2006-07 Winter Season Forecast

[Larry_Cosgrove]

Overview

I have observed that many forecasters of the upcoming winter continue to use a multitude of climatic indices to try and forecast the upcoming nature of the season. However, comparisons of this 2006-07 period to other times may prove futile, in that we have a fairly unique arrangement of parameters involving SSTs, MJO fluctuations, and blocking signatures.

The ongoing El Nino episode may be near peak; witness the cooling of the Pacific Ocean near Malaysia and Indonesia. Waters in the Gulf of Alaska are warming on one end (oceanward) and cooling in the other (near shore), so PDO measures may not really be viable. A scenario of occasional +PNA incursions and sometimes vigorous southern branch jet stream development seem likely. This means that rather than widespread mild or warm weather across the lower 48 states, we would have to expect at least some intrusions of cold air to the right of any mP regimes or build-ups of ridging.

During the fall we have seen impressive cases of cPk to cA vortices close to the Hudson Bay position but also in the St. Lawrence Valley and near Newfoundland. In colder winters, these positions can tend to migrate even further southward. And since phasing with southern stream energy has been present in about 50 percent of important cyclogenesis cases, with time heavy precipitation events will develop snows in proximity to the cold air generated by nearby motherlodes and gyres.

The MEI Analogues: How Valid?

In recent research on the Multivariate ENSO Index or MEI, Dr. Klaus Wolter has compared the ongoing +ENSO event (deemed weak to moderate) to four seasons with a similar measurement of the variable (1951-52, 1963-64, 1976-77, 2002-03). If your review the actual thermal outcome of those winters, you see a distinct similarity in the structure of the temperatures around North America. A strong cold dome looms over what is now NU, and drains into the Midwest and Northeast while an existing warm pool south of Central America contributes to mild readings in the Deep South and in the Florida Peninsula. The west to east nature of the upper flow will allow for incursions of mP values into the western states and Great Plains. Note that the coldest core region was seen in 2002-03, which might seem against the trend of global warming. I feel that the Wolter hypothesis of this coming winter falling somewhere into an average of the four examples is reasonable.

Importance Of El Nino Event

While essentially best termed a weak, basin-wide +ENSO episode over the equatorial Pacific Ocean, there are some defining tendencies of the oceanic temperature field. The western sector has cooled somewhat, while upward spikes are noted in the central and eastern domains of the oscillation character. With cool anomalies very noticeable west of California and Mexico, it is highly probable that a recurrent moist and energetic subtropical jet stream will be noted from the Baja California Peninsula into the lower Mississippi Valley. When this flow pattern phases with the northern branch, as has been seen four times this autumn, heavy precipitation events will be noted from eastern Texas all the way into New Brunswick and southern Quebec. A persistent subtropical high, often seen in El Nino years, has been observed in the vicinity of the Bahamas and western Sargasso Sea, and occasionally may retrogress to near Florida. This anticyclone may strengthen the moisture fetch while also bringing bouts of warm to the Deep South.

Because this is a weak +ENSO display, with occasional jumps in warmth of SSTs toward the moderate range, other anomalies such as NAO and PNA styled blocking can occur, delivering cA regimes into the Midwest and Northeast.

First Key Element Of The Forecast: Southern Branch Phases With Polar Jet Stream

While the often-observed "El Nino Cutoff Low" has been an erratic presence off of the coastline of California, the subtropical jet stream has found a home from near the Equator into northern Mexico and eventually Texas. Several times in this energetic flow has phased with the northern branch, bringing widespread severe weather, heavy rain and powerful straight-line, gradient derived winds to parts of the Midwest, Northeast, and eastern Canada. In October, two such storms actually pulled in enough cold air for heavy snows in lake-related events. So with the coming of winter and a lower sun angle against a growing snow field, frozen precipitation could be a major threat to those regions which recently have had excessive rains.

Second Key Element Of The Forecast: Never-Ending Supply Of Moisture

The astounding pattern seen all across the Northern Hemisphere has been the relative ease with which midlatitude cyclones have been able to tap deep tropical (i.e. equatorial) moisture. While the aforementioned +ENSO association helps this condition in the U.S., notice that the intake of higher dewpoints continues even as the deep storms move north and east. Unless this moist advection occurrence eases or ends during the winter, there will be cases of high precipitable water with colder air, a sure formula for snow and ice within cold and overrunning sectors of cyclones.

Third Key Element Of The Forecast: The Tendency For NAO-Styled Blocking

We are only now coming out of a long-duration example of NAO-styled blocking. A few times during the fall PNA-styled thumb projection ridges have built into BC and AK, but the two and a half week show of a positive height anomaly building over Iceland before retrogressing to NU has been noteworthy. With very strong closed low still taking shape west of the British isles, and Atlantic Ocean SST anomalies favoring a warm "trap" east of NL, the stage is set for at least a few more displays of Rex signatures in the NAO positions. The formation of such anticyclones can only mean colder thermal profiles across the eastern half of Canada and a good chunk of the U.S. to the right of the Rocky Mountains.

Temperature

In a "worst-case" scenario, where +PNA and -NAO derived ridges create deep troughs across eastern North America, cAk regimes will occupy a good portion of the U.S. to the right of the Rocky Mountains. Another example of cooler air impacts may be in the Southwest, where El Nino derived closed lows work inland from offshore of California and Baja California. The ongoing strength of subtropical jet stream flow and a heat ridge (near the Bahamas) will act to keep the immediate Gulf Coast and Florida warm for most of this winter, while a tendency for thumb-projection ridging along the western rim of the continent should bring a milder and drier than normal tendency to weather in the Pacific Northwest. Breakdowns in the amplified jet stream tendency (with importation of mP and ImP regimes) should keep seasonal averages tolerable in the Great Plains and Tennessee Valley.

Precipitation

The storm track patterns during the autumn have been refreshingly simple: digging shortwaves from Alberta; occasional inputs from the Southwest resulting in Colorado/Trinidad cyclones; and southern branch systems that move through the Old South before turning northward along the Atlantic Coastal Plain. In about half of all storms this fall, there has been a phasing of the northern and southern jet streams resulting in high volume of precipitation, severe weather and even bombogenesis.

With ongoing overall SST anomalies, and a weak El Nino still present through the winter, we can probably expect some shift of storm tracks to the south. That would mean any phased systems might redevelop in classic cases of secondary cyclogenesis off of the shoreline of the Carolinas, and cPk or cA input into the northwest quadrant of the lows. In the cases of the deepest storms, lake-related and lake-effect squalls may develop. The best potential for excessive precipitation will be along the subtropical jet stream and its phased presence with the polar westerlies, roughly along and 300 miles either side of an axis from Houston TX to Rimouski QC.

Snow And Ice

Were this a strong or a truly moderate El Nino, with resultant warmer temperature profiles along apparent storm tracks, frozen precipitation types would be much less likely. But this particular +ENSO episode falls within the weak category, and is already showing erosion on its western flank. With warmer subsurface readings about to "max out" and blending into the top oceanic layer, the stage will be set for an active southern stream interlocking with and often phasing with northern branch flow over the Midwest and Eastern Seaboard. This set-up allows for cold air infiltration, hence offering the potential for snow and ice production in the cold and overrunning sectors of storms. Because the lower latitude flow may have periods of particular strength, warm advection may keep frozen types out of the Deep South entirely while bringing about change or mixing to the Mid-Atlantic region and New England shoreline. I suspect there may be a passing resemblance to the events of 1993-94 (although that season is not a particularly good match in terms of ENSO or other climatic indices) from lower Appalachia into the Northeast. But where cold advection is prevalent (which should be fairly often if thermal trends seen this fall continue), the Great Lakes snow belts, the middle and upper Appalachian Mountains and a good portion of the Interstate 95 and 81 corridors could be hit with several high-impact snow events.

Summary

Combined a weak El Nino episode that may decay in late winter, thus shutting down or diminishing the vaunted southern branch moisture feed, and recurrent strong NAO-styled blocking in locations stretching from Nunavut AR to Iceland (which should suppress the polar or Arctic jet streams in latitude), and the stage is set for a colder than normal, fairly snowy winter season across the Midwest and Northeast.

With occasional +PNA positive height anomalies in place over the West and a subtropical high likely to be centered near or just east of the Bahamas, then it becomes evident that temperatures will stay moderate or warm across the Intermountain Region and along the immediate Gulf Coast, including Florida. Precipitation may be sparse in these same vicinities, owing to the strength of the anticyclones. However, with the pulsations of the El Nino energizing the lower latitude jet stream, precipitation can expected to be well above normal from parts of Texas into the upper St. Lawrence Valley. Where moisture intersects the incoming cold air, such as in the Ohio Valley, Appalachia, and the Northeast, snowfall can be expected to run above average. In some cases, icing may be an issue with the very warm mT fetch overrunning the cold dome (in a manner similar to 1993-94).