Orographics & Rainfall

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.

6 Responses to Orographics & Rainfall

  1. Kirk says:

    I am starting to think a forecast more than 48 hours is completely irrelevant. I was a forecast surfer until about a week ago now I only look at Marks forecast or the NWS!!! I am sick of watching every forecast change so drastically every 12 hours. Even the NWS climate prediction center today changed from favoring cooler weather to a warmer pattern. I need a 12 step program for my cold weather addiction. I was going to say snow addiction but I did not want all of you to get the wrong idea. I have lived here all my life so I should just realize our winter is really just 6 months of spring. OK I am done venting thank you.

  2. Andrew says:

    We had snow at the 500 foot level at 6:40 am this morning in Portlands west hills.

  3. Kirk says:

    It is 6am Sunday snowing hard and 35deg. I know it is just a shower but still nice to see. I live at 1000ft north of Camas near Jones Creek. Drove to the top of my road and the grass was white. Nice job Mark you talked about lower snow levels for this morning a couple days ago. Still wishing for a real cold snowy period.

  4. Andrew says:

    Just something ineresting i thought i would note. I live at the 850ish elevation in portlands west hills and we just had a rain/snow mix shower move through. This was around 9:35, will be interesting to see if the temps drop a few more degrees.

  5. Andrew says:

    Well, mark might have some better answers then me, but i know a few things here and there. Depending on how cold it is outside, 1 inch of rain can be about a foot of snow or snow. so if the temps hovered around 25 degrees since the beginning of this month, you could have as much as 4+ feet of snow on the ground. However it is unlikly that this would ever happen. Since generally when we do get cold enough temps in the metro area, we tend not to have a strong string of moisture, since the heavy rains from the sw or w did to be warmer.

  6. Deb says:

    I’m tired of whining about the rain and the mud up to my shoetops in the yard. Help me appreciate the fact that it’s just water.
    How many inches of snow would we be looking at if these last 26 days of wet had been flakes instead of drops? How often would we have had to shovel a path out? How much weight would that be on my flat roof? Approximate guesses are OK. ;>
    I’m south of Salem, but Portland metro numbers would be close enough.

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