We’re heading into yet another raw, wet, and overall miserable weekend. Make that five weekends in a row without the sun, including this Saturday and Sunday. The last time the Boston area saw a dry weekend with some sunny moments was March 8-9.
Why can’t we seem to get a nice, warm weekend this spring? Is it bad luck, or are we stuck in a crummy weather pattern?
We saw a similar pattern develop last year with most of New England starting 2024 with rainy weekends right through May.
So what’s going on?
A natural pattern
Naturally, this kind of wet weather is part of spring. The season brings a more active weather pattern across the country because the sun angle increases over the region.
The additional solar heat makes for larger temperature differences between cold and warm air masses. These air masses are divided by the jet stream high up in the atmosphere and by frontal boundaries at the surface. With more variation between adjoining air masses, more frequent and sometimes more intense storms can develop.
We all know the saying, “April showers bring May flowers,” and that holds mostly true for the spring season, especially across New England with the Atlantic Ocean sitting just to the east, which can influence storm track, duration, and intensity.
Slightly more than three-quarters of an inch of rain has fallen over the past four weekends in Boston. Roughly the same amount is expected from this weekend’s system.
‘Stuck’ spring westerlies pattern
The “prevailing westerlies” are winds moving from west to east across the globe within the mid-latitudes, including the Lower 48 of the United States. They are the underbelly of the jet stream and move along in a wavy pattern, directing surface storms from west to east across the country.
The bending in the westerlies is due to geography — the sun heats differing terrains unevenly, which results in varying air mass size, temperature, and movement rates. The bending and bowing are more dramatic in the spring because of the increasing sunshine, even if the sun doesn’t always get through to us.
The southward bending within the westerlies are also called “Rossby waves” and help set up high-pressure ridges and low-pressure troughs.
At times, when the westerlies bend drastically, wind speeds correspondingly slow down, and the pattern may get “stuck,” resulting in persistent storm development and tracks for extended periods. Think of trains on a track, taking the same path in intervals.
A pattern like the one below develops. Storms often live in the bottom (the southernmost point) of the bends (or troughs), allowing meteorologists to gauge storm frequency over a given location.
La Niña speeding storms up
A weak La Niña emerged in early December and has played a small role in the type and frequency of weather New England has seen so far this year. The influence has been minor but still pretty pertinent.
El Niño and La Niña are the warm and cool phases of a naturally occurring climate phenomenon across the tropical eastern Pacific that oscillates back and forth every two to seven years, greatly influencing and disrupting the weather across the US and worldwide.
La Niña makes the jet stream even more variable than normal during spring. But most importantly, La Niña amplifies the jet stream, making for more frequent storms and cold air outbreaks — which is spot on with what New England has seen so far with storms passing through or near the region every couple of days this spring.
The precipitation records in February show that every single weekend during the month saw rain and/or snow at some point.
The National Oceanic and Atmospheric Administration (NOAA) recently declared La Niña over as the pattern in the equatorial Pacific transition to more “neutral conditions” this spring and summer, which should restore a bit more balance and breaks between storms.
It is spring, after all. It would be nice to see some sunshine during the weekend in mid-April, where a typical sunny day can deliver highs in the mid-50s.
Ken Mahan can be reached at ken.mahan@globe.com. Follow him on Instagram @kenmahantheweatherman.