It’s no secret if you’ve seen my comments around the blog. I have struggled with this forecast more than any over the last few years. Examination of indices that help determine the long term pattern have been mixed to the point where I have not felt confident leaning too much in any direction. In such a case, I place more weight on a correlation of the weather pattern of November helping to determine the upcoming winter overall. This in itself, while a decent correlation, has been nowhere close to usable on many occasions. Taking all this into account, and placing a little faith in the November-to-winter correlation, the following paragraphs are my overall thoughts, month-by-month breakdown, and wrap-up summary of the what I expect to take place during the coming winter.
Drought is not so much a factor now as it was going into last winter. Although we continue to run a long term precipitation deficit, short term moisture was decent enough during a good portion of 2017 and from a couple events during the autumn to eradicate the ongoing drought and then put out a drought “flare up” that occurred during our warm and fairly dry stretch in early to mid autumn. I have examined current and expected indices including ENSO (which is expected to be weak to moderate La Nina), QBO (expected easterly), PDO (expected negative), AO/NAO/MJO among a few others. We can talk more about these as we get into the season on the blog on a daily basis. I also realize that these letters may mean nothing to some of you but just know that they have something to do with the large scale pattern in the atmosphere, and in some cases ocean circulation, and impact weather in certain ways. In addition, fairly rapid advancement of snowcover in Siberia, as well as eastern Canada, sets up plenty of cold air north of the border to be tapped. So even in an overall mild pattern we can tap the cold air and experience brief very cold shots of air behind departing low pressure areas. So these, along with the correlation for November mentioned above, leads me to the feeling that the winter overall will end up not all that far from normal in both temperature and precipitation, with a slight leaning toward the mild side of normal for temperature and a slight leaning toward the wetter side of normal for precipitation. Either of these, especially the precipitation, can be skewed significantly by 1 or 2 significant events. And even though we can say this about any seasonal forecast, I feel that the “bust potential” for this particular winter is a little above the average. How we will arrive at “near normal / near normal” will not be by seeing daily weather that fits the average for the date, but by high variability in temperature and frequent weak storm systems with the occasional larger one. Heading into the first 1 to 2 weeks of meteorological winter it looks like the current pattern will hold, and that is one of fairly dry weather overall with changeable temperatures, mild with brief periods of cold, a little milder overall than November had started out. The tendency for the atmosphere to develop a blocking pattern will be there, especially during the first half of the season. One trigger, stratospheric warming, which then disrupts the stable state of the polar vortex and sends it into fragments that then migrate further south, may very well take place toward the end of the first 10 days of December. With somewhat high stakes my leaning, based on fairly reliable guidance with these types of events, is to forecast a shift to colder with enhanced risk for snowfall during the middle and end of December. This pattern will probably not hold much longer than that and will probably adjust into a more zonal pattern similar to that of the early to mid portion of November, which was on the chilly and fairly dry side. This would be the dominant pattern for January’s early and possibly middle portion, but La Nina is going to want to put more ridging in the Southeast, and over time, we should see the pattern revert to the one that we currently have. Assuming La Nina peaks in late January and February, along with the continued negative PDO, during episodes of zonal flow with no blocking, the pattern would be fairly mild and on the drier side overall. Blocking can still occur with little warning and result in 1 or 2 storm systems of any precipitation type, dependent on air mass in place and synoptic conditions evolving during the event. Even with this leaning toward a milder overall pattern during the second half of the winter, we can still be subjected to brief periods of intense cold as Canada will continue to be on the colder side of normal with extensive snowcover. One last thing I am concerned about is icing events. Of course these can occur in any winter, but if we have a pattern that features mild air aloft and a storm system moving through, or even just a precipitation event with a frontal boundary nearby, the availability of cold air in nearby southern Canada can set up a situation where we have a warm atmosphere in all but the lowest levels, resulting in icing situations. The number, magnitude, and location of such events cannot be determined in the longer range, but just know that the possibility of such events this winter, in my opinion, is a little higher than average.
DECEMBER
Breakdown: Weak La Nina pattern. Zonal pattern overall first 10 days, blocking potential higher middle of month possibly into late month but may relax to zonal but more northwest flow before the month ends. Systems during the first third of the month would be more apt to produce rain versus snow, with the snow threat higher thereafter. A significant amount of the seasonal snowfall can occur in a pattern like the one expected in mid to late December, assuming storm threats line up to produce it.
Temperature: Near to below normal, arrived at by balancing a mild beginning of the month with a colder mid to late month.
Precipitation: Near to above normal.
Snow: Above normal.
JANUARY
Breakdown: If the pattern expected in mid December should try to repeat, as we sometimes see, it would likely be during the first half of the month in an otherwise colder and drier regime as late December’s pattern should be more dominant (zonal northwest flow). With time, expect this to relax and become more of a west southwest flow with milder dominant air and brief shots of cold as more of a ridge occurs in the US Southeast. We can see clipper type systems with the northwest flow, and a couple of Pacific systems when the pattern relaxes. Rain/ice/snow would be determined as always by the conditions in place at the time, but the tendency here would be for a somewhat wetter pattern by month’s end
Temperature: Near normal, arrived at by a balance of a colder start with a milder finish.
Precipitation: Near normal.
Snow: Near normal but high stakes as a big swing between Pacific systems producing rain or snow can mean many inches difference.
FEBRUARY
Breakdown: La Nina in its strongest state at this point of the winter should put a more persistent ridge in the US Southeast with the late January pattern continuing, the mild dominating and the cold shots brief but potentially intense.
Temperature: Near to above normal.
Precipitation: Near normal.
Snow: Below normal.
MARCH
Breakdown: Uncertainty abounds not only from it being several months away but the status of La Nina and other indices being somewhat in question. For now going to lean toward a milder and drier than average pattern, but with low confidence.
Temperature: Near to above normal.
Precipitation: Near to below normal.
Snow: Below normal.
WINTER SEASON OVERALL
Temperature: Near to slightly above normal.
Precipitation: Near normal.
Snow: Near normal.
-Boston 40-50 inches
-Worcester 50-60 inches
-Providence 30-40 inches
-Hartford 40-50 inches