It has been a persistently cold winter thus far for much of New England. While it certainly won't go down as a historically cold or harsh winter, it could be said this winter has been a reality check. The past few winters in New England have generally featured multiple major thaws and long stretches of above average temperatures. This winter remains on track to be coldest since the 2017-18 winter for Boston. In Boston, the number of days with a high temperature at or below 32° this winter hasn't been this high since 2014-15.
This week has been no exception to the persistent cold with highs in the teens and 20s, lows in the single digits and wind chills well below zero across the north. This will be changing heading into the weekend and next week, albeit temporarily. A gradual warming trend is now underway across New England, with each day likely getting a notch milder than the last through the early next week.
For many locations in central New England (south of the mountains, north of the Mass Pike), this could lead to the first days with a high temperature at or above 40° in over a month by early next week. While this won't be a significant late-winter thaw, even the northern tier could push above the freezing mark early next week.
This break in winter's chill will come as an increasingly progressive flow across the United States takes hold. After brief downstream blocking for New England, the NAO (North Atlantic Oscillation) Index is poised to rise back to a positive phase. The PNA (Pacific-North American) Index will also remain positive. When both of these index values are positive, it is an indicator of zonal flow across the United States, when no major troughs or ridges are present and the jet stream lays mostly flat across the country.
The EPO (Eastern Pacific Oscillation) is also trending toward a positive phase after being persistently negative in February. When EPO is in a negative phase, it supports deep Arctic air pushing into the eastern and central United States, which has been seen multiple times in January and February. With the EPO, PNA and NAO all shifting to a positive phase, it points toward a Pacific influence spreading across the United States.
Below: NAO and PNA Index values since mid-October. The present day is where the solid line turns dotted (the dotted line represents the upcoming forecast):
When influence from the Pacific increases, it often means more mild air over the Pacific Ocean can spread across the country, especially within a fast, zonal flow, which will be setting up amid the more progressive pattern. This will help temperatures bounce back from being persistently below average.
With that said, a big thaw doesn't look to be coming as a general ridge-in-the-west-trough-in-the-east remains broadly in place across the mid-levels. This points toward truly warmer weather remaining to the west and New England (and the eastern US in general) seeing temperatures closer to seasonal averages. Still, it may feel like a significant warm-up after this week. This pattern can be seen on the Climate Prediction Center's 6-10 day outlook with broad warmth across the west with generally average temperatures favored in the east.
A progressive, zonal flow also tends to support weaker and lower impact storm systems moving across the country. A series of moisture-starved northern stream disturbances will likely cross New England during this time, bringing periods of lower-impact precipitation. The most widespread precipitation event is likely for Monday to Tuesday, when a clipper system crosses the region. The next more organized, large-scale system will likely hold off until at least later next week.
Below: Current 7-day precipitation forecast, continuing to show minimal accumulations for New England through next Thursday morning:
We're now into the final week of meteorological winter, but winter overall is likely not letting go amid this mini-thaw. Arctic Oscillation (AO) is currently in a negative phase. The Index recently bottomed out near -5 a few days ago. This is very strongly negative and supported the major Arctic outbreak across the center of the country this week (New England has been more on the fringes of this outbreak this week). The Index is expected to climb out of this deep negative phase, before quickly dropping once again.
Below: AO Index, which works the same as the NAO and PNA charts above:
This points to a fresh supply of Arctic air to be readily available away from the North Pole. With a pool of Arctic air away from the pole, likely to New England's north heading toward the beginning of March, more cold air intrusions will be likely sometime after the middle of next week. This would come as a broad area of low pressure may lead to general troughing over the northeast toward the start of March.
Below: 500mb geopotential height around the start of March showing a deep trough north of New England. This will be readily available to send colder air southward:
This return to more cold air intrusions is supported by the Climate Prediction Center's 8-14 day outlook (which currently covers February 27 to March 5). It currently shows a generally colder east coast as troughing is likely to become more pronounced for the east. The pattern may become more amplified as well. We may need to watch the timing of temperatures and a storm system around Thursday to Friday of next week (February 27-28).
