THE OFFICIAL 2012 FALL FORECAST
By Sam Ebby of Northeast Weather
Setpember 27th, 2012
As summer fades to autumn, we are reminded that winter lies right around the corner. Much can be said about any season’s winter during the Fall Season. Atmospheric patterns tend to spill over into the winter from autumn. Last year’s Fall season proved to be quite diverse in nature. A historic “freak” paralyzing snowstorm struck the Northeast in late October, only to be followed by an unseasonably warm November across the Northeast and much of the Nation. November of 2011 was the 11th warmest November of all time. These conditions, to the disappointment of many, spilled over into the Winter. This year, the planet is in a completely different atmospheric pattern; one dominated by a weak El Niño. This is essentially the opposite of the Moderate La Niña that we experienced last year. The 2012 Fall season will develop an entirely different pattern than that of last year, potentially leading us into a far snowier winter this year in comparison to last.
While there are many contributing factors into this seasons forecast, among the most influential is the current phase of ENSO (El Niño Southern Oscillation). As of September 24th, 2012, A weak El Niño is expected to develop, and remain present throughout the Winter months. El Niño is characterized by unusually warm sea surface temperatures in the equilateral Pacific ocean, or an ONI (Oceanic Nino Index) value of +0.5 or greater. During the Fall months of a weak El Niño, the southern branch of the Jet Stream historically becomes very active; producing large rainstorms that primarily impact the southeast and occasionally the Mid Atlantic/Northeast. This is something I expect to become existent during this year’s Fall Season. In addition to the chance for above average precipitation, A weak El Niño historically bring slightly above-average temperatures during the autumn months of September, October and November.
However, the current phase of ENSO is not the only factor contributing to the fall season’s forecast. There are three other prominent factors that dictate our climate here in the Northeast and across the world. The N.A.O or (the Northern Atlantic Oscillation), being the first. The NAO is a climactic fluctuation, located in the Northern Atlantic Ocean. There are two phases of the NAO: the positive phase, and the negative phase. During the positive phase, low pressure centered around Iceland and high pressure centered around the Azores Islands intensify. The stronger the systems in these areas, the more positive the NAO becomes. Throughout the positive phase, the Jet-Stream retreats to the north, allowing warmer air to move farther north, having an impact on the Northeast with well-above average temperatures. Historically, large-scale storms are less common during positive phases of the NAO.
During the negative phase, low pressure centered around Iceland weakens significantly, and moves over Greenland, while the Azores high weakens. This creates a “blocking” effect, forcing the Jet stream to dip far to the South, allowing cold Arctic air to drop down onto the Eastern United States. This periodically allows large-scale Nor’easters to move up the eastern seaboard, and cause heavy rain and/or snow during the fall and winter months. While it is nearly impossible to predict the NAO for an entire season, let alone four weeks, I can tell you this: I have noted recent trends are beginning to point towards a more negative NAO this fall season in comparison to last year. This would allow large-scale storms like Nor’easters to strengthen and move up the eastern seaboard in greater frequency than last autumn. Unlike last year’s fall season, when the NAO stayed positive for the majority of the fall and winter seasons, resulting in an unusually warm fall, with very few Nor’easters. This was a pattern followed us into winter. While I cannot eliminate this scenario, it is far less likely to occur once again this year.
The second major factor or “teleconnection” contributing to the fall season’s forecast is the A.O or (Arctic Oscillation). The AO, like the NAO has two phases, and tends to fluctuate a great deal. When in it’s negative phase, High pressure builds in over the arctic, forcing the Jet Stream south, and with it, cold arctic air. During the Positive phase of the AO, low pressure builds in the arctic, trapping Arctic air up in higher latitudes, while a zonal flow tends to develop across the United states, spreading warm Pacific air across the nation. Along with the NAO, the AO is virtually impossible to predict no further than 1.5 - 2 weeks out at a time. Recent trends suggest that for the most part, we are in a primarily positive phase. However, this can change dramatically over the course of a few weeks.
The Third major contributing factor is the P.N.A or (Pacific/North American). The PNA like the AO and NAO, has two phases, and is a variable. The Pacific/North American can have a large Impact on the National weather pattern during the fall and winter seasons. During the PNA’s positive phase, high pressure centered around the Rocky Mountains strengthens, while low pressure centered around the Aleutian Islands does the same . This creates a large ridge across the eastern Pacific extending into the western United States. In response to the western ridge, a trough develops in the east.
The negative phase of the PNA is essentially the exact opposite. High pressure centered over the Rocky Mountains weakens, as high pressure centered over the northern Gulf Coast strengthens. This creates a trough in the West, and a large ridge in the East. On occasion, a negative PNA can form a stubborn “southeast ridge” continuously pumping warmth into the eastern seaboard for an extended period of time. This was something that was very persistent last year, and greatly contributed to the exceptionally warm fall and winter of 2011-2012. However, this year I do not see a southeast ridge developing and persisting for as long as it did last year. As far as the PNA goes, I am forecasting it to fluctuate often, wondering in between positive phases, and negative phases. Neither phase more dominant than the other. However, this will likely change as we head into winter.
Overall, I do expect the weak El Niño to have a substantial impact on the nations pattern as we move towards winter. Extensive research has been completed in order to create the maps you see in this forecast. Using analogue years of similar ONI values & patterns, current 30 to 90 day temperature anomalies, climate forecasting models, and much more, I have crunched in the numbers, and what I have found is the following. I will begin with the Temperature Outlook issued for September, October, November and into early December of 2012
In the forecast, I am calling for a good portion of the Northeast to experience slightly to moderately above average
temperatures during the fall season of 2012. The probability of above average temperatures begins in Allentown, PA and extends north, throughout the rest of the Northeast. While to the South & west of Allentown, there are “Equal Chances” for both above, and below average temperatures. This is where I anticipate either “near normal” or a less than 20% risk of either above of below average temperatures. Either departure is possible, however nothing significant. To the south, Starting in North Carolina, and extending throughout the Southeast and portions of eastern Texas, I am forecasting below average temperatures. This is backed up by both history, and 30-Day and 90-Day trends. Negative temperature anomalies are beginning to develop in these areas.
Meanwhile, off to the Northwest, I am calling for among the greatest departure from normal to setup to the southwest of the Hudson Bay in Canada. Nearly every year with a similar pattern to that of this year has had above normal temperatures to the south of, or along the southwestern shores of the Hudson Bay. While off to the west, I am anticipating much of the west coast to experience below average temperatures. Especially along the western portions of California. In this area, I am forecasting a 60% chance of below average temperatures. Looking back in history, this idea is supported time and time again. Nearly every weak El Niño Fall, this area recorded below average temperatures.
Second, I have created a Precipitation Outlook issued for September, October, November and into early December of 2012. This year, I expect an active southern branch of the Jet Stream to produce large rainstorms across the Southeast & Gulf coasts. Periodically, if the conditions are right, the southern branch may phase with the Northern branch, creating Nor'easters that move up the east coast. If we are to see a pattern favoring frequent Nor’easters over the course of the next 70 days, this may very likely carry over into Winter. However, at the same time, it is very possible that the Northern branch of the Jet Stream stays split, and rainstorms slide off the Southeastern coast instead of up the East Coast. As of now, I expect at least three Nor'easters to impact the Northeast this fall. My forecast calls for the heavily populated eastern I-95 section of the Northeast to receive slightly above average precipitation, while farther inland, the probability for normal if not below average precipitation increases. South of Philadelphia PA, the probability of above average precipitation increases significantly. This is a result of a moisture-rich active southern Jet Stream expected to fire up this Fall and early winter. During a typical weak El Niño fall, the Gulf & Southeast coasts experience above average precipitation as a result of an active southern Jet Stream. Off to the Great Lakes & interior Northeast region, where a long stretch of below average precipitation is forecasted. This area typically receives less than normal amounts of precipitation during an weak El Niño Fall season.
As a result of El Niño, the Pacific Northwest is also expected to receive below average amounts of precipitation this Fall. Storms will be less frequent in the Washington/Oregon/Vancouver region of the Pacific Northwest.Most of the powerful Pacific storms that typically form during the fall and winter months, will track farther to the north, impacting central and northern British Columbia along with Alaska.
Drought conditions in the south will likely see some relif. This year’s pattern favors a very wet season in the south. The wettest since the fall/winter season of 2009-2010. While I do not expect the same pattern to setup, similar effects may be felt.
In conclusion, expect a very different season this year in comparison to last. We must study the patterns that setup during the autumn season; as history has shown they may very likely follow us into the winter season. I am forecasting a pattern immensely different from what we experienced last year; perhaps a pattern favoring heavier snowfall along the east coast in comparison to last year. This is an example of one of the many questions can only be answered by studying the nature of our global weather pattern during autumn.
Thank You for reading The Official 2012 Fall Forecast written and published by Sam Ebby of Northeast Weather. For more on the upcoming Winter of 2012-2013, stay tuned to Northeast Weather over the next few weeks, as the Official 2012-2013 Winter Forecast is expected to be released in Mid/Late November. Have a wonderful season!
-Sam Ebby of Northeast Weather
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