By: William R. Deedler, Weather Historian, National Weather Service Detroit/Pontiac Mi- 5/99
Now that the winter of 1998-99 is over, a study of La Niña's effect on Southeast Lower Michigan's weather, as well as the overall global atmospheric pattern, can be made. First off, it is necessary to review Detroit's climate statistics of past La Niña winters in order to make a comparison. In addition, reference to my previous article, "El Nino's "Sister" La Niña, Comes This Winter For A Visit (But What Of Her Past Visits)...Will History Repeat Itself" is needed, along with a comparison to the winter of 1998-99. The following chart reflects the statistics of the past La Niña winters studied, along with the winter of 1998-99:
Winter Data (December-February)
|Month/Year||Temp/Depart||Winter Ave* /Depart (1870-1990)||Precip/Depart||Snow/Depart|
|01/87 Pk:Dec-Jan||23.0/+0.1||1.31/-0.45||12.8/ +2.3|
|12/3 (Strong)||22.5/-5.8||18.7||1.83/-0.99||12.3/+ 2.3|
|01/25 Pk: Feb||22.5/-0.4||1.08/-0.68||10.5/0.0|
|12/28 (Strong)||32.9/+4.6||26.0||1.38/-1.44||2.6/- 7.4|
|01/29 Pk: Feb||21.8/-1.1||4.27/+2.51||22.1/+11.6|
|01/71 Pk: Dec/Mar||20.7/-2.2||1.03/-0.73||8.7/-1.8|
|01/74 Pk: Nov/Jan||26.5/+3,6||3.26/+1.50||14.1/+3.6|
|12/98 (Moderate -||35.3/+7.0||30.3||1.16/-1.66||1.2/-8.8|
|02/99 Pk: unkwn||32.7/+7.3||+3.8||1.98/+0.24||7.8/-1.4|
Though temperatures for the month of March were not included in the previous La Niña winters statistics, these statistics do have some relevance to this past winter, especially in portraying the temperature and snowfall cycles. When examining this past winter's climate, one of the most striking statistics is the overall warmth of the winter. None of the other winters average temperatures were even above normal, let alone nearly four degrees. While the winter average temperature was mild, the other consideration that must be made, when comparing the 1998-99 winter temperatures to the La Niña winter temperatures is the overall temperature cycle. To get a better idea of this past winter's temperature cycle, one must look at the actual upper wind flow that prevailed this past winter and compare it to the normal (or average) La Niña winter upper air pattern. One of the most common characteristics of the La Niña winter patterns, though not exclusive to La Niña, is a rather transient amplified upper air (jet stream) pattern, rather than a zonal flow, which is more typical of an El Nino phenomenon. This amplified pattern is commonly referred to as a "progressive" pattern, meaning the position of the overall wind flow pattern generally moves along from west to east and varies considerably. Therefore, upper troughs and ridges generally do not lock into place for any extensive period of time. This classic, progressive and amplified La Niña pattern did, in fact, dominate the U.S. throughout much of the1998-99 winter season. The progressive trough-ridge-trough pattern brought considerable variability in the winter weather to Great Lakes region, including some extremes. When reviewing this past winter climate statistics, one can actually visualize the transient "sine-wave" upper air pattern cycle and its resultant weather.
At winter's start, most of December's weather was unseasonably sunny and warm with precipitation less than half the normal and snowfall almost nonexistent. A strong ridge of high pressure in the upper levels of the atmosphere that dominated the eastern half of the country through much of the month was mainly responsible for the mild weather. The upper ridge kept polar intrusions at bay until Christmas week, when a notable change took place. Ironically, this abrupt change in the jet stream and its resulting weather coincided very well with the findings of the other La Niña winters around the holidays. Note the following excerpt from the previous La Niña study article:
"The most notable weather trend change in the studied La Niña patterns usually occurred during the transition period of fall into winter. In all cases, temperature averages went from being above (or well above) normal to near normal or below. This transition occurred mainly between mid November and early January, or basically during the "holiday season" of Thanksgiving into Christmas and New Years Day.
A trend change from the preceding fall is evident in the majority of the winters from the mild and dry weather of the fall, to a PERIOD of cooler than normal temperatures and above normal precipitation. The word "period" is stressed as the majority of the winters indicate a period of time in which temperatures average below normal and precipitation above. While this period of time may generally vary a month or two, it does not mean the winter as a whole will be cold and wet. In fact, when looking at the overall winter departures, one can see quite a range in the results."
This past winter's change to cold and snowywas quite dramatic in early January and was heralded into the area by a classic blizzard New Years weekend. By mid month, at least a couple of feet of snow had been dumped on the region and temperatures averaged just slightly better than eleven degrees, nearly twelve degrees below normal! A strong, intense upper low pressure trough plunged south out of the Arctic into the eastern half of the country replacing the high pressure ridge that had dominated much of the fall and early winter. Then, just as abruptly as the Arctic upper low pressure plunged into the area early January, it was booted northeast out of the area by late January by the persistent strong Pacific jet stream. This returned much milder conditions to Southeast Lower Michigan with temperatures averaging above freezing (34F)! These "key players" in La Nina's upper air pattern, among others, were also discussed in the previous La Niña study:
"Usually by December, the positioning of the upper air Hudson Bay Low (a regular winter feature) became more apparent. During the majority of the La Niña falls, this semi-permanent annual feature initially developed and grew east of Hudson Bay in eastern Canada, over the Canadian Maritimes and, either retrogressed with time toward Hudson Bay or, developed a trough or extension of the center westward into Hudson Bay. The development and positioning of this low was primarily responsible for delivering the cold outbreaks during the winter, in varying intensities.
The last main "key player" was the development and positioning of an upper air mid-Atlantic ridge of high pressure. The positioning of this ridge tended to influence the positioning of the Hudson Bay Low. When the ridge became established just west of, or over western Europe, generally Michigan's resultant winter weather was more variable and usually not as severe as opposed to when the ridge formed and became anchored further west, in the Greenland/Iceland region. When a strong ridge of high pressure became established over that area, it would tend to displace the Hudson Bay Low further south, generally oscillating between James Bay, Canada and the Great Lakes. The cold air in the circulation would tend to be "bottled up" much like a freezer when its door remains primarily shut, only letting out minimal amounts of cold air at times. Thus, only small amounts of the cold air gets dispersed or "escapes" during much of the winter from its cyclonic flow over much of central and eastern Canada and the Great Lakes."
The Atlantic Ridge did become established early winter and, for the most part, remained just west of Europe during the remainder of the winter. On a few occasions, the ridge did back (or retrogress) to the central Atlantic near Greenland and Iceland, and thus, helped temporarily "lock in" the Hudson Bay Low further south into the Great Lakes. This combination, along with a highly amplified eastern Pacific ridge, helped deliver the most brutal cold of the winter to the area during early and mid January. The ridge however, never became a semipermanent feature in the central Atlantic like was noted in our more severe La Niña winters. This ridge, working in conjunction with a deep polar trough over central and eastern Europe, also helped in delivering some very cold outbreaks along with very heavy snows to portions of central and eastern Europe, during the winter.
The upper air pattern during this past winter was well reflected by the dominant storm tracks also found in the La Niña study, note the following:.
"The upper air pattern brought several Alberta Clipper low pressure systems (named in the old clipper ship days in which these clippers moved along at a brisk speed with the aid of a strong wind, much like a sailboat). As the Arctic storm track became more active as the winter evolved during the La Niña winters, the Pacific jet stream continued active and routinely brought storms into the country. Some of these storms intensified in Colorado or the Texas Panhandle area and are known in meteorological vernacular as "Colorado Lows" or "Texas Hookers". These terms refer to their favored breeding ground and subsequent movement or "hook" to the northeast across the Plains and Midwest, and finally into the Great Lakes."
The above storm track scenario was quite evident during this past winter. In fact, our overall winter storm track pattern was almost exclusively dominated by Alberta Clippers and Texas Hookers. The oscillation of control between the Arctic Jet Stream and Pacific Jet Steam basically caused the sometimes wild fluctuations in temperatures during the winter (including March) and was still an influence at least into early May.
The "sine wave" temperature pattern of above normal to below normal continued through the remainder of the winter with February being well above normal and March, below. In addition, the majority of the snowfall this season occurred during the months of January and March and nearly all of it during the first two weeks of each month. It is also interesting to note that the overall temperature/snowfall patterns of both months were ironically quite similar. The coldest of weather, along with nearly all the snowfall, occurred during the first half of January and March. On the flip side, in keeping with our "sine wave" or oscillation pattern, February held a fairly strong resemblance to December. In both months, temperatures averaged well above normal (7.0+) with not surprisingly, below normal snowfalls. Even in the month of April, this trend continued with above normal temperatures returning. While the majority of months do generally display a pattern of above normal and below normal temperatures, it is the exaggerated amplitude here that is most noteworthy.
In my previous article, I equated the fall of 1928 and secondly, 1973 to this past fall as far as temperature and precipitation trends. The subsequent winters that followed, 1928-29 and 1973-74 also both contained several similarities to this past one. The monthly (and intra-monthly) temperature oscillations reflected well a progressive, amplified upper air pattern both winters. In addition, while the snowfall total of the 1973-74 season was nearly duplicated this past winter (49.2 vs 49.5), the pattern of snowfall was quite similar to that of 1928-29 and the severe winter of 1903-04. There was also the familiar "sine wave" pattern in last winter's snowfall where snowfall peaked in January and this too, was present in the winters of 1928-29 and 1903-04 (the winter of 1886-87 also reflects the same familiar pattern, but the snowfall maxed-out earlier in December).