nycwinterheader

Five things to expect during the upcoming winter

Just about a month ago, we released our 2015-2016 Winter Forecast to the public. The forecast featured a tremendous amount of information, research, and data, most of it very technical in nature. With winter only a few weeks away, there is no better time than now for us to lay out the ideas we gathered in a more simplistic form. After all of our work to compile the forecast, there are five general things you should expect during this upcoming winter season.

1) The Winter of 2015-2016 will start off warm

Over the past few weeks, much has been made in regards to the warm pattern setting up for the month of December. Truth be told, confidence has never been higher that December will be a warmer than normal month with less snow than normal as well. Much of this can be attributed to the development and effects of a strong El Nino in the Equatorial Pacific. But some, also, has to do with the surrounding global circulations.

This month, the stratospheric polar vortex (way up there) tightened and strengthened over the North Pole. While this doesn’t totally eliminate any chance for our area to experience cold air, it does make it more difficult. A tighter and more consolidated stratospheric polar vortex means less atmospheric disruption in that region; i.e: Less high latitude blocking, or ridges, to displace the cold air farther south. This vortex is expected to weaken over the next 30 to 50 days, eventually moving and/or splitting and promoting high latitude blocking during the second half of the winter.

CFS model showing 500mb heights during the month of December. The pattern is warmer than normal throughout much of the Northern 1/3 of the United States.

CFS model showing 500mb heights during the month of December. The pattern is warmer than normal throughout much of the Northern 1/3 of the United States.

Additionally, the pattern across the Pacific and Atlantic Ocean’s during the month of December is modeled to promote the development of warmer than normal air across much of the Northern 1/3 of the United States. A developing trough in the Gulf of Alaska and Northwestern United States will allow milder Pacific air to flood eastward into the United States. On the Atlantic side, a progressive pattern with no blocking ridges or highs will support warmer than normal air as well.

With the subtropical jet strengthening over time, December looks very likely to be an active month in terms of precipitation, but a warmer than normal month in terms of temperature.

2) Frigid cold snaps are likely to be less frequent, and less intense

The severity of a winter, in retrospect, has a lot to do with the intensity and duration of cold air. Think about some of the winters in the last 5 years. While snowfall was absolutely above normal, and that is obviously memorable, i’m sure you also remember the stretches of below normal temperatures. Uncomfortably — and record setting — cold air has been a theme over the past several winters, especially the last few. Temperatures have fallen near zero for multiple days and there have been several months with below normal temperature anomalies.

This year, the atmospheric orientation is unlikely to support prolonged periods of much below normal air — or frigid, record breaking cold. While such airmasses may still impact the area in a more transient nature, we aren’t expecting these to be prolonged or sustained. Much of this can be attributed to the aforementioned stratospheric orientation — but additionally, a weaker and less prevalent EPO ridge in the Pacific and Western part of North America will have an impact as well.

GFS model showing a very organized and large stratospheric polar vortex during the next few weeks.

GFS model showing a very organized and large stratospheric polar vortex during the next few weeks.

An EPO ridge has a major impact on the atmospheric pattern. As the ridge strengthens, cold air near the poles and arctic is dislodged farther south. This displaces very cold, anomalous air over Canada and this air can often move south toward the Northern 1/3 of the United States for prolonged periods of time. Without this ridge, it will be more difficult for these airmasses to make it to our area.

3) Precipitation will be above normal

El Ninos, while known for their early season warmth, are equally famous for their anomalous precipitation and active subtropical jet. This is especially true for Southern parts of the United States. Convection and active weather patterns with an anomalous number of disturbances typically occur, with storm systems moving into California and tracking eastward through the South-Central and eventually South-East United States.

In our area, the precipitation anomalies depend more on the surrounding meteorological pattern. But this year, the El Nino is expected to provide a much higher than normal frequency of storm systems, with precipitation anomalies likely to average above normal in a large majority of our area. More of that precipitation is expected to be wet, than white, especially early in the season during the month of December.

El Nino precipitation anomalies during all events since 1951.

El Nino precipitation anomalies during all events since 1951.

4) A pattern change is very  likely in January, possibly earlier

Here’s the big wild card of the winter, and something that will be talked about more frequently with each passing winter week. A significant pattern change is currently forecasted sometime from late December into the middle part of January. The warmer than normal weather and less-snowy that normal regime is expected to take a stranglehold for much of December and, quite possibly, some part of January as well. But a multitude of factors — including a slowly weakening El Nino, increased atmospheric blocking, and a retrograding low in the Gulf of Alaska, will eventually readust the pattern across the Northern Hemisphere.

In overly simplified terms, think of it as a wave pattern. To start the winter, a trough over the Gulf of Alaska and very minimal or non-existent ridging in the higher latitudes (over Canada and the pole) will lead to higher than normal heights, on average, in our area. But as winter progresses, imagine that trough in the Gulf of Alaska retrograding toward the Aleutian Islands. This opens up room for ridging to build on the west coast of the United States. With an increased tendency for high latitude blocking expected, colder than normal air is more than likely to eventually be shunted southward toward the Northern 1/3 of the United States.

CFS model ensembles showing a general trend toward a more negative Arctic Oscillation as winter progresses.

CFS model ensembles showing a general trend toward a more negative Arctic Oscillation as winter progresses.

The active precipitation pattern is expected to continue, and thus leads to increasing confidence in a pattern that will turn colder and more snowy for the second half of the winter.

5) There will be the potential for “a big one”

While some winters end up more mundane, with no real “big” storm threats, many feature at least one opportunity for a large “blockbuster” storm. These are the storms that make or break a winter — what the season will essentially be remembered for. This year, the second half of the winter is likely to feature the opportunity for one or two “Big ones”.

The juxtaposition of colder than normal air, an active pattern with plentiful precipitation, and high latitude blocking is expected — at some point — during the second half of the winter. During this time frame, likely from late January into February, the potential will exist for a significant winter weather event.

Exactly how the event plays out? Nearly impossible to predict at this point. Small nuances in the pattern can change a snowfall forecast even 12 hours in advance. So at this range, we can only use our knowledge to give us an idea of the best potential time frame. As winter progresses, however, forecasters will certainly have their eyes peeled to this time frame to see if the “big one” does in fact come together.

Stay tuned for additional updates and information in a series of posts explaining our winter forecast, as well as updates with new information during the first week of December.

Comments

comments