The recent June-like warmth across much of the Eastern US will feel like a distant memory by this weekend. Temperatures will tumble almost everywhere east of the Mississippi River throughout the remainder of the week as the Polar Vortex steadily dives toward the Northeastern US. Its arrival will generate snow showers across portions of the Great Lakes and Northern New England as it establishes a weather pattern that will feature a persistent chill through at least the middle of May.
Cool weather will already begin trickling into the East throughout the week. Temperatures will be 5-10°F below normal each day through Friday, after which temperatures will dramatically drop behind a much stronger cold front associated with the arrival of the Polar Vortex. The arctic air mass will reach the Upper Midwest Friday and the Eastern Seaboard Saturday.
How dramatic will the polar plunge be? According to the North American Ensemble Forecast System (NAEFS), an international blend of 40 different weather models, the cold will break climatological precedent since 1979. Depicted in the below graphics from the NAEFS are mean temperature (contours) and temperature percentiles from 1979-2009 climatology (shaded) for 5 different vertical levels of the atmosphere. 1000 mb is just above or below the surface whereas 200 hPa is generally 7-8 miles above the surface.
Light purple shading represents areas where the mean temperature forecast of the NAEFS is below all temperatures in the 1979-2009 climatology for the three week period centered on May 9. In other words, the light purple represents temperatures lower than all temperatures observed at the corresponding pressure level from April 29 and May 19 between 1979 and 2009. The dark purple shading represents temperatures colder than those ever observed only at 12:00 UTC or 8:00 am ET.
What all this translates to is that cold records will be smashed this weekend across Michigan, Ohio, Pennsylvania, New York, and New England. Clouds and precipitation will keep temperatures warmer near the coast Friday night and Saturday morning. High temperatures Friday in the Midwest and Saturday in the Northeast away from the coast will struggle to climb out of the 30s and 40s, representing anomalies of 15-25°F below normal. Growing season has already begun across most of the afflicted area so unprotected crops will suffer extensive damage each night with the plunge of temperatures well below freezing. Lows will dip into the 20s across the Upper Great Lakes, the interior Northeast, and southern Appalachia as far south as North Carolina’s Blue Ridge Mountains.
As if temperatures well below freezing were not unusual enough for May, the threat of accumulating snow across the Great Lakes, Upstate New York, and New England could make for an extremely rare white Mother’s Day. May snow is rare but not unheard of in Upstate New York or New England, where on May 9-10, 1977 parts of Massachusetts and Connecticut observed one to two feet of snow. The snow expected this May 8-9 is not expected to be nearly as heavy but it will nonetheless be equally startling, as most of the snow-stricken area failed to observe a white Christmas last year.
The snow will follow a period of steady rain Friday and Friday night. As the arctic front pushes through the Eastern US, temperatures will quickly drop to allow for rain to mix with or completely changeover to snow from west to east in northeast Ohio, northern and central Pennsylvania, Upstate New York, and interior New England, including western Massachusetts and northern Connecticut. The low will redevelop off the New England Coast Saturday and spread heavy snow across portions of Northern New England through Saturday night. While the snow could be heavy at times, it will not accumulate on roadways until late overnight, mainly in higher elevations.
Behind the low pressure system, strong northwest winds will blow over the Great Lakes, generating Lake Effect Snow showers across Michigan, northeast Ohio, western Pennsylvania, and western and central New York away from immediate lake showers. These snow showers will not accumulate much during the daylight hours due to the strong sun angle but they could reduce visibility, making for hazardous driving conditions.
The Polar Vortex is an uncommon visitor in the winter, let alone the spring. Why has it drifted so far south when the pattern should be favoring warmer weather? It has to do with the conditions that support the development of the Polar Vortex in the first place. The Polar Vortex is a large-scale upper-level area of low pressure residing near the arctic that rotates counter-clockwise. The cold temperatures in the arctic result in low pressure in the upper layers of the atmosphere. Since cold air is more dense than warm air, the colder it is at the surface, the lower the pressure is above the surface.
This winter the arctic was colder than it has been in recent years. According to NASA, there was “persistent arctic cold” in the Arctic which led to the highest sea-ice extent since 2013 (although still the 11th lowest on record). Sea ice radiates heat faster than water, so that there was more sea ice helped the arctic cool faster throughout the months’ long polar night compared to open ocean. Since surface temperatures cooled faster, so too grew stronger the Polar Vortex earlier in the winter.
Meanwhile, mid-latitudes experienced a very mild winter. The arctic’s close proximity to warmth created a strong temperature gradient that bolstered the Polar Jet stream, which essentially “locked” the Polar Vortex in place over the arctic this winter. With the sun finally rising again over the Arctic in March, solar heating gradually warmed temperatures and weakened the Polar Vortex and thereby weakening the jet stream, allowing it to “buckle” southward in a wave-like fashion into the mid-latitudes of North America. Strong high pressure over Western North America and over Greenland has essentially forced the buckling of the polar jet stream to continuously focus over eastern North America since there is nowhere else for it to go. This weekend will be especially cold compared to normal because this will be the first time this spring that the Polar Vortex itself will be displaced over the Northeast.
Unfortunately the blocking pattern that has focused the buckling of the jet stream and influx of cool air into the Eastern US is expected to remain in place for the next few weeks. That doesn’t mean every day will be cool, but it does mean it will be a few more weeks before persistent warmth returns.