October comes and the “water year” ends

Every year it gets to September and I wonder what October will be like. Will it be one of those that's gray and wet? Or will we be able to enjoy lots of sun and blue skies with these colorful leaves? This year it seems to be the latter, at least for the next 1-2 weeks.

Just today I thought: “It's been a really nice fall so far!“. So much sunshine, but we've had a bit of a soaking moment at times so we're not too worried about fires. We haven't experienced record heat in the last few weeks, but it's definitely been warmer than normal. I would argue that this is just about perfect for late September and early October.

But what lies ahead? A quick summary:

• Drier and warmer than normal Is the story for at least the first 10 days of October, possibly the entire first half of the month
• There will be some rain for a few hours on Friday, otherwise the next good chance for showers is mid-week (next week)
• While temperatures will not be record warm, as we saw in early October 2022, they will be warmer than normal.
• There is no sign that the rainy season will begin in early October this year

SEPTEMBER STATISTICS

September was warmer than normal in most parts of the region. In fact, almost the entire U.S. was warmer than average.

For many of us in northwest Oregon and southwest Washington, September 2024 was in the top five warmest temperatures. Much of this was due to a few unusually warm nights, not so much the daytime highs

As for rain, officially 0.74″ at PDX was about half of normal last month. But many parts of the metropolitan area gained more than 1.00″. This tool provides a rough approximation; But you can certainly see where the downpours poured through the Central/East subway several times. I suspect the grasses are a little greener in Gresham vs. Vancouver at the moment

WATER YEAR 2023-24 ENDS

In much of the western United States, most precipitation occurs during the cool season. This means that when we look at annual rainfall, it is actually a mix of the second half of one rainy season and the start of the next

That's why water resource managers, climatologists and weather forecasters use a little trick called a “water year” to assign each rainy season to a year. We call October 1st the beginning of a new water year, and this water year lasts until the following September 30th. See how this makes analyzing each rainy season much easier? Pretty neat, right?

Last year and the previous two years were about normal or slightly wetter than average in Portland. Much of Oregon is experiencing relatively mild to moderate drought, although this could easily be eliminated by a wetter than average winter. We'll see!

WHAT IS HAPPENING?

The beautiful fall weather will last for at least another week. There is only a weak weather system moving through late morning/midday Friday with maybe 1/3″ or so. Then a dry weekend is planned for this first weekend in October. Why so mild and dry? This is due to higher than normal upper level elevations, also known as upper ridges. Check out the next 7-day 500 MB height anomaly using the ECWMF model ensembles. Generally, there are waves over the western United States and troughs far out in the Gulf of Alaska. This may not be a completely dry pattern, but not many rainy weather systems make it into the PACNW in this setup

Then 8-14 days from the same European model ensemble average, not a big change

The result is dryer than average weather in the first half of October. Both the GFS and ECMWF ensemble averages are “dry”, this chart is the GFS

The jet stream, which becomes stronger as October begins, remains to the north. You see the much wetter conditions along the Canadian coast. The two big messages?

1. The rainy season in the Pacific Northwest will not arrive until mid-month at the earliest.
2. There should still be enough time to implement outdoor projects before the persistent rains arrive in late fall.

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