10pm Tuesday…

We’re into summer now (at least weather-wise) and things are relatively slow for meteorologists in the Pacific Northwest. Of course we had the hot weather last week and now we’re back below normal with the temps in the 60s the past few days

Today we’ve seen showers roaming throughout the area as an upper-level low sits just offshore. It’s strange because we can’t see them well or show on TV. That’s because the NWS is doing some maintenance on their radar = no coverage over our area for a week. Better now than in January…

Looking ahead, expect the same partly cloudy sky tomorrow. Just a few showers may pop up in the cool air overhead. We have an upper-level low just offshore that should weaken and push inland Thursday. But a new trough is dropping down into the Eastern Pacific

So we get a dry day Thursday (or nearly dry), then the leading cold front with the next system gives us a wet day Friday. That’s the easy part of the forecast. Less than 0.25″ rain now through late Friday.

Then things get much trickier…the next upper-level low digs straight south, pushing up a hot ridge just east of us. This could push Eastern Oregon into the 90s by Sunday, while a lingering front and warm southerly flow keeps skies cloudier west of the Cascades. By Monday the pattern is quite “meridional”, meaning flow is south-north instead of west-east. A slight pattern shift west or east this weekend will make a huge difference down here where we live.

And models are pushing a LOT of moisture north. Check out the Saturday evening map of “precipitable water”. Up to 1.50″ or so.

This lingers through Monday. Expect mild/humid weather (regardless of rainfall totals) Saturday evening through Monday; we haven’t seen that yet this season.

How much rain? Very tough call right now. This much moisture around means someone in the region COULD get a big soaking, which would be nice! The 18z Euro ensembles (51 members averaged on this map) showed at least 0.50″ for ALL of western Oregon and SW Washington north of Eugene. About 1/3rd of the members produce more than 1″ rain in Portland…that would be nice, although it would mess up any weekend outdoor plans

We will see how this pans out, but I think the big message is that there is no stretch of guaranteed dry weather the next 7 days. Wait until after we get past this weekend to see if we’ll enter a warmer/drier pattern heading into the 2nd half of June.

Chief Meteorologist Mark Nelsen

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.

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…

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

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

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

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.

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.

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

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:

And (a very colorful) TV version…

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