Meteorologist Mike Daniels
When I was given the charge to produce a forecast for the upcoming winter season, like a typical meteorologist my thoughts immediately went to the state of the ENSO (El Nino Southern Oscillation). And for good reason, since this climate phenomenon perhaps more than any other is likely the most talked about, and most well known, of the many meteorological players that impact the Nation’s weather. Indeed, what hurricane forecast was issued this past year that didn’t cite the fact that a La Nina was developing, which would increase the likelihood of more Atlantic hurricanes this past season?
WINTER FORECASTS: Northeast | Southeast | Midwest | South Central | Northwest | Southwest
Alas, if seasonal forecasting were that simple so as to just glance at the ENSO, and be able to predict with any certainty the outcome of the hurricane season, or whether the winter will be extremely cold, etc. As important as the ENSO is, there are many other cyclical climate factors that are believed to have an influence on weather not only nationally, but globally as well. Other known factors are the Arctic Oscillation (AO), North Atlantic Oscillation (NAO), Atlantic Multidecadal Oscillation (AMO), Pacific Decadal Oscillation (PDO), Pacific North American Teleconnection (PNA), Eastern Pacific Oscillation (EPO), Madden-Julian Oscillation (MJO), and the Quasi-Biennial Oscillation (QBO), among others.
In addition, the amount of snow and ice coverage in the Northern Hemisphere, as well as solar activity, is often taken into account. Comparisons are made with prior years where they climatologically appear similar to what is occurring now, and these are used as analogs to determine outcomes. And nature itself is sometimes used (think Old Farmer’s Almanac), and I recently read a winter forecast on the web where the fact that the squirrels are much fatter than is typical was used as part of the basis for predicting a colder than average winter! So as you can see, the amount of work that goes into making a seasonal forecast is large, and the amount of data that can be used is enough to make the best forecaster’s head spin.
While I have reviewed several of the parameters mentioned above (even taking the squirrel situation into account), this forecast will focus on the current and predicted state of the ENSO, as well as what the NAO may be doing overall this coming winter; more on the NAO a bit later. Other factors will be mentioned only as they correlate to the ENSO and NAO. Forecasts have been made for temperature and precipitation, and each encompasses a four-month period (December, January, February and March) for six regions; Northeast, Southeast, Midwest, South-central, Northwest and Southwest, and presented in order from what I perceive as being the simplest (relatively speaking, of course), to the most difficult. A snow/ice forecast has also been issued.
The Reasoning
According to NOAA’s Climate Prediction Center, as of November 13, 2007, the ENSO is currently in a moderate La Nina (colder) phase, and this La Nina is expected to continue into early 2008, but may then weaken as we approach spring. Given the fact that La Nina’s occur due to upwelling of cool sub-surface ocean waters, and the fact that these sub-surface waters are now quite a bit cooler than average, a forecast of continued, and possibly intensified, La Nina conditions seems quite reasonable. Overall, La Ninas tend to bring a great deal of variability to the weather across the northern states since the Polar Jet becomes dominate, hence the changing configurations of the jet can bring very quick and dramatic weather changes, i.e., cold to mild, dry to wet. Across the southern states, changes tend to not be so dramatic or frequent since the Polar Jet typically resides to the north, and a weakened Sub-tropical jet does not generate Gulf storms as readily.
One important aspect of La Ninas is the blocking high that tends to develop over the North Pacific somewhere from right along the west coast of North America to the Aleutian Island Chain. Where this develops is very important to the positioning of the jet stream. If it tends to form south of the Gulf of Alaska, it would cause the jet to thrust northward over Alaska, then southward over the Northern Rockies and into the Plains. This would allow the jet to tap colder arctic air and drag it southward into the western and central US. Additionally, a split jet stream would likely form, with a piece flowing eastward to the south of the block, which would allow storms to at least occasionally affect the Pacific Northwest, especially when the block weakens somewhat and temporarily shifts to the west a bit. Conversely, if the block forms over the Aleutians, it would then force the jet to the north over Russia, and then southward over Alaska and into the Pacific, entering the US anywhere from northern California northward to Washington State.
Additionally, the split jet may tend to take storms more into the west coast of Canada, missing the Pacific Northwest to the north. Right now, my belief is that the block will be more of a Gulf of Alaska type because the colder sea-surface temperature anomalies associated with the La Nina appear to be setting up more eastward, rather than westward, across the equatorial Pacific. The regional effects of this jet positioning will be discussed below. There will likely be occasions when the Pacific jet is very strong, and it will plow into the Northwest and flow unimpeded across the northern states, which will act to cut the supply of cold Canadian air off from time to time. During these periods, much of the country may experience milder conditions.
As a side note, while the ENSO is the more frequently discussed climate factor in the Pacific, another important factor is the PDO (Pacific Decadal Oscillation). We can think of ENSO as a smaller and faster changing variation of the PDO, for while ENSO encompasses a relatively small region near the equatorial Pacific, with warm (El Nino) and cold (La Nina) phases that usually last from one to perhaps a few years, the PDO involves a much larger portion of the North Pacific, and warm and cold phases last for decades (30-40 years or more). The last cold phase of the PDO occurred from the late 1940’s to the late 1970’s, when a warm phase began that appears to still be ongoing. There appears to be a correlation between the phases of the two; in other words, stronger and longer lasting El Nino’s tend to occur during warm phases of the PDO, and vice versa, which may help explain the extreme El Nino’s of the 1980’s and 1990’s. However, there appears to be some indication that the current warm phase of the PDO may be waning, and that a new cool phase may begin sometime in the not too distant future. If this occurs, we may be looking at stronger and longer lasting La Nina’s rather than El Nino’s, which may have an effect on future hurricane seasons, drought patterns, etc.
NOA
The NAO is an atmospheric pattern that exists over the Atlantic that can have a dramatic effect on the weather, mainly over the eastern and central US. There are two phases; positive and negative. When the NAO is positive, wintertime temperatures tend to be warmer than average since the jet stream can reside over the northern US or over Canada, and since the jet tends to flow in an almost straight west-to-east fashion (zonal flow), any cold air intrusions that do occur tend to be short-lived. Precipitation is typically not very heavy as any storms that develop tend to move quickly, limiting the amount of time that a storm can affect any given area. When the NAO is negative, temperatures can be very cold for long periods of time as the jet stream may be pushed far to the south, allowing cold air from the arctic to spill southward. Also, storms tend to intensify and move slower due to blocking that sets up in the Atlantic, which allows precipitation events to be heavier and longer lasting. Major eastern snowstorms often occur when the NAO is in a negative state, since colder air is available when big storms are affecting the region.
Unfortunately, the state of the NAO is very difficult to predict more than a couple of weeks in advance, which certainly makes a seasonal forecast challenging at best.. However, like the PDO, persistent trends in the NAO tend to occur, usually over many years in which one phase or another tends to predominate, so clues as to which state may predominate in any given winter can be gleaned. Nevertheless, this method is certainly not foolproof as the wild winter of 1995-1996 proved when the NAO was primarily negative, at a time when the predominate state over the years was positive. In the recent 2007-2008 winter forecast out of the Detroit, Michigan NWS office, some interesting facts regarding the NAO was presented which appears to show that it may be trending toward an overall more neutral state, and perhaps this will eventually lead in future years toward a predominately negative phase. Time will tell! For this forecast, a more neutral NAO regime will be assumed, which means even greater variability may occur in the eastern and central states resulting from the toggle between positive and negative.
