While the East Coast has dealt with three nor’easters in the last two weeks, it appears a fourth is on the way. With the atmospheric pattern favorable for a coastal storm set-up and major global forecast models in agreement that a storm will form, confidence is very high in a high-impact storm next week. And unlike recent past systems, next week’s storm could be a multi-day event across a much broader region.
A nor’easter is a storm along the East Coast of North America, so called because the winds over the coastal area are typically from the northeast. While these coastal storms can form any time of the year, the most frequent, potent storms usually form from the fall through the spring. Beyond the three nor’easters that have hit the eastern US hard this month, other noteworthy nor’easters include the Blizzard of 1888, the “Ash Wednesday Storm” in 1962, and the ’93 “Superstorm”. All of these noteworthy nor’easters have formed in March and it appears at least one more is on the way.
Nor’easters usually develop in the Eastern United States, between Georgia and New Jersey, typically within 100 miles of the coastline. Over time, these storms move north and east up the east coast, reaching maximum intensity off the New England coast. These storms go through a period of organization and intensification known as cyclogenesis. If the intensification is rapid enough and the area of low pressure “bombs” out, it’s known as “bomb-cyclogenesis” or “bomb cyclone” for short. The recent nor’easters have been technically considered bomb cyclones because of this.
These bomb cyclones bring wind-whipped heavy precipitation. Depending on the availability of cold air, the common precipitation type in the winter is snow. Nor’easters also produce strong, potentially damaging winds; the wind field and force of winds in that field are dependent on how deep the low pressure becomes and how close nearby high pressure systems are; the greater the pressure difference across a smaller area, the higher the winds will be. These winds are typically strongest over coastal waters and nearby coastal communities; however, as a nor’easter in early March showed, damaging wind gusts could be felt well inland. Beyond damage, the winds can also toss heavy falling snow or blowing fallen snow about, reducing visibility. If the winds are high enough and visibility low enough, blizzard criteria could be met.
Officially, the National Weather Service defines a blizzard as a storm which contains large amounts of snow OR blowing snow, with winds in excess of 35 mph and visibilities of less than 1/4 mile for an extended period of time (at least 3 hours). When these conditions are expected, the National Weather Service will issue a “Blizzard Warning”. When these conditions are not expected to occur simultaneously, but one or two of these conditions are expected, a “Winter Storm Warning” may be issued. In the last nor’easter to strike New England, blizzard criteria was reached across much of the coastal region including the city of Boston. It is possible the next system for next week will also create blizzard conditions for some along the Mid Atlantic and/or Northeast regions.
A complex weather pattern will set the stage for the next nor’easter. On Monday, high pressure is forecast to extend southeastward to over the northern Mid Atlantic. While the high does that, significant energy ejecting eastward from the Central Plains will drive a surface low pressure into the lower Ohio Valley by later Monday. This will back the flow aloft some in the East. Clouds are expected to increase toward daybreak Tuesday with the increasing chance for some precipitation from southwest to northeast. As significant energy rolls eastward, the upper-level trough is forecast to amplify into the East with the potential for some phasing. This will take low pressure across the Mid-Atlantic region northeastward off the coast through late Tuesday. The amount of short wave energy interaction, which also involves a more organized system aloft undercutting and moving in tandem with a progressive ridge moving through southern Canada to its north, will ultimately determine what happens during the middle of the week. The National Weather Service’s Weather Prediction Center is forecasting that a secondary low will develop near the North Carolina coast early Tuesday morning. The track and timing of this storm will depend on how much upstream blocking is still in place. It is this blocking that helped form the recent nor’easters and raced them up the northeast coast. The upstream blocking will impact the thermal fields and therefore precipitation types and amounts for the storm. For Wednesday and Thursday, computer forecast guidance is suggesting that the storm will slow down as an upper level trough becomes more amplified in the East. This would take low pressure off the coast northeastward with precipitation lingering into Wednesday. Low pressure is currently forecast to be off the New England coast to start Thursday with cyclonic low-level flow occurring across the Mid-Atlantic. The strength of the storm will determine the magnitude of the winds across the northeast later Wednesday and especially Thursday.
As more data is evaluated, meteorologists will be better able to forecast the storm outcomes. Currently, morning runs of the American GFS, Canadian GEM, and European ECMWF all show a major winter storm unfolding. The American and Canadian forecast models put the Mid Atlantic area, especially New Jersey and Pennsylvania, under the bullseye for heavy snow in excess of a foot. The European model has a somewhat different storm set-up, with the heaviest snow stretching from West Virginia to Boston, with a widespread one foot or more snowfall along and to the north of the I-95 corridor. As the event nears, it is likely that these forecast models will become better aligned with each other, giving meteorologists more confidence in storm specifics.
Until confidence in storm specifics increases, it is still too soon to determine where the rain/snow line will set-up, the precise start/end times for precipitation, and how bad wind and coastal storm flooding threats will be. It is likely those details will come into focus by the end of the weekend.