EL NIÑO’S "SISTER" LA NIÑA, COMES THIS WINTER FOR A VISIT . . . BUT WHAT OF HER PAST VISITS . . . WILL HISTORY REPEAT ITSELF?

By: William R. Deedler, Weather Historian, National Weather Service Detroit/Pontiac Mi   12/98

No sooner do the effects of El Niño on our weather leave us, but then his "sister" La Niña, follows in his footsteps. La Niña, which means "the little girl" (as opposed to El Niño’s, "the little boy") refers to a cooling of the waters in the mid Pacific to below normal (again, opposed to El Niño, in which the water warms above normal). El Niño and La Niña patterns generally oscillate between one another on an average of about four years, but La Niña does not necessarily have to follow immediately an El Niño. During a La Niña, normal easterly trade winds along the equator strengthen and thus, colder water deep down is brought up to the surface. This in turn, causes the sea surface temperature to drop, sometimes as much as 7 degrees F below normal, along the equator. The cooling of the ocean surface temperature impacts the atmosphere and climate patterns around the world. In the United States, drier than normal conditions are usually found in the southwest by late summer and generally last into the winter months. Drier and nicer weather also occurs in the Central Plains during the fall, and extends into the southeast U.S., along with warmer than normal temperatures, during the winter (start planning that trip to Florida, now)! On the other hand, colder than normal winters generally extend from the Pacific Northwest, east across the Northern Plains to portions of the Great Lakes. Wetter than average conditions also dominate the Pacific Northwest during the fall and this extends east, across the Northern Plains, to portions of the Great Lakes and Ohio Valley by winter. Because this winter’s La Niña is expected to be moderate to strong, the aforementioned weather pattern is, more or less, typical of what is expected this winter across much of the U.S.

Two important factors of La Niña that influence the resultant weather are its strength and, just as important, the timing of that strength (or its peak). The strength and timing of the La Niña frequently "fine tunes" the effects on the subsequent winter weather and generally continue into the following spring as well. While most La Niña episodes last close to a year, some have been known to last a few years. The interval of time between a La Niña and El Niño varies between two and seven years. It is interesting to note that since 1975, only about half as many La Niñas have occurred as El Niños. Is it possible there are El Niño/La Niña cycles of dominance?

Data on past La Niñas go back into the late 1800s, and while we know of general weather trends across the country in past La Niña episodes, what about specifically in the Great Lakes area and Southeast Lower Michigan? Have past winters been similar when this years’ predicted La Niña trend is correlated with nearly identical La Niña strengths and peak timings of the past? In other words, have past La Niñas of similar strength and timing created similar weather conditions and trends (cyclone and anticyclone tracks, temperatures, rainfall and snowfall) across the Great Lakes and particularly in Southeast Lower Michigan?

Years of La Niñas dating back into the late 1800s include 1886, 1903, 1906, 1909, 1916, 1924, 1928, 1938, 1950, 1954, 1964, 1970, 1973, 1975, 1988 and 1995. While there are slight variances in intensities and timing of cycle peaks, five past La Niña cycles strongly resemble the predicted La Niña of 1998-99. These La Niñas occurred in 1903, 1928, 1970 and 1973. In addition, while the La Niña affecting the winter of 1917-18 also has similarities to this and the five La Niñas listed above, the timing was different. The cycle of that particular La Niña actually began early in 1916, well over a year before the winter of 1917-18. The winter of 1917-18 was brutally cold across the Great Lakes and Southeast Lower Michigan and had an early start the preceding fall. In fact, the fall of 1917 remains the fifth coldest fall ever recorded in the metro Detroit area and the following winter of 1917-18 was the fourth coldest winter. The average temperature of 19.9 that winter (normal winters average around 26.5) was 6 ½ degrees below normal, although snowfall was not especially heavy at 38.4 inches, which is about normal. In any event, the winter of 1917-18 was not considered, due to the extended length of the La Niña beforehand and timing differences, although an occasional reference may be warranted.

Another point that must be considered is the availability of some of the data during the winter seasons of 1886-87, 1903-04 and 1928-29. In addition, upper air data were nonexistent for1886-87 and national surface data are at best, limited. Detroit’s records go back to the 1870s, thus, locating local climate data is not a problem. Though partially estimated, the Monthly Weather Review surprisingly does offer some upper air data at the 850 MB level (5,000 ft) and 700 MB level (10,000 ft) for 1903-04. It is puzzling, however, that in the same records, upper air data during the winter of 1928-29 are absent. Finally, data from the later fall and winters of 1970-71 and 1973-74 are the most thorough and complete. The available information presented therefore, offers interesting comparisons: First off, exploring the climate statistics and trends of the preceding falls of 1886, 1903, 1928, 1970 and 1973 from the Detroit metropolitan area shows the following:

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                        MID-LATE FALL DATA (October-November) --------------------------------------------------------------------------------------------------------
MONTH/YEAR - TEMP/DEPART       PRECIP/DEPART         SNOW/DEPART
(0CT NORM)          (51.2)                           (2.10)                            (1.2)      (Based on 30 year
(NOV NORM)        (40.2)                           (2.67)                            (2.9)        normals: 1961-90)
-------------------------------------------------------------------------------------
10/1886 -               55.5/+4.3                      1.04/-1.06                      0/-1.2
11/1886 -               38.7/ -1.5                      2.17/ - .50                   0.6/-2.3
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10/1903 -                53.1/+1.9                       1.67/ - .43                       T/-1.2
11/1903 -                36.5/ -3.7                       1.18/-1.49                      2.3/-.6
-------------------------------------------------------------------------------------
10/1928 -                55.4/+4.2                       1.82/ - .28                       T/-1.2
11/1928 -                42.5/+2.3                       2.69/+ .02                      1.0/-1.9
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10/1970 -                54.2/+3.0                       1.91/ - .19                        0/-1.2
11/1970 -                40.0/ - .2                        2.73/ +.06                     1.7/-1.2
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10/1973 -                56.2/+5.0                       2.01/ - .09                       0/-1.2
11/1973 -                41.4/+1.2                       3.21/ +.54                    0.1/-2.8
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10/1998 -                53.8/ +2.6                      1.34/ -.76                       0/-1.2
11/1998 -                43.8/ +3.6                      1.36/-1.31                      0/-2.9
                                                             CHART - 1

Looking at Chart-1, the trend that stands out immediately is the mild and dry October months. Every one of the five October average temperature was above normal with the mean departure at about +3.8 degrees. Precipitation amounts all averaged drier than normal with the mean at just below half inch. Also predominant, near or above normal temperatures as well as dry conditions were prevalent in September (September’s data were not included in the fall chart because it is generally more of a transitory month from summer to fall; also, at least calendar-wise, more of a summer month). The September and October temperature data strongly support a dry and mild fall as a precursor to the La Niña winters. This idea also gives credence to one or more "Indian Summers" during the fall, especially during October. This is the predominant Indian Summer month anyway considering that an Indian Summer is generally a spell of unseasonably mild and dry weather after a killing frost. It must be remembered though, Indian Summers more often than not occur across the region anyway whether in a La Niña or an El Niño cycle. The idea of a relatively dry and mild fall is the main trend that was noted and should be considered. By the way, rainfall during the preceding summers for the most part, did not begin the dry trend of the falls; in fact, unlike this past summer over much of the Great Lakes area, normal to above normal rainfall occurred.

Looking over the five November months, however, a subtle change is noted in the temperature averages and resulting departures. The change is from the above or well above normal readings seen earlier in the fall to around the normal November average of 40.2. The averages ranged from over two degrees above normal to nearly four degrees below normal. In other words, one can visualize a change taking place in the established fall temperature pattern. The actual numbers above or below normal are not as important as the CHANGE in the pattern or trend. Three out of the five November months averaged near normal to above normal precipitation. These temperature and precipitation TREND CHANGES hinted to the upcoming winter trend. Finally, another persistent trend noted was the lateness or lack of snow. In all the October and November months listed, below normal snowfall was registered with three out the five having none or negligible amounts.

Now that this fall’s data (Oct-Nov) can be entered, a comparison between this fall and the five La Niña falls can be made. One can hardly argue the weather similarities between the fall of 1998 and the dominating weather trend of the other falls. The weather this past fall was indeed mild and dry and overall, exceptionally nice. Also, like the other falls, snowfall was negligible. In fact, the fall of 1998 set a record for lack of snow since officially there was no snow observed at Detroit Metro Airport until December 16th. The two falls (Oct-Nov) that show striking similarity in above normal temperature departures are 1928 and 1973. These autumns, along with the autumn of 1998, averaged better than six degrees above normal. While neither fall was as dry as this past one, the fall of 1928 is the drier of the two comparisons. September’s data in 1928 makes even a stronger case for a drier fall than 1973 with only 1.68 inches rainfall. This was 1.21 below normal (September of 1973 recorded just a half inch below normal). Looking at the overall weather trend of the La Niña falls to this past one, as far as temperature and precipitation, one is led to the conclusion that the fall of 1928 was most like the past one, with 1973 close second.

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. Therefore, proceeding into the winter months, Chart - 2 depicts the climatic statistics of the winter months from the Detroit metropolitan area for the five main La Niña winters. In addition, two other La Niña winters were included: the cold winter (especially December and January) of 1917-18 discussed earlier and, a moderate La Niña that took place during the winter of 1924-25. While the strength of the 1924 La Niña is similar to the present, during December and January the 1924 La Niña weakened...only to strengthen once again late winter and spring. Therefore, the La Niña of 1924-25 was included as an example of a generally weaker La Niña that peaked in the moderate category. Also included in Chart - 2, next to the month/year column, is the La Niña strength and timing (peak:Pk) of that strength. In the years of 1970-71 and 1973-74, two peaks were actually noted, while in 1886-87, the peak lasted for two months (Jan-Feb).

All seven winters displayed a change in both temperature and precipitation trends, although not necessarily at the same time. As an example, the 1970 average temperature went from being above normal in December to below normal in January. At the same time, the below normal precipitation in December carried through January. Wetter conditions did arrive but not until February, when the average temperature also went above normal. When comparing the average temperatures of the fall months to the winter months however, a definite change is noted in the overall temperature trend from above normal, to around normal and then by mid-winter, to below normal. In two instances (1887, 1971) conditions did moderate by late winter.

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                                                 WINTER DATA (December-February)
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MONTH/YEAR-  TEMP/DEPART    WINTER         PRECIP/DEPART    SNOW/DEPART
                                                        AVE*/DEPART
                                                          (1870-1990)
___________________________________________________________________________
(DEC NORM )               (28.3)                Aprx                   (2.82)                    (10.0)            SEASON
(JAN NORM)                (22.9)                (26.5)*                (1.76)                    (10.5)             AVE: 41"
(FEB NORM)                (25.4)                                           (1.74)                    (   9.2)
----------------------------------------------------------------------------------------------------
12/86   (Moderate)        23.9/- 4.4           25.0                    2.21/ - .61           20.8/+10.8        TOTAL
01/87   Pk:Dec-Jan        23.0/+ .1                                       1.31/ - .45           12.8/+ 2.3
02/87                            28.2/+2.8            -1.5                    4.16/+2.42            6.9/ - 2.3         52.9/+11.9
-------------------------------------------------------------------------------------
12/03   (Strong)             22.5/-5.8           18.7                      1.83/- .99            12.3/+ 2.3
01/04    Pk:Apr             17.6/-5.3                                        3.34/+1.58           20.1/+ 9.6
02/04                            16.1/-9.3           -7.8                       2.55/ + .81            5.8/ - 3.4         57.0+16.0
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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
02/29                            23.2/ -2.2         - .5                        1.36/  - .38            10.8/+ 1.6         38.9/-2.1
--------------------------------------------------------------------------------------
12/70   (Strong)            29.0/+ .7             25.7                       .61/-1.21             9.8/ - .2
01/71    Pk:Dec/Mar     20.7/- 2.2                                        1.03/- .73             8.7/ - 1.8
02/71                             27.4/+2.0            - .8                      2.68/+ .94            5.9/ - 3.3            35.4/-5.6
---------------------------------------------------------------------------------------
12/73    (Strong)            28.7/+ .4              26.3                    3.51/+ .69           16.4/+6.4
01/74    Pk:Nov/Jan      26.5/+3.6                                        3.26/+1.50          14.1/+3.6
02/74                             23.6/- 1.8            - .2                      2.37/+ .63           11.2/+2.0           49.2/+8.2
---------------------------------------------------------------------------------------
12/17    (Strong)            21.9/-6.4             19.9                     .88/- 1.94            9.2/ - .8
01/18    Pk:Apr/Dec     13.2/-9.7                                       2.53/+ .77             8.6/- 1.9
02/18                            24.5/- .9             - 6.6                    3.56/+1.82            4.8/ -4.4            38.4/-2.6
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12/24    (Moderate)    23.2/- 5.1            25.5                     3.54/+.72              7.4/-2.6
01/25    Pk: Feb         22.5/- .4                                          1.08/- .68            10.5/0.0
02/25                         30.8/+5.4             -1.0                     2.10/+.36              2.7/-6.5             30.0/-11.0
                                                            CHART - 2
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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. The departures from normal of the extremely cold winter of 1917-18 and even more outstanding, the coldest winter ever recorded in Detroit, (1903-04), clearly demonstrate the worse case scenario. In that winter, the temperature averaged a pitiful 18.7 degrees, while during the winter of 1917-18, the temperature averaged just 19.9, which places it as the fourth coldest winter since 1870. Averaging a 120 years of winter temperatures (1870-1990) gives the Detroit metro area an winter average of approximately * 26.5 degrees, while the 30 year average from 1961-90 is slightly colder at 25.5 degrees. This slight skew downward may be explained by the abnormally cold winters in the mid 1970s to early 1980s when four winters placed in the top 20 coldest: 1976-77/3rd,1977-78/7th, 1978-79/11th, and 1981-82/16th. Therefore, obviously averaging 120 years of winter data would present a better representation of "normal" winter temperatures rather than just 30 years.

Studying available upper air and surface patterns during these La Niña falls and winters was VITAL to drawing some sort of conclusions as to possible weather pattern development this fall and winter. That is to say, were there common features and similarities in both the upper air patterns and surface patterns that evolved with time in the past La Niña seasons? In reviewing these upper air and surface patterns during the fall and winter, several "key players" in determining the overall pattern evolvement became apparent:

The fall months:

1) A broad Pacific low pressure trough with a stronger than normal jet stream was the first upper air development in nearly all the cases during the fall (Sep-Oct).

2) As the strong jet stream took shape over the northern Pacific, a deep Aleutian Low usually evolved by mid fall (Oct-Nov).

These two features combined to bring a nearly continuous procession of surface low pressure systems into the Pacific Northwest and western Canada. The majority of these passed to the north or west of the Great Lakes. Some were very intense as they made landfall in the west, weakened, and then re-intensified in the Colorado/Texas Panhandle area and shot northeast into the Lakes. Since the main storm track remained north and west of Southeast Michigan, the mild and drier weather established early in the fall persisted, for the most part, into late fall (Nov-Dec). As a result, the Indian Summer regime was commonplace.

The late fall and winter months:

3) 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.

4) 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.

Another interesting feature which occasionally "fed" the Hudson Bay Low was the development and strength of what could be referred to as an "Arctic feeder." Simply put, this is an important ridge of high pressure that sets up from near the North Pole, south into mainly the Canadian Northwest Territories (near the Alaskan/Canadian border). When established, this "Arctic feeder" feeds cold Arctic air southeast into the Hudson Bay Low regularly. The strength of this ridge, combined with the strength, and more importantly, the position of the Atlantic ridge and the Hudson Bay Low, acted as sort of a conveyer belt for the Arctic air. Thus, Arctic air was routinely fed into the Hudson Bay Low, this combined with the blocking Atlantic ridge near Greenland helped keep the "freezer" charged and the cold air in place. This 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). These clippers generally form in or around Alberta, Canada and head quickly southeast into the northern U.S. A stronger, more amplified version of this upper air pattern led to the most severe La Niña winters in this study, specifically 1903-04, and in 1917-18, when several cold outbreaks flooded the region.

Meanwhile, 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 Panhandle Low." 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. Once this pattern established itself, the normal to below normal temperature and above normal precipitation pattern generally played out across Southeast Michigan. These storm tracks (Alberta Clipper/Colorado Low/Texas Panhandle Low) were the most common paths of low pressure systems in all the La Niña winters studied. While Gulf (of Mexico) Lows, Ohio Valley Lows and East Coast Lows or "bombs"(a drop of 12 MB/12 hrs at/above 45 degrees latitude) did exist, they were not as common as the others mentioned, and tended to be more active mid-late winter, when the coldest of air dominated the northern half of the country from the Rockies to the East Coast. Subsequently, the storm tracks gradually shifted south and east with time.

After reviewing the data for several La Niña winters, a case could be made that the majority of La Niña winters are basically a "normal winter" in Southeast Michigan. For the most part, that’s true, especially when just looking at climatic data. Some of the winter months did have above normal temperatures, however, the majority were near normal or below, and some months also had less than normal snowfall, although, still better than half, were above. Thus, this study backs and gives credence to the National Weather Service’s winter outlook for Southeast Lower Michigan which calls for normal to below normal temperatures and normal to above normal precipitation/snow.

Curiosity about the winter FOLLOWING the initial La Niña winters studies led to an interesting and somewhat surprising fact. While snowfalls for the subsequent winters did vary, they still averaged about 10 more inches of snow than the initial La Niña winter. Maybe something to keep in mind for next winter!.

 

 


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