Now THIS is the November weather I remember growing up with west of the Cascades…day after day of rain and mild weather. Some southerly breezes, then a bit of mountain snow. Annoying if you want to get outside, but great to see the ground saturating, reservoirs refilling, and (eventually) a building snowpack in the Cascades. Especially after a brutally hot summer and record dry spring. It’s pouring in past of the metro area right now and we are under a Flood Watch for local areas of flooding through tomorrow. What’s ahead? Read on…
- Showers continue in the metro area and south tonight, while heavy rain continues most of night in a line from Tillamook to Longview and northward.
- Steadier rain develops later tonight in the metro area and south, continuing through Friday afternoon. The morning commute and midday will be a soaker again!
- Rain comes to an end all areas west of the Cascades around sunset Friday
- Saturday should be dry
- Rain returns north of Portland Sunday, we’ll be right on the edge of that rain line in the city. Definitely dry south and east of here Sunday too.
- Expect another 2-4″ Coast, Coast Range, and Cascades. 1.00-1.50″ metro, and 2-3″ Longview up to Olympia the next 24 hours
This is what we’ve seen so far; a big dousing central/south Willamette Valley last night, but then heavier rain shifted north of Portland most of today. Of course mountains both east and west of us have been significantly wetter
Notice Portland is one of the driest spots. That’s not unusual. In fact check out the typical yearly rainfall in the Portland/Vancouver area. Charlie Feris retired from BPA many years ago, but continues to maintain a precipitation network of observers. He’s been doing it since about the time I was born!
Precipitable Water chart shows that we are most definitely under an atmospheric river (pineapple express) right now. Let’s just call it an “AR” so I don’t have to type it out each time. Look at that slug of tropical moisture stretching 2,500+ miles from near Hawaii to Washington!
A tool we use to analyze this situation is Integrated Water Vapor Transport (IVT). Basically how much water vapor is being transported in any one location through a cross-section of the atmosphere. Chart below is 4pm today. Numbers show total mass of water vapor passing through each square meter of the AR each second. Got that? Simpler: Bigger numbers equal more moisture moving through any one spot overhead.
You may notice there isn’t much “IVT” going on east of the Cascades. Where did all that water go? It was converted into rain. That “river of water” in the atmosphere slams into the Coast Ranges, then the Cascades. Rising air cools, and moisture must condense into droplets = rain. The heaviest rains fall where high IVT encounters a mountain range perpendicular to the flow. Westerly flow squeezes maximum rain out of these ARs with our north/south oriented mountain ranges. That’s why so much more rain falls west and east of downtown Portland compared to the city itself.
Alright, so the AR is aimed at Washington right now and through much of tonight. By tomorrow morning it has weakened just a bit, but dropping south over Oregon. But don’t worry, there’s still plenty of rain to go around
Then it has fallen apart by Saturday morning, ending the rain over most of Oregon
An even more intense AR sets up Sunday, but this time it’s aimed at Vancouver and NW Washington. We will be right on the edge of that one.
How much rain is ahead? I think the 18z ECMWF shows a good approximation of what we can expect the next 24 hours. Less than 2″ for most areas south of Longview in the lowlands. Up to another 4″ in the wettest parts of the Cascades and Coast Range
We will probably end up with 2″+ in Portland out of this event, pretty good model forecasts so far…
The final part of this AR moves south over us Monday, followed by colder air; snow levels will go below the passes Monday night. But then a relatively dry and cool-ish pattern follows much of next week. The GFS model’s ensemble forecast for Salem high/low temps gives you the general idea.
There’s no sign of a stormy pattern OR low elevation cold/snow in the next 10 days. That can happen once we hit mid-November.
That’s it for now. I’ll be on TV at 8/9pm on FOX12Plus and 10/11pm on FOX12, see you there!
Chief Meteorologist Mark Nelsen