Special Climate Summary April-July 2004 Hot in Alaska, Cool Over Central North America, Wet in South-Central U.S
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Special Climate Summary April-July 2004 Hot in Alaska, Cool over Central North America, Wet in South-Central U.S. by Gerald Bell Climate Prediction Center/NOAA/NWS/NCEP Table of Contents: 1. Overview p. 1 2. Surface Temperatures and Precipitation during April-July 2004 p. 2-4 3. Mean Atmospheric Circulation over North America during Apr.-Jul. 2004 p. 5 4. Mean Jet Stream over North America during April-July p. 5-8 5. Links to Hemispheric Circulation p. 8-10 6. No Link to the Madden-Julian Oscillation (MJO) p. 10 7. Summary p. 11-12 1. Overview Surface Temperature Departures a 70N The period April-July 2004 featured exceptionally warm and dry conditions from 60N Alaska to California and below-normal tempera- 50N tures across the eastern half of Canada and the 40N central United States. It also featured above- average precipitation from Texas northeastward to 30N New England (Fig. 1). These conditions were 20N 180 160W 140W 120W 100W 80W 60W 40W associated with a persistent blocking ridge over Alaska and northwestern Canada, and a west- -4 -3 -2 -1 -0.5 0 0.5 1 2 3 4 (C) Precipitation Percentiles b ward shift and strengthening of the mean trough 70N over central/ eastern Canada. Three factors that contributed to the 60N longevity of these conditions are discussed. These 50N include 1) A persistent zonal wave-3 pattern at 40N high latitudes; 2) A pronounced stratospheric extension of the high-latitude height anomalies, 30N with the blocking ridge over Alaska and the 20N 180 160W 140W 120W 100W 80W 60W 40W downstream amplified trough extending past the 30-hPa level; and 3) Anomalous wave and jet 10 30 70 90 (% ile) stream conditions over the western North Pacific, Fig. 1. April-July 2004: mean 300-hPa heights (contour including a reduced eastward extent of the East interval is 60 m) overlayed with (a) surface temperature departures (oC) and (b) precipitation percentiles. Asian jet core. 1 2. Surface temperatures and precipitation Surface Temperature Rankings 70N during April-July 2004 60N Most of western North America experi- 50N enced exceptionally warm (Fig. 1a) and dry (Fig. 40N 1b) conditions during April-July 2004, while 30N central North America experienced unusually cool 20N 180 160W 140W 120W 100W 80W 60W 40W conditions and the eastern United States experi- enced above-average precipitation. The most 1 3 5 10 15 20 40 45 50 53 55 anomalously warm and dry conditions were Fig. 2. April-July 2004 surface temperature rankings. A ranking of 1 indicates that April-July 2004 is the coldest observed in Alaska, northwestern Canada, and the such period, and a ranking of 55 indicates that it is the Pacific northwestern United States. In Alaska daily warmest such period, in the 55-year record 1950-2004. mean surface temperatures averaged 2°-5°C April-July Area-Average Temperature Departures Alaska a above normal, and precipitation totals were 3 generally in the lowest 30th percentile of occur- 2 rences. In northwestern Canada and southeastern 1 0 Alaska temperatures averaged 2°-3°C above -1 normal, and precipitation totals were in the lowest -2 th -3 10 percentile of occurrences. 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Central Canada b Mean temperatures during April-July 2004 3 2 were among the warmest in the 1950-2004 1 record for much of Alaska and western Canada 0 (Fig. 2). For Alaska as a whole area-averaged -1 -2 temperature departures reached 1.8°C, exceeding -3 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 the previous record departure of 1.5°C observed Fig. 3. Time series of April-July surface temperature in 1993 (Fig. 3a). deparures (oC) from 1950-2004 in (a) Alaska and (b) central Time series of daily temperatures (Fig. 4) Canada. Departures are calculated from the 1950-2004 base and precipitation (Fig. 5) at Fairbanks in south- period means. Daily Mean Temperatures and Normals central Alaska and Yakutat in southeastern Alaska Fairbanks, Alaska a 25 25 illustrate the persistence of the exceptionally warm 15 15 and dry conditions. Above-normal temperatures 5 5 -5 -5 °C first appeared at Fairbanks in early April 2004 -15 -15 -25 -25 (Fig. 4a), and at Yakatut in early February 2004 -35 -35 -45 (Fig. 4b). This anomalous warmth continued at Sep Nov Jan Mar May Jul both stations through early August, with no sub- 2003 2004 Yakutat, Alaska b 20 20 stantial cold-air outbreaks during the period. 15 15 Significant precipitation deficits began in 10 10 5 5 September 2003 at both stations (Fig. 5), with °C 0 0 Fairbanks receiving no measurable precipitation -5 -5 -10 -10 for the 7 ½ month period from mid-September -15 -15 Sep Nov Jan Mar May Jul 2003 through early May 2004 (Figs. 5a, b). 2003 2004 Fig. 4. Daily mean temperatures and normals at (a) Beneficial rains were then observed during May, Fairbanks and (b) Yakutat, Alaska during September 2003 - and again in late July and early August (Fig. 5b). July 2004. Temperatures are shown by thin solid line and At Yakatut, the total September 2003- normals are indicated by dashed line. Departures from July 2004 precipitation deficit was 1400 mm (Fig. normal are shaded, with red (blue) indicating above (below) normal temperatures. 2 5c). Rainfall was particularly Precipitation Fairbanks, Alaska suppressed during May-July, inches mm inches mm 10 250 1.0 25 Accumulated Oberved: thick line Daily Observed b when daily totals reached 25 8 Accumulated Normal: thin line 200 0.8 20 mm on only three occasions 6 150 0.6 15 (Fig. 5d). 4 100 0.4 10 2 a 50 0.2 5 Looking at longer time 0 0 0 0 Sep Nov Jan Mar May Jul Sep Nov Jan Mar May Jul 2003 2004 scales, April-July surface 2003 2004 Yakutat, Alaska inches mm inches mm temperatures in Alaska have 160 4000 5 125 Accumulated Oberved: thick line Daily Observed d exhibited considerable low- 128 Accumulated Normal: thin line 3200 4 100 96 frequency variability since 2400 3 75 64 1600 2 50 1950. For example, during the 32 c 800 1 25 0 0 0 0 19-yr period 1959-1977 only Sep Nov Jan Mar May Jul Sep Nov Jan Mar May Jul one season (1969) was 2003 2004 2003 2004 above average and sixteen seasons were below average. During the following 27-yr Fig. 5. Daily and accumulated precipitation at (a, b) Fairbanks and (c, d) period (1978-2004) sixteen Yakutat, Alaska during September 2003 - July 2004. Accumulated departures from normal are shaded, with brown shading indicating precipitation deficits. seasons (59%) were above- average and nine seasons were below average. June 2004 a In contrast to the exceptionally warm and 55N dry conditions in Alaska during April-July 2004, daily mean temperatures were well below average 50N across central North America. The largest tem- 45N perature departures were observed across central 40N Canada (2°-3°C below normal) and the north- central United States (1°-2°C below normal). The 35N largest southward extent of the cool conditions 30N occurred during June and July, when maximum temperatures were below average across nearly 25N 120 110W 100W 90W 80W 70W the entire United States (Fig. 6). In central Canada, many regions experi- 55N July 2004 b enced their coldest April-July period in the 1950- 50N 2004 record (Fig. 2). Overall, area-average temperatures in central Canada were 1.4°C below 45N normal during April-July 2004, making this the 40N coldest such period since 1967 and the third coldest since 1950 (Fig. 3b). 35N A time series of temperature departures at 30N Waco, Texas (Fig. 7) illustrates the character of the cold conditions over the south-central United 25N 120 110W 100W 90W 80W 70W States during June and July. Temperatures at this station were below-average throughout the period, -4 -3 -2 -1 -0.5 0 0.5 1 2 3 4 (C) with the cold conditions even extending into mid- Fig. 6. Mean maximum temperature departures (°C) August. during (a) June and (b) July, 2004. Departures are calculated from the 1971-2000 base period normals. 3 Several significant cold-air outbreaks WACO, TEXAS ÊC Daily Average and Normal Temperatures ÊC occurred during mid-June through mid-August, as 35 35 cold fronts penetrated into the deep south. These 30 30 events also brought well below-average tempera- 25 25 tures to much of the eastern United States (shad- 20 20 ing, Fig. 8). 15 15 These cold-air outbreaks were associated 10 10 1APR 1MAY 1JUN 1JUL 1AUG 1SEP with an amplified upper-level ridge (contours) over 2004 western North America and an amplified trough extending southward into the central or eastern Fig. 7. Daily mean temperatures and normals at Waco, United States. In Texas, the exceptional south- Texas during Apr.-Aug. 2004. Temperatures are shown by ward penetration of the cold fronts during these thin solid line and normals are indicated by dashed line. Departures from normal are shaded, with red (blue) periods was also related to a strong southward indicating above (below) normal temperatures. funneling of cold air along the eastern slopes of the Rocky Mountains. This funneling is 300-hPa Heights and 1000-hPa Temperature Departures related to up-slope flow along the 18-24 June 2004 a 25 June - 1 July 2004 b southern (or leading) edge of the 80N surface pressure ridge, which is 70N generally located upstream of the 60N mean trough axes.