Snow has begun falling from New York City to Massachusetts, where blizzard warnings are flying in anticipation of the arrival of one of the most severe and dangerous Nor’easters in U.S. history. The great storm, dubbed “Nemo”, has just emerged into the waters off the coast of Virginia, and is predicted to “bomb” to a central pressure of 975 – 980 mb by Saturday afternoon. Cold, Arctic air spilling southwards behind a strong 1038 mb high over Canada will collide with warm, moist air over the Atlantic, where ocean temperatures are unusually warm–about 5°F warmer than average over a large swath from New Jersey to Nantucket, Massachusetts. The contrast between the cold and warm air will help intensify the storm, and the unusually warm waters will pump large quantities of moisture into the air, which will be capable of feeding record-breaking snows over New England. The latest NWS forecast for Boston calls for 22 – 30″ of snow by Saturday morning, with additional snows though Saturday afternoon. Since Boston’s all-time heaviest snow storm is 27.5″ (February 17-18, 2003), Winter Storm Nemo has a chance of exceeding that. According to NWS, here are the top snowstorms since 1936 for Logan Airport:

1. February 17-18, 2003 27.5″
2. February 6-7, 1978 27.1″
3. February 24-27, 1969 26.3″
4. March 31-April 1, 1997 25.4″
5. January 22-24, 1945 22.8″
6. January 22-23, 2005 22.5″
7. January 20-21, 1978 21.4″
8. March 3-5, 1960 19.8″
9. February 16-17, 1958 19.4″
10. February 8-10, 1994 18.7″
11. January 7-8, 1996 18.2″
11. December 20-22, 1975 18.2″
11. December 26-27, 2010 18.2″

The weight of all that heavy snow on rooftops will create the danger of roof collapses. In addition to the heavy snow, the storm will bring coastal wind gusts over hurricane force, and moderate to major coastal flooding. During the peak of the storm, Friday night into Saturday morning, snowfall rates of 2 – 3″ per hour can be expected. These intense bursts of snow may be accompanied by lightning and thunder. The cites of Hartford, Providence, and Portland are all likely to get more than a foot of snow, and two feet of snow will probably fall along a swath from South Central Connecticut to Southwest Maine, with isolated amounts of 3′. Ferocious sustained winds near 50 mph will occur at the coast, with wind gusts in excess of hurricane force–74 mph. The combination of heavy snow and high winds will make travel extremely dangerous or impossible, with near-zero visibility in white-out conditions. The snow and high winds are likely to cause many power outages.

Figure 1. Predicted snowfall for Winter Storm Nemo from Friday’s 00Z run of the European (ECMWF) model. The highest snowfall amounts (> 24″) are predicted for Long Island, Southern Connecticut, and Eastern Massachusetts, including Boston. This forecast assumes that the ratio between liquid water equivalent and snow depth will be 10:1. In some areas away from the coast, this ratio may be closer to 15:1, leading to snow amounts near 36″.

Figure 2. Predicted wind speeds in knots at 7 am EST Saturday, February 9, 2013, from the 00Z February 8, 2013 run of the European (ECMWF) model. The model is predicting sustained winds of 50 knots (57.5 mph) will be just offshore of Cape Cod and Nantucket Island, Massachusetts. Multiply by 1.15 to convert knots to mph.

Serious coastal flooding expected in Massachusetts
The high winds from the storm will drive a damaging storm surge of 2 – 4′ along the coast of Eastern Massachusetts Friday night and Saturday morning. High tide Friday night will occur between 9:30 – 10 pm EST, and minor to moderate coastal flooding is expected along east and north-facing shores, when the storm surge of 2 – 3′ rides in on top of the tide. Battering waves of 8 – 17′ will hit the coast south of Boston in Cape Cod Bay, causing severe beach erosion. Of greater concern is the flooding that will occur during the Saturday morning high tide cycle, as that is the time of the new moon, which will bring the highest tide of the month. The ocean’s height near Boston varies naturally by about ten feet between low tide and high tide, so it matters greatly when the storm surge arrives, relative to the tidal cycle. Thus we speak of the “storm tide”–how high the water gets above the high tide mark, due to the combination of the storm surge and the tide. During Hurricane Sandy, on October 29, 2012, a potentially very damaging storm surge of 4.57′ hit Boston, but arrived near low tide, so the water level during the peak surge did not rise above the normal high tide mark. Fortunately, it appears that the peak storm surge from Nemo will arrive at the time of low tide early Saturday morning, and the surge will have fallen about a foot by the time the high tide arrives near 10 am EST Saturday. As of 9am EST on February 8, 2013, the latest storm surge forecast from the GFS model was calling for a storm tide of about 3.4′ above high tide (MHHW, Mean Higher High Water) in Boston on Saturday morning. This would cause minor to moderate flooding in the city, and would be approximately the 10th highest water level on record. The official top 5 storm tides since 1921 at the Boston tide gauge, relative to MHHW, are:

1. 4.82′ – February 7, 1978 (Blizzard of 1978)
2. 3.92′ – January 2, 1987
3. 3.86′ – October 30, 1991 (Perfect Storm)
4. 3.76′ – January 28, 1979
5. 3.75′ – December 12, 1992

More serious flooding is expected in Cape Cod Bay to the southeast of Boston, where the northeast winds from the storm will pile up a higher storm surge. A storm surge of 3 – 4′ is predicted from Scituate to Sandwich Harbor Saturday morning. The surge will be accompanied by battering waves 18 – 26′ feet high, and major flooding and significant coastal erosion is expected. Major coastal flooding is also expected on the east end of Nantucket Island.

Severe beach erosion is also expected along the north and northeast facing shores of Long Island, NY, where a storm surge of 3 – 5′ will combine with 4 – 8′ breaking waves. Minor to moderate coastal flooding is expected along western Long Island Sound. New York City is expecting a 2 – 4′ storm surge, which will cause mostly minor flooding, with a few areas of moderate flooding.

Figure 3. Coastal flooding hazards during the high tide cycle on Saturday morning, February 9, 2013, as predicted at 5 am EDT Friday, February 8, 2013, by the NWS Boston.

from:    http://www.wunderground.com/blog/JeffMasters/article.html