The weather chart from the Great Appalachian storm of 1950. (National Weather Service)

It’s one of the most intense 20th-century storms you may have never heard of. And it consistently makes it to meteorologists’ top 10 lists of all-time greatest storms — including those published by the National Oceanic and Atmospheric Administration and the Weather Channel.

East Coast flooding, widespread wind damage, record snowfall and low temperatures conspired to make the Great Appalachian Storm a meteorological powerhouse. Upward of 383 deaths are attributed to this storm, which impacted 22 states and levied nearly $700 million in damage, after adjusting for inflation.

You may also know it by its other name, the Great Thanksgiving Storm, as it started over the Eastern United States on the Friday after. If this same storm hit now, it would probably be called the Great Black Friday Storm.

The storm initiated over western North Carolina on Nov. 24. After moving north along a stalled cold front, it met up with a trough in the jet stream, causing it to rapidly deepen over Washington. Then it hooked westward (an odd track, which we discuss later), while continuing to rapidly deepen or “bomb out” over West Virginia, western Pennsylvania and Ohio. The system stalled for nearly a day over Ohio as a massive and very intense vortex before rapidly weakening and drifting into Canada on Nov. 28 to 30.

The graphic below presents a timeline of this storm’s evolution, based on surface analysis charts, highlighting several key features and the general life cycle outlined above. (Upper atmospheric, i.e., jet-stream level features played a major role in the development and intensification of the surface storm; however, for length considerations, I chose only to focus on the major surface features.)

Three days capturing the structure and evolution of the Great Appalachian Storm, highlighting minimum central pressure, storm movement, winds and frontal structures. (Modified after Kocin and Uccellini, Snowstorms of the Northeast, Volume 2, AMS)

The storm generated a huge wind field with hurricane-force wind gusts across much of the Northeast. The winds over New York blew from the unusual direction of southeast, and this is why the term “Southeaster” is coined for this rare type of event. Wind gusts ranged from 110 mph at Concord, N.H., to 108 mph in Newark, 83 mph in Albany, N.Y., 94 mph in New York City, and 88 mph in Bridgeport, Conn.

Unlike a hurricane, which generates its strongest winds along the coast, the high winds of the 1950 storm were projected broadly and deeply inland. The effects of fallen trees were particularly massive across Upstate New York. There were nearly a million power outages in total — a very large number for a single storm in 1950.

Looking back at Figure 1, these winds were generated not just because the storm’s central pressure was exceptionally low. Rather, it was the size of the pressure gradient — the pressure difference — between the storm’s center, and high pressure building north of Maine. Early on Nov. 26, this difference was an extremely large value of 56 millibars between Maine and Ohio (1045-1050 millibars in high pressure north of Maine, vs. 987 millibars in the storm’s center).

The extent of the extreme pressure gradient is shown below. A pocket of intense southeasterly winds (magenta zone) developed into a “low level jet stream” feeding into the low. In fact, Mount Washington Weather Observatory in New Hampshire (elevation 6,288 feet) was positioned inside this jet stream, where a peak gust to 160 mph (!) was recorded.

Anomalous 5,000 foot level winds circulating around the Great Appalachian Storm, from the NCEP/NCAR 20th Century re-analysis data set. Magenta zone of intense southeasterly flow represents a wind speed four standard deviations higher than average. (Richard Grumm, National Weather Service)

Storm with an identity crisis: Coastal flooding and record-breaking snowfall

The storm produced very different warm side and cold side hazards: nontropical cyclones entrain disparate air masses, i.e., tropical air from the south and east, polar air from the north and west.

Two very different facets of the same cyclone: Extreme snowfall over Pittsburgh (left) vs. heavy rains and coastal storm surge over New England (right). (AP)

While the Great Thanksgiving Storm inundated much of the region stretching from Northern Virginia to New England with 2 to 6 inches of rain, serious coastal flooding unfolded from New Jersey northward. The powerful, sustained push of wind from the southeast generated many hours of battering ocean waves, which coincided with astronomical high tide. The coastal damage in Connecticut was particularly severe, with so much beach sand deposited on streets that snowplows were required to clear the roads.

On the cold back side of the storm, more than 20 inches of snow blanketed West Virginia, western Pennsylvania and eastern Ohio. A whopping 62 inches fell during blizzard conditions at Coburn, W. Va. In the diagram below, the region of incredibly deep snowfall is small, but the Thanksgiving Storm stands as the greatest snowstorm on record along the western Appalachians. Based on the NOAA Regional Snow Impact (RSI) scale, it ranks as a rare, Cat 5 or “Extreme” level event.

Condensed but extremely deep “snowprint” of the Great Appalachian Storm, illustrating a 40- to 50-inch bull’s eye over the Allegheny Plateau. (Kocin and Uccellini, Snowstorms of the Northeast, Volume 2, AMS)

Additionally, the massive storm swept record-breaking cold temperatures into the South, including 5 degrees in Birmingham, Ala.; 3 degrees in Atlanta and minus-1 degree in Louisville. These were all due to the huge sweep of counterclockwise-blowing, Arctic winds circulating around the back side of the vortex. The figure below shows the extent of record cold for November, which caused considerable damage to crops in Georgia and South Carolina.

Anomalous 5,000-foot level cold air plunging over the Deep South, courtesy of the Great Appalachian Storm, from the NCEP/NCAR 20th Century re-analysis data set. Lime green zone over Georgia represents temperature values six standard deviations below average. (Richard Grumm, NWS)

Some oddities: Retrograde motion and an inverted warm sector

Weather aficionados will be interested in a couple bizarre attributes of this storm: It tracked westward from the District to Ohio, and the warmest air in this storm was located on its north side, over New England … with the most extreme cold circulating into the Deep South.

In 99.99 percent of cyclones, the warm air surges northward, while cooler air is pulled in from the northeast, and particularly cold and dry air circulates down from northwest. The system’s wedge-shaped “warm sector” lies ahead (to the east of) the cold front, and south of the warm front.

Classic cyclone model (left panel) with warm sector to the south and east of the center of low pressure. Contrast with the Great Appalachian Storm (right side), on Nov. 25, which reveals an “upside down” temperature pattern. (Modified after Kocin and Uccellini, Snowstorms of the Northeast, Volume 2, AMS)

In the Great Thanksgiving Storm, the system’s pressure became so deep (978 millibars over Ohio, Nov. 26), and the wind circulation so wound up or “coiled in” on itself, that the push of Arctic air swept first southward … then eastward … then northward. Meanwhile, the extreme pressure gradient over New England swept in relatively mild oceanic air, on southeasterly winds, pushing warm air as far westward as Ohio. The result was an “inverted warm sector” as depicted in the diagram above.

As a consequence, some unexpected — if not downright crazy — weather changes happened. For instance, in Pittsburgh, heavy snow was falling in 21-degree air … then as winds shifted from northwesterly to southeasterly … the temperature plunged to 9 degrees, with the arrival of the cold front from the southeast! And while Pittsburgh was at 9 degrees (and heavy snow), hundreds of miles to the north, Buffalo was experiencing a balmy 54 degrees, heavy rain and hurricane-force gusts. It’s hard to believe that the District’s temperature was nearly 50 degrees warmer than Atlanta’s!

Another bizarre behavior was this storm’s westward track. Again, 99.99 percent of cyclones move generally west to east across the United States, as they are embedded in the westerly jet stream of winds aloft. But the jet steam became extremely contorted over eastern North America late in November 1950. As shown below, the core of the jet became distorted into a massive, cut-off vortex (purple region) swirling counterclockwise, over much of the East. Meanwhile, a “blocking high” or ridge of extreme high pressure, became established over eastern Canada (red region), with these winds spinning clockwise. The end result: The surface low, originally over central North Carolina, was drawn inland, toward the northwest, over Ohio.

Anomalous 500 millibar pressure level “heights” (approximately 18,000 feet above the ground) from the NCEP/NCAR 20th Century re-analysis data set. Purple zone is a huge zone of significant height falls, marking the center of low pressure in middle levels of the atmosphere. Red region shows exceptional height rises, building a ridge of high pressure. Arrows show mid-level direction of flow — steering the surface low toward the northwest. (Richard Grumm, NWS)

Some of you may recognize parallels between the 1950 storm and Superstorm Sandy of 2012. Sandy’s tropical vortex co-evolved with a trough in the jet stream, carving out a massive vortex over the eastern United States. At the same time a Greenland Block became established. Like the 1950 storm, post-tropical Sandy (a large nor’easter at landfall) was drawn westward, but a bit further north, across New Jersey and then into Pennsylvania.

Sandy was also characterized by a somewhat inverted warm sector, with mild air surging westward across the northern Mid Atlantic and New England, and subfreezing air plunging further south across West Virginia and the Tennessee Valley. In fact, recall that the remnants of this storm dumped up to three feet of snow in this southern cold zone!

A storm that helped improve weather models

Finally, the Great Appalachian Storm helped impel numerical weather forecasting (a.k.a. the “models”) to its present state. Back in the day, manual forecasts did not anticipate a storm of this size, intensity or anomalous track; the storm was simply was not predicted more than a day in advance. But it became a test case for the first two-level, operational forecast model, and was thus at the forefront of modern weather prediction’s infancy.

The Great Appalachian Storm was one of the most destructive and anomalous middle latitude cyclones ever documented. Perhaps the best way to end this story is to quote the words of two of the most eminent East Coast cyclone researchers of our day, Paul Kocin and Louis Uccellini, who state that this storm “represents perhaps the greatest combination of extreme atmospheric elements ever seen in the eastern United States. We feel that this storm is the bench mark against which all other major storms of the 20th century could be compared.”