A comparative retrospective on the Perfect Storm
by Walter Drag - Senior Forecaster WFO Taunton

1. Introduction

The following paper has not been peer reviewed, and is personalized into a somewhat less formal scientific format to facilitate understanding. It continues to serve as an accurate reference for what forecasters know about this storm and incorporates post event contributions from the meteorological community, including selected research deemed pertinent regarding this "powerful, extensive, long lasting, westward translating, mid-latitude northern hemispheric extra-tropical ocean storm". Please direct any correspondence to Walter Drag.

This compilation of data and research, in probable final web form 14 July, 2000, was a result of the many inquiries this Taunton office received since 1991. References ( ) are listed at the end.

Official National Climatic Data Center storm summary and satellite imagery is found in
http://www.ncdc.noaa.gov/ol/satellite/satelliteseye/cyclones/pfctstorm91/pfctstorm.html

Animated analysis of surface pressure, ~2500 ft wind and 500 mb features are found in
http://www.mpc.ncep.noaa.gov/perfectstorm/mpc_ps_an.html

In addition to the coastal weather forecast offices of eastern North America, another primary player in forecasting of this ocean storm was the Marine Prediction Center
http://www.mpc.ncep.noaa.gov/perfectstorm/mpc_ps_intro.html

A web search will show a number of media and private sector meteorology sites that contributed additional Perfect Storm related information and comment via word and/or imagery.

2. 1991

In 1991, the NWS southern New England forecast office was located at Logan airport in Boston.

We could see from our Mass Tech Center office rooftop the huge waves breaking against Massachusetts Bay landmarks Wednesday afternoon October 30.

We did not have access to the web (including model displays), nor the sophisticated analysis and forecast platforms/tools of Automated Weather Information Processing Systems (AWIPS) and the Buffalo Forecasting Toolkit (BUFKIT).

Nevertheless, the storm for southern New England and the adjacent ocean waters was generally well forecast, in part due to several days (model runs) of reasonably accurate long range weather outlooks and, in part due to the impressive developments that we saw occurring in the waters south of Nova Scotia on Monday and Tuesday October 28 and 29--a day or two before translating WESTWARD toward our coast.

 The title for the book "Perfect Storm" was coined in the relay of meteorological information during post event conversation between our then NWS Boston Deputy Meteorologist - Robert Case and author Sebastian Junger.

Officially, the U.S. Department of Commerce (DOC), National Oceanic and Atmospheric Administration (NOAA) NWS Natural Disaster Survey Report titled this as The Halloween Nor’easter of 1991 (1).

3. Comparison

The table below is comparison of approximate values along the southern New England coast for 1991, against well known and relatively recent historic storms, the Blizzard of ‘78 and the unnamed Hurricane of ‘38.

1991 1978 1938
Movement ~20 knots ~20 knots then nearly stationary ~45 knots
Max Surge ~5 feet ~4 feet ~20 feet
Max Wind G 78 MPH G 92 MPH G 186  MPH
Max Precipitation 5.5" rainfall se MA 25-35" paralyzing snowfall ~15"rainfall
BOSTON Storm Tide 14.3 feet 15.2 feet
Max WH BOSTON ~30 feet Estimated ~15-25 feet estimated
Ocean wind FETCH ~700 miles ~350 miles ~350 miles
Duration ~3 days ~1 1/2 - 2 days A few hours
Pressure Gradient ~70 MB ~64 MB ~57 MB
Sea Surface Temp (SST) ~52F (11C) ~35F (2C) est 64F (18C)
Deaths 12 99 564
1998 Dollar damage <1 billion1 2.3 billion1 19 billion

Movement references the forward motion toward southern New England.
Max surge was listed for Boston in 1991 and 1978, for the south coast of New England in 1938.

The storm tide listed above is for the Boston National Ocean Survey (NOS) location and is the maximum water level recorded at that location during the event. If the maximum surge does not coincide with the high tide, potential maximum storm (highest) tide is lessened. Major tidal flooding begins at Boston and nearby points when the Boston storm tide reaches 13.6 feet.

Flood damage is greatly aggravated by wave battery and therefore the subsequent listing of wave heights (WH) that ride atop the storm tide! Again, they must coincide with the highest storm tide to create maximum (worst) impact.

Buoy data references the wave height. Boston represented by 44013 is located approximately 16 miles east of Boston harbor http://www.ndbc.noaa.gov/station_page.phtml?$station=44013 Data has been available from buoy 44013 since 1984.

1For simplification purposes, the normalized 1998 dollar estimate for the extra-tropical events, Blizzard of 78 and the 1991 storm, was calculated in a manner far different from Pielke (2), using only an inflation calculator http://www.westegg.com/inflation/ maintained by S. Morgan Friedman, and by this method the extra tropical event data is financially conservative. This formula is much simpler and does not take into account landfall, counties etc as in Pielke. No matter whether you use Pielke or the inflation calculator, the Hurricane of 38 was far more costly; deaths, surge, maximum wind gust, and precipitation all attesting to this!

Hurricane Bob in August 1991, the December 1992 and the March 1993 superstorm were not compared to this 1991 event, though this seemingly was quite an active 19 month period for major events affecting southern New England. The historic Columbus Day 1962 storm that lashed the West Coast of North America from northern California to southern British Columbia, is considered the most damaging mid-latitude cyclone to hit the U.S. in the 20th century (3).

Much of the information presented so far is available from the NWS Disaster Survey report (1), NWS forecaster analysis, Normalized Hurricane Damages In The United States (2), Snowstorms Along The Northeastern Coast Of The United States 1955-1985 (4), and A Centennial Review Of Major Landfalling Tropical Cyclones In Southern New England (5).
http://www.nws.noaa.gov/er/box/tropical_cyclones.htm

4. Waves

The basics of ocean wave generation can be found in this, to date unpublicized url http://polar.wwb.noaa.gov/waves/pres/primer/primer_1.html   (6). Note slides 2 through 4.

Waves build in response to wind speed, duration and fetch. For example, if the wind suddenly increases to 50 knots (58 MPH) and holds for 5 hours, a flat sea will build to19 feet. After a total of 20 hours of 50 knot winds, waves will have risen to about 40 feet! (7)

Data, courtesy Environment Canada for the 1991 storm, from buoy 44139, started flowing in 1988. This buoy was closest to the last known position (~60 miles west) of the Andrea Gail and about 650 miles northeast of Nantucket. The buoy (44139) showed "peak" wave heights of about 11 feet early Monday evening October 28, which rose dramatically to about 50 feet 3 hours later. A total 10 hour rise of ~73 feet to a peak of about 84 feet occurred by early the 29thAND, this storm was still in its rapid intensification stage, not to reach its most intense-mature state until Wednesday morning the 30th.

Buoy 44137, about 200 miles southwest of 44139 and due south of Sable Island, recorded PEAK

wave heights to nearly 100 feet on that Wednesday morning the 30th.

Derivation of PEAK wave heights for the Canadian buoys can be found in Appendix 1 of A Meteorological Overview of the Halloween Storm of 1991. (8)

It is important to note that the huge envelope of 50 knot (9) sustained winds covered portions of the Atlantic south of Nova Scotia for almost 24 hours. Usually, huge storms at our latitude and longitude move faster, with the maximum sustained winds of 50 knots (58 MPH) or greater, lasting less than12 hours. "Peak" waves in the vicinity (Buoy 44137) of the worst of the storm, were greater than 60 feet for at least 24 hours.

Supposedly, this was to be a 100 year event for southern Scotian shelf buoys, yet the March 1993 super storm also produced "peak" wave heights to near 100 feet!

If you cannot believe the wave heights listed, please check this URL from the 24th Conference on Hurricanes and Tropical meteorology 29 May-2 Jun, 2000 http://ams.confex.com/ams/last2000/24Hurricanes/abstracts/12652.htm

This URL also highlights similar Marine Prediction Center experience that at higher latitudes, very strong storms occur with huge waves that rival the storm of 1991.

Dolan and Davis (10) in their research determined by means of a relative storm power index referenced to a point off Cape Hatteras (5 foot or greater wave height combined with duration known to create measurable beach face erosion), that this storm was the greatest in the period of record dating back to at least since 1950, and possibly back to the early 1940s. This surpassed the mid Atlantic coast Ash Wednesday storm of 1962. Wave heights were highest in the period of record and duration was 6th longest.

Buoy records for this storm were established at the Boston buoy (44013) for highest significant wave heights, and at the Georges Bank buoy (44011), about 180 miles east of Nantucket, for maximum wind and significant wave height.

5. Surge

Surge, how is it created for "extra (non) tropical" systems? Gradient wind is the primary contributor to surge, generally acknowledged to be twice that of the pressure contribution. That is why the pressure gradient is listed in the table. The greater the pressure gradient (difference between the lowest pressure and highest pressure), the stronger the wind and consequently the larger the surge. Additionally, lowering barometric pressure contributes a foot of surge for every inch it falls (a fall of 34 MB). Other primary contributing factors to surge are coastline configuration and bathymetry.

Surge for "tropical" systems works differently. The contributions of pressure and wind stress (speed) vary for differing forward motions of the tropical system (TS), but are more equally distributed. The literature (11) indicates that maximum surge occurs at about 35 knots (40 MPH)

with less surge for storms moving parallel to the coast as opposed to crossing the coast. This information is presented to give a better sense of the varying contributions to surge. Hurricanes are much smaller than the huge non tropical storms, and consequently have much greater change in pressure over a short distance. A 25 millibar (MB) change in pressure (~0.75 inches of mercury) over only a distance of 30 miles will cause a 3 foot surge, already much more contribution than the extra tropical system listed in the previous paragraph. A 50 MB (1.5" barometric pressure) change in pressure causes a 6.5 foot surge. Add to this the wind speed, and you can see why tropical systems have much larger surges. Finally, bathymetry and direct fetch funneling into bays, inlets or a suspected (but yet unproven) sloshing from Long Island Sound can combine to further increase surge and create a major coastal forecast concern.

6. Conclusion

Could the October 1991 storm have been worse for the southern New England coast?

Analysis of the pressure fields associated with this system showed that it essentially spared New England far greater damage, its center at its closest point, having passed about 125 miles southeast of Nantucket and the storm was WEAKENING! This can also be partially visualized in Environment Canada research hindcast analysis of the significant (highest third) waves (not peak) found at http://www.oceanweather.com/perfectstorm/  Note the late fade to the south.

Additionally, this storm occurred during perigean neap tides when the normal Boston forecast high tide at 433 PM October 30 1991 was 10.2 feet. Normally, our office begins considering issuing Coastal Flood Warnings (CFW) when combined surge and tide exceed 12 feet at Boston, especially if wave heights are to be 15 feet or higher. The base storm tide value of 13.6 feet is our Major Flood episode level and as you saw in the table in the beginning, our maximum tidal level (storm tide) was 14.2 feet. Had the storm of 1991 occurred a week earlier or later, high

tide, astronomically, would have started nearly a foot higher (about 11 feet), without the surge forcing contributions of wind or pressure falls!

These previous two paragraphs highlight how the potential impact along the southern New England coast could have been far worse.

As for a comparison between the October ‘91 storm and previous New England historic events, there is no doubt this was a highly unusual benchmark storm for the open waters of the northwest Atlantic in the second half the 20th century. Never making a U.S. landfall, it has to be regarded as a recent extreme and record setting event for this portion of the Atlantic Ocean. However, for the southern New England mainland, while a devastating storm for those lives lost at sea - their families and friends, and for the homes destroyed along the coast, other events in the 20th century had equal or more severe impact. The harshness of the paralyzing Blizzard of ‘78 along with its comparable coastal flooding to that of ‘91, and the extensive rampage of the Hurricane of ‘38 (surge, flash flooding and damaging wind) probably will be considered more severe than the coastal limited impact of The Halloween Nor’easter of 1991, also known as the "Perfect Storm".

Acknowledgments. The considerable efforts of dozens of forecasters from New England to our national centers in Washington are gratefully acknowledged with too many to name here. Special contributions to this summary were received from Environment Canada, specifically the Maritimes Weather Centre in Bedford Nova Scotia inclusive of their critical BUOY data as well as their meteorological overview, research and perceptions of this storm. Conversation with the NWS Marine Prediction Center (MPC), specifically Joe Sienkiewicz, and email correspondence with Professor Clifford Mass at the University of Washington was helpful in placing this storm in perspective to others in the higher latitudes of the northern hemisphere. Dr. Vince Cardone’s isotach analysis and gracious sharing of research assisted the post event demonstration of the power of this event. Thanks to Hendrik Tolman for his web primer. An excellent long range discussion from the NWS medium range branch, Frank Rosenstein, seen very early in this storms evolution (referencing a near record anomaly for southeast Canada) lent considerable support to southern New England forecasters visions of an impending unusually strong storm threat to our coast. The efforts of scientists who develop our long range models cannot be underestimated. At that time, that included the MRF, ECMWF, UKMET and Navy model information, verbally relayed to our office.

REFERENCES

(1) US Dept of Commerce NOAA NWS, Silver Spring Natural Disaster Survey Report
      The Halloween Nor’easter of 1991.
(2) Pielke Jr., R.A. and C.W. Landsea, 1998 Normalized Hurricane Damages In The United
      States: 1925-1995, Weather and Forecasting, 13;621-631
(3) Mass., C.F. (email Columbus Day 1962 correspondence), Professor Dept of Atmospheric
      Sciences, Univ of Washington, Seattle, Wa
(4) Kocin., P.J. and L.W. Uccellini, Snowstorms Along the Northeastern Coast of the United
      States
1955 to 1985. 211-220.
(5) Vallee., D.R., A Centennial Review of Major Land Falling Tropical Cyclones in Southern
      New England, 24th Conference on Hurricanes and Tropical Meteorology. Amer. Meteor.
      Soc, Ft. Lauderdale, FL, 29May-2 June 2000 (linked earlier and also on our web page)
(6) Tolman, Hendrik, web page primer, UCAR visiting scientist Environmental Modeling Center
      NOAA-NCEP, Camp Springs, Md.
(7) Kotsch., W.J. Weather for the Mariner. 135
(8) Cameron, D and Georges Parkes, A Meteorological Overview of the Halloween Storm of
      1991, Maritimes Weather Centre, Bedford, Nova Scotia, 4
(9) Cardone., V. J (email isotach-wave height correspondence)
(10) Dolan., R and R.E. Davis, Rating Northeasters, Mariners Weather Log, Winter 1992 Vol
      36 #1, 4-11.
(11) Tropical Cyclones - Their Evolution, Structure and Effects, 166-169


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