9:30pm Sunday…

Ahhh, spring break in Oregon. One thinks of…exactly what our 7 day forecast shows. A mix of cool, showery days plus some warm sunshine too.

It’s raining this evening as a cold front approaches the coastline.

Satellite PACNW.png

The front should pass by Astoria by midnight, then through the metro area sometime between 2-4am. Behind this front the steady rain changes to our usual “sun & showers” spring weather pattern. Tomorrow won’t be all that wet, just light showers that come and go. Snow levels stay relatively low and I expect another 6-10″ snow around Mt. Hood above 3,000′. The snow level forecast based on the ECMWF model…

ECMWF Snow Level From 850mb Temps LONG TERM.png

We get a break from the rain Tuesday, then another weak system moves inland Wednesday. That does give us a beautiful spring day Tuesday, but then it’s back to onshore flow and gloom Wednesday. Rain has been running below average this month, and we’re slowly making up a bit of ground. But it appears we’re headed back to drier weather again Thursday through next Sunday. See the upper-level ridging developing just offshore Thursday

thursdayecm.PNG

Then it’s right over us next Saturday…could be well into the 60s with sunshine!

saturdayecm.PNG

Then by NEXT Monday a cold trough drops in over us, crashing the warm/sunny party.

mondayecm.PNG

That should leave us with a bit of a dry spell. Most likely Thursday-Saturday, or possibly Thursday-Sunday. Or if we get leftover sprinkles Thursday, it would just be Friday-Sunday. The point is that you should be aiming for Friday/Saturday for guaranteed dry days. Thursday & Sunday will be bonus days if we can stay dry for those.

Dry Streak Forecast.png

Hopefully no one missed the “big tornado” Friday evening? Portland NWS says a very weak EF-0 touched down very close to the Ilwaco airport. That’s near the mouth of the Columbia River NW of Astoria. 65 mph wind and damage could only be tracked for about 1/3 of a mile.

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This is the latest in a series of very weak tornadoes in our area the past few years; all EF-0

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On another subject, I spent almost two hours tonight “digitizing” a metro rainfall map.

Huh?

It goes back to Charlie Feris, a local Oregon AMS Member. Charlie worked at BPA as a meteorologist for many decades. During that time he set up a network of rain gauges around the Portland metro area. People would send in monthly rain totals and he would tabulate the numbers. I believe Charlie retired in the early 2000s but he has continued to “accumulate” the monthly rain readings. Over the years he has created a contoured map, showing annual average rainfall in our area. I remember first seeing this 20+ years ago. Then last March, at our last in-person AMS meeting, he passed out some copies. I told him I’d love to get it in graphical form so it’s saved for future use. That’s what I did this evening. Here’s the original version:

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And (a very colorful) TV version…

SAVE Mark Average Yearly Rain Metro Map from BPA.png

Let’s talk about it. I don’t think many people are aware how much variation we get in annual rainfall here. A few points:

  1. In general it’s driest in the lowest elevations and wettest higher up. That makes sense because as clouds rise over terrain, they cool, moisture condenses, and raindrops form.
  2. There is a bit of a rain shadow coming off the Coast Range, possible off Chehalem Mtn. in southerly flow as well.
  3. The driest parts of the metro area are around Bethany and Tanasbourne areas westside, and Sauvie Island to I-205 bridge along the Columbia River. Some of these areas see less than 38″ in an average year. In fact the 30 year average at PDX is around 36-37″.
  4. Wettest (by far!) is the west slope of the Cascades. The eastern suburbs of Portland into the Cascade foothills pick up 50-60″ rain each year.
  5. Terrain has a huge influence not only on daily weather, but yearly precipitation.

Of course these numbers are averaged over a year. In certain patterns the variations on these maps can erased, or even enhanced. For example a strong westerly flow at 3-5,000′ can leave an inch of rain at Troutdale or Corbett, but almost nothing falls in Hillsboro. OR, a strong south-southeast flow in the first few thousand feet of the atmosphere can really soak the west metro (Banks, North Plains, Hillsboro), but leave those usually wet eastern suburbs significantly drier.

Alright, that’s it for now. Keep your fingers crossed for some warm sunshine at the end of the week.

Chief Meteorologist Mark Nelsen

 

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