In our previous article, we discussed some of the features in the upcoming hemispheric pattern that could support a winter weather event. These features were particularly evident during the period from January 16th to 20th. As we have gotten closer to this time period, forecast models have come into much better agreement on exactly how these features will evolve.
Winter has finally arrived. Despite potentially record breaking warmth for one day this weekend (Sunday), and the return of rain this weekend as well, we will continue to transition into a colder pattern with an arctic frontal passage on Monday. The atmospheric global circulation pattern is trending much more favorably for winter weather lovers — with teleconnections favoring cold being dislodged into the Northern 1/3 of the United States. With high latitude blocking also in the forecast on all medium range models — the question now becomes: What about the snow?
In our winter forecast, we discussed certain signals that supported high-latitude blocking developing for the middle and late periods of this upcoming winter. High-latitude blocking often comes with negative phases of the North Atlantic Oscillation (NAO), the Artic Oscillation (AO), and/or the Eastern Pacific Oscillation (EPO). High-latitude blocking is one most important factors in any winter forecast, as it typically supports colder temperatures and larger snowstorms for the Northeast US. This is an even more important factor in during a strong El Nino winter, as the Pacific or Subtropical jet is generally more active and stronger.
Research from Al Marinaro (@wxmidwest ) brought to light a strong correlation between the sea-level pressures in North Pacific and the NAO modality, during +ENSO (El Nino) winters. North Pacific sea-level pressures less than 1013 mb had a -NAO value on average for the December, January, February, and March period (DJFM). Sea-level pressures above 1013 mb during the month of October had either a positive NAO value or very close to netrual in the following winter. An official pressure for North Pacific reading has not been released for October yet. However, NCEP/NCAR reanalysis tells us that sea-level pressures were on average around 1010 mb over the region 65N-30 & 160E-140W in the North Pacific. That would be well-below the 1013mb threshold for a -NAO DJFM.
It has been a while since the meteorological community has had the chance to analyze the potential for higher latitude blocking. It has also been a while since we’ve had the opportunity to analyze a synoptic heavy rain event. Both of those look to come to fruition, in multiple facets, over the next five to seven days. A dramatic pattern change will unfold across North America this week, with anomalously strong ridging and surface high pressure building into Canada and the Northwestern Atlantic Ocean. Precariously timed with the formation of a tropical cyclone, this blocking high pressure will lead to a forecasting headache — and the potential for heavy rains and impacts from a Tropical Storm along the East Coast.
For those without a technical background, high latitude blocking is a broad term for higher then normal pressures/heights in the higher latitudes. These “blocking” ridges of high pressure to our north, sometimes over Canada and the Atlantic and sometimes as far north as parts of Greenland, slow down the weather pattern closer to our area. The slower weather pattern can allow disturbances to interact and phase — forming much larger, more powerful storms that otherwise would have continued on their own way if the pattern was moving at a normal progressive speed.
This week, forecast models are in agreement that higher latitude blocking will develop over much of Canada into the Northern Atlantic Ocean. Ridging builds into these areas in the mid levels of the atmosphere, and a very strong surface high pressure builds east and southeast into Canada and even parts of New England. This is one important piece to the forecast headache, and one reason why meteorologists are slightly more concerned than normal at this range: The tropical system, or storm system that forms, cannot simply escape north or northeast. The blocking will slow down the pattern considerably.