Fall Fronts Ahead

Of all the seasons, autumn is typically the quietest when it comes to weather.  Winter has lots of strong storms in the jet stream.  Spring and summer are more unstable, creating thunderstorms.  But autumn typically has none of that.  The atmosphere is stabilizing (surface is cooling while the levels aloft remain warm).  And the jet stream is just starting to strengthen again.  Also, the temperature contrasts over different areas aren't very extreme yet, so weather fronts tend to be weaker.

We've seen a few weak fronts pass by our area recently.  Most just bring some clouds, maybe a sprinkle or light shower, and perhaps a little breeze.  But overall, the first half of September has been rather quiet.  The weather computer forecasts are indicating a trend towards cooler and wetter weather for the second half of the month.

Here's what the weather pattern looked like last weekend.  Warm high pressure over the western US with cool weather systems in the Gulf of Alaska.  

One of these Gulf of Alaska systems moved through our area on Tuesday, but was very weak as it came through the strong high pressure.  So we mainly just had clouds, along with a little cooler temperatures.

Now, high pressure has built back into the Northwest for a rather warm weekend.  Temperatures will reach into the mid-80s by Saturday, which is about 15 degrees above normal for this time of year.

But the heat doesn't last long, as yet another Gulf of Alaska wave moves through BC by Monday.  This system is a little stronger/cooler than its predecessors. 

 The computers have been struggling a bit with this one.  Yesterday's computer forecasts were painting a rather wet day for Monday/Tuesday.  

But they've changed their tune today to a dry cold front, more of a wind event for Monday.  So either scenario is still possible.

By the middle of next week, another Pacific weather system has moved in on the heels of the Monday storm.  This will take temperatures down another notch, probably into the 60s, with a better chance of rain showers.

Looking farther out into the following week, it's impossible to pin down exact storms and timing that far out.  But the overall pattern is quite a bit different than what we have now.  The warm high pressure areas are now over eastern Canada and out in the Pacific.  The western US now has the cooler trough pattern.  This would bring showery weather to our area with highs in the 60s. Wouldn't be surprised to see our first frost in the middle of next week. There's a chance that this weekend will be the last of the 80s for this year.

Here's the week 2 outlook from the Climate Prediction Center:

And for weeks 3 & 4 (last week of September, first week of October), CPC also sees things being wetter than normal for the West.  This doesn't mean it's going to rain hard every day.  But it does indicate that we probably won't have warm sunny dry weather for the end of the month.