The wretched polar vortex responsible for last week’s deadly arctic blast is retreating back to the North Pole. In its departure temperatures are on a soaring rebound as the Gulf of Mexico opens its reservoir of balminess for parts of the Eastern US to bask in for up to six days. Some areas will undergo a temperature upswing of more than 60°F early this week compared to last week. Though Punxsutawney Phil did not see his shadow Saturday, this week’s warmth is not indicative of an early spring. Winter cold will return by next weekend.
The active Pacific jet stream is expected to continue to dominate this week as it has all winter. The brutal cold faced by much of the contiguous US over the last few weeks was instilled mainly by the polar jet stream, but influences from the Pacific jet were not wholly eliminated. Storm systems in either branch of the jet stream were often deflected north and/or west of the coastal Northeast due to an eager subtropical Pacific jet phasing (merging) with the polar jet too early and too far west to bring heavy snow to the coast. This tendency has typically favored a storm track across the inland Northeast, a track responsible for the replacement of cold arctic air by warm subtropical air just in time for precipitation along the Northeast coast, with temperatures remaining cold enough for snow along and west of the Appalachian mountains. Consequentially, the I-95 corridor has narrowly escaped several severe winter storms while the interior Northeast and Midwest have been buried by 1-3 feet of snow since the start of the new year.
Now with the arctic cold in retreat, the Pacific jet will temporarily regain status as the only dominant driver of weather across the contiguous US. Its first order of business is to perpetuate southerly flow of balmy air from the Gulf of Mexico across the eastern half of the US. High temperatures will generally be 40-70°F warmer than lows of the recent cold outbreak.
Both the Mississippi River Valley and Upper Midwest have already recovered from the polar plunge. High temperatures in St. Louis Sunday reached 64°F. Just four nights earlier the mercury dipped into negative territory to -6°F, representing a +70°F change since Thursday. Further north, Chicago recently made headlines for cold so bitter that the CTA resorted to burning sections of its railways to thaw frozen switches and to reduce stress on its tracks from compressing steel. Temperatures in the Windy City bottomed out at -23°F Wednesday morning. Sunday temperatures climbed to 45°F and are expected to briefly crack 50°F by Monday, representing a change of at least +73°F.
The warm up has been most dramatic in parts of the Upper Midwest. Cotton, MN, a small town located 36 mi north of Duluth, recorded a high temperature of 23°F Sunday. As this is still below freezing, this temperature is a far cry from true warmth for most Americans. But considering that the actual ambient air temperature–not the wind chill temperature–collapsed to -56 °F Wednesday morning, Sunday’s temperatures represented a nearly 80°F upswing. Elsewhere in the state, temperatures bottomed out at comparatively modest values in the -30s, but the frigid temperatures were disruptive enough to close schools and for energy companies to implore customers to limit their thermostats. So even considering an ongoing snowstorm in the Upper Midwest, any temperature on the positive side of 0 likely feels quite balmy for Minnesota residents.
Unfortunately a cold front will knock temperatures back down to seasonable levels in the Mississippi and western Ohio Valleys Monday. But seasonal will still be quite warm compared to the dangerous cold of the polar vortex.
Temperatures are on the rise along the Eastern Seaboard too, although the temperature swing won’t be nearly as dramatic. Pittsburgh’s middle 50s Sunday follow subzero single digit low temperatures last week and precede temperatures in the low 60s Monday. New York City and Boston, both of which recorded minimum temperatures just a few degrees above 0, will follow a similar temperature pattern as Pittsburgh Monday and Tuesday. It will be even warmer in Washington, DC this week, where low temperatures Thursday morning of just 5 °F will be contrasted with middle 60s Tuesday afternoon.
Mild air will last longer on the East Coast than it will in the Midwest. A weak low pressure system will bring light rain across much of the Midwest through Monday night. It will leave a seasonably cold airmass in its place before departing northeast into Canada along with the core of the seasonable cold. High temperatures will return to the low 30s in Chicago to mid teens in Green Bay by Tuesday. Simultaneously, the light rain will impact the Northeast Tuesday, but only a slight down-tick in temperatures will follow as a result. High temperatures will remain in the low to mid 40s Wednesday from Boston to Washington.
Another surge of mild air will surge northward ahead of a more intense storm poised to develop on the forefront of a large-scale trough in the Pacific jet stream midweek. Warm air will surge one last time ahead of this trough along and east of the Mississippi River from St. Louis to Boston between Wednesday and Friday. Temperatures on the East Coast will be on an uptrend Thursday and Friday, but an easterly flow from the ocean could inhibit the temperature upswing Thursday. This second warm surge will likely be at least as intense as the first warm surge earlier in the week.
The Pacific trough will swing northeastwards throughout the week, bringing a heavy snowstorm to the West and Upper Midwest while delivering a soaking rainfall elsewhere across the eastern half of the US. As the upper-level feature is expected to propagate northeastward, the heaviest precipitation will be focused on the country’s northern tier. The rain may be followed by a brief changeover to snow across the Ohio Valley and inland portions of the Northeast before the storm’s associated cold front ends the week-long sample of spring. Seasonal cold will likely hold until at least the first half of next week.