm discussion, with analysis of computer model forecast accuracies: http://nws.met.psu.edu/severe/2006/12Feb2006.pdf