UPDATED 4 p.m. ET: The National Weather Service upgraded the blizzard watch to a blizzard warning for Boston, which is in effect from Saturday at 7 p.m. ET to Sunday at 11 a.m. ET. The blizzard warnings and watches stretch from Cape Cod all the way to the border between Maine and Canada. The NWS is forecasting between 10 to 14 inches of snow in Boston on top of the three to four feet already on the ground, and is also warning of a life-threatening combination of powerful winds and cold temperatures during and in the wake of the storm through Sunday.

The powerful Valentine's Day storm set to blast eastern New England this weekend with roaring, frigid winds, heavy snow and pounding surf will be so strong that it can be compared in some ways to a Category 2 hurricane.

Fortunately, though, it will not bring the same impacts as a hurricane of that intensity, but its effects on multiple locations — from Providence and Boston to Portland and Bangor, Maine — will be similar to a winter hurricane, with power outages, tree and structural damage, and coastal flooding. Depending on the storm's exact track, it could dump a foot or more of additional snow in the Boston area, with even more snow in coastal New Hampshire and Maine.

See also: Yeti spotted roaming the streets of Boston during blizzard

Snow statistics for Boston, including snow totals, assuming another 12 inches falls this weekend. Image: Bob Al-Greene/Mashable

The storm is set to sock a region that is reeling from a nonstop barrage of epic snowstorms, with about 100 inches of snow on the ground now in parts of Maine, and 3 to 4 feet in the Boston area. These snow depths have never been seen before, since records in these areas began in the late 19th century.

In many parts of eastern Massachusetts, the relentless pace of snowstorms have caused two- to three-lane roadways to narrow into one-lane games of chicken between drivers going in opposite directions, and Boston road crews have been frantically trying to widen streets before the next storm arrives.

Boston, which has never experienced a winter with more than one storm that had verified blizzard conditions, is now gearing up for its third such storm in just the past three weeks.

In the first 12 days of the month, Boston set a record for the snowiest February. What's more, the next storm system — which may dump up to a foot of snow on the city — could vault the city into the top five snowiest years, with plenty of winter left to come and no signs of a weather-pattern change. This storm will almost certainly lead Boston to set the snowiest month on record, beating January 2005.

The National Weather Service has issued blizzard watches from Cape Cod to the Maine coast, all the way to the Canadian border. Blizzard conditions are defined as a minimum of three straight hours of heavy snow and/or blowing snow, with visibilities of one-quarter mile or less, along with winds of at least 35 miles per hour or greater.

A windier, wilder storm

The upcoming storm will set itself apart from previous storms in Boston's snow blitz of 2015 by being far windier than the others were, moving through the region faster, and being accompanied by frigid, downright dangerous cold temperatures.

Much of the snow will fall at a time when wind chills will be below zero. What's more, winds that may gust to hurricane force (74 miles per hour) along the coast could cause power outages that would knock out heat to many area residents just when they need it most.

Trucks dump snow in a "snow farm" maintained by the city of Boston due to the mounting snow on area roadways. Image: Kit Block for Mashable

The storm will not likely dump as much snow in Boston as the previous 20 inch-plus storms did, with up to a foot of snow expected, rather than 2 feet-plus accumulations. Two feet of snow is possible, however, in areas from Portland, Maine, northeastward into the Canadian Maritimes.

Due to its intensity, the storm will also have a large footprint, with winds gusting to 60 miles per hour possibly all the way south to Virginia, including Washington, D.C., Philadelphia and New York. So while these cities will escape heavy snow, they won't be immune to all of the storm's hazards.

[Sunday] Pressure avg @ 970 mb, typical w/ cat 2 hurricanes; dangerous winds which would cause structural damage / widespread power outages — NWS Boston (@NWSBoston) February 13, 2015

After the storm moves past New England by late Sunday, the coldest air of the season will plunge southward out of the Arctic, leading to the coldest temperatures seen in 20 years in parts of New England and the Mid-Atlantic states. New York City, for example, may see an overnight low temperature of 0 degrees Fahrenheit or colder on Sunday night — the first time that's happened since 1994.

Meanwhile, most locations in New England will see overnight lows below 0 degrees Fahrenheit, with daytime highs barely making it out of the single digits.

@ericfisher @NWSBoston avalanche watch w/ blizzard warnings ? this is behind our office today in Gloucester, Mass. pic.twitter.com/jNXTIXu9XF — Deven (@Bellhorn12) February 13, 2015

What is causing these storms?

Mother Nature's shock-and-awe campaign of snowstorms in New England has largely resulted from a stubborn weather pattern featuring a broad area of high pressure over the West, which has led to record warmth there, and a deep dip, or trough, in the jet stream across the eastern U.S.

Disturbances embedded in this jet stream, which is the high altitude river of air that blows from west to east across the Northern Hemisphere, steering and giving rise to storm systems in the process, have repeatedly led to the explosive development of low pressure areas at the surface off the coast of Long Island and Cape Cod.

These storms are powered by temperature differences across short distances, which meteorologists call baroclinic low pressure systems. Aiding in the rapid development process have been unusually mild sea surface temperatures off parts of the East Coast, although it's entirely likely the storms would have formed anyway, and been intense, even without those temperature anomalies.

The Gulf Stream current, which carries warm water off the East Coast and into the North Atlantic, has long been an ignition source — like a match thrown onto a gasoline spill — for northeastern winter storms in particular. This year has been no exception.

Sea surface temperature anomalies for Feb. 13, 2015, showing the presence of milder than average water off parts of the East Coast.

The weekend storm will result from an unusually powerful packet of upper-level energy descending into the U.S. from Greenland. A low pressure area at the upper levels of the atmosphere will produce large-scale lifting of air, which will help form clouds and precipitation, and spark a strong low pressure area at the surface by Saturday afternoon.

The storm will be aided in its development by a phenomenon known as a tropopause fold, which occurs when the boundary between the troposphere and the stratosphere is bent downward in the presence of an area of extremely strong jet stream winds known as a "jet streak." A tropopause fold can leading to large-scale ascent of air nearby, which encourages the rapid strengthening of a low pressure area.

Computer models have been hinting at this for days, lending confidence to forecasters who are calling for an intense blizzard for coastal New England.

A global warming role?

It's also possible that the unprecedented snow blitz in the Boston area and other parts of New England is being assisted in part by manmade global warming, which is known to increase the odds for precipitation extremes, including major snowstorms.

In the past several decades, the Northeast has seen the sharpest uptick in the number of extreme precipitation events, both rain and snow, of any area in the country. This increase has been tied in part to global warming and the related worldwide increase in water vapor in the air.

Some climate scientists say that the milder-than-above-average ocean temperatures off parts of the East Coast this winter have helped this give the winter storms an extra boost, turning what would have already been potent storms into record-breaking ones.

However, no peer reviewed studies have yet been produced to tease out global warming's possible influence in the record snows so far this winter. Some studies of trends in these types of storms, including findings published in the U.N. Intergovernmental Panel on Climate Change's 2014 report, have not turned up statistically significant trends pointing to a global warming influence on Northeast snowstorms.

A more recent study suggests that a warming climate may lead to fewer cold season storms in the Northeast, but that each storm may drop more precipitation on average than currently occurs.

Global warming is already leading to more severe extreme-weather events in general on larger geographic scales, and this trend is expected to continue to ratchet up in the coming decades. So, although the evidence is somewhat lacking now, it may not stay that way for long.