(Undated)  --  Authorities are urging caution again today as a dangerous heat wave grips the Pacific Northwest.  Excessive heat warnings have been put into effect for about 15 million people in Oregon and Washington, where temperatures could reach record highs.  It was over 100 degrees in Portland yesterday.  The Weather Service is reminding people throughout the region to stay hydrated, limit outdoor activities and check on the elderly, especially those without air-conditioning.