Ph.D. Student - Earth System Science (UC Irvine), B.Sc. - Atmospheric Sciences (Cornell University)
By: Zachary Labe , 7:26 PM GMT on October 01, 2011
The first snowflakes have begun to fly in a few of the higher elevations of the east coast. This weekend an anafront and associated ULL (Upper Level Low) will allow cold air to filter in near the 850hPa low with heights dropping to near 0C or -1C. This will allow the elevations above 2500ft particularly in West Virginia and southern Pennsylvania to see some light snow accumulations anywhere from a coating to upper to 4in in West Virginia. Also a few weeks ago, snowflakes were reported up on Mt. Washington in New Hampshire and covered the ground for a day or two after before milder air moved in. Given the beginning of October, the start of winter threats is just a tad over a month away and even some early season snowstorms have occurred before in mid October. Looking ahead at the long term pattern though does favor above normal temperatures. The long range GFS shows little to no frontal passages around mid month with upper level heights well above normal. This will keep ridging across much of the Midwest and Northeast keeping temperatures on the mild side. I think October will end up with monthly anomalies near +2C for many locations. Drier conditions also are possible during this period with a very inactive weather pattern. The ECMWF is a bit more aggressive with some cooler air mixing in the pattern every now and then, so there are still some uncertainties in the long range.
Winter 2010-2011 was characterized by a moderate La Nina under another very negative NAO (North Atlantic Oscillation) mean. This allowed for relatively snowy conditions across much of the Northeast especially the farther east one traveled with record snows particularly towards New York City. Lake effect snows were also well above normal with 174in before reported at Laurel Summit in Somerset County, Pennsylvania. Areas along the I-81 corridor received below normal snowfall due to the abnormality of the amount of coastal storms affecting only eastern areas, especially the I-95 corridor. This winter will be dramatically different for much of the country as we are beginning a more active solar cycle and moderate La Nina.
The atmosphere is a fluid with differential heating across the entire planet. Given the energy budget feedback of unequal heating from the equator the poles, mechanisms such as convection, conduction, and latent heat balance temperatures globally. These currents of energy help to maintain the balance of the fluid making everything interconnected. Therefore weather patterns over areas such as Europe and Asia are equally important in forecasting weather conditions over North America. It is critical to look at the entire circulation to get a better understand for forecasting long term weather patterns. I will refer to a series of indices throughout the context of my forecast. This indices help to give a general idea of weather patterns in different regions of the world. Many of these indices offer correlations relating back to weather pattern conditions more closer to home over the eastern United States. The following explanations are defined by Paul Kocin and Louis Uccellini's "Northeast Snowstorms Volume I (2004). The Southern Oscillation (SO) is one of the most important indices noting short term climate patterns over the equatorial Pacific by using sea level pressure anomalies to correlate to cooler or warmer than normal sea surface temperatures. This is the determining factor for El Nino vs. La Nina. El Nino's are characterized by warmer sea surface temperature (SST) anomalies over this region strengthening the Pacific jet stream causing a series of related weather patterns across North America and the rest of the globe. El Ninos are noted with warmer air flowing into the western United States given the active jet. The subset subtropical jet remains abnormally active given a period of storminess over the southern United States. La Nina's on the other hand weaken the Pacific jet with colder air spilling south over the northwestern United States favoring ridging and dry conditions over the southern United States.
Fig. 1 shows the effects of El Nino/La Nino on surface temperatures.
The SO index has been kept since 1882 utilizing sea surface temperature anomalies. El Nino episodes tend to have strong correlations to above normal winter precipitation over the eastern United States courtesy of the active subtropical jet while La Ninas generally have the opposite relation. Strong cases of either El Nino/Southern Oscillation (ENSO) events tend to produce relative warmth and lack of snowfall over the entire Northeast.
Fig 1.1- This chart courtesy of the Mt. Holly NWS tells an interesting story of Nina vs. Nino seasonal snow totals.
The most important short term index is the North Atlantic Oscillation (NAO) measuring pressure anomalies across the Atlantic Ocean. A negative NAO is associated with a weak subtropical high and Icelandic Low. The Icelandic low is also displaced to near Newfoundland associated with blocking patterns over the eastern Atlantic and Europe. This pattern favors milder air over Greenland with colder air over the Northeast. A positive NAO is noted by lower pressures over the Arctic with higher pressures over the Atlantic resulting with a strong westerly and southwesterly flow over the eastern United States.
Fig. 2 presents the sub sequential effects of each subset NAO status.
A strong NAO status can override the effects of the current ENSO status. Therefore the NAO is the most critical index in determining winter patterns. Unfortunately the NAO is a variable index and results in short term changes in patterns. This makes long term predictions quite difficult. Current ensemble models predict NAO deviations generally two weeks in advance. Several theories have arisen in the idea of long term NAO trends. Unlike the Pacific Decadal Oscillation (PDO) index, the NAO is not often looked at in terms of 10 year trends. But risen correlations do suggest several periods show trends towards a more negative NAO regime and vice versa. Given the increasing negative NAO trends, I do believe we have entered a decadal period favoring an increased negative NAO regime. The 1960s seemed to have favored a negative NAO regime while the the late 90s and early 2000s favored a positive regime. This does not always mean the NAO is positive for the entire period; remember it is a weekly variable index.
In a case study by Kocin and Uccellini, 18 sites were subdivided to reflect the impact of the NAO on cities with seasonal snowfall averaging less than 20 and greater than 40in. The results indicated the impact of the NAO on seasonal snowfall is greatest along the I-95 corridor including all of the major metropolitan areas. Quoted by "Northeast Snowstorms," "Since the seasonal snowfall within this region is significantly influenced by the occurrence of moderate to heavy snowfalls, an important relationship between the NAO and the occurrence of significant snowstorms is indicated." Also Kocin and Uccellini uncovered another relationship noting the transition periods from negative NAO to positive NAO characterized by a significant Northeast winter storm. For further information on this correlation note the 1950 Appalachian Storm, 1962 Ash Wednesday Storm, 1979 President's Day Storm, 1983 Metropolitan Storm, 1993 Superstorm, and 1996 Blizzard.
it is also important to note is the Pacific Decadal Oscillation (PDO).This index monitors SST anomalies in the northern Pacific and can be closely followed based on 10-year trends. A negative PDO is often associated with a warmer pattern for the central and eastern United States with cooler conditions to the west and vice versa for a positive PDO. It is closely in correspondence with the shorter term Pacific/North American Oscillation (PNA). Typically La Ninas are often correlated to negative PDO regimes while El Ninos are noted with positive regimes.
Fig 3. 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.
Other indices looking at monsoonal patterns across the Indian Ocean include the Atlantic Multidecadal Oscillation (AMO) and the Madden-Jullian Oscillation (MJO). The MJO is noted in another cyclical pattern favoring waves from 1-8 each with different effects on the position of the jet stream noting troughs and ridges across North America. The Quasi-biennial Oscillation (QBO) is a cycle period noting the equatorial zone winds between the easterlies and westerlies in the stratosphere. The QBO has correlations for the Northern Hemisphere during the winter months helping to identify stratosphere warming events which often correspond to colder periods over the Northeast.
The over aching theme in this quick explanation of several global teleconnections is the concept that everything is related in the atmosphere. Many of these correlations noted above are in inexact science due the volatility of the atmosphere. The sun is the ultimate controlling force in the resultant weather. Solar radiation enters a series of interactions with the Earth's surface and atmosphere creating an extreme complexity. Air masses form when air takes on the characteristics of the region its sitting on with cold air masses moving towards the tropics and warm air masses moving towards the poles. The Earth system is trying to reach an equilibrium. This balance that is trying to be reached causes the extreme weather. Because the sun varies in activity, it is likely effects on the Earth's weather patterns also occur as a result. But science in this field is very uncertain.
Fig. 4 shows the current sunspot cycle dating back to 1850.
For the time being we are entering a period of reduced solar activity despite a recent uptick in sunspots.
Because of the complexity in the Earth system taking account for a multitude of variables and factors, long term weather predictions are difficult. In any case, using a few of these indices above help to paint a sketch into the world of long term seasonal predictions. And using climatological means, forecasters can hope to try to make an 'educated guess' in seasonal predictions.
The winter 2011-2012 will be characterized by a moderate La Nina carving the template for temperature and precipitation trends over North America. Current Oceanic Nino Indices (ONI) indicate anomalies falling below 0C during the last three months indicating a cooling of sea surface temperatures in the equatorial Pacific. Once ONI readings drop below -0.5C, a cool ENSO period is defined. Last winter was also defined by a La Nina as ONI readings dropped to -1.4C at its max during the November, December, January period Link.
Current ensemble and global model predictions indicate a continued drop in SSTs, especially in Nino region 3.4
Given the upcoming Nina, we can already note the higher chances for a weaker subtropical and Pacific jet favoring drier and warmer weather over the eastern United States. But the next index on the list is the NAO, which at times can override the ENSO status.
This figure above strongly supports the decadal NAO trend theory. Please note the increasing abnormality in negative NAO values during the last few years. In fact record daily -NAO values occurred in 2010. During this past summer, the NAO also was generally negative throughout June, July, and August. This often correlates to a negative NAO regime during the winter. A negative NAO features higher than normal heights over northeastern Canada and Greenland creating a 'blocking' pattern helping to instill colder air over the northeastern United States with a trough-type pattern. It physically 'blocks' the warm air from moving north and from storms penetrating to the west over the Ohio Valley and Midwest. La Ninas often correlate to positive NAO regimes, so given the variability in the index, I do expect the -NAO regime to continue on average for the winter, although there will be warm periods.
It is very important we establish the -NAO given the high likelihood in a southeast ridge formation. If we do not establish this regime, then the Middle Atlantic will likely be toast this winter. Looking at the state also of the -PDO emphasizes the La Nina theme with the formation of the Aleutian Low over Alaska which helps to keep troughs over the Pacific Northwest. Very few large coastal storms have occurred during this type of setup. Given the weaker subtropical jet, the threat of Miller A coastal storms is smaller than normal. A miller A cyclone in one which forms in the Gulf of Mexico or so and travel up the entire eastern Atlantic coast. A miller B is the cyclone forms as a redevelopment off the primary low, typically off the North Carolina coast and really undergoing bombogenesis off of southern New England.
Looking at the current precipitation anomalies and monsoonal trends in the Indian Ocean, we can determine several patterns in the MJO and there effects on the long term pattern. It appears several wavelengths remain open favoring phases 6-8 this winter especially early towards December.
Current snow depth and ice coverage across the globe remain below normal climatological values, but yet above long term lows such as 2007.
It is important to follow the snow coverage over parts of Siberia during the month of October with strong correlations to winter temperature patterns. So far numbers appear encouraging.
North Atlantic sea ice also has been steadily increasing in coverage the last few winters with the buildup over cold air over that region lately.
In quick summary, this year's winter forecast relies on the importance of the NAO regime in response to the moderate predicted La Nina. Also wavelengths from the MJO/QBO will remain important. Looking at these indices, a general summary results in a winter characterized by the importance of latitude and proximity displaced from the Atlantic Ocean. Given the La Nina, I do expect the formation of the dreaded southeast ridge. This will bring milder periods especially to the Middle Atlantic region particularly south of 40N. The storm track may be focused to the west of the Northeast, but given the negative state of the NAO, redevelopment in terms of Miller B storm systems are very likely. For those along and north of 40N, the heaviest snows will fall in this region in these cases while areas in the Delmarva sadly watch from the dry slot. The likelihood of large-scale miller A storms will remain below normal, but cannot be ruled out. I am expecting snowfall to near normal for areas along I-95 with above normal snowfall for those 100mi inland. The likelihood of another well above normal snowfall season for the major cities remains low given climatology. There have never been three consecutive winters of over 40in of snow for Philadelphia for instance. Temperatures will average slightly above normal for Pennsylvania, New Jersey, Delaware, Maryland, Virginia while normal to below normal temperatures are possible for New England given weak troughing over this region.
Winter 2011-2012 Anomalies
KDCA- (+2.0F) (90-110% of normal snowfall)
KBWI- (+1.5F) (100-110% of normal snowfall)
KPHL- (+1.5F) (100-125% of normal snowfall)
KMDT- (+1.0F) (110-130% of normal snowfall)
KNYC- (+1.0F) (100-125% of normal snowfall)
KBOS- (+0.5F) (125-130% of normal snowfall)
I think the winners in the snowfall department will generally be those farther inland with some additional latitude favoring southern New England. The I-81 corridor could do well this winter if the southeast ridge remains relatively benign. The colder temperatures anomalies will be saved for northern New England across Maine, New Hampshire, upstate New York, and Vermont. I am expecting December to average below normal region wide while January and February are warmer with then perhaps a cooler March. Snowfall will generally occur in the form of overrunning precipitation, secondary lows, and clippers. The threat for ice storms is heightened due to upstream blocking locking in the colder air at times as cyclones track through the Ohio Valley. A few may be wondering about the prospects of this wet pattern continuing... And to be honest often wet patterns in the summer can quickly swing to dry patterns in the winter. But really this correlation is pretty weak. I expect precipitation to be near normal for much of the region.
For the final section, I thought it would be interesting to post some archived maps of major nor'easters of our past courtesy of Penn State Meteo. EWall...
12 February 1983...
7 January 1996...
14 March 1993...
17 February 2003...
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"Lower Susquehanna Valley Doppler"
(Courtesy of WGAL)
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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