Long Range Pattern Outlook #6

Brief Overview:

Tonight’s long range pattern outlook focuses on the medium range pattern going into the second half of the month, as some changes in the pattern do not result in a much colder pattern but do increase the probability of seeing at least one snowstorm by the end of the month.




Pattern Outlook Archive

<< Pattern Outlook #5 – November 21, 2012
Pattern Outlook #6 – December 10, 2012
>> Pattern Outlook #7 – December 24, 2012


Expected Pattern Teleconnections:

  • NAO – slightly negative

  • AO – slightly negative

  • PNA– negative

  • EPO – positive

Medium-Long Range Outlook

Scroll down to the bottom for a brief summary

There were some changes in the pattern early in the month, but as expected from the previous pattern outlook, these changes failed to bring in a cold and snowy pattern. Instead, however, the first half of December is even warmer than expected from the last pattern outlook posted before the winter forecast, leaving the area on track to likely end up with warmer than average temperatures in December. Out of the first ten days of the month, three days had temperatures reaching 60 degrees somewhere in the area, with today one of those days as a cold front is approaching the area, bringing cooler temperatures for the rest of the week but still ending up overall near to slightly warmer than average.

The pattern so far remains unfavorable for snow, but is different than the pattern last year that resulted in the complete lack of winter. Unlike last year, when blocking or even ridging was almost entirely absent from the higher latitudes near Alaska and Greenland, there has been occasional blocking in these regions, and the AO, while strongly positive last year, has been negative this month, strongly negative at times. Some of the aspects of the current pattern that are preventing a winter-like pattern for the region include only weak ridging mostly east of Greenland, along with the persistent lower 500mb heights near the western US and Alaska, as the PNA has been persistently negative with little variation since early November. The PNA did slightly rise closer to neutral today with a strong ridge near the northeastern Pacific, but this feature is short lasting as the ridge shifts west north of Hawaii with lower heights returning to the NE Pacific. Ridging continues over the southeastern US with the stronger cold air masses that do manage to drop into the US relatively transient and mostly staying west of the Northeast.

There are changes coming up in the pattern, however. While this will not be a significant pattern change with persistent cold and snow locking into the region, there will be gradual changes that will result in at least somewhat cooler temperatures along with a few snow potentials before the end of the month. Most notable of the changes in the medium range is ridging and/or blocking extending into Greenland, with a central to possibly west based -NAO expected to develop already within the medium range. Around the same time, the strong polar vortex will shift out of North America and towards northeastern Russia. The Pacific pattern remains not too different with lower heights persisting near the western US and the northeastern Pacific, which will help to prevent a persistent cold pattern from developing in the East, although some MJO activity around phases 1-2 is expected. Normally, a MJO in phase 1 tends to result in above average heights in the western US; while this will not be the case here through at least the end of the month, having the MJO closer to phases 1-2-3 rather than the warmer 4-5-6 phases is just one of many differences from last year that suggest winter will actually arrive this time.

One of the several features to consider with this pattern is the lack of strong cold as the polar vortex shifts over to Russia with ridging and/or blocking taking place near Greenland. While a more active pattern is expected, no strong cold outbreaks are expected along with it, which along with the continued lower heights near the western US is likely to act as an obstacle for any sustained strong cold and snow pattern for parts of the region. However, especially with the expected ridging over Greenland, more suppressed heights are expected for the region as shown with the GEFS above from the PSU e-Wall, with below average heights further south in the region as opposed to over Canada, which does support cooler but not persistently cold temperatures for the region, and along with a likely stormier pattern, depending on the storm track, some of these are expected to produce snow for the region.

Summary: With some changes expected to take place in the pattern, chances will increase for slightly cooler temperatures and snow for the region starting from the 16-18th through at least the end of the month, with less frequent warmth surges than what was observed during the first ten days of the month until today. Stormier conditions are expected with this pattern than the first half of the month, with a weak rain event expected on the 16-17th with another storm sometime around the 19-21st. This is not expected to be a persistently cold and snowy pattern, with temperatures in December still likely to end up slightly warmer than average at this time. This pattern, however, will be more favorable for at least a few snow events in the region, certainly more favorable than last year was at this time, and the potential is there for at least one snowstorm to affect the area between the 20th and the end of the month. This potential may or may not end up verifying, but unlike last year, the potential is actually there.

Pattern Outlook #3 Verification

Link to Pattern Outlook #3

Pattern Outlook #3, posted on November 5, expected a -PNA and +NAO pattern to develop for November 10-13, with highs in the 50s and a day or two of 60s in NYC. Ridging would rebuild behind a cold front on the 13th, with a dry and warmer than average pattern through the 18th. Temperatures would then moderate closer to average afterwards.

Above, I posted the GEFS initialized 500mb height anomalies across North America from 11/11 at 18z, from the PSU e-Wall, the same time frame from the posted GEFS image in the pattern outlook. As modeled, this shows a strong ridge over the eastern US and a trough in the central-western US. Temperatures ended up in the 60s for November 11-13, with Central Park peaking at 66 degrees on the 12th. Temperatures did cool down behind the cold front, and while ridging did rebuild over the majority of the US through the 18th along with above average 500mb heights, surviving through the 23rd, lower heights persisted over the Southeast with a high pressure over the Northeast instead of the Mid Atlantic, keeping the region with NE winds and slightly below average temperatures instead of above average temperatures. With this development, which was not expected in the pattern outlook, November ended up with below average temperatures instead of near-slightly above average temperatures, even though this time period lacked any strong cold air mass.

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