12Z GFS loop of 500 millibar winds, heights and vorticity levels in the period from March 2nd to March 10th (at 12-hour increments); courtesy NOAA/EMC
The overall weather pattern for the first couple weeks of March certainly looks colder than current conditions in the Northeast US which isn’t saying much as we’ve been experiencing record-breaking warmth. Nonetheless, there are signs for multiple cold air outbreaks into the Northeast US during the first part of March and these outbreaks will likely be accompanied by “clipper” low pressure systems. “Clippers” are officially known in the meteorological community as “Alberta Clippers” and defined as follows: “a fast moving low pressure system that moves southeast out of Canadian Province of Alberta (southwest Canada) through the Plains, Midwest, and Great Lakes region usually during the winter”. This low pressure area is usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is sometimes called a "Saskatchewan Screamer". We better get used to the term "clipper" around here in the Northeast US because it looks like several of them will head this way during the first ten days or so of March.
“Clipper” systems are generally weak low pressure areas with limited moisture content as they travel along at a high speed and don’t have much opportunity to gather abundant moisture along their usual path (i.e., not near the Pacific Ocean, Atlantic Ocean or Gulf of Mexico). This time of year, however, the contrast in temperatures from the southern US to the northern US typically intensifies as warmer and warmer air masses build in the Deep South with the increasing amounts of sunshine while - at the same time – winter-like cold air masses continue in abundance up to the north. This tightening temperature gradient can, in some cases, contribute to significant intensification of a “clipper” system as it heads towards the Northeast US.
For a “clipper” system to generate snow in the I-95 corridor region from DC-to-Philly-to-NYC during the month of March, it is generally required that the surface low pressure system tracks to the south which allows for the penetration of cold air into the big cities. Often, the “clipper” system will stay to the north of DC, Philly and perhaps even NYC and in these situations the snow and cold is primarily confined to New England – and, by the way, they could get slammed up there during this first ten days or so of March. Every once in a while, however, a “clipper” system will indeed drop far enough to the south and east to produce snow in the big cities along the DC-to-Philly-to-NYC corridor.
Winds in the upper part of the atmosphere over the northern US will soon evolve into a pattern featuring a persistent northwesterly direction and a high rate of speed. This unfolding upper-level wind pattern will change our unusual warm spell of recent days to one with near normal or even below-normal temperatures during much of the first half of March. In addition, this type of fast-moving wind flow in the upper atmosphere will push low pressure systems to the southeast from southwestern Canada into the Northern Plains, Great Lakes and ultimately the Northeast US.
The loop of 500 mb winds using 12Z GFS model data shows a consistent northwesterly flow in the period from March 2nd to March 10th with wave after wave (shown by orange areas) of upper-level energy (i.e., vorticity) flowing right along with the winds from southwestern Canada into the Northeast US. The 12Z GFS surface forecast maps in the same time period feature four separate “clipper” systems – often just a day or two apart – all of whom are currently projected by the GFS model to slide to the north of DC, Philly and NYC. However, it is not impossible for one or more of these systems to end up dropping farther to the southeast than currently predicted which would increase the chances for snow around here.
Meteorologist Paul Dorian