Thursday December 3 2020 Forecast (7:27AM)

127 Comments

DAYS 1-5 (DECEMBER 3-7)

A westerly air flow aloft and high pressure south of New England combine for a very nice early December day today with relatively mild air and plenty of sun, but some high cloudiness will make an appearance in the sky only to increase the chances of a photogenic sunset in the 4 p.m. hour. 🙂 Tonight, more of those high clouds will steam in ahead of an approaching trough and cold front, which will bring even more cloudiness on Friday. Previously, I thought that some rain showers may accompany the passage of this front during the day on Friday but right now it looks like the front will come through dry, but moisture streaming northeastward ahead of our weekend storm threat may mean that some rainfall arrives by the end of the day or Friday evening, shortly after the front has gone by the area. And now onto the storm threat for the weekend – something we’ve been eyeing with great uncertainty for days. I wish I had a clearer picture of how this will unfold for today’s update, given that it’s not that far away, but there are still some questions to be answered. It does look like we’ll be looking at a system that impacts us throughout the day Saturday, making an exit Sunday. It will be a system that starts as rain through the entire WHW forecast area (southern NH, central and eastern MA, eastern CT, & RI). But there is going to be cold air available just to the northwest, as well as aloft, and we’re very likely to end up with a rain/snow line as this system moves through. The big question is how quickly and how far south and east does this rain/snow line get. I can say that the greatest threat for accumulating snow will be across interior southern NH and central MA based on current information, and the least of the threat will be across the South Coast and Cape Cod. But what happens between these areas? Further refining of this forecast is obviously needed, but my initial feeling is that these areas will at least mix with if not turn to snow at some point Saturday night or early Sunday, before precipitation departs, with some accumulation possible… Other impacts of the system not to be forgotten will be some strong wind and coastal flooding potential. More on that in the next update. Regardless of the details of the storm, behind it comes a period of windy and colder weather with a few lingering snow showers possible Sunday afternoon, and continued breezy and chilly but mainly dry weather Monday, with just a possibility of a few snow flurries migrating southeastward from the hills and mountains to the northwest.

TODAY: Mostly sunny. Highs 47-54. Wind W to SW 5-15 MPH.

TONIGHT: Increasing high clouds. Lows 35-42. Wind SW 5-15 MPH.

FRIDAY: Thickening high to middle overcast. Light rain possible by late-day mainly southwest of Boston. Highs 45-52. Wind SW up to 10 MPH shifting to NW.

FRIDAY NIGHT: Cloudy. Periods of rain. Lows 38-45. Wind N to NE up to 10 MPH.

SATURDAY: Overcast. Rain likely, with some mix/snow possible higher elevations of southwestern NH and north central MA by later in the day. Temperatures steady 38-45 then slowly falling. Wind NE 5-15 MPH inland and 15-25 MPH coastal areas with higher gusts especially in coastal areas and higher elevations.

SATURDAY NIGHT: Overcast. Rain southeast, rain/mix I-95 corridor, mix/snow to the northwest with accumulation possible. Lows 28-35 except 35-42 South Coast. Wind NE to N 15-25 MPH, higher gusts.

SUNDAY: Overcast with rain/mix/snow ending from west to east early – additional snow accumulation possible I-95 belt north and west. Becoming partly sunny with a few lingering snow showers thereafter. Temperatures generally steady mainly in the 30s. Wind N to NW 15-25 MPH, higher gusts.

SUNDAY NIGHT: Partly cloudy. Chance of passing snow flurries. Lows 18-25. Wind NW 15-25 MPH, higher gusts possible.

MONDAY: Partly sunny. Chance of passing snow flurries. Highs 30-37. Wind W 10-20 MPH, higher gusts possible.

DAYS 6-10 (DECEMBER 8-12)

Keeping an eye on a possible offshore storm December 8 – should stay offshore with dry/chilly weather here. Fair, cold start then a little milder December 9. Disturbance from the northwest brings a rain and/or snow shower risk December 10 then a shot of colder air with dry weather through December 11. Next jet stream disturbance may threaten the region with rain/mix/snow by the end of the period.

DAYS 11-15 (DECEMBER 13-17)

Precipitation threats early and again late in the period. Obviously with recent model performance and typical uncertainty this far in advance anyway, can’t elaborate on much at this point. Temperatures near to below normal.

127 thoughts on “Thursday December 3 2020 Forecast (7:27AM)”

  1. Good morning and thank you TK.

    I wouldn’t want to be responsible for this weekend’s forecast, that’s for sure. Based on what I have seen, looks like you
    have done the very best with the information available.

    Here’s hoping for a faster changeover, but not holding my breath.

  3. Thank you, TK. Great read. I know it isn’t dec 9 but sure sounds like the storm on the 9th in 1978. Rained all day and changed just after 4;00 and was significant

      1. Not at that level. Certainly not for Boston. 🙂

        The exclamation point was the best I could do for this one.

        Now watch it go poof with the 12Z run, coming out now.

  5. Thanks TK. I am going with my mom and stepdad to pick up the Christmas tree for their place. I am hoping the change to snow happens sooner with the snow falling just like it was in 2017. It added a nice touch.

  7. 12Z NAM has 850 mb temps BELOW freezing in Boston prior
    to precip onset. A good sign there might be a quick changeover.
    We shall see.

  9. Putting on my grinch hat this morning ……..

    The phasing vs non-phasing vs some phasing aspect of this next system has been changing a lot run to run.

    The one thing that hasn’t changed at all : not a lot of cold air to work with.

    This is another …… would enough heavy precipitation fall into marginal 850 mb temps to produce a decent period of heavy wet snow ??

    It’s another everything has to work out perfectly to produce some snow closer to the coast.

  10. Southeastern CT and eastern MA get more snow on the 12z NAM run then I do? I am hoping this is a bad NAM run so I could be in on the snow party like I was with the 6z NAM run.

  11. You “may” Get a COWABUNGA out of me with the 12Z NAM.
    Looking promising. It will, however, still be a rain to snow scenario, but the heaviest precip will be in the form of snow.

  12. The 12z NAMnest to me looks like the more likely scenario. With the low tracking like that you don’t see southeastern CT get more snow than northwest CT.

  13. Well still not worthy of a COWABUNGA, but not too shabby just the same. IF this were to verify, I would happily take it.

    Snow maps shortly. Waiting for one more panel.

  15. The snow is still continuing in your area on that run as the model only goes out to 60 hours so you could add a little bit more to those snow totals.

  16. Thanks TK.

    Yeah, this is as tough a forecast as I can remember. There’s a million possibilities on the table… up to and including a fairly major snowstorm even at/near the coast. I think TK has nicely lined out the most like scenario, but a lot remains TBD.

    1. Thank you WxWatcher.

      Wish you were at home for this one?

      I wonder how very different the GFS, CMC and Euro will be OR
      if we get some consensus.

      1. Ha, I do miss snowstorms. I really haven’t seen one since March 2018. Like I’ve said before, down here the balance of active weather events tips a little bit away from winter and towards severe weather, but the past couple years it’s been a much greater tilt than normal 🙂

        Not envious of the SNE forecasters though, we have some challenges to figure out down here with this one but it’s a nightmare forecast for you guys. My inclination is that you (in Boston) will see some degree of accumulating snow from this, if only a little on the back end, but lots of questions and the 12z guidance so far is only providing some partial answers. Expect some surprises.

    1. Can’t see the 300 and 200 mb flow, but my guess is
      that this would pass somewhere around Nantucket or thereabouts, perhaps just SE of Cape or even over
      the Cape. But IF the 200 mb flow is a bit more flat, then
      we could be in business.

      I found the 250 mb flow at another site. This would make it close to the Cape/Nantucket or over the Cape?Se MA. Hard to tell for sure.

      https://imgur.com/a/Gh0dI7o

    1. Still just a little too tight to the coast. Snow will eventually make
      it to the coast, but plenty of rain first. Inland snow bonanza.

  24. Tough forecast and I could get really technical but my time is short at the moment.

    Treat is a bit like the spring. Areas of elevation north and west have the best chance of accumulating snow. Valley, urban, and coastal areas, less likely. Sneaky snow accumulation area, maybe the hills of NE CT, NW RI and Southeastern Worcester County.

    As for model output products. I might try the snow depth / on ground / positive change maps in addition to 10:1, Kuchera, Cobb, etc. Also, I will probably cut by 1/3 to 1/2 depending on location the ECMWF precip amounts which had been running in the 2.5-3.0″ range yesterday.

  31. Regardless of model – I think there are limiting factors here that have to be considered when thinking about snow potential – yes the lack of cold air which has been mentioned a lot and then just as often mentioned, the intense storm will make its own cold air. That is a lot harder when dew points are above freezing, The ocean temps are not marginal, they are warm. Upwards of 50 degrees.

    TK and WXW may disagree, but I see those as significant limiting factors for snow in the urban and coastal areas.

  33. Thank you TK!
    I’m liking your preliminary thoughts on southern Worcester county JMA, that would be nice. With an intensifying system maybe we can get some wet bulbing to help us out with the marginal temps.

  36. This would get a cowabunga indeed if these weren’t 10:1 snow amounts. Regardless, I like where Vicki, SC, and I are sitting for this one. NE CT to Worcester County could be the jackpot for this one!

    1. I smiled when I read JMAs comment and now yours. Seems to have happened a few times

      Sadly, Saturday night is Gillette’s drive through Christmas light festival.

  37. Eric Fisher
    @ericfisher
    1h

    Seems like we’re getting closer to a consensus track over Cape Cod/Islands. Would keep mild air/rain largely in control east of ~495 but would favor at least some snow farther inland. Will have all the maps/info on #wbz this eve

  38. Could that area of higher snow totals come a little bit more to the west like the 12z NAMNest showed.

  39. So we basically have the Euro and NAM for more snow totals
    and all of the rest for mainly a rain event.

    Most interesting. Curious to see the 18Z HRRR out to 48 hours, ie 1PM Saturday.

      1. That map matches up well with the Pivotal map I posted above but both of those maps are straight up 10:1 with no algorithm so are definitely overdone.

        On the other end of the spectrum, I have noticed those F5 weather snow maps with the special algorithm are typically on the low side.

        Does weathermodels have a Kuchera snow map for the Euro?

  41. From Ryan Hanrahan
    An extremely challenging forecast for Saturday. Nor’easter develops but small changes in the track mean difference between a heavy accumulating snow and very little.

  46. It’s rather “fun” (read, somewhat frustrating at times haha) to see even the short range guidance struggling between runs just hours apart.

    No definitive feeling when taking the guidance into account – simply not enough agreement for me to be comfortable. After I look at all 18z when available, we’ll see if that changes at all.

    1. More adventurous than I am. I have the luxury of waiting until tomorrow morning’s update for amounts. 😉 Very high bust potential, which goes without saying given what we were discussing about variation between different models let alone run to run with the same model. 😉

    2. Interesting.

      Which guidance does one trust in times like this?

      Euro and 3KM NAM?

      Hrrr is trying to get rain to changeover.

      I dunno. Glad I don’t have to make the forecast.
      And I sure as hell wouldn’t put a map out this early.
      Pete has brass ones.

      1. None of them have proven trustworthy in recent times, and the one time they did it was against the opinion of a lot of meteorologists (go figure). 😉 I think this is yet another learning process and I wonder how many more of these we’ll have to endure. From a meteorology standpoint, I’m looking forward to the challenge, as I am going to take something positive out of this – i.e., hopefully sharpening the skill through having to pay greater attention to basics than ever, and is easy to let slide in “the technology age”. 😉

        1. So well said, and as tough as this forecast is from the standpoint of synoptic diagnosis, I think the now over-prevalence of forecast model guidance decays forecaster confidence even more.

      2. In Pete’s defence, he may have been told to have #’s, and if not, it’s his decision, which I respect. This can probably be said of any of the on-air folks for tonight’s evening news. If it were I, I’d probably beg the program director to please let me wait until the late-night news to hopefully gain advantage of 00z data, but they’d say no. 😉

          1. FWIW if I had to put a map out right now myself, I’d basically be nearly identical to the NWS snowfall forecast. We will see if I still feel this way come tomorrow morning.

  53. Really good NWS map, all things considered. I would have gone more Putnam CT and into adjacent areas of MA and RI, less in Boston if they are using Logan as verification.

    1. NWS Boston has been great at snow forecasts for a number of seasons running now. Granted, they got surprised by the October snow, especially regarding the amounts right on the coast including Logan, but so did everyone else. 😉

      1. I prefer their range map vs the pinpoint map. Everyone but the snow mafia blew the October snowfall. They have blown some CT River Valley of MA forecasts , but as Bob Thompson and Walt Drag once said to me, that area is where good snow forecasts go to die.

        1. Thompson and Drag are definitely two of the best that Boston has ever had, if not the entire country.

          Walter and I had several great conversations at a few of the southern New England weather conferences. Even if I hadn’t seen him for a couple of years when we ran into each other the next time he would remember my name..

      1. Mark and everyone else enjoy this snowfall as it looks like I am going to be watching this one from the sidelines unless this could shift just a tad west.

    2. That’s the only model from the 18z data that shows anything like that. It’s an automatic toss. 🙂

    1. That’s why I laughed.

      I happened to catch some of the 6pm newscasts and wow, there would need to be some serious backpedaling on the 11pm weather segment.

      1. I have been told by some that they rather people be prepared for the possibility of a larger storm and have it bust than they saying its not going to be big and it does become big.

    1. Quite true.

      I feel like when I look at some of the models snowfall projections, I´m expecting there to be a lot more cold air around.

      I understand the whole idea of an intensifying storm cooling off the mid levels enough to produce snow under higher intensity precip ……. but, its just very marginal and really requires a lot to go right to get the amounts some models have been displaying.

  63. Its kind of a catch 22 on this particular setup.

    Since its cold air that the storm needs to produce, you want a stronger storm, which requires better phasing, but then you end up with a storm track closer to the coast.

    A less phased system in this instance, I actually don´t think would bring more snow closer to the coast, because then the less dynamic system wouldn´t produce as much mid level cold to overcome too warm a boundary layer,

    I´ll get more excited about snow prospects when I see a blue H somewhere in SE Canada and Burlington, VT and Caribou, ME being near or below 0F.

    2. Telling though on the radar projection that the northwest flank under the lighter precipitation is mixed rain/snow which implies a really marginal scenario.

      In these precip intensity cases, coastal sections and lower elevated areas usually don’t make out too well.

  67. If Boston can achieve 4+ inches of snow in late October, just based on climatology, even early December it “should” be a piece of cake.

    1. That’s not how it works. The October event is HIGHLY anomalous. How often has Boston received 4+ inches of snow in October since record keeping began?

      What is Boston’s average seasonal snowfall through December 5? No, it’s not a piece of cake. 🙂 Not even close.

  69. I like yesterday’s NWS Map better.

    Elevation will be the key to achieving the higher end of any range. coastal locations , adjacent urban areas, and deeper valleys may struggle to hit the low end of the snowfall ranges.

    Precip amounts will have sharp cut offs north and west, further complicating the end result.

    1. But if that storm goes under bombogenisis further east, then that amount for Boston would certainly verify…and maybe then some.

      Certainly would be ironic if this behaved just like in late October.

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