| Year | Mar-Apr extent | March–April | March–April (13 year average) | March–April (linear trend) | June extent | June | June (13 year average) | June (linear trend) |
|---|---|---|---|---|---|---|---|---|
| 1967 | 1806 | 118 | 84 | 485 | 168 | 75 | ||
| 1968 | 1451 | -236 | 79 | 149 | -167 | 69 | ||
| 1969 | 1697 | 10 | 75 | 0 | 63 | |||
| 1970 | 2134 | 447 | 70 | 441 | 125 | 58 | ||
| 1971 | 1874 | 186 | 66 | 392 | 75 | 52 | ||
| 1972 | 1299 | -389 | 61 | 273 | -44 | 46 | ||
| 1973 | 1286 | -401 | 56 | 625 | 308 | 40 | ||
| 1974 | 1371 | -316 | 15 | 52 | 213 | -103 | 7 | 34 |
| 1975 | 1480 | -207 | 102 | 47 | 366 | 49 | 37 | 28 |
| 1976 | 1978 | 291 | 84 | 42 | 336 | 20 | 50 | 23 |
| 1977 | 1614 | -73 | 50 | 38 | 355 | 39 | 35 | 17 |
| 1978 | 1923 | 236 | 38 | 33 | 492 | 176 | 29 | 11 |
| 1979 | 2037 | 350 | 113 | 29 | 323 | 7 | 36 | 5 |
| 1980 | 1984 | 296 | 156 | 24 | 233 | -83 | 7 | -1 |
| 1981 | 2589 | 901 | 238 | 19 | 544 | 228 | 15 | -7 |
| 1982 | 1459 | -228 | 255 | 15 | 166 | -150 | 4 | -12 |
| 1983 | 1685 | -2 | 187 | 10 | 242 | -74 | 0 | -18 |
| 1984 | 1723 | 36 | 145 | 6 | 313 | -3 | -14 | -24 |
| 1985 | 2268 | 581 | 98 | 1 | 365 | 48 | -31 | -30 |
| 1986 | 1848 | 161 | 41 | -4 | 259 | -58 | -36 | -36 |
| 1987 | 2438 | 750 | 5 | -8 | 315 | -1 | -29 | -42 |
| 1988 | 1698 | 10 | -89 | -13 | 217 | -99 | -46 | -47 |
| 1989 | 1103 | -584 | -65 | -17 | 286 | -30 | -36 | -53 |
| 1990 | 1060 | -627 | -17 | -22 | 174 | -143 | -30 | -59 |
| 1991 | 1311 | -376 | -34 | -27 | 271 | -45 | -17 | -65 |
| 1992 | 1306 | -381 | -82 | -31 | 259 | -58 | -22 | -71 |
| 1993 | 1509 | -179 | -105 | -36 | 328 | 11 | -28 | -77 |
| 1994 | 1364 | -324 | -167 | -40 | 314 | -2 | -34 | -82 |
| 1995 | 1776 | 89 | -170 | -45 | 300 | -16 | -36 | -88 |
| 1996 | 2304 | 617 | -157 | -50 | 316 | 0 | -52 | -94 |
| 1997 | 1510 | -177 | -100 | -54 | 493 | 176 | -52 | -100 |
| 1998 | 1640 | -48 | -92 | -59 | 297 | -19 | -62 | -106 |
| 1999 | 1547 | -140 | -52 | -63 | 173 | -143 | -66 | -112 |
| 2000 | 1635 | -53 | -6 | -68 | 248 | -68 | -75 | -117 |
| 2001 | 1658 | -29 | -13 | -73 | 185 | -131 | -83 | -123 |
| 2002 | 1270 | -417 | -44 | -77 | 80 | -236 | -101 | -129 |
| 2003 | 1812 | 125 | -106 | -82 | 174 | -142 | -123 | -135 |
| 2004 | 1412 | -276 | -88 | -87 | 133 | -183 | -158 | -141 |
| 2005 | 1818 | 131 | -99 | -91 | 208 | -108 | -178 | -147 |
| 2006 | 2113 | 426 | -98 | -96 | 216 | -100 | -183 | -153 |
| 2007 | 1277 | -410 | -61 | -100 | 215 | -101 | -198 | -158 |
| 2008 | 1372 | -315 | -114 | -105 | 56 | -260 | -204 | -164 |
| 2009 | 1494 | -193 | -110 | 34 | -282 | -170 | ||
| 2010 | 1740 | 53 | -114 | 44 | -273 | -176 | ||
| 2011 | 1505 | -183 | -119 | 33 | -283 | -182 | ||
| 2012 | 1553 | -134 | -123 | 109 | -207 | -188 | ||
| 2013 | 2114 | 427 | -128 | 49 | -267 | -193 | ||
| 2014 | 976 | -711 | -133 | 104 | -213 | -199 |