Water Year Ends Slightly Wet

September 29, 2010

Thursday should be the warmest day in 3+ weeks here in the Metro area with temps at least in the mid 80s, possibly upper 80s in a few spots too.  The 00z Salem sounding was up to +18 deg. C. at 850mb and models forecast a +20 to +22 by tomorrow afternoon/evening.  Then we add a good 4-5 millibars easterly gradient (it was flat today) through the Columbia Gorge and over the Cascades.  That should give us far better surface heating, all the way out to the North Coast.  We’re going for 84 degrees at PDX, but I think anywhere between 83 and 88 is possible.  88 IS the record for the day at PDX too.  A nice, warm (and dry!) end to a strange month.

So what is a “Water Year”? 

Quite a foreign concept to any normal person, but let me attempt to explain…

Here in the Pacific Northwest a vast majority of our yearly precipitation (rain and snow) comes from Pacific frontal systems that move in regularly during the cool season; that would be October through March.  In other parts of the country (especially east of the Rockies) it’s the opposite; most rainfall comes from summertime thunderstorms or active frontal boundaries in the warm season.  Generally winters are the dry season in those areas.

So if you want to measure how much precipitation fell each “rainy season” , it doesn’t do you much good to measure the calendar year rainfall.  It makes more sense to start the “Water Year” at the beginning of each rainy season.  Most meteorologists in our area use October 1st as the start of that year.   Sometimes September 1st is used instead.

Tomorrow will be the end of the 2009-2010 water year.  Now this year is a bit strange because we had such a huge amount of rain in May and June, plus September to a lesser extent.  It was actually a pretty dry “wet season” due to the El Nino conditions last winter, but our water year number was salvaged by the wet Spring and early Fall.  So we ended up slightly above average.   Most of the Pacific Northwest was above average as well.  Notice that only 3 of the last 10 years have been at/above average…seems as if the first decade of the century has been a bit drier than normal.

Chief Meteorologist Mark Nelsen