WEATHERAmerica WINTER SEASON OUTLOOK: December 1 2007 - April 1 2008
LarryCosgrove | October 30, 2007 at 02:28 amby
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Yes, it is that time of the year again.
With the long, hot summer of 2007 now a memory and its only slightly cooler and wetter autumn cousin nearing a midpoint, many are clamoring for some insight into the low sun period of 2007 and 2008: winter! For purposes of this forecast, I define the winter season as being from December 1 through April 1, sort of a hybrid between what many think as being the calendar time block and the "meteorological" (i.e. for easy record keeping) time frame. This fits nicely, as you can see frozen precipitation types and what can be construed as "cold" temperatures as early as 12/01 and as late as 04/01.
Key elements in the assemblage of this forecast are the current state of the El Nino Southern Oscillation (ENSO), which is now classified as a near-moderate La Nina; other SST anomalies in the Northern Hemisphere; growth of sea ice, glaciation, and snow cover; prevailing trends in the 500MB longwave pattern over and near North America; and comparison of the upcoming winter to past analogues.
La Nina: Current And Forecast Intensity
If you buy into the theory that the ongoing La Nina episode is one of, if not THE key player in determining the outcome of the 2007-08 winter, then it makes sense to check the current strength and forecast trends in measures of the -ENSO status. There appears to be some (as in relatively minor) weakening in the cool SST anomaly over the central and eastern equatorial Pacific Basin over the past two weeks. While it is hard to say if this represents a definitive moderation in hydrothermal values between the Galapagos Islands and Indonesia, the pocketed and small areal coverage of the warming waters seems to fit nicely with the packages of the computer forecasts that cover the El Nino Southern Oscillation, which seem to agree on an ever-so-gradual warming toward neutral levels (that is, between an El Nino and La Nina) by April.
If indeed this is the case, then some of the classic synoptic signatures linked to La Nina will begin to slowly wind down as the winter progresses. In most sharply defined -ENSO events, strong, unified polar westerlies dominate with placement across the northern Pacific Ocean into the Columbia Basin through the Great Lakes and Gulf of St. Lawrence. Colder temperatures and heavier precipitation often line up from the Intermountain Region, Upper Midwest, and southern Canada. Unseasonably mild values, meanwhile, rule the roost from the Desert Southwest through the central and southern Great Plains, Old South, and the Northeast. But if the mechanisms which drive the jet stream character and placement begin to break down in the second half of the low sun period, then it would be wise to look at the possible inclusion of the occasional +PNA configuration, suppressed cA vortex position, and more southern relocation of the surface storm tracks.
Since there are other signals (climatic indices as well as SST anomalies in different oceanic zones) that point toward a tendency for semizonal rather than hyper-amplified flow aloft, the safe position is that any changes in storm track and cold air penetration will not occur until about two-thirds of the winter is over. So if you look at the +/- 120 day period, we would not expect to see a remarkable shift from what is expected during a weak-to-moderate La Nina episode in North America until perhaps the week of February 18, 2008. Until that time, climatology and persistence should be the best tools in forecasting apparent weather across much of Canada and the U.S.
How Can Climatic Indices Be Used To Predict The Winter?
Every year (or so it would seem), a new climatic index seems to gain favor in the meteorological community as an indicator of expectant seasonal weather. In many cases, the initial research was done many years ago, and forecasters scouring the scientific landscape will seize on a measure and proclaim it to be a solution to figuring out the predictive puzzle for winter.
I always urge caution before embracing any index or anomaly of ANY kind in finding a complete solution to longer term temperature and precipitation outlooks. The reasoning is simple: the data base, familiarity and understanding of indices is usually lacking and in-depth correlations are not available. Besides, if one predicts the weather without first "looking out the window", the results may be alternately at odds with reality or so vague as to be worthless. The best possible solution, it would seem, is to blend ongoing trends (persistence) with well-defined associations (climatology) and historical comparison checks (analogues).
Some climatic indices have made news recently as possible indicators of seasonal weather. The quasi-biennial zonal wind oscillation or QBO measures the tendency of stratospheric winds near the equator to flow west (negative) or east (positive) in near-yearly variation. If, for example, the QBO measures an extremely negative number late in the fall season (such as this year and in 2005), the theory goes the powerful westerly flow aloft will tend to negate the formation of blocking signatures at higher latitudes. And if near-zonal or semizonal flow dominates in the upper and middle troposphere, then mP and ImP regimes should dominate day to day conditions in North America (that is, mild and dry). Unfortunately for users of the QBO, there is no foolproof way to find out when the index will "switch" in character and if a negative phase will allow for special blocking patterns created by unique atmospheric conditions (such as cross-equatorial fetches, superstorms, or inputs from non-forecast solar phenomena).
The GLAAM (Global AAM; the AAM index measures the ratio of east /west versus north/south winds on the planet that affect the rotation rate of the earth) index is looked at in much the same way, with a negative measure favoring a -ENSO atmospheric course and the positive phase pointing toward split flow configurations normally associated with El Nino events.
While the Madden-Julian Oscillation is believed to be an important player in transfers of energy from the tropics to the higher latitudes, no credible way has been found to predict the outcome of the MJO beyond the 11-15 day period. So while we know that higher amplitude impulses (and Kelvin waves) associated with the oscillation can link with the westerlies, to create cases of EPO or PNA styled blocking (witness the famed Pineapple Express that is common in El Nino years), there is no way to measure vertical velocity extensions from the equator on a seasonal basis. But MJO activity tends to be more chaotic, or weaker, in a pronounced La Nina episode, which would appear to be the case this year.
Ongoing Trends In The 500MB Longwave Pattern And Sensible Weather
Despite an unusually warm summer which virtually eradicated snow and ice cover in the higher latitudes outside of Greenland, October has seen glaciation and snowfields expand with some rapidity over the far northern portions of the continent and surrounding waters. As storms off of the Pacific Ocean track close to the Canadian border, it is expected that stretches of bare ground in the Prairie Provinces, Ontario, and Quebec will build a viable snowpack, necessary for the chilling and broadening of cold air masses that may reach the lower 48 states.
Weak signatures of a -AO and -NAO measures are seen in the positive height anomalies near Iceland and above Alaska. Winter climatology favors expansion with retrogression of higher latitude ridges, and deepening and southward reformation of cold lows (negative height anomalies). If this is the case, much of Canada may come under an elongated Rex signature (see three related graphics), promoting relatively mild and dry conditions over much of our northern neighbor. The impressive storm sequence over the northern Pacific Ocean, meanwhile, will have its first and greatest impacts from BC into OR, spreading through the northern part of the Intermountain Region. With time, the stronger upper winds and intrusions of cold pools should suppress the familiar heat ridge over the eastern half of the U.S., with this feature eventually taking shape as a flat subtropical high over the western Sargasso Sea. The stable ridging along the Mexican border, which occasionally expands into the Great Salt Lake Valley, may enable more high-profile Santa Ana/Canyon Wind events across central and southern California during parts of this winter.
Analogues To Note
If one were to look at climatic indices and the 500MB structure of the ENSO episode of the evolving late 2007 season and previous years, then 1970-71 stands out as the most viable analogue. But sensible weather the summer and fall previous across the U.S. was not at all similar to recent conditions (such as the landmark drought in the Corn Belt and Old South), and there were also important upstream differences in Eurasia with the placement of positive and negative height anomalies. So again, caution is the word in using this analogue.
In addition, the tendency for warmer temperatures in recent times must be measured against any comparison with years previous. If we take the past five La Nina winters (which were warmer continent-wide) and meld the results with the 1970-71 December - March synoptic pattern, we should come up with a reasonable preview of the coming winter. Note on the above depictions the extensive negative height anomaly over the Gulf of Alaska, which matches well with the hydrothermal dipole across the northern Pacific Ocean and also is a good teleconnection for the evolving active storm sequence there. And while there are many continental locations with warmer than average temperatures (most notably the Southwest and central/eastern Canada with the respective ridge complexes), much of the U.S. shows a "normal" thermal signature. With this method, the coming winter can be shown to have the storm track undercutting the Canadian ridging and likely taking on a pattern consistent with the formation of Colorado/Trinidad cyclones. The small, but important, negative height anomaly just off the coastline of New York and New England signifies a potential for secondary redevelopment and with it, some cold advection into the Great Lakes and Northeast.
What Could Change (Or Go Wrong) With The Outlook
There are three possible "gremlins" which could disrupt the results of the winter season forecast. These are, as following:
1) The highly energetic, yet fairly amplified, jet stream and storm sequence across the Pacific Ocean
If this fast flow configuration encounters any blocking signals downstream, cold pooling and temperature diversity could sharply increase. Plus, production of warm sector thunderstorms in the Old South could increase along with snowpack in the Upper Midwest and Great Lakes. It is also possible that the fast flow may not allow for much in the way of significant blocking, at least in the NAO positions.
2) An unforeseen collapse of the ongoing La Nina episode
Last winter saw the complete erosion of what was a moderate El Nino during later January. Perhaps as a result, the trend toward mild weather nationwide vanished and seasonably cold readings (in some cases, lower) took over much of the continent to the right of the Rocky Mountains. Caveat: the numerical models which monitor ENSO are not foolproof, and some La Nina weather associations are not what could be termed ironclad.
3) Chaos theory
The famed "Butterfly Effect" says that events which are so small (like a butterfly flapping its wings, or mesoscale events that are not documented) that we cannot see can have great impact on apparent weather. Consider the differences in the 500MB longwave pattern between this October and that of 1970 (or for that matter 2005, which looks surprisingly similar). While the basic outline of features is the same, there are deviations. And taken downstream, we could have a far different result in terms of apparent weather. Chaos theory is the main reason why index-based outlooks can fail; there are ALWAYS other factors to consider!
I am sure that many who follow predictions and examinations of the upcoming winter have heard cries of "another warm one...". And, truth be told, there are good reasons for throwing the approaching season into the "wait until next year" bin. First and foremost being the La Nina episode, which IF it were in the moderate to strong category, would probably favor everyone outside of the Pacific Northwest being placed in an almost routine procession of mP and ImP regimes with little moisture and occasional ventures into record warmth.
That said, there are signs that this winter will not be "wall to wall warm". As we have seen, sea ice and land snow cover is increasing steadily and will soon nudge into lower latitudes. This makes generation and maintenance of cPk and cA regimes easier to accomplish. Secondly, there is enough evidence among numerical models and SST observations to indicate that the current weak-to-moderate La Nina episode will gradually wind down into neutral ENSO territory. As a last point, note that the parade of storms across the northern Pacific Ocean are very much amplified (longwave or hybrid type vs. shortwave). As the low sun period takes hold, the potential is there that some of these lows will close off and pump up blocking signatures to the north and east (probably in the AO and NAO positions).
Most likely, the best chances to see below normal readings will be along and north of the mean storm track. Positive thermal anomalies may be virtually guaranteed in higher latitudes of Canada and also close to the flat subtropical highs straddling the Sonoran and Sargasso positions. But, in the proverbial middle, mPk, cPk and the rare cA intrusion to the rear of departing and intensifying storms will likely bring times of chilly temperatures and the occasional snowfall.
Expected Storm Tracks
Since we understand that the jet stream configuration will be semizonal or only suppressed by -AO and -NAO styled Rex signatures, the argument is presented that cyclones will mostly occur in the mid-latitudes. In a -ENSO period, the heaviest influx of storm energy usually occurs over the Pacific Northwest. So if you are looking for truly extreme snow and wind episodes, head for the area between BC, WA, and OR into the Front Range. In many cases, leeside cyclogenesis will take place, and new low centers will organize in S CO before embarking on a trajectory into the Great Lakes (non-blocking situations) or toward the Virginias (and reforming off of the VA Capes if the NAO is in a defined negative state (featuring high latitude ridging in Greenland, the Davis Strait, Baffin Island or Quebec). In cases where enough cold air is present and the upper low is strong, the coastal secondary lows may dump heavy snows on the northern third of the Interstate 95 corridor. This is what is known as a "Miller B" scenario and may deprive locations below the Potomac River of meaningful snows, should reformation or center jumping be too far north (such as near or east of Cape May NJ).
In warmer phases or when no blocking is present, shortwaves may deepen and accelerate from the northern High Plains into the St. Lawrence Valley, or simply move close to the International Border with Canada into ME and the Maritime Provinces. In such cases, temperatures will turn quite mild in much of the lower 48 states and precipitation may be minimal.
Precipitation (Including Snow And Ice) Potential
Because of the sometimes semizonal component to the upper flow associated with La Nina, and the periodic presence of Rex signatures across much of Canada, there will probably be wide stretches of below normal precipitation across North America this winter. The tendency for positive height anomalies near and just below the Arctic Circle should depress the mean storm track to along or below the Canadian Border, so the best overrunning and lifting mechanisms should occur above 35 N Latitude and below 55 N Latitude.
The scenario for the winter is not really that bad for snow, in the sense that the Canadian provinces and territories are virtually guaranteed some snowfall during the times when ridging breaks down and that U.S. locations to the north of cyclone tracks will see significant snows when the Gulf of Mexico and northern Pacific Ocean moisture sources are open to cyclonic inflow. Where subtropical highs appear, however, in the Sonoran and Sargasso positions (common in La Nina episodes), dewpoint flow will be restricted below 40 N Latitude. The accompanying chart for snowfall potential is based on two simple premises: the farthest south location of the snow cover and overall chances to see accumulating snow. In the cold and overrunning sectors of a low pressure center, if any cP component is present there should be some snow which accumulates. Unless a surprise southern storm track forms (and that would likely be later in the winter or possible early spring), odds on a significant frozen precipitation event are far better over the northern half of the lower 48 states this winter.
It would appear that the period from December 1 through April 1 will be relatively mild across much of North America, excepting portions of the Intermountain West and the Northeast sections of the U.S. Dominant influences on apparent weather will be a steady, mainly semizonal, progression of high amplitude storms from the northern Pacific Ocean to the Grand Banks; a tendency for weak to moderate Rex blocking in the AO (Alaskan, Yukon north coast) and NAO (Quebec into Greenland); the ongoing La Nina episode over the equatorial Pacific Basin, which should undergo a gradual weakening to neutral ENSO status by the first week of calendar spring. When the climatological effects of these elements are put together, and recent trend sin the 500MB longwave pattern are factored in, a transition from mostly mild and dry conditions in the lower 48 states in November will give way to a series of Colorado/Trinidad type storms in the heart of winter. Since +PNA configurations in the jet stream will likely be rare, any turn toward colder readings must come from two sources: mA and mPk regimes from the Gulf of Alaska into the western states, and cPk or ImA air masses that are drawn south from Ontario and Quebec behind departing lows strengthening over the western Atlantic Ocean.
Drought and heat may continue to cause problems in the southwestern states and in Florida. While much of the Old South may trend dry due to a preponderance of westerly flow aloft and a storm track between 35 and 40 N Latitude, a few more amplified disturbances could bring convective or even stratiform rainfall to Dixie between now and the first week of April. Locations expected to see above average precipitation will be the Pacific Northwest, northern Intermountain Region and central High Plains, Great Lakes, and the Northeast.
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