Orographics & Rainfall

January 13, 2006

A nice and slow Friday evening weatherwise.   Last week at this time the weather was just beginning to get crazy.  But now we just have a lackluster cold front moving through.  Much cooler air comes in this weekend.  The moisture supply is pretty limited for Sunday even though the snow level will fall below 1500′.  So don’t get too excited.  But the skiing/snowboarding will be excellent with the fresh and dry powder.  Next week will be seasonally mild.  No sign of any major windstorms or cold weather.  The threat for a significant freeze (arctic air) ends in about 4 weeks in Portland.

I just received an e-mail today…someone wanted to know what OROGRAPHIC refers to in weather discussions.  TV weatherpeople usually to avoid words like that on the air.  Mainly because it tends to make our bosses a bit queasy and uptight…for good reason!   Apparently most viewers would be absolutely clueless as to the meaning of the word.  I agree.

Orographics refers to "orography" or the science of mountains.  As air moves into mountains it has to rise right?  It can’t go anywhere else.  And rising air cools, moisture condenses, and you get rain or snow.  At the very least you often get cloud cover.  So when air flows perpendicular to a mountain range, that’s the best set up for squeezing the maximum amount of moisture out of an airmass.

Apply that to the Northwest landscape and it’s pretty obvious what direction you want the winds in the atmosphere to flow from to get maximum mountain snow…Westerly.  The Cascade and Coast Ranges are oriented north/south, so the westerly flow at 2000′-6000′ runs right into them.  Southerly flow will often give the same rainfall totals for the valleys AND Cascades because the winds are running parallel to the mountains which isn’t very helpful for extra mountain rain.  By the way, easterly winds are great for certain spots too.  Hood River, The Dalles, Bend, Madras can all get soaking rains (heavy snow in winter) in the rare case where the airflow is coming in from the east.