Bomb Nor'easter Bringing Major Flooding, Damaging Winds

The pressure was plummeting and the damaging wind gusts near hurricane force were ramping up early Friday afternoon, as a huge and powerful nor’easter rapidly intensified off the Northeast U.S. coast. “Bombogenesis” occurs when the central pressure of a storm drops at least 24 millibars (mb) in 24 hours, and Friday’s “bomb” cyclone—dubbed “Riley”—achieved that standard, with the central pressure falling 30 mb in 24 hours, to 970 mb early Friday afternoon, when its center was southeast of Nantucket Island, Massachusetts. The storm’s central pressure will fall just a few more millibars through Saturday, but the winds will continue to increase through Friday night as they adjust to the rapid pressure fall that occurred Friday morning.

That yellow dot is Lt Gillan of our Marine Unit. Just to give you an idea of the depth of this water - he’s over 6ft tall. pic.twitter.com/IV31fQqPh2

— Quincy Police (@quincymapolice) March 2, 2018

Dangerous coastal flooding

Riley’s winds on Friday were gusting in excess of 50 mph over a huge swath of the coast, from Maine to Virginia, and these winds piled up a large storm surge that caused widespread coastal flooding during the late morning/early afternoon high tide cycle. The flooding was exacerbated by the highest tides of the month, due to the effect of the full moon. A storm surge of three feet along with a high astronomical tide more than a foot above average combined to bring a water level in Boston that peaked at 4.4’ above Mean Higher High Water (MHHW) at 11:12 am EST Friday, during the morning high tide cycle. This was the third highest water level ever recorded in Boston, and was close to a 1-in-100-year event. According to NOAA, a 1-in-100-year storm tide with a 1% chance of occurring in a given year in Boston is a water level of 4.59’ above Mean Higher High Water. Since records began in 1928, the only higher storm tides in Boston were 4.88’ above MHHW on January 4, 2018 in Winter Storm Grayson, and 4.82’ above MHHW on February 7, 1978, during the infamous Blizzard of ’78. The water level was falling on Friday afternoon thanks to the tide going out, but the surge appears to be slowly increasing, and two more near-record storm tides are expected in Boston--late Friday night, and near midday on Saturday.

During the late morning/early afternoon high tide cycle along the U.S. East Coast, a total of 29 tide gages from Portland, Maine to Cape Hatteras, North Carolina were in “high water condition”—water levels at least 1.5’ above the expected water level, due to the tide alone. The highest storm surge was at Chatham, Massachusetts, on the east side of Cape Cod, where a storm surge of 3.55’ was observed at 11:42 am EST. Here are a few storm tide readings from Friday's late morning high tide cycle (see NOAA’s Quicklook page for a nice summary):

Portland, ME: 3.21’ (10^th highest on record)
Nantucket, MA: 3.05’ (6^th highest on record)
Montauk, NY (east end of Long Island): 2.67’
King’s Point, NY (west end of Long Island Sound): 3.12’
New York City, NY: 2.15’
Atlantic City, NJ: 2.17’
Cape Hatteras, NC: 2.72’

Figure 1. Predicted water levels in Boston due to the combined effects of the tide and storm surge (black line with “X” marks), along with the expected water level due just to the tide (blue line), using a storm surge model driven by the 6Z Friday, March 2, 2018 winds from the GFS model. The difference between the black curve and the blue curve is the storm surge (gold line). MLLW, MSL, MHHW, and MAT stand for Mean Lower Low Water, Mean Sea Level, Mean Higher High Water, and Maximum Astronomical Tide. MAT is the maximum tide that will occur in a 19-year span, and there is probably flooding if the total water level crosses MAT. Boston has already seen one storm tide of 4.4’ above MHHW, which was the third highest water levels ever recorded. Boston is predicted to see two more storm tides about 4 – 4.5’ above MHHW. Fortunately, it appears the highest storm surge of about 3.8’ will arrive Friday afternoon as the tide is falling. Boston's record-high water level was set earlier this year--4.88’ above MHHW on January 4, 2018 (previous record 4.82’ on 2/7/1978 during the infamous Blizzard of ’78). Boston’s period of record (POR) extends back to 1928. Image credit: NWS.

Damaging winds, heavy rain, and heavy snow

The nor’easter’s strong winds and heavy, wet snows combined to cause power outages to over one million customers from the Great Lakes to New England by Friday afternoon. Winds gusted to 71 mph at Washington-Dulles and 62 mph at DC's Reagan National Airport Friday morning, and a 74 mph gust was clocked atop Chickaree Summit, Pennsylvania. In Massachusetts, a wind gust of 78 mph was recorded at White Crest Beach on Cape Cod, 77 mph at Woods Hole, 72 mph at E. Falmouth, 71 mph at Marstons Milles, 64 mph at Barnstable, 64 mph at Nantucket, 63 mph at Newbury and Aquinnah, and 56 mph at Boston’s Logan Airport.

The town of Wyoming, New York, east of Buffalo, picked up 24 inches of snow in just 24 hours as of Friday morning. Other notable heavy snow totals as of Friday morning included 14 inches in Harborcreek, Pennsylvania, and Windham, New York, 12 inches near North East, Pennsylvania, 12.1 inches at Buffalo-Niagara International Airport, and 10.2 inches just west of Albany, New York. Riley is also bringing heavy rain to Rhode Island and Southeast Massachusetts; storm total rain amounts of 2 – 4” will cause some area rivers to go to at least minor flood stage over the weekend.

Please avoid Arsenal St until further notice. pic.twitter.com/sCKFJK8aqt

— Watertown Police (@WatertownPD) March 2, 2018

A long-duration nor’easter

Unfortunately, a strong and slow-moving ridge of high pressure (a “blocking high”) over Greenland, which has brought an incredible flow of warm air northwards into the Arctic with above-freezing temperatures at the North Pole, will keep this weekend’s nor’easter from making much progress eastward. As a result, the Northeast U.S. coast will receive a punishing assault from a large storm surge and high waves that will last through a total of three high tide cycles.

In Eastern Massachusetts, major flooding is expected to occur for two more high tide cycles: near 11 pm Friday and 12 pm Saturday, when waves of 25 – 35 feet are possible just offshore. This stretch of coast is unusually vulnerable to erosion, thanks to the terrific pounding wrought by the storm surge and wave action of Winter Storm Grayson on January 4. In one case, Winter Storm Grayson helped wash away 60 – 75% of 110,000 cubic yards of sand added in 2016 to Town Neck Beach (about 90 miles southeast of Boston). This leaves the beach highly vulnerable to a renewed assault by this weekend’s storm.

Minor to moderate coastal flooding is expected along most of the Mid-Atlantic coast, from Rhode Island to Cape Hatteras, North Carolina, and along much of the coast of Maine.