Synoptic Evolution
A very fast and progressive mid to upper tropospheric pattern prevailed across the mid latitudes of the U.S. on Thursday, Friday, and Saturday, March 13th, 14th, and 15th. Two strong and energetic short wave troughs moved quickly across the central and then southeastern states during this weekend and interacted with moist unstable atmospheric conditions to produce severe weather. The first of these two short waves moved across the southeastern states Friday evening, accompanied by a broad surface low, and then turned northeastward across the northwest Atlantic, where it quickly deepened to a 990 mb low. The mid latitude upper level flow across the Atlantic immediately prior to this time was depicted by a relatively low amplitude two wave, very progressive pattern, as a positive NAO pattern had dominated the Atlantic for most of the winter season . However, as the second of these two short waves moved into the western Atlantic, the upper flow transitioned to a broad omega blocking pattern across the entire north Atlantic, and effected a brief negative phase of the NAO.

Figure 6: Animated loop of 500 mb heights revealing an omega blocking pattern in the North Atlantic.

The second of these short waves exited the Outer Banks of North Carolina overnight Saturday the 15th, accompanied by a 998 mb surface low and moved east northeastward, directly over the Gulfstream’s warm waters, and continued to deepen through Sunday morning.

Figure 7: Sea surface temperatures showing the Gulf Stream influences in the Atlantic Ocean.

During this time, this deep layered and intensifying low evolved into the western low of a North Atlantic omega blocking pattern. This allowed for explosive deepening of the low Sunday afternoon through Monday evening as it became cut off from the main flow and moved more slowly northeastward, continuing directly over the Gulfstream. By 00Z on Tuesday the 18th this low was analyzed at 964 mb near 41 North 55.5 West.

Figure 8: Animated loop of surface analysis charts depicting the deep low pressure system in the Atlantic.

A broad zone of cold Arctic air was in place from the Great Lakes region north and eastward across North America prior to the passage of the two short waves. This cold polar air, supported by a 1038 mb high over eastern Canada, advected south and southeastward behind the deepening low and contributed to a broad zone of very strong surface winds that exited the northeastern U.S. and spread quickly south southeastward across the northwest and western Atlantic. This also forced a cold front to sweep off the eastern seaboard, across the West Atlantic, through Florida and into the Bahamas. The deep low pressure center then drifted very slowly northeastward over the next 36 hours between 58 and 54 West, while curving cyclonically within the larger deep layered cyclonic circulation. The very slow motion of this low during the period from Monday morning through Tuesday evening, supported by the strong and dense polar air spilling into the northwest Atlantic on its backside, produced a broad zone of 35-50 kt winds that spread from the Canadian Maritimes and coastal New England, south southeastward into the west central Atlantic to 30 North. Within this large wind field, and near the center of the low, remotely sensed wind observations revealed that a narrow zone of hurricane force winds rotated through the western semicircle.

Figure 9: Satellite-derived winds from QuikScat imagery reveal strong counterclockwise rotation around the deepening low pressure system.

The high pressure ridge to the west and northwest of the low shifted slowly east and northeastward in tandem with the low during its passage across the Atlantic to maintain a very tight pressure gradient across the western semicircle of the low. This pressure pattern configuration allowed for a broad and dynamic fetch to set up from the Canadian Maritimes and coastal New England, spreading south and southeastward across the western Atlantic to near 25 North by Wednesday the 19th.

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