"Highs in the upper 60s" means that the temperature will be 67, 68, or 69. The phrase "lows in the upper 50s" means that the low temperature will be 57, 58, or 59. "Highs in the low 6os" means that the high will be 60, 61, or 62. The phrase "lows in the low 50s" means that the low will be 50, 51, or 52. The
s in those phrases means that there is more than one possibility.