The last month of 2017 has arrived and for those of us in the weather world, it means the start of meteorological winter! Meteorological winter lasts the duration of December, January, and February, but as you all know real winter can hit before or after that period.
Continuing a SCW favorite, I’ll be coming back every two weeks (or just about) to provide you with a broader and longer term outlook on the weather in the region. This will be my third year doing it, and overall, I’ve done well with these. As an added incentive to do well, I grade my performance. Here’s to a lot of A’s this winter!
Unlike the usual forecasts which use operational models, the main tool used my outlooks are the ensembles. Ensembles are the operational runs of the major models which are tweaked with different conditions. This (ideally) helps resolve the inherent uncertainties that grow in time during a forecast period.
Although I’m not making the usual sensible daily weather forecast, it is my hope that I’ll be able to provide enough detail about potential to help you plan ahead.
The first question that everyone wants to ask is wrong. We don’t know if it will snow. There are no strong signals in the guidance for a significant snow event, and that’s usually the case this early in the season. A switch does not flip just because the calendar says December.
To save yourself time and stress, it is important to understand the potential a weather pattern brings, and what normal, or “climo” is. Let’s get started.
When I say climo I am simply referring to weather conditions averaged over a period of time, usually using the data between 1981-2010. Climo gives you a sense of what you could expect on average. It keeps us that don’t like hype tethered to reality. That said, things can and do fluctuate, sometimes significantly as patterns change.
Overall in December, CT usually sees its first significant snowfall (greater than 1”). It not always a guarantee and the second half of the month tends to be the best shot for wintry weather. Again, even in a great pattern in December, a lot of Connecticut (especially at the coast) can end up with events that are rain or mix.
BDL (Bradley Airport)
Dec 1 avg high—45
Dec 1 avg low—28
Dec 31 avg high—35
Dec 31 avg low—19
Dec avg snow—7.4”
Dec record snow—45.3” (1945)
Dec 1 avg high—47
Dec 1 avg low—33
Dec 31 avg high—38
Dec 31 avg low—24
Dec avg snow—5.1”
Dec record snow—16.0” (1948, when records are first available at this location)
With that refresher, it is time to talk about the first half of December.
Two Week Outlook Summary
Week One—Friday December 1 to Thursday December 7
Temperatures—above average (moderate confidence)
Precipitation—average (moderate confidence)
Wintry Precipitation—below average (moderate confidence)
The first week of December will have temperatures tilting warmer than normal, but not awfully so. The biggest precipitation chance comes midweek as a significant cold front traverses the region. We have to watch that front as it could serve as a focal point for wintry precipitation at the very end of the period, but right now, I see that front as giving us mostly rain.
Week Two—Friday December 8 to Thursday December 14
Temperatures—below average (high confidence)
Precipitation—average (low confidence)
Wintry Precipitation—above average (moderate confidence)
If the overall pattern begins to reveal itself as the guidance is currently suggesting, week two should bring a real winter blast, at least in terms of temperatures. By Friday, we should see temperatures slightly below normal as cold begins to push in. Reinforcing shots of colder air look possible. The pattern is less certain for precipitation chances however, as we wait to see what the dominant storm track will be and whether we see a lot of upper level activity.
The first week of December could be interesting, but I think it’s more likely that it merely heralds the arrival of what could be a significant winter pattern heading toward the middle of the month. There is nothing particularly special about the next few days, as ridging develops overhead into early next week and brings us a period of warmer than normal days.
For winter lovers, what happens at the end of the period is significant. A deep trough is set to barrel into the region by the end of the week, bringing colder than normal conditions. You’ll see this on the ground in the form of the cold front, which should bring rain showers midweek. I’m thinking the Tuesday-Wednesday timeframe. Even at the end of the forecast period (Thursday) if we’re below normal, it probably isn’t by much.
Precipitation, and wintry precipitation in particular becomes a bit trickier. I went with “normal” precipitation because the cold front looks as if it could drop some meaningful rain. Not a washout by any means. I think we have below normal chances of wintry precipitation because even though some guidance has shown some snowfall possible after the front passes, anafrontal snowfall tends to be fool’s gold—something that models play with but reality rarely entertains.
Still worth watching.
The second week (Friday forward) is the period I will be watching with a closer eye. If the guidance is correct, we roll into what looks like a great pattern. There is rather strong agreement that we will see a deep trough settle over the eastern portion of the US, allowing for colder than normal temperatures, potential for reinforcing cold shots, and a potentially active storm track.
As you roll forward to the end of the animation you see red shading and a hint of blocking over the Arctic region. That’s the start of the blocking we’d like to see for continued cold and increased snow chances.
Right now, the guidance is taking stabs in the dark at pieces of upper level energy rotating through the east coast. Don’t get caught chasing ghosts. What matters right now is that the pattern looks potentially ripe for increased winter precipitation chances by next weekend and through the second week of my forecast period.
If I were to hazard a guess, I’d say that the most likely way for us to produce a winter weather event would be through a clipper type event—a weak low that comes from Canada and provides a light snow event on the heels of a reinforcing cold shot, or a system that grazes the coast, which would be an all types of precip event (including rain).
Overall, this looks like the best pattern to open December we’ve seen in quite some time. I have high confidence that the pattern will change, but the duration and strength of the pattern remains unclear. We’ll watch midweek, and keep a close eye on if the pattern allows anything to pop that could impact us. Of course, this early in the season, it is best to proceed with caution (speaking to you snow lovers!). A great pattern does not guarantee snow for anyone in Connecticut this early in December.
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Our winter forecast is coming next week!
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