Long Range Pattern Outlook #8

Minor revisions were made on January 12.

Brief Overview:

Today’s long range pattern outlook focuses on the medium range pattern going into late January. Pattern changes are underway which will result in a colder pattern for the region, but the question remains how cold the pattern will be and how much storminess is associated with it.





Pattern Outlook Archive

<< Pattern Outlook #7 – December 24, 2012
Pattern Outlook #8 – January 10, 2013
>> Pattern Outlook #9 – January 29, 2013


Expected Pattern Teleconnections:

  • NAO – slightly negative
  • AO – negative
  • PNA– negative becoming neutral
  • EPO – negative becoming neutral

Poll: With a colder pattern upcoming, two polls have been opened in the right side of the blog. The first is for temperature departures relative to average in NYC for the last 10 days of the month, and the second is for total January snowfall, taking into consideration the first 15 days of January will end up with no measurable snowfall. The polls will close on Tuesday night, with the results posted in the next pattern outlook.

Overview of Current Pattern

Through January 10, wintry weather has not been frequent across the region. The only wintry pattern so far since the start of meteorological winter, on December 1st, has been from the late month through early January, with two major storms affecting the region in the last week of December; the first was on the 26-27th, with significant snow in the Northeast and the interior parts of the NYC area, with mostly rain from NYC and further south/east. The second storm was on the 29th, which developed into a strong nor’easter a bit too late, with widespread snow in the Northeast and heavy snow in southern New England, but mostly rain in NYC and further south. Despite the snow staying mostly west and north of NYC, however, snow cover across the US was significantly higher than previous years following the stormy late December pattern. Even so, the active last 10 days of the month were not enough to prevent December from ending up significantly warmer than average.

The initial outlook for a cold early-mid January, as mentioned with the daily discussions from that time period, did not verify; the first few days were a bit colder than expected, but a mild pattern quickly developed, and since January 6 temperatures have been above average across most of the region. With a strong southeast US ridge building, even warmer temperatures are expected this weekend, with the majority of the region 15-25 degrees above average. By January 15, month to date temperature departures could end up as high as 7 degrees warmer than average in the NYC area. While the last half of the month will be colder, it is not expected to be enough to completely erase the warm departures of the first half of the month, with January still likely to end up slightly warmer than average as a whole.

Medium-Long Range Outlook

Scroll down to the bottom for a brief summary

The mild pattern currently affecting the region won’t last for long. There are pattern changes unfolding in the short to medium range which will be more significant than what was observed earlier this winter as well as last winter. Strong ridging in the northeast Pacific will gradually shift east through the medium range, reaching the western North America coast. Significant stratospheric warming has been observed this month, with a negative AO pattern developing for the rest of the medium-long range as the polar vortex remains over Canada, shifting south towards central-southern Canada in the medium range. A weak negative NAO will continue, generally east-central based.

Briefly summarizing the above for the region: It will get colder. Following a wave of low pressure on Wednesday potentially producing snow for parts of the Northeast, north of NYC, temperatures will gradually cool down and end up at least slightly below average by the late week. Meanwhile, the polar vortex will be over the Hudson Bay in Canada, quickly dropping towards the US-Canada border. There have been some model outlooks early on for this polar vortex to drop close to the region with a widespread eastern US arctic outbreak, but this is not expected to happen – the polar vortex is compact, and without enough amplification over the US as a result of the flattening of the western US ridge, and a lack of strong west based Greenland blocking to keep the polar vortex far south enough for a longer period of time, the polar vortex will fail to result in an arctic outbreak across the eastern US, and will only clip the northern US while remaining transient.

While a -NAO, -AO and a PNA trending positive may make it seem as if a significant cold pattern is about to settle into the eastern US, this will not be the case through the next 10-15 days. The negative NAO blocking is relatively weak and east based, with the polar vortex initially dropping south but not enough for the majority of the East to enter a frigid pattern. This is illustrated in the 0z GEFS posted above, from the PSU e-Wall, with the polar vortex near the Hudson Bay but still too far north and too compact for a widespread arctic outbreak into the US. The weak east based -NAO is also highlighted by the orange-red colors north and east of Greenland, with a relatively flat western US ridge shown as well. With the above taken into consideration, a strong arctic air mass will quickly drop towards the northern US, likely resulting in frigid temperatures around Friday into next weekend for the Midwest, northern Great Lakes, and northern to possibly central New England. The NYC area will observe near to slightly above average temperatures on most days, as the polar vortex will remain too far north at least in this time frame for the cold to affect the area. This time frame will be generally dry without much, if any precipitation.

Beyond the 20th to the 23rd, however, is when the potential for a more wintry pattern is likely to increase for the northern US outside of the US-Canada border with more sustained cold. As will be discussed in the next section, a sustained cold pattern is not certainly guaranteed, and there is room for error that would instead result in temperatures remaining seasonable without much cold air. Several factors in the pattern need to end up in the right position for a sustained cold pattern to develop, which currently are at least partially or mostly likely to verify; should ridging continue in the western US with a MJO near phase 7, along with at least central based -NAO blocking, the likelihood of a colder pattern will be greater towards the last 10 days of January, with a cold pattern fluctuating between near-slightly below average temperatures and several surges of strong arctic air into the region, mostly focusing on the Northeast, Great Lakes and Midwest, although the cold will be more widespread and will reach further south than the initial cold surge on the 18-20th as the polar vortex is closer to the northern US. Depending on the setup, the pattern may also be more favorable for at least some snow events before the end of the month. With these occasional strong cold surges, significantly below average temperatures are likely, although the significantly below average temperatures are not expected to be sustained over the region; the true arctic air is still likely to remain settled over Canada for at least most of this time period, occasionally dropping into the region, as opposed to a sustained frigid pattern over the entire region.

Overall, following the initial cold surge in the early weekend, temperatures will moderate briefly towards average but likely still below average, followed by a much colder and occasionally fluctuating pattern for the last 10 days of the month between seasonable-slightly below average temperatures and occasional significant cold surges. Temperatures should end up slightly to moderately below average for the last ten days of January, but at this time a sustained, frigid pattern is unlikely to settle over the eastern US.

As with every long range forecast, there is room for error. There has been a strong tendency for a negative PNA this season, and some of the latest models show a persistently positive PNA. While a rise in the PNA is expected towards at least neutral or slightly positive, some of these models may be overdone with the +PNA. Ridging is currently modeled to remain over the western US through the medium range, although it is a possibility, given recent patterns, that the ridging may shift back west into the NE Pacific, which would be less favorable for a cold pattern and more favorable for at least seasonal temperatures. The MJO outlook is also uncertain for the longer range, with some models showing the MJO stalling in phase 6, which favors above average temperatures for the region, other models collapse the MJO wave, while others bring it into phase 7, which is somewhat more supportive of colder temperatures. A MJO remaining in phase 6 would be less favorable for a colder pattern as opposed to a phase 7-8 MJO. While the current outlook is for slightly to perhaps moderately below average temperatures for the last ten days of the month, the main bust potential is that this pattern fails to fully materialize, and instead temperatures end up near to slightly warmer than average towards the end of the month, with arctic air still struggling to enter the US, as it has for the majority of the winter so far. More information on this will be posted with the upcoming daily discussions, as well as the next pattern outlook.

Summary: Following the continued mild temperatures through early next week, on the 14th, a colder pattern will gradually develop. For late next week and next weekend, there will not be a significant arctic outbreak across the eastern US; instead, the frigid temperatures will remain north of the NYC area, with temperatures through the 20-23rd near to slightly above average. Afterwards, a much colder pattern will develop for the region; occasional strong cold air surges are expected, with temperatures ending up slightly to moderately colder than average, significantly below average at times with the cold surges, although the magnitude of this cold pattern is not expected to be record-breaking. There is still some uncertainty, however, and the main bust potential is for the cold pattern to fail to fully materialize, with temperatures instead near to slightly warmer than average.

Pattern Outlook #5 Verification

Verification for Pattern Outlook #5 will be added soon.


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