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The 1995-96 winter season (December-February) featured recurring and unusual weather patterns
(Figure 1, top) throughout the Northern Hemisphere middle latitudes in association with an abnormal
planetary-scale pattern of storm tracks and jet stream winds that extended from the North Pacific Ocean
eastward through Eurasia. These weather patterns were in many respects opposite to those observed during
the 1994-95 winter season (Figure 1, bottom), particularly during the December-January period. Two
important contributing climate factors to these differences were: (1) a pronounced transition over the tropical
Pacific during 1995 from warm episode conditions [also known as El Nino] to cold episode conditions [also
known as La Nina]; and (2) short-term variability inherent to the climate system.
This Special Climate Summary highlights selected notable weather events during the 1995-96 winter over North America (Section 1). A comparison is then presented of the atmospheric circulation, temperature, and precipitation patterns during December-January 1994-95 and December-January 1995-96 over the Pacific/North American sector (Section 2) and the North Atlantic/Eurasian sector (Section 3). Finally, the transition from warm to cold episode conditions in the tropical Pacific during 1995 is described (Section 4). For additional details regarding the circulation, temperature, and precipitation patterns over North America during the 1994-95 winter season, refer to the CPC: Special Climate Summary 95/1 (CPC, 1995a).
1. Selected Notable Weather Events over North America during the 1995-96 Winter
a. The Blizzard of 1996: January 6-9, 1996
The winter season featured several episodic, severe weather events throughout North America. In the East, the most notable of these events was the Blizzard of 1996" which occurred during January 6-9 (Figure 2). During this event, 20-48 inches of snow covered the Eastern Seaboard from the middle Ohio Valley eastward to eastern Virginia and northward into southern Massachusetts. Snowfall totals topped 30 inches at Philadelphia, PA, establishing a new single-storm record, while totals reached 35 inches in west- central New Jersey and 48 inches in Pocahantas County, WV. Most areas from Washington, DC northeastward to Providence, RI reported one of the five biggest snowfalls of all time. The extreme snowfall forced many businesses and most governments to close for several days. It also shut down most airports on the East Coast from Virginia northward, and made other modes of transportation virtually impossible.
In sharp contrast, the eastern United States experienced very mild conditons the following week. This rapid warm-up was accompanied by widespread heavy rain (2-6 inches), resulting in rapid snow melt and extensive run-off that engendered severe flooding along many rivers and tributaries. The major rivers in Pennsylvania, Ohio, West Virginia, Virginia, Maryland, New York, and Vermont generally crested 4-12 feet above flood stage during January 19-24, which for many locations was their highest crest since the flooding caused by the remnants of Hurricane Agnes in June, 1972.
b. Major Cold-Air Outbreak over North America: January 29 - February 6, 1996
Much of western and central Canada and the northern tier of the United States experienced substantially below-normal temperatures and several major cold-air outbreaks during the 1995-96 winter. The most notable cold spell occurred during January 29-February 6 (Figure 3) when temperatures averaged 20F to 30F below normal from the southern Canadian Prairies southeastward through the northern and central Plains and western Great Lakes, and more than 10F below normal throughout virtually the entire United States except the Southwest.
Temperatures dropped below -40F throughout interior Canada during the event, and dipped under -50F over portions of central Canada (Figure 3, top). In the U.S., all-time record low temperatures were set in four states (-60F at Tower, MN; -47F at Elkander, IA; -35F at Elizabeth, IL; and -25F at Greene, RI), over a dozen other stations (mostly in Iowa) established new all-time record lows, more than 50 locations recorded new February minimum readings, and nearly 400 daily record lows were either tied or broken during this period. These conditions were accompanied by strong winds that produced extremely dangerous wind chill temperatures reaching -60F to -90F over much of Canada and large portions of the northern United States (Figure 3, bottom). Interestingly, this cold-air outbreak was followed by much milder air which moved into the central and eastern United States during February 6-10. During this 5-day period, temperatures in the Plains reached 70F at many locations, nearly 100F higher than the record low readings observed the week before at some locations.
c. Flooding in the Pacific Northwest: February 5-9, 1996
Surplus precipitation dominated the Pacific Northwest during the 1995-96 winter (Section 2a), with the largest anomalies observed in western portions of Washington and Oregon. Two particularly heavy rainfall events (November 22-December 3 and February 5-9) occurred over western Washington and Oregon this season, each triggering significant flooding. During the latter event, precipitation fell as rain at all but the highest elevations, with rainfall totals averaging 10-16 inches at many locations (Figure 4). This excessive rainfall, coupled with extremely mild temperatures, caused rapid snow melt (in excess of 50 inches over a few days) and massive run-off at many mountain locations. Collectively, these conditions resulted in severe flooding west of the Cascade Mountains, particularly along the Willamette and Columbia Rivers.
d. Heat Wave and Wildfires in the Southern Plains: February 18-27, 1996
During mid- to late February, summer-like heat, with temperatures reaching 100F in central Texas and 90F in southern Kansas, low humidities, gusty winds, and long-term dryness (Section 2a) provided ideal conditions for wildfires in the southern Plains. According to the National Interagency Coordination Center, wildfires in Texas and Oklahoma have consumed approximately 129,000 and 54,000 acres, respectively, of rangeland from February 18-March 20. In Texas, this 4-week value is nearly 8 times greater than the entire 1995 total of 16,422 acres. Much colder air, higher humidities, and scattered precipitation towards the end of February allowed firefighters to contain most of the blazes; however, new wildfires were sparked during the second full week of March, and the current potential for additional wildfire development remains very high.
The above conditions, along with extreme temperature fluctuations, resulted in a highly unfavorable environment for the hard red winter wheat crop in the central and southern Plains this winter. According to the U.S. Department of Agriculture's National Agricultural Statistic Service, 44%, 61%, and 60% of the winter wheat crop in Kansas, Oklahoma, and Texas, respectively, was rated poor or very poor for the week ending March 17.
2. Pacific/North America: Seasonal Circulation, Temperature, and Precipitation
Large amplitude circulation, temperature, and precipitation anomalies have been observed over the Northern Hemisphere during the past two winter seasons, with significant differences clearly evident between the two seasons (Figure 1) and (Figure 5). These differences are attributed to large changes in the patterns of jet stream winds and attendant storm tracks between the two seasons, and in the relative variability of these features during each season.
a. The 1995-96 Winter
During the 1995-96 winter, the mean Pacific jet stream and storm track over western North America were centered over the Pacific Northwest in approximately their climatological mean position (Figure 1, top). However, there was considerable variability in the eastward extent of these features during the season, with episodes of extremely strong winds and heavy precipitation over the Pacific Northwest, as well as periods of minimal winds and precipitation. For the season as a whole, the Pacific Northwest recorded exceptionally heavy (125-175% of normal) precipitation (Figure 6a), and experienced two locally severe flooding episodes (Section 1c).
Farther south, below-normal jet stream winds and reduced storm activity brought abnormally dry conditions (Figure 6a) to the Southwest and southern Plains during December and January. Fortunately, the precipitation deficits were partly alleviated in southern California during February as a series of storms moved into the Southwest. For the season as a whole, precipitation totals in the southern Plains were less than 50% of normal. These conditions exacerbated soil moisture deficits that began to develop during the fall (Section 1d). On a long-term basis, this was the second driest October-February period on record since 1895-96 in the hard red winter wheat area of the south-central Plains, according to preliminary data from the National Climatic Data Center. Summer-like heat also occurred in the southern Plains during late February as temperatures reached 90F on several days, and briefly hit 100F in central Texas. This extreme warmth further aggravated dry conditions throughout the region.
Farther north, there was also considerable variability during December-January in the height and wind patterns throughout western and central Canada and across the northern tier of the United States. One prominent aspect of this variability was a recurring pattern of enhanced northwesterly flow, which extended from northern Alaska and the Bering Sea to southeastern Canada (Figure 1, top) and (Figure 5a). This pattern was associated with well below-normal temperatures throughout the region (Section 1b). Overall, December- January temperatures averaged 2-5C (4-10F) below normal in these areas (Figure 7a). This northwesterly flow pattern also brought a series of major storms to the eastern United States, resulting in near-record to record snowfall for the season (Figure 8); Section 2c). Another aspect of this circulation variability was evident in the occasional large swings in temperatures from one week to the next over large portions of the central and eastern United States (Section 1b). A prime example of this was when new February extreme minimum (-11F) and maximum (90F) records were set within 18 days of each other at Tulsa, OK.
b. The 1994-95 Winter
In contrast to the large variability observed during 1995-96, the December-January 1994-95 period featured relatively persistent patterns of jet stream winds and storm tracks. For example, a persistent pattern of below-normal heights dominated the eastern North Pacific and Pacific Northwest during December- January 1994-95 (Figure 5b). This pattern was associated with an eastward extension of the Pacific jet stream to central California (Figure 1, bottom), along with a southward shift of the jet stream by approximately 18o latitude from its normal position over southwestern Canada and the Pacific Northwest. This jet stream pattern directed a series of major storms across the North Pacific Ocean and into California, producing excessive precipitation (125-200% of normal) throughout California and the Southwest (Figure 6b). Additionally, this jet stream pattern directed an abnormally strong flow of marine air into the western United States and western Canada, resulting in abnormally warm conditions across western North America (Figure 7b). These precipitation and temperature distributions differ from the 1995-96 winter patterns of enhanced precipitation in the Pacific Northwest, suppressed precipitation in California and the Southwest (Figure 6a), and abnormally cold conditions throughout western Canada (Figure 7a).
In the East, above-normal heights dominated central and eastern Canada during period (Figure 5b), and the polar jet stream was shift well north of normal toward southern James Bay (approximately 15o latitude north of normal). This flow pattern inhibited the formation of intense cold air masses, and directed weaker weather systems quickly across central and eastern Canada and then eastward over the western North Atlantic. As a result, the entire eastern two-thirds of North America also experienced significantly warmer than normal conditions [3-6C (6-10F) above normal] during the season (Figure 7b). Again, this warmth contrasts with the subnormal temperatures observed throughout Canada and the northern and northeastern United States during December-January 1995-96.
c. Snowfall: Comparison of the 1994-95 and 1995-96 Winter Seasons
The large differences in circulation, precipitation, and temperature between the last two winters were accompanied by large differences in the observed patterns of snow cover and total snowfall. In the United States, well above-normal snowfall covered the northern Plains, the Northeast, and the mid-Atlantic states during the 1995-96 winter (Figure 8). Many of these areas received 200%-350% of normal snowfall, with the largest surpluses observed over the west-central Appalachians, eastern Maryland, southeastern Pennsylvania, and New Jersey. Approximately fifteen major cities and several smaller locations established new all-time record seasonal snowfall totals (Table 1). In contrast, much of the northeastern United States and the Great Lakes region experienced significantly below-normal snowfall during the 1994-95 winter, with large portions of the Appalachians and the mid-Atlantic states recording less than 25% of normal snow cover (CPC, 1995a).
Table 1. All-Time Record Snowfall Totals for Selected Locations as of March 10, 1996. Location Total (Inches) Date Previous Record Broken Sault Ste. Marie, MI 202.9 January 30 Elkins, WV 126.7 March 8 Windsor Locks, CT 107.7 February 16 Boston, MA 100.3 March 7 Charleston, WV 94.7 February 2 Providence, RI 93.2 February 16 N.Y. Central Park, NY 70.8 March 2 Philadelphia, PA 63.1 February 16 Baltimore (BWI), MD 62.5 February 16 Wilmington, DE 59.2 February 16 Lynchburg, VA 56.4 February 16 Jackson, KY 52.1 February 16 Washington (DCA), DC 46.0 February 16
3. North Atlantic and Eurasia: Seasonal Circulation, Temperature, and Precipitation
Large-scale circulation anomalies also dominated the North Atlantic and Eurasia during both the
1994-95 and 1995-96 winter seasons. During the 1995-96 winter, the circulation (Figure 5a) was dominated
by above-normal heights over the high latitudes of the North Atlantic and Scandinavia, and by below-normal
heights over the middle latitudes of the North Atlantic, western Europe, and north-central Russia. This
pattern was associated with enhanced jet stream winds throughout the central North Atlantic, northern
Africa, and southwestern Europe (Figure 5c), and with weaker-than-normal jet stream winds farther north. The
intensity and duration of this overall circulation pattern has not been observed since the late 1970's.
This flow pattern reflected a marked southward shift of the normal Atlantic jet stream and storm track, and resulted in abnormal temperature and precipitation patterns throughout Eurasia. The largest negative temperature departures [-2C to -4C (-4F to -8F)] were observed across northern Europe, southwestern Russia, and south-central Russia during the season (Figure 7c) in association with enhanced northerly flow and a reduced transport of marine air into the region from the North Atlantic. This southward shift in the jet stream and storm track also brought significantly above-normal precipitation to southwestern Europe and northern Africa, alleviating moderate-to-severe drought conditions that had affected these areas for the past several years. It also brought above-normal snowfall to much of eastern Europe and western Russia.
In contrast, the circulation during the 1994-95 winter (Figure 5b) was dominated by below-normal heights over the high latitudes of the North Atlantic, and by above-normal heights over the central latitudes of the North Atlantic, western Europe, and northwestern Russia. This pattern and accompanying jet stream wind anomalies (Figure 5d) are nearly opposite to those observed during the 1995-96 winter season. For example, enhanced westerlies dominated the eastern North Atlantic and western Europe during the 1994-95 winter, while enhanced southwesterly flow persisted throughout western and central Russia. This jet stream pattern brought an enhanced flow of marine air into western Eurasia, which subsequently spread eastward to cover large portions of northern Russia. These conditions resulted in record or near-record warmth throughout Eurasia (Figure 7d), with temperatures averaging 6-10C (12-20F) above normal. This extreme warmth persisted through May 1995 and was associated with significantly reduced snowfall throughout western Eurasia (CPC, 1995a; CPC, 1996). Again, these patterns were nearly opposite to those observed during the 1995-96 winter.
4. The 1995 transition from warm episode (El Nino) to cold episode (La Nina) conditions
a. 1994-95 Warm Episode (El Nino) Conditions
The short-term climate system witnessed a return to the mature phase of warm ENSO conditions (commonly referred to as El Nino) during the 1994-95 winter for the third time in four years. This frequency of occurrence is unprecedented in the last 50 years, and is comparable to the prolonged 1911-1914 warm episode.
These warm episode conditions are identified by five prominent features: (1) abnormally warm ocean water throughout the central and east-central equatorial Pacific (Figure 9, bottom); (2) enhanced tropical thunderstorm activity throughout the central equatorial Pacific (Figure 10, bottom); (3) below-normal strength of the low-level easterly winds across large portions of the equatorial Pacific (Figure11, bottom); (4) above- normal surface pressure over Indonesia, much of Africa, and the equatorial Atlantic Ocean (Figure11, bottom); and (5) above-normal atmospheric temperatures throughout the tropical Pacific (not shown).
b. 1995-96 Cold Episode (La Nina) Conditions
Following the 1994-95 winter, there was a rapid decline in sea surface temperatures throughout the central and east-central equatorial Pacific. This decline resulted in a complete disappearance of all warm episode conditions by June, which was followed by the development of weak cold episode conditions during September-November. During winter 1995-96, these cold episode conditions strengthened, with the following conditions dominating the tropics and subtropics: (1) abnormally cold ocean waters throughout the central and east-central Pacific (Figure 9, top); (2) below-normal thunderstorm activity throughout the central equatorial Pacific and enhanced rainfall throughout Indonesia (Figure10, top); (3) enhanced equatorial easterly winds at low levels throughout the Pacific (Figure11, top); (4) a persistent pattern of below-normal pressure over Indonesia, Africa, and the equatorial Atlantic, and above-normal pressure over the eastern equatorial Pacific (Figure11, top); and (5) below-normal mean temperatures throughout the global tropics and subtropics (not shown). Collectively, these conditions reflected a reversal from the atmospheric and oceanic anomaly patterns that had dominated the tropical Pacific since late 1990.
c. El Nino and La Nina as Important Contributors to Year-to-Year Variability in Weather Patterns
The El Nino and La Nina phenomena are well known contributors to the observed year-to-year climate variability in the tropics and middle latitudes. The transition from warm episode to cold episode conditions during 1995 included: (1) a dramatic increase in hurricane activity over the North Atlantic, following four consecutive years of significantly below-normal hurricane activity (CPC, 1995b); (2) above- normal rainfall throughout Indonesia, northern Australia, and southern Africa, following a prolonged period of below-normal rainfall and periodic drought; (3) drier-than-normal conditions over southern Brazil, northeastern Argentina, and Uruguay (CPC, 1996); and (4) a stronger-than-normal Indian monsoon (CPC, 1996).
In the middle latitudes, the direct effects of the transition to cold episode conditions during 1995 were most evident over the eastern North Pacific. In this region, there was a pronounced northward shift of the jet stream and storm track during the 1995-96 winter compared to the past several winter seasons (three in the last four) in which a significant strengthening, southward shift, and eastward extension of these features toward the southwestern United States prevailed (CPC, 1996). As a result, these changes in the jet stream position and storm track directly forced changes to the downstream atmospheric circulation, temperature, and precipitation patterns throughout eastern North America, the North Atlantic, and western Eurasia compared to those observed during the 1994-95 winter.
CPC, 1995a: Special Climate Summary 95/1: ENSO Winter Impacts: California Flooding, Mild to the East, February 1995. NOAA/NWS/NCEP/CPC, Camp Springs, MD 20746.
CPC, 1995b: Special Climate Summary 95/3: Climate Conditions during the 1995 Atlantic Hurricane Season, November 1995. NOAA/NWS/NCEP/CPC, Camp Springs, MD 20746.
CPC, 1996: Climate Assessment for 1995, March 1996. NOAA/NWS/NCEP/CPC, Camp Springs, MD 20746.