COMMENTARY
On December 1, the 5-day period that I am about to forecast just below in the DAYS 1-5 section was in the DAYS 11-15 section. This is what it said: “Fast-flowing jet stream but more of a southwesterly flow and a milder trend expected with a couple minor precipitation threats during this period.” This vaguely-written medium range outlook will verify, somewhat. It’s not a perfect outlook by any stretch, but this 5-day period will be warmer, on average, than the 5-day period that preceded it, and we do have a fast-flowing jet stream pattern, which starts out from the west southwest then goes more westerly in response to high pressure ridiging centered to the southwest. Our first “minor precipitation threat” is today’s event. Ok, maybe this is not “minor” in the sense that there is some moderate rain pockets around with a warm front passing by as I write this commentary, and we are expecting some gusty wind today and this evening along with another round of rain showers, some of which may be heavy. But it’s also not a major storm in the sense we often think of them at this time of year. So the wording I used, while vague, did cover the chance of something happening in the weather pattern we are in, and the use of the phrase “milder trend” would have hinted that we’d probably have at least a slightly better chance of rain than we would snow. Ok, yes, there has been some icy roads over portions of interior MA and southern NH early this morning, due to surface temperatures having fallen below freezing before the onset of the rain, but do you think that could have been predicted in a day-11 forecast? Not likely, since model guidance didn’t have anything until at least the next day, and was showing it as just rain. Even anticipating error in a model still doesn’t make it a wise idea to try to get too detailed that far out. The reason I bring this up is because if I tried to just go with what the easy access model that goes out that far then there would have been too much day to day detail, more than is possible in this situation, and it would have been wrong, as noted above, since the guidance at that time had dry and cool to mild weather for today (Dec 11), unsettled weather moving in Dec 12-13 with warmer temperatures, and drying weather with a slight cool-down for Dec 14-15. As you often hear myself and other meteorologists say, in one way or another, you can’t really use medium range model guidance for day-to-day detailed weather beyond just a few days out, as the error increases with time, and most especially so as we have noted in a pattern such as this. I just wanted to cite another example of this as a reminder. Oh yes, my self-quote of my vaguely written 11-15 day outlook did mention a “couple” minor precipitation events, so what about the second one? Is there a second “minor threat”? Read on to find out… 😉
DAYS 1-5 (DECEMBER 11-15)
Today we experience a weather system somewhat similar, but not quite as potent, to the one we had go through here on Monday. That system was low pressure coming out of the Midwest, through the Great Lakes, and into southeastern Canada, quite a strong low with a significant pressure gradient. This one tracks similarly, but is not as strong, however after its warm front goes by here and our periodically rainy / showery and foggy patches morning comes to an end, we’ll find ourselves in a breezy warm sector again, with temperatures pushing or exceeding 60 as winds pick up this afternoon. Before that happens we may even see some thunderstorm activity due to some very unstable air moving into the region. But at least we’ll spend a fair amount of the afternoon free of rainfall in the region, so if you don’t mind a lot of clouds and a gusty breeze, it’s going to be quite mild out there, not bad for running errands or taking a walk, etc. It is this evening when we will see rain showers return from west to east ahead of and along a sharp cold front. There is the risk of isolated tree damage and power outages, but to a slightly lesser degree than earlier in the week. We may start with some cloudiness on Sunday, otherwise expect a dry, breezy, and seasonably chilly December day. The breezy weather will continue Monday as high pressure slips off to the south of the region, but this will allow a temperature moderation so it’ll be a little bit milder. But a media-advertised “big warm up” with implied staying power is not really in the cards for us here. The center of high pressure ridging is too far west to allow that to take place here. We’ll have a cold front quietly slip through the area Monday night and a bubble of high pressure move across southeastern Canada Tuesday and Wednesday, and this will keep us from continuing a warm up here. It won’t be all that cold, but don’t expect to be breaking out your Bermuda shorts either. After fair weather Tuesday along with a gusty breeze, we should be seeing an increase in clouds and the chance of some light precipitation before Wednesday is over as a warm front approaches the region ahead of the next low pressure area the jet stream is carrying.
TODAY: Cloudy with areas of fog and periodic rain, along with embedded heavier showers and possible thunder, ending from west to east midday. Mostly cloudy afternoon. Highs 57-64. Wind S to SW increasing to 10-20 MPH with gusts above 25 MPH likely and gusts to around 40 MPH possible, especially South Coast and higher elevations.
TONIGHT: Mostly cloudy. A shower, possibly heavy, including the slight chance of thunder and small hail, from west to east late evening to early overnight hours. Lows 38-45. Wind SW 10-20 MPH frequently gusting to 30 MPH and occasionally gusting stronger, shifting to W from west to east.
SUNDAY: Mostly cloudy start, then increasing sunshine. Temperatures steady 38-45. Wind W 10-20 MPH, higher gusts.
SUNDAY NIGHT: Mostly clear. Lows 22-29. Wind W 5-15 MPH, higher gusts.
MONDAY: Sun/cloud mix. Highs 45-52. Wind W 5-15 MPH, higher gusts.
MONDAY NIGHT: Partly cloudy. Lows 23-30. Wind NW 5-15 MPH.
TUESDAY: Mostly sunny. Highs 43-50. Wind N 5-15 MPH.
TUESDAY NIGHT: Mostly clear. Lows 24-31. Wind NE up to 10 MPH.
WEDNESDAY: Increasing clouds. Chance of light snow/mix/rain at night. Highs 42-49. Wind E up to 10 MPH.
DAYS 6-10 (DECEMBER 16-20)
While we continue to see a pattern of fast-moving systems, there will be a transition starting to take place during this period as it appears right now. Two more systems should pass by to the north with brief warm-ups, more confident in the first one December 16, maybe one more around the middle of the period, and by the end of the period a shot of much colder air is possible. Again with iffy guidance at best, this doesn’t help confidence in a medium range forecast, but the signs are for pieces of colder air to make it further south and east in Canada and some other atmospheric readjustment to make our air flow a little more northwesterly by the end of the period. Obviously lots of fine-tuning to come.
DAYS 11-15 (DECEMBER 21-25)
As we approach Christmas we’ll eye the possibility of a colder pattern and while initially it looks fairly dry, we’ll have to keep an eye on jet stream disturbances which are not well-seen by guidance this far in advance. I cannot offer any detail this far in advance with too many forecast unknowns.