Portland’s Changed Snow Climatology

March 14, 2012

Portland’s latest snowfall of spring has just now been moved forward at least 10 days.  Sounds mildly interesting for regular folks, but if you’re a weather geek, read on.

During the Oregon AMS Meeting last night, a brief discussion about Portland’s latest snowfall revealed some slight disagreement over what the date is.  Remember, we’re talking MEASURABLE SNOW, which is 0.1″ or more.  A trace is recorded even if it snows but nothing sticks, which could happen anytime in an extremely heavy hailstorm, even in May or October.  On the NWS website it says the latest date is March 20th, 1995, which you see in my graphic below.  I made this graphic a few days ago.  Of course I heard that and said “what about March 25th, 1965”?  According to the Western Regional Climate Center site which I use, it shows that as the latest measurable.

So I actually went and looked at the hourly observations…the LOW temperature on the March 20th date was 43 and the high was 55!  Obviously it didn’t snow on March 20th, 1995.  NWS folks agree so that one is gone and they will correct their records.  They also did some checking on March 25th, 1965.  The temp was 33 early and 43 in the afternoon.  Sure seems like it could have snowed briefly that morning. 

But now that date is out as well…why?  It was hail on both those dates.  In the past, snow and hail were both recorded as the same thing, or at least in the same column.  I’ve known that for quite a few years, alerted to it once when I heard another TV person and the Oregonian claim it snowed in the first week of October here in Portland.  I looked it up and found the low temperature on that date was something like 51 degrees!  It must have been hail.  Also, watch out for lots of “traces of snow” in March-April-May in the record books.  Many of those in the past may have been hail.

The NWS guys are still checking on the new “latest snow on record” of March 15th, 1946.  I think I saw Vancouver had a few tenths of an inch of snow that day too, so maybe it’s real.   Either way, just remember, the latest recorded is in mid-March now.

Keep in mind that these records are for PDX (1940-1995) and the Portland Forecast Office (1996-Now).  Earlier daily records (a bit sketchier) downtown date back to 1928, and even earlier records than that go back to the late 1800s.  It did snow the first few days of April, 1935 downtown, and the latest measurable down there is 1.4″ on April 7th, 1933.

Now all this discussion of March and April snow in Portland makes it appear that it’s relatively common, but it isn’t.   Looking through the records, I noticed large periods of time where little or no snow fell after late February in the Downtown records. 

1875-1890:  15 years with no measurable snow except 0.1″ in one March, 1 month out of 30

1907-1919:  13 years with just 3 months measurable, none more than 1.9″.  3 months out of 26

1921-1935:  15 years with one month of 1.4″, a .2″ and a .1″ two other months.  3 months out of 30.

1968-1988:   21 consecutive years with none

1990-2011 or 2012:  22 or 23 consecutive years with none.

This also shows how much elevation plays a role with marginal snow events.  There are many years where at least an inch or two falls up in the hills (even in the city) in March, but it doesn’t stick all the way down to the floor of the valley, or city in this case.

Chief Meteorologist Mark Nelsen

For those of you that complain about the local NWS sometimes here on the blog?  Give them a break; they have put a tremendous amount of effort put into local climate information (and their web page) the past 10 years or so.  And they’ve been wonderful at correcting errors as well when they show up.  Most of them are into weather just as much as many of you visiting this blog.