First things first: happy meteorological winter! Although real winter will not arrive for another few weeks, December 1st is the start of winter for us in the weather world and a number of you saw a taste of it with the first winter advisory of the season for Hartford and Litchfield Counties this morning. Depending on your preference, this is either the most wonderful time of the year, or the most terrible. Now that we are in December and a new “season”, I want to take a moment to talk about Connecticut climatology (climo). For our purposes, when I discuss 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, but we all know that things can and do fluctuate, sometimes wildly as conditions and patterns change.
So what is normal? Below I have the average high, average low, and average monthly snowfall at selected Connecticut locations. We are at the month where historically CT sees its first significant snowfall (greater than 1”), though it is not always guaranteed.
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)
Of course, this is just a snapshot of two locations, but with this data you can get at least a general sense of what to expect on average. If you are in the hills of the state, you can probably expect cooler temps relative to everyone else and more snow, and if you are in a city center or coastal areas—especially in the southeast corner of the state, you can expect warmer temperatures and less snow.
New Medium Range Outlook Feature at SCW
As the newbie, I am still tweaking my writing style, and in an effort to provide all our subscribers with as much information as we can reasonably predict to help you plan ahead, I am going to try to do a two week outlook every two weeks during the winter months. Generally, forecasting specific sensible weather conditions (i.e. the exact high temperature or specific amount of precipitation) two weeks out is an exercise in futility. The atmosphere is so dynamic, no one has really acquired the skill to do that kind of forecasting outside of 7 days, though outlets do try. What you can do however with reasonable skill, is look at the pattern and get a sense of what things could look like in the longer term. The further out in time you go, the greater the risk that you will be wrong, so I will be using confidence levels to give you a better sense of where we stand on the forecast. I will have a bare bones summary of each week for those looking for a quick read, and a technical discussion for those interested in the meteorology behind the forecast. Let’s dive in…
Two week outlook summary
* Note—high confidence (70% or greater belief of event occurring), moderate confidence (36-69%), low confidence (0-35%); nothing significant (less than 1” snowfall and .25” ice)
Week one—Tuesday December 1 to Monday December 7
Temperatures—near to slightly above average (high confidence)
Precipitation—average (high confidence)
Wintry Precipitation—nothing significant (high confidence)
The start of this week brings an unsettled period to the state, as precipitation dominates the time between Tuesday and early Thursday. The Monday 18z GFS painted between .5-1.75 inches of rain falling throughout the state during that period. Temperatures will start off close to average, with the exception of early today when we saw cooler temps and light freezing rain in isolated spots, but we are likely to be near to slightly above normal afterward. Under this regime, chances of widespread and significant frozen precipitation in the state are very low moving forward. After the precipitation ends early Thursday, high pressure is likely to build in and bring much calmer weather to the state into early next week.
Week two—Tuesday December 8 to Monday December 14
Temperatures—above average (high confidence)
Precipitation—average (moderate confidence)
Wintry Precipitation—nothing significant (moderate confidence)
As you will see in the technical discussion, there is growing confidence that we see greater positive departures in the temperature department in week two, as a warmer pattern is established and floods us with warmer than normal conditions. Odds of wintry precipitation look low at this time.
As the plots above show—we’re likely to warm up quite a bit over the next two weeks. To be clear however, positive departures in December may not feel as good as the ones we experienced when average high temperatures were higher last month. The pattern that allowed for that very warm November is looking persistent into at least mid December, and confidence is higher than usual because of the run to run consistency of the American and European ensembles. Using the ensembles—the GEFS and EPS (Euro ensembles) are particularly useful because they are run with individual members attempting to account for inherent forecasting errors in time by slightly changing initial conditions within each individual member. This ideally makes it a bit easier to see large scale pattern changes further into the future, but just like an operational model, the further out in time you try to predict the more likely you are to see change, especially if the pattern looks unstable.
If you like wintertime temps and frozen precipitation, the ensembles for both week one and two are not your friend. On the flip side, if you like mild weather, you’re in luck! The two things that forecasters look at closely during this time, the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) both look to be positive, and significantly so, in the near future. As mentioned in our winter outlook, the AO is an index measuring pressure anomalies in the Arctic, and is correlated with temps for our area for the winter months. +AO suggests above normal temps, -AO below. The NAO is a close relative of the AO, and it measures roughly the same thing in the North Atlantic. The NAO has a correlation with blocking in the atmosphere for storms; -NAO usually implies more blocking is available, +NAO represents a faster pattern with less room for storms to amplify.
In terms of precipitation, the models show potential storminess between the 8th and 10th, but it is too far out to really get a sense of if that materializes and what shape it takes if it does. The operational GFS also shows storminess at the very end of this outlook period, but that isn’t worth serious discussion at this point. Track matters, and the projected pattern favors cutters—storms that cut to our west and up through the Great Lakes, usually leaving us in the warm sector. Even if we did manage a coastal (as currently projected in the 8-10th period), warm air seems to be in place, preventing significant wintry precipitation. As a result, I have the overall two week outlook showing no significant frozen precipitation on the horizon and near normal precipitation—meaning a mildly active period.
There are hints beginning to show that this regime could break down at least a bit during the second half of the month. Right now, any change looks to be gradual and a shift back to normal temperatures, rather than a big shift toward cold and snow. We’ll see what things look like in two weeks, but at the moment, we are likely to be above normal as we reach the middle of the month.
In addition, we are including a graphic with even more concise thoughts for those that are in a hurry and are looking for top line information. As always, don’t forget to like us on Facebook and share our discussions, and follow us on Twitter @southernctwx!
As this is a new feature, I encourage you to provide feedback so I know what I can do better.
Thanks for reading SCW.