DAYS 1-5 (APRIL 29 – MAY 3)
A frontal boundary will be spending most of its time in or at least close enough to have significant impact on the weather in the southeastern New England region (aka the WHW forecast area) during this entire 5-day period. It’s already played havoc with my own forecast made just yesterday morning, within hours of me posting it, when it became sunnier and warmer than I thought just a few hours in advance. I don’t think too many people will complain about a nicer spring day than had been expected, so there is that! My idea had been that today would turn out like yesterday did, and yesterday would have been more like today is likely to be. I don’t think I’m going to get surprised with nice weather today though, as the overcast will be hanging around and there will be an increasing chance of rain showers as we go through the day, so that by later this afternoon wet weather is pretty widespread across the region. This in response to the third in a series of disturbances / low pressure waves riding along the frontal boundary, which separates Canadian coolness from springtime warmth to the south. Model indications have been that if we’re going to bust into the warmer air on the other side, that we may have to wait until Monday, but yesterday that was a day 6 forecast, and do I need to repeat how those can go? Today’s guidance is not so sure about the Monday warmth, with some models “deciding” that the frontal boundary is going to stay down to the south. From a pure temperature forecasting standpoint, this could make a difference of at least 20 degrees in the temperature at some locations. Just for an example, a warm push on Monday with a west or southwest wind could drive Boston’s temperature toward 80, while being on the other side of the boundary with an east or northeast wind could hold them closer to 55 or even lower, depending on cloud cover. So not only does the forecast challenge include timing and coverage of any precipitation, but also taking into account temperature and wind direction based on the forecast position of that front. Rewind a bit to Friday. After today’s general onshore flow keeps it cool, we get “milder” for a while as the wind flow shifts to a strong land breeze during Friday, with many areas breaking 60 for high temperatures. But did you notice I said “precipitation” and not just “rain” earlier? Yes, there is a reason. On Friday, when the third disturbance pulls away from the region, it will be intensifying and there will also be a lot of cold air aloft with an upper level disturbance that still has to make its way through the region. A lobe or two of energy rotating around the back side of the departing low will create some showers, and these can fall as rain, mix, or snow during the late night hours of Friday and early morning of Saturday. This will likely be dependent on elevation and/or intensity of precipitation. So while it’s highly unlikely anybody will see accumulating snow, it is distinctly possible that parts of the region may see very early May flakes. During the day Saturday we’ll see nothing more than a passing additional rain shower in a few locations, otherwise it should be dry but breezy and rather chilly for the start of May. Many times when we have a day like that, the next day is likely to be tranquil and somewhat milder as high pressure builds in. Nope. Not this time. This is such a fast-moving pattern right now and the frontal boundary is not getting that far south of us Saturday that it’s ready to be pulled back northward Sunday by another approaching disturbance. It’s going to be tough to time this one for a couple days, until we get closer. Some guidance has it moving more rapidly than other guidance. But the idea is that Sunday will likely feature a fair amount of cloudiness and possibly the threat of some light rainfall at some point. And this leads us to the day 5 dilemma for Monday. A faster-moving disturbance would drag the boundary back to the south during Sunday and yet another approaching disturbance from the west could push it back to the north Monday, at least giving us a shot at getting into the warm air. A slower-moving disturbance would drag the front back to the south as it departed Sunday night into Monday, making it more likely we’d be on the cooler side for Monday. This is one of those times when I am just going to say I have no idea how this is really going to play out yet, so I’ll write the forecast below to cover this uncertainty as best I can and then re-evaluate for the next update. That’s about all I can do from a forecasting standpoint right now…
TODAY: Cloudy. Isolated to scattered rain showers through early afternoon. Scattered to numerous rain showers becoming widespread by late day including a slight chance of thunderstorms. Areas of fog. Highs 48-55 eastern coastal areas and Cape Cod, 56-63 elsewhere. Wind E up to 10 MPH.
TONIGHT: Cloudy, areas of fog, widespread rain showers with a chance of thunderstorms evening. Mostly cloudy with scattered rain showers overnight. Lows 45-52. Wind E up to 10 MPH evening, N 5-15 MPH overnight.
FRIDAY: Variably cloudy. A passing rain shower possible. Highs 58-65. Wind NW 10-20 MPH but gusting 30-40 MPH at times.
FRIDAY NIGHT: Variably cloudy. A passing rain or snow shower possible overnight. Lows 35-42. Wind NW 15-25 MPH, higher gusts.
SATURDAY: Sun/cloud mix. A brief rain shower possible. Highs 52-59. Wind NW 10-20 MPH with higher gusts.
SATURDAY NIGHT: Mostly cloudy. Light rain possible overnight. Lows 40-47. Wind variable 5-15 MPH.
SUNDAY: Mostly cloudy. Slight chance of rain. Highs 52-59. Wind variable 5-15 MPH.
SUNDAY NIGHT: Variably cloudy. Slight chance of rain. Lows 43-50. Wind variable to W 5-15 MPH.
MONDAY: Partly sunny. Highs 60-67 southern NH and northern MA, 68-75 to the south except cooler parts of South Coast. Wind variable to SW 5-15 MPH.
DAYS 6-10 (MAY 4-8)
Oh goodie! More uncertainty as it appears we’ll have a frontal boundary in our vicinity much of the time. Best chance of “warm” is very early in the period, otherwise leaning cooler with episodes of unsettled weather.
DAYS 11-15 (MAY 9-13)
Overall pattern similar, frontal boundary tends to hang nearby with episodes of unsettled weather and variable temperatures averaging not that far from normal.