New England finished up the month of March on a cool note, under the influence of a sprawling upper level storm that has been locking in cool air with periodic showers spread over several days and that doesn’t look to change until this weekend. Of course, that raises the concern: what if this is the story for April?
It would be classic New England for us to have fairly quaint winter and follow it up with a chilly, gray spring. In my monthly forecast delivered on NBC10 Boston and NECN Wednesday morning, however, I shared I don’t see that as being a sure thing this time around. In fact, not only does our exclusive First Alert Weather Team 10-day forecast show our current cool pattern breaking after Saturday, but our team also sees temperatures crossing slightly above normal as soon as Sunday for some, and then staying slightly above normal for most of next week.
Our forecast for a slightly milder than normal April comes despite a fast-moving, active northern branch of the jet stream dipping over the Northeast in a weak “trough” – the name given to dips in the jet stream. Because the jet stream is the fast-flowing river of air, high in the sky, that separates cold air to the north from warm to the south and steers storms near its path, to see it dip south means cool air is usually dipping south, too.
So why not forecast colder than normal temperatures this April? My thinking is this: we end up close enough to the jet stream that disturbances will move fairly quickly, and the most significant moisture available from the Gulf of Mexico will generally get swept south of us, because the fast flow of air aloft over New England will prohibit long stays of moisture.
This time of the year, temperature is dependent heavily on two things: amount of sun and wind direction. With fast-moving disturbances and limited stays of deep moisture, this argues in favor of more periods of sun between disturbances – perhaps frequent showers passing through given our proximity to the jet stream and the disturbances it steers along, but periods of sun between them. The second factor is a bit more tricky. Though fast-moving disturbances often mean a changing wind direction, a jet stream close to New England in a trough in April definitely can favor “backdoor cold fronts” – cold fronts that come in from the north and east instead of north and west – delivering pushes of cool air into New England, particularly Eastern New England, and does make the forecast for above normal temperatures in places like Boston, Portsmouth and Portland a bit less confident.
Some of the answer to our near-normal precipitation forecast can be found in the temperature reasoning above. April is among the top five wettest months of the year for most of New England, so even a forecast of near-normal precipitation means we should be in the mindset for some rain – after all, “April Showers Bring May…” well, you know the saying.
As mentioned above, a fast-moving jet stream dipping over New England is likely to shunt periods of extended deep moisture necessary for prolonged heavy rain events to our south, instead replacing those with rounds of passing showers or moderate rain events. If we don’t see many backdoor cold fronts, described above, the warming atmosphere with frequent fast-moving jet stream disturbances aloft will create favorable conditions for thunderstorms, too, particularly later in the month, if the timing of disturbances should line up with warmest time of the day and this would add to our precipitation totals, though perhaps in more of a spotty or scattered fashion.
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In short, barring potential incursions of cool ocean air explained by backdoor cold fronts, above, there appears to be good reason not to take the last few days of March as a sign of the entire month of April.