Long Range Pattern Outlook #2

Posted on October 16, 2012 by NYC Area Weather

[notice]At this time, ONLY the pattern outlooks will be posted in the new proposed posting format. A decision will be made next weekend regarding the posting format of the daily discussions.[/notice]

Tonight’s long range pattern outlook focuses on the second half of October, regarding the medium range pattern and the potential for October to end up cooler than average, along with some thoughts going forward into the rest of the fall and the winter. The final winter outlook will be posted towards early-mid November.

Pattern Outlook Archive

<< Pattern Outlook #1 – October 6, 2012
Pattern Outlook #2 – October 16, 2012
>> Pattern Outlook #3 – November 5, 2012

Expected Pattern Teleconnections:

NAO – negative, then slightly rising
AO – negative, then rising
PNA– negative, then rising
EPO – positive becoming negative, then rising

Medium Range Outlook:

Following the collapse of the western North America block last week, a less amplified pattern returned to the US as a low pressure moved through the central US and into the region early this week, moving out yesterday. Currently, the pattern consists of a brief block over Greenland, with a trough near southwestern Canada expected to drop into the central US before cutting off and becoming a strong closed low with lower than average 500mb heights over the Great Lakes region. The trough will weaken before reaching the East, however, and will fail to produce any significant cold, keeping the region not far from average.

Behind the trough next weekend, a stronger, more persistent trough is expected near the NW US and the northeastern Pacific, with ridging developing for the eastern half of the US accompanied by a surge of warmth. Unlike the previous long range outlook when the longer range was easier to forecast due to model agreement regarding a strong block off the western North American coast, however, in this case the longer range pattern becomes more complex beyond this time frame as model solutions diverge beyond 6-7 days out. While the earlier model guidance developed a strong eastern US ridge from next weekend (10/21) through the rest of that week, the GFS and the latest ECMWF run quickly bring in another trough into the region while the strong ridge builds into the central US. At this time, I am siding with this solution of a brief surge of warmth for the 10/21-10/23 time frame but with a strong ridge failing to build in.

By 10/23, a week from today, strong ridging will be located near Alaska and the Aleutian Islands, with above average 500mb heights stretching through northern Canada towards Greenland, where some ridging is also expected. With the -NAO in place and a trough in SE Canada, the developing ridge early next week will quickly be suppressed, but is expected to be replaced with a generally closer to average pattern instead of a cold pattern. The pattern will remain unfavorable for any solid cold pattern to develop for the eastern half of the US especially with the strong persistent trough off the coast of the NW US; the persistently cooler time frames in mid September and early October featured a persistent ridge in that region as opposed to a trough. Heading towards the remainder of that week, there could be an additional warm up, although no significant warm pattern is expected to develop, with occasionally cooler temperatures but no persistent cool pattern. The storm track is expected to stay to the north and west of the region, towards the north central US.

Towards the very end of October, the PNA, which will be negative in the medium range, is expected to rise, along with a generally persisting negative NAO although with some moderation towards neutral possible. The EPO is currently modeled to be slightly negative, a considerable drop from the current positive EPO. A positive EPO, which was dominant during last year’s winter, favors lower than average heights over the NE Pacific and Alaska with above average temperatures and heights for the northern US and parts of Canada; a -EPO is often more favorable for a colder pattern in the region. Although at this time, no significant persistent cold pattern is expected to set up, the potential is there for a colder pattern towards the end of October or early November. More on this will be posted with the next pattern outlook.

Tonight’s Focus: Will October finally break the consecutive 18-month stretch of near-above average temperatures?

Most of the NYC area, including other parts of the region, have observed an unprecedented 18-month stretch of warmer than average monthly temperature departures. Some months came close to breaking that stretch, and June barely did for Central Park with average temperatures in Newark, but there has not been a single month in this stretch with widespread colder than average departures since the ending of the winter of 2011. With a colder pattern in the first half of October, the question at this time is if October will finally manage to break that spell.

As of October 15, most of the NYC area is slightly warmer than average, more so in southern Connecticut and Long Island than NYC and NE NJ, which are for the main part equal to or less than 0.5 degree above average so far. Central Park is currently 0.2 degrees cooler than average, although it is the only NWS station in NYC that is colder than average for the month. For the United States, the entire central United States is colder than average, most locations more than 3 degrees colder than average, while the eastern US is generally seasonable and the western US is slightly warmer than average. Given the expected pattern for the remainder of the month, no significant warm pattern is expected for the area, although not many significant cold departures are expected either, with the warmth likely overshadowing any cold departures through the last full week of the month. As stated in the original October monthly outlook, in the Long Range Forecasts page, October will generally end up close to average, and at this time it still appears that for at least most of, if not all of the area, the final monthly departure should end up slightly above average. With the proximity to average, the potential is there that some parts of the area could be slightly cooler than average, especially if much colder temperatures are observed towards the very end of the month. More information on this will be posted with the daily discussions.

Poll: Earlier this month, NYC Area Weather had a poll about the monthly departure of October and whether it will break the 18 month stretch of near-warmer than average temperatures. Out of 19 votes, below are the final results:

7 votes – Yes; colder than average (<-1 degrees)
7 votes – Yes; barely cooler than average (-0 to -1 degree)
1 vote – No; slightly warmer than average (+0 to 1 degree)
4 votes – No; warmer than average (>1 degrees)

Longer Range Thoughts: El Nino Struggling To Develop

Current sea surface temperature anomalies in the ENSO regions; Nino 1+2 and Nino 3 are slightly negative, while regions 3.4 and 4 remain positive, but not as much as earlier in the fall.

So far, numerous winter outlooks have been issued, and some are expecting a colder and snowier than average winter for the East. While that may not be an unreasonable expectation, many of the outlooks from earlier this fall, however, were based on the expectation of an El Nino to form. While El Nino conditions did develop in the ENSO regions in the equatorial Pacific, the El Nino development has stopped, and sea surface temperature anomalies have dropped recently, with the western ENSO regions still slightly positive, although most importantly the eastern ENSO regions have dropped more sharply and are now slightly negative; other than a brief negative dip in the Nino 1+2 region in late August, both regions have not been negative since the spring, when last year’s La Nina was weakening. While the eastern ENSO regions tend to fluctuate more frequently than the western region and may still rebound to become slightly positive, the El Nino is still struggling, and at this point it is increasingly less likely that this winter will end up as an official El Nino, even if there is some recovery with the SST anomalies. Some of the recent climate models, such as the CFS, are also suggesting that the anomalies continue to trend slightly downwards without an official El Nino this winter, with a likely result being a neutral positive winter, in which SST anomalies are neutral or slightly positive but are not enough to be qualified as an official El Nino. Some La Nina-like influence may perhaps be possible going into the winter, although a La Nina will not develop. With the absence of a solid El Nino, signs are pointing to another year of difficulty with making the winter outlook.

Instead of a preliminary winter outlook, I will be posting occasional thoughts on the winter with the pattern outlooks until the final outlook is issued. Next week’s pattern outlook will contain a section about the winter focusing on the pattern and the differences and similarities compared to last year, and how this could impact the winter outlook. Stay tuned for this year’s winter outlook, which will be issued around early-mid November.

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