On Monday, a powerful storm tore through New England with hurricane-force wind gusts, pounding rain and coastal flooding. When the storm was over, a half foot of rain had fallen in places, over 50 river gauges in New England had reached flood stage and about three quarters of a million customers lost power. Here's how this storm came together and why it was so impactful:
The main storm system took shape in the Gulf of Mexico over the weekend. While this system formed, another disturbance was tracking across the Central Plains. These two systems would interact while the main storm system moved up the east coast. The storm would deepen as it pushed north along the coast. This would allow showers to break out well ahead of the main storm's arrival.
Weather map for Sunday evening:
This storm affected the entire east coast, bringing 7-13 inches of rain to parts of Florida and South Carolina. The storm set new December record low surface pressure across parts of the south.
When the storm complex arrived in the northeast, it would take an interior track across western New England. This would ensure a mild storm with all rain and no snow. The storm would become a "double-barrel low", meaning two areas of low pressure created an elongated center of the storm. This would ensure that rain and wind would last for an extended period of time as opposed to if the storm had one, tightly packed center.
The orientation of the elongated center was also oriented north to south, ensuring that the peak of the storm would last for many hours instead of quickly racing through like the storm in the previous week. The entire storm complex as a whole was strengthening as it moved through the northeast.
The storm complex's central pressure dropped to around 975mb. This came just after a strong area of high pressure was set up over New England. This high had a central pressure of around 1,040mb. Some areas saw a 50-60mb drop in pressure from Saturday to Monday.
Along with the double-barrel low setup, the storm also had other factors that came together to create this historic storm. A big player for New England was the track of the storm. It's already been stated that the track put New England on the "warm side" of the storm. The track would play a pivotal role in both how strong the winds would get and how much rain would fall. In our forecast post for this storm, we wrote:
"A more westward track would push the winds further inland while slightly reducing rainfall totals in western areas. A more eastern track would bump up rainfall, but limit winds."
The storm's track trended a bit further west in the lead-up to the storm, which increased the wind threat across New England. This also placed New England into the warm sector of the storm, which would allow for very heavy rainfall rates across eastern New England.
Around the storm, there were very strong winds aloft. The low-level jet had increased to around 100mph at about 2,500 feet above sea level. This jet's flow was out of the south-southeast. This orientation allowed very warm, moist air from the Gulf of Mexico to push into New England during the storm.
Wind speed forecast from Saturday, December 16 at 2,500 feet above sea level (925mb low-level jet):
This setup not only allowed the very heavy rainfall rates, but also made it easier for some of those strong winds aloft to mix down to the surface. This is evidenced by the 80-90mph wind gusts that were reported in Massachusetts and Maine.
So, it was the combination of the storm's track and setup that allowed for both extensive wind damage and flooding. Along with this setup, other factors went into just how bad the flooding would get. As discussed, very heavy rainfall rates were experienced, with 6.15 inches falling in Jackson, New Hampshire.
On top of this, parts of northern New England had a snowpack. The strong southerly flow sent temperatures soaring to 50-60°, with daily record highs for December 18 set in cities across the region. This allowed for rapid snowmelt adding water to the multiple inches of rain falling from the sky.
The ground was also frozen in places, which made it much more difficult to absorb water. This led to as many as 52 river gauges to reach flood stage across New England, with many seeing top five crests. At least three rivers in Maine set new records for highest crests.
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