NY NJ PA Weather Forecasts

Winter Outlook for 2009/2010

It’s that time of year again where forecasters are thrown into a battle royal for best winter forecaster and that battle begins with the seasonal winter weather forecast.  To be perfectly honest, I have never really considered myself a long range/seasonal forecaster.  My love and my strength (I guess one feeds the other) has always been the medium and short range territory.  However, if you don’t challenge yourself and put yourself outside of your comfort zone, you don’t grow.  That’s true no matter what you do.  I think that’s why I took poetry in college.  Oh boy, what a disaster.

So as we move into November and look towards this winter, I’m sure we have all been watching the headlines blaring about the coldest winter in a decade or the most active ever.  My favorite forecasts are the 150% snowfall forecasts, which stretches from New England to the Mid Atlantic.  I have not nor never will be a big fan of the % above or below normal snowfall forecasts.  The problem with these forecasts is that how do you take into account sleet or freezing rain?  Do we have a % above normal sleet map?  The point is that these maps are used for one thing and one thing only, to get you excited and keep on visiting that particular website or news outlet.

I know YOU will come back here for the best weather information because the forecast has a solid foundation and is presented in an honest fashion.  So you won’t find any hype here.  Just my thoughts in as clear as possible a way to present them.

There are several factors I looked at to make this forecast.  I want to discuss the data I used and the thought process before diving into the forecast.

Method:

The process of developing the forecast took around two months to formulate.  I studied the various years of weak and moderate El Nino ENSO states and their impacts on the North American cold season pattern.  I did not use the seasons as a copy for this season, but I wanted to examine the range of possibilities from El Nino years.

I also examined extensively into studying the development of the stratosphere in terms of magnitude and location of anomalies.  I also studied years that had similar height anomalies in the fall with El Nino cold season years.

Finally, I’ve been studying the progression of Sea Surface Temperature anomalies and the impact they have had on the pattern thus far.

My forecasting method was not an exercise in matching this year to previous years.  While learning from past cold season pattern evolutions is an important learning tool, it is not a forecast.

Other considerations included the development of the Fall pattern, sea surface temperature gradients, and an examination of the pattern evolution from La Nina to El Nino over the past 6 months.

The Forecast:

The best way to describe the winter of 2009/10 is active.  Very active.  There are several players on the field that suggest that there will be no lack of moisture nor disturbances that will move through the northern Mid Atlantic this winter.  The key as to whether those disturbances produce snow, ice, or rain will depend on the interaction of the Sub Tropical Jet Stream and the Polar Jet Stream.

Sea Surface Temperatures:

PACIFIC:

I first want to discuss the evolution of the Sea Surface Temperatures in the Pacific and Atlantic.  To the left I have posted the SST anomalies on October 1 and then October 26.  There are important changes in the SST anomalies that gives me a direction on where to take this forecast.

The first feature we need to discuss is the El Nino that has developed.  El Nino is approaching a moderate level with impressive warming occurring in the 3.4 region this month.  The latest guidance (CFS Ensemble and other guidance) are trending towards this El Nino peaking at a moderate level around 1.0C above normal.  However, this would place this El Nino clearly on the boarder between weak and

moderate.  This El Nino is currently west based, centered over region 3.4, however there are indications of expansion to a basin wide El Nino, which would have an impact on the development of the 500 MB pattern over the Pacific.  As the El Nino transitions into a basin wide event, support will grow for lower heights to migrate to and from the Gulf of Alaska back to the Aleutian Islands.

The Tropical Pacific will also influence the winter with the emergence of Kelvin Waves as an active MJO will keep the Sub Tropical Jet Stream very energetic.

Moving northward, we can now key on the northern Pacific.  I’ve been studying the progression of the sea surface temperatures over the Gulf of Alaska and the northern Pacific since September.  The trends clearly show warming SST over the Gulf of Alaska over the past 60 days and significant cooling has transitioned towards 160 W/40N.  It is clear that the PDO is now in a weakly positive state.  Given the focus of the best upper level divergence over the central Pacific, the feedback on SST will continue to support a weakly positive PDO.  A positive PDO fosters a ridge that builds over the west coast of North America and a trough over the eastern third of North America.

The overall feel I get from the Pacific is an environment that supports an active Sub Tropical Jet Stream and the support for a negative EPO pattern for the Polar Jet Stream.  These two jet streams will at times be called the northern and southern branches.  Either way, I don’t expect a zonal “water hose” signature in the Pacific this year as the 500 MB pattern will be significantly amplified.

ATLANTIC

The changes in the Atlantic Sea Surface Temperatures Anomalies were not nearly as drastic as seen in the Pacific this Fall.  The main change I noticed is the continued cooling off the Newfoundland coast of Canada, also known as the Canadian Maritimes.  There is some question as to whether the SST in the northern Atlantic foster the NAO or the other way around.  My forecast on the NAO is more focused on stratospheric conditions, which I will address shortly.  However, the development of a “horse shoe” signature over the northern Atlantic does support the potential for a feed back environment where an upper level low can become established.

The most interesting features in the Atlantic are found along the Mid Atlantic and Southeast coasts and the Gulf of Mexico.  Temperatures are significantly above normal over the Gulf Stream over the past several months.  The above normal waters over the Gulf Stream and the Gulf of Mexico point to an environment of rising air off the coastal waters of these coastal locations.  Where air rises, colder air tends to move in to fill in the rising air.  Beyond all the complicated numbers and calculations, sometimes it is important to go back to simple meteorological physics.  The above normal SST anomalies strongly point to an environment that is supportive of cyclone genesis in these locations.

TOP OF THE WORLD!- Stratospheric Thoughts

One of the features that I don’t see utilized enough is understanding the evolution and mechanisms of the stratosphere.  Think of the stratosphere like an air conditions.  When temperatures warm, the air in these locations press down on the troposphere, creating colder temperatures in the mid and lower levels.  Conversely, colder anomalies in the stratosphere leads to warmer temperatures in the lower and mid levels of the troposphere.  The key to understanding the impacts of the stratosphere on the troposphere in my opinion is not just the magnitude of the temperature anomalies, but where the anomalies were established.  A perfect example of this would be in 2006 where significant warming of the stratosphere developed, but was primarily focused over Asia.  The result?  One of the worst winters China has had in recorded history with well above normal snowfall and well below normal temperatures.  The warming of the stratosphere over Asia supported strong blocking that kept a cold pattern in place.

Now, here is where the study of the stratosphere gets interesting.  The sun has been rather quiet over the past several months however of late some new solar flares have developed.  These flares have a significant impact on the upper layers of the Earth’s atmosphere, like the stratosphere.  Increase activity from the sun means more energy and more the potential for warming in the stratosphere.

Note the latest trends to the left.  Up until the past two weeks, the temperatures at the stratosphere have taken on an almost average trend in temperature rises and falls this year.  However, the latest activity has caused temperatures to spike to above normal levels.  At this same time, a strong upper low has formed around the Aleutian Islands as temperatures have been rising over this region at the stratospheric level.

I believe we are about to enter a more active period in the output of solar flares from the sun and the result will be a warming event at the stratosphere.  The current trends suggest that the anomalies should be focused over the north Pacific, which will lead to a significant amplification of the jet stream over the North Pacific.  This amplification will lead to a sustained upper low over the Aleutians through the winter.

As for the North Atlantic Oscillation, there is a trend to have the lowest anomalies to the north and east of Greenland, basically around northern Europe.  I don’t find much support for a strong NAO signal this season and would lead towards a neutral to slightly negative NAO through the winter.  Stratospheric temperatures around Greenland should average near normal for much of the winter, which would continue to lend support to a neutral, wavering NAO signal.

What Does A Neutral NAO Signal Mean?

There has been a lot of misunderstanding on what the NAO means for the northern Mid Atlantic.  The main misunderstanding is that a negative NAO is a must for an active, stormy winter.  This is not the case at all.  I turn to the great research work of Paul J. Kocin and Louis W. Uccellini in the book Northeast Snowstorms, which proved that winter storms developed when the NAO was in a state of change.  When the NAO was strongly negative, the result was a suppressed jet stream which supported cold and dry conditions, while a strongly positive NAO supported a progressive and usually warmer pattern over the Northeast and Mid Atlantic.  A neutral NAO however, supports a pattern where the NAO is in a constant state of change, which will support an environment favorable for storms along the Mid Atlantic coast.

Over the past several weeks as the changes in the stratosphere begin to take hold, we’ve seen a change in the NAO indices from a strongly positive NAO in the early Fall is a more neutral signal heading through the late Fall.  There has been several case studies that suggest a strong signal in the NAO through October lends to a strong opposite signal for December, January, and February.  However, little connection can be made when weak neutral signals are analyzed.

My support for a neutral NAO is based on the positive of stratospheric anomalies, which will support normal stratospheric temperatures around Greenland and northeastern Canada, which will lend to weak blocking over the northern Atlantic.  The supportive cyclone genesis environment (above normal SST of the Gulf Stream) off the Mid Atlantic will lend to the potential for several upper lows to develop in and around 50N/50W through the winter, however the lack of support to keep these upper lows locked in place will lend to an ever fluctuating signal in the NAO index.

Been Kind Of Wet Lately:

For those that have been following the section of the New Jersey drought coverage, you’ll note that a significant change in the amount of precipitation over the Mid Atlantic has increased.  In fact, since April of this year, the drought deficit over southwestern New Jersey has dropped from 14 inches below normal to only 3.73″ below normal as of this writing.  This is a significant drop in the rainfall drought deficit.  In fact, precipitation over the past 7 months have averaged 1.64 inches above normal and in the past 3 months has averaged an amazing 3 inches above normal per month!  These rainfall amounts above normal are very impressive for southern New Jersey, and I’ve found similar trends in the precipitation throughout the northern Mid Atlantic that mirror southwestern New Jersey.  What has been observed is a change in the pattern influences of that of a La Nina pattern with a strong Southeast ridge in place to one of the El Nino were the Sub Tropical Jet Stream is more active.

QBO Is Negative:

Another feature I’ve been keeping an eye on is the QBO or the Quasi-Biennial Oscillation, which recent date suggest is now in a negative state.  A negative QBO lends leads to a more favorable environment to stratospheric warming (Holton and Tan (1980)), which would lend support to the idea of an above normal stratospheric temperature regime over the Aleutian Islands and northern Pacific.  A negative QBO also supports a higher potential for blocking in the northern latitudes, which given the latest developments in the stratosphere could lend more support to a Cahir ridge over eastern Europe.

Snow Growth:

Naturally, one of the keys to forecasting for the winter is to examine the environment favorable for cold air masses to develop, namely snow growth in the northern Latitudes of Siberia and northern Canada.  Even with all the blocking in the world, if cold air can not be supported then frozen precipitation is going to be hard to come by.

As of October 27, 2009; the amount of snow cover is above normal over Siberia and much of Canada.  The only exception is over Alaska where a warm marine environment from the deep 500 MB low just to the west is preventing snowfall.  What this means is that a supportive environment exits for cold air masses to develop and remain sustained.  This also means that the amount of moderation with these air masses will initially be limited as these air masses slide south.  In other words, when an Arctic connection develops expect a strong punch of arctic air to follow with little in the way of moderation from northern Canada into the Mid Atlantic.

The Forecast:

NOVEMBER

Now that all the data is before us, how will these different factors combine going forward this winter.  I expect a transient pattern to continue through much of November as the positive PDO takes hold and influences the Polar branch of the Pacific jet stream.  The basin wide moderate El Nino will lead to a broad trough over the central Pacific as well.  The support of the ridge over the Bahamas will continue to weaken, but not without a fight.  Expect model guidance beyond day 3 to continue to change wildly as November continues, this will be a constant theme for this winter as the Sub Tropical Jet Stream strengthens.  With no clear blocking taking hold over the northern Atlantic and a still evolving Pacific pattern developing, expect troughs to continue to dive and then lift out of the East.  This will support cold shots of Polar air to dive into the northern Mid Atlantic, but not able to establish itself as a dominant air mass.  The transient nature of these troughs will lead to progressive storm tracks, however the strengthening Sub Tropical Jet stream will add a moisture source from the South Pacific and Gulf of Mexico to these developing storms.  The potential for another month of well above normal precipitation can be expected.  Two distinct storm tracks will develop in November.  The first storm track type would support low pressure development over the southern Plains and track towards the eastern Great Lakes, much like what has been seen in late October.  This storm track would represent a reload pattern.  The second storm track, which I expect to start November and end November would feature a deep trough over the eastern two-thirds of North America and several clipper systems diving south towards the Mid Atlantic.  These storms could support rapidly coastal redevelopment depending on the state of the NAO at that time and the strength of the upper level disturbance.

DECEMBER, JANUARY, AND FEBRUARY

Given the transition of the PDO to a weakly positive state, a moderate basin wide El Nino, a negative QBO, and a neutral NAO; all indications point me to this winter being a very active winter for storm development.  Above normal precipitation can be expected throughout the Philadelphia and New York City metropolitan areas as an energetic and moisture loaded Sub Tropical Jet Stream enhances disturbances over the Gulf Coast and Southeast, which will eventually impact the Mid Atlantic.  The neutral state of the NAO suggests that Arctic nor Polar air masses will not be able to lock in over the Mid Atlantic for long stretches, however the above normal Sea Surface temperatures off the East coast and Gulf Coast suggest that cold air will not have a problem reaching the coast when in a supportive

atmospheric environment.  The development of a negative EPO (the Aleutian Low) and the Cahir Connection over eastern Europe will allow plenty of cold Arctic air to move into the eastern two-thirds of North America, however the focus of the cold air will remain from the extreme eastern Northern Plains through the Great Lakes and south towards the Tennessee Valley.

The return of the Gulf Low and the Nor Easter storm tracks will make a big return to the winter pattern this year.  However, the pattern will at times result in a storm track over the interior towards the eastern Great

Lakes due to a positive phase in the NAO at that time.  As November comes to an end, a shift should take place in the storm track towards the Mid Atlantic coastal waters as wave lengthens increase, as is normal for cold season months.

Conclusion:

Given the facts stated above, I believe the New York City and Philadelphia metropolitan area or also known as the northern Mid Atlantic can expect a very active year for stormy conditions.  Temperatures will average near to slightly below normal with a temperature range of 0 to -1°F below normal for December, January, and February.  A few significant cold shots are expected this winter, but the brunt of the cold air will be focused further west.  A storm track will develop from the Gulf Coast to southern New England, which will continue the trend of precipitation averaging above normal for the winter.  The combination of near normal temperatures and above normal precipitation lends support to an environment of above normal snowfall for the region.