Consequences and Trend Implications of a Prolonged Dry Spell in Southern Coastal British Columbia of Canada

From approximately mid-July until mid-October, 2012, a very persistent ridge blocked storm systems from traversing southern coastal British Columbia (BC) in Canada. During that period, approximately 15 percent of normal rainfall was measured at several locations along the BC coast, including Vancouver at 18%, Victoria at 13% and Comox at 14%. Although lengthy, this nearly rain-free interval is not unusual if it happens earlier in the season. Recent data suggests that a prolongation of the typical summertime dry period may be occurring, as has already been noted for the southern interior of the Province. The extended dry spell of this year has had several consequences for rainfall-sensitive sectors. For example, one river on Vancouver Island, the Cowichan, nearly completely dried up by the first week of October, 2012. Not only was the fall salmon spawning season put at serious risk of failure, but paper mill operations, recreational reservoir usage and a local communities' water supply was threatened. A recent study suggests that one consequence of Arctic warming may be the increased persistence of blocking circulations due to a reduction in the phase speed of the Rossby Wave. The eastern Pacific, just west of the North American coast, is a favoured location for blocking ridges during the summer. Prolonged summertime blocking due to Arctic warming will increase the risk of dry periods extending into the fall. In turn, this will heighten the risk of hydrologic drought and may create problems for a large number of economic and natural sectors in southern coastal British Columbia that were developed in a different hydrologic-climatic regime and so depend on an earlier onset of rain. Meteorological early-warning systems can be used to help water and river management agencies prepare for these eventualities; some prototypes and approaches to early warning are being explored this year by the Meteorological Service of Canada