Cornell University- Atmospheric Sciences Undergrad; Research Assist.- Onset of Spring Indices Toolbox; Interests- Small spatial scale climatolology
By: Zachary Labe , 12:25 AM GMT on January 18, 2009
The Middle Atlantic Winter...
(December 21 Ice Storm - Blue Mountain, PA)
Introduction to Typical Middle Atlantic Winters...
For quite a few years the Middle Atlantic has been suffering from under a snow drought. While many are quick to answer this snow drought with a few little words such as climate change, it is not quite that easy to characterize this reasoning. Many global features and synoptic patterns have been favoring an unusual and extended jet stream formation that has characterized areas farther north to see above normal snowfall with sharp gradients located across Pennsylvania between snow and no snow. While there is no easy answer to the snow drought, this blog will examine the reasoning behind much of this pattern along with examining past climatological means (finding relationships), and examining case studies of past major east coast snowstorms to show the variables necessary for these major events. This is my 100th blog special edition and I hope you enjoy...
Middle Atlantic snow seasons are characterized by long periods of snow droughts followed by large anomaly years, which tend to make up for all of the snow droughts. Therefore much of the region has snow averages that are much higher than they should be for typical winters. Exclude memorable winters such as 1993-1994, 1995-1996, and 2002-2003 and you are left with snowfall means nearly 10inches less than current historical seasonal averages. Take this graph for example for snowfall seasons from 1984-2008 for State College, PA...
(Courtesy of NOAA)
A majority of the seasons fall below climatoligical means with only a few near seasonal norms and only a handful of large anomalies. With most human minds programmed to recall only the large anomalies, therefore people are disappointed when those large anomalies do not come to fruitation. Here are a few Philadelphia winter seasons. Keep in mind they average 20inches per season. This is a random chosen decade from 1949-1959...
The years that fall below the seasonal mean have been bolded. Note that most seasons fall well below the average, but only a few are near average with one large anomaly. The Middle Atlantic region is characterized as a relatively non-snowy location. Snowy winters are rare and far in between. Several teleconnections and ENSO patterns make these snowy winters more of an anomaly than a common occurrence. I will examine these synoptic setups below.
Also there are clear trends that during the snowy seasons, there are usually one or two large snowstorms that occur. During the less snowy seasons, climatology favors little if any chance of a significant snowstorm. Take these statistics from Baltimore, MD...
Snowiest season on record (95/96)- 62.5inches
Fourth largest snowstorm (96)- 22.5inches
Winter of 95/96 without snowstorm- 40inches
Seasonal Average Mean Snowfall- 18inches
Snow anomaly- +22inches
Fifth snowiest season on record (60/61)- 46.5inches
Ninth largest snowstorm (60)- 14.5inches
Winter of 60/61 without snowstorm- 32inches
Seasonal Average Mean Snowfall- 18inches
Snow anomaly- +14inches
This is just one random example to show that seasons characterized by major nor'easters still manage to have above normal snowfall even without the major snowstorm. Few correlations can be made between less snowy winters accompanied by major snowstorms. So basically odds are significantly higher to have a major snowstorms during already snowy winters. Therefore I am sorry to say, but the winter of 08/09 has a much lower chance of holding a historical east coast snowstorm.
Setups to these Snowy Winters...
The snowy winters found across the Middle Atlantic region are characterized by favorable conditions across the global oceans and jet stream orientation. A first look at one of the indices is the MJO. The MJO stands for the Madden-Julian Oscillation which is an area of disturbed weather characterized by the Indian Ocean tropical influence resulting in heavier than normal precipitation. These transitions and movements take place from west to east resulting in eight different phases that have a direct impact on the weather in North America. Certain phases such as 7 and 8 result in troughing across the eastern US and ridging across the West. Here is a composite mean of phase 8, Dec, Jan, and Feb, anomalies...
(Courtesy of Raleighwx)
Now other phases such as 3 and 4 result in troughing over the West and ridging over the East. Here is a composite mean of phase 3, Dec, Jan, and Feb, anomalies...
(Courtesy of Raleighwx)
Another common index to look at which I will examine later on is the NAO. This is a very important feature. The North Atlantic oscillation (NAO) is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea-level between the Icelandic Low and the Azores high. Through east-west oscillation motions of the Icelandic Low and the Azores high, it controls the strength and direction of westerly winds and storm tracks across the North Atlantic. It is highly correlated with the Arctic oscillation, as it is a part of it. Link. Winters are highly characterized by the negative and positive phases of the NAO. During positive phases arctic air is kept from plunging southward across North America and usually bottled up towards Greenland. During negative phases strong Canadian Highs build down in North America resulting in troughing across the east coast and ridging across the west coast...
(Courtesy of NOAA)
That graphic is a perfect example of the relationships the NAO shares. Snowy winters are typical accompained by a negative phase for a majority of the winter, but it does not always mean snowy. Take the winter for 08/09 as example. Here is a composite of the past few months and the NAO phase. Notice that we have been under a negative phase for quite a while...
(Courtesy of NOAA)
Yet we do not have anything snowy to show for it. But looking at our temperatures we have average below normal for November, December, and January. Here is the climate station of KMDT which is located just south of Harrisburg...
January (as of the 17th)- (-2.6)
The NAO is much more highly driven to affect temperatures than precipitation. But when everything falls together correctly it can produce large snow occurrences.
The PNA is another index which highly affects winter time weather sometimes even more than the NAO. During positive phases ridging occurs across the west and troughing across the east. Take a look at the average PNA mean from 1950-2008...
(Courtesy of NOAA for chart)
Notice that during positive phases the snowier winters tend to occur. Take Washington, DC snowfall and look at the snowy seasons of...
Note that on the PNA chart in the circled locations, the winters have a positive PNA are also found to be quite snowy. Another important index for the Pacific ocean is the PDO which highly affects the snow drought periods and the snowier periods. The PDO is a decadal index which examines Sea Surface temperature trends in the Northern Pacific which is also closely related to the ENSO/SO. Negative phases are characterized by above normal temperatures while positive phases are characterized by below normal temperatures. Positive phases are closely related to a more favorable east coast storm tracks while negative phases favor western troughing. Here is the PDO from 1900-2000. Note that snowy periods such as the early to mid 90s were found with generally positive indices...
(Courtesy of NOAA)
Blocking is an important feature that occurs typically across Greenland and helps to result in non-Great lakes storm tracks. High pressure systems or cut-off lows can result in these blocking scenarios across the northern Hemisphere. Large blocking features typically result in snowy conditions downstream across the eastern US along with colder weather. Here is a composite for this year's blocking index. Note the higher amount of blocking towards early January 2009 resulted in a generally colder regime in the east...
(Courtesy of NOAA)
Lastly the greatest impact in Middle Atlantic Winters is the Southern Oscillation patterns. The SO is the sea level pressure anomalies in the Pacific located near the equator, which result in temperature anomaly patterns with below normal temperatures correlating to La Nina and above normal temperature correlating to El Nino. During El Nino phases the Pacific jet is stronger bring an active storm track to the west along with warmer than normal temperatures. La Nina brings a weak jet stream along with a cooler than normal temperatures across the western US and north central US. Both Nina/Nino are unfavorable for significantly snowy seasons in the Middle Atlantic but there are a few differences. El Nino years have been accompanied at times by large storm systems sometimes resulting in heavy snows such as the winter of 1957-1958. During La Nina patterns especially strong ones, favor dry and warm air over the southeast therefore the infamous southeast ridge. Winters such as the non-snowy winter of 1998-1999 can be associated with a strong La Nina. We can thank the last few winters of the 21st century being non-snowy due to La Nina.
Recent Snow Drought...
From about the winter of 2003-2004, many areas across the Middle Atlantic have experienced below normal snowfall. This likely can be closely related to the indices quickly highlighted above. We have been generally under the influence of moderate El Nino patterns and strong La Nina patterns. Here are the mean equatorial Pacific sea surface temperatures for those periods of time during the months of December, January, February... Moderate to strong phases have been bolded.
Note that for the most part Pacific sea surface temperatures have been highly volatile and during moderate to strong SO patterns we typically do not do well in the snow department. So lets take snowy winters for example with mean equatorial Pacific sea surface temperatures for those periods of time during the months of December, January, February... Moderate to strong phases have been bolded.
Out of the sample note that only one season favored moderate SO patterns and generally that season of 95/96 was characterized as a weak Nina pattern. Generally these periods of moderate Nina and Nino have resulted in the recent snow drought. Some are quick to blame global warming but lets take a look at some statistics. The following are the least snowy seasons on record for Middle Atlantic seasons...
Philadelphia (1972-1973)- Trace
Baltimore (1949-1950)- .7inches
Washington DC (1972-1973) .1inches
Wilmington (1931-1932)- Trace
Harrisburg (1937-1938)- 8.8inches
Allentown (1931-1932)- 5.0inches
Trenton (1918-1919)- 2.0inches
Note that all of these least snowy winters have occurred greater than 35 years ago. Also many of the least snowy winters were found in the 1930s. Lack of snowfall and global warming has little to no correlation in the Middle Atlantic region. Therefore this likely proves we are headed for a snowy period soon. Also some of the snowiest seasons on record such as 1993-1994, 1995-1996, and 2002-2003 have all occurred within the past 15 years. So based on this data is seems that if you want to blame anything, blame it on the unfavorable Pacific. Another index not in our favor is the PDO which has been negative for quite a while, but showing signs of slowly inching more neutral or even positive. Overall I think expectations are a bit too high for snowfall each season especially from the Mason-Dixon line on southward. Areas north of the Mason-Dixon line have significantly higher seasonal snow totals than areas just to the south. For example the seasonal average at Middletown just south of Harrisburg is 36inches, while the average at Baltimore is 18inches with a distance of 88miles. Now take that seasonal average of 36inches at Middletown and look farther towards Williamsport with an average of 41inches and a distance of 90miles. Note the larger gradient to the south with different seasonal totals.
Analysis President's Day Snowstorm...
This winter is one of recent memory in which there was consistent snows throughout the winter along with one historical east coast snowstorm. So what made this winter stand apart? Well it is because it had a neutral ENSO status along with a negative NAO for the majority of the winter. Two distinct major snowstorms stand out with one being the President's Day Storm and the other being the Christmas Snowstorm. So here is my analysis of the President's day storm. This will detail what to look for in the coming days for a major snowstorm....
(Courtesy of NOAA)
Looking on a global index level, the PNA was at a favorable positive level along with a negative NAO trending positive. Almost all significant east coast snowstorms have been found that the NAO trends towards a positive level right during the storm. There is no clear definition reason why, but it is always a near correlation. Here are a few examples and I circled some significant blizzards...
(Courtesy of NOAA for the chart)
Looking the surface map it is evident to see a anticyclone to the north of the storm system which helps to keep the cold air funneled in especially east of the Appalachians. The presence of a high to the north is a must of a major east coast snowstorm...
Looking at the 500mb chart you can see the clear upper level troughing beginning to become negatively tilted therefore resulting in the coastal forming off the coast of North Carolina...
For major snowstorms cyclongenesis must occur downstream of the trough axis with the jet streak located to the north. A large region of ascent then occurs combined with isentropic lift forming precipitation likely in the form of snow to the north of the system. Initially precipitation remains light. As warmer and moist air rises aloft over the surface low, a rapid deepening of pressure then occurs making the shield of precipitation much larger and heavier. As this occurs, frontogenisis takes place creating the heavier embedded snow bands...
(Courtesy of The Weather Channel and DEWX at Easternwx)
As the deepening of the low occurs, heavier precipitation continues to break out before the low lifts out to sea. So overall for major snowstorms there must be presence of anticyclone favorably located over Ontario or Quebec, source of Gulf/Atlantic moisture, rapidly deepening low or transfer of low pressure energy, source of strong frontogenisis, fresh batch of cold arctic air resulting in cold air damming, an upper level trough with a central trough axis resulting in a negative tilt for North Carolina low formation, and a favorable negative trending positive NAO.
Well as you can see Middle Atlantic snowy winters are quite fickle and difficult to predict. Large snows only come together when synoptics are all near perfect. When missing part of the equation, it completely throws out a snowy solution. Also I hope you see that it is likely not climate change resulting in this period of non snowy weather across for the past few years in the Middle Atlantic. It is impossible for me to see if this pattern continues through next winter, but it will definitely be interesting to watch. ENSO patterns seem to play the most significant roles in our snowy v. non snowy winters so that is definitely important. One of these years we will have a neutral year. I think many of us are just frustrated with this current winter because we have the cold air. In fact much of North America has experienced the coldest air since 1994 with even Maine breaking the all time record low for the state which is now -50degrees. Parts of Pennsylvania dropped in the negative 20s with even areas such as State College dropping to -15. Many areas also have a little bit of snow on the ground so it definitely feels like winter. In fact it is currently lightly snowing and 16degrees here as I type this. It just seems that every time we get a big storm system it tracks to the west of us putting the eastern US in the warm sector. Then as the secondary low forms, it forms too far north of us to keep the Middle Atlantic cold. Ice storms seem to be quite common during these La Nina events. I hope you have a new appreciation for our snowy winters. This is my 100th blog here at Wunderground and I hope that one of these days we can get that significant east coast storm system to track here. Have a wonderful evening!!!
"Here northeast of Harrisburg 2008-2009 winter statistics"
Current Snow Cover- 1.5-2.5inches
Monthly Total- 3.50inches
Seasonal Total- 13.95inches
Winter Weather Advisories- 6
Winter Storm Warnings- 1
Ice Storm Warnings- 1
Blizzard Warnings- 0
Freezing Rain Advisories- 0
Winter Storm Watches- 3
Lowest High Temperature- 14degrees
Lowest Low Temperature- -3degrees
Wind Chill Advisories- 0
Wind Chill Warnings- 0
(Snow Storms Stats)
First Snow - October 29 - Trace
First Snow on Ground - November 18 - Coating
Lake Effect Snow - November 21/22 - 6.00inches
Synoptic Snow - December 16 - 3.5inches
Clipper - January 17-19 - 1.5inches
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