Whoever coined the term "tossing a monkey wrench" to mean gumming up a process was obviously not a weather forecaster, because if they were, the common phrase would go something along the lines of "Wow, that flat tire sure tossed a cut-off low into our dinner plans."
That's because nothing -- and I mean not even the finickiest of convergence zones -- can make a weather forecaster's hair go grey faster than a cut-off low. (OK, I take that back. A marginal 500-foot snow event does a good job of zapping hair color too.)
These pesky lows are trouble because forecast models have a difficult time dealing with the random nature of these systems. The mathematics involved in solving future weather patterns are most happy when you have something going at a certain speed and you can really apply a "distance equals speed times time" formula but cut-off lows are just that -- cut off from the main stream of weather and are prone to just wandering around, much like trying to predict with any skill what direction a spinning top is going to drift to next.
And of course, where that low goes, so goes the surrounding weather and future weather development.
Bottom line, we're gonna be washing that grey right out of our hair...
The forecast this week is prone to frequent changes as a cut off upper level low sits and meanders off our coast for a while. We're fairly confident in the short-term forecast, but by about Friday and into the weekend, all bets are off.
Just to illustrate the challenges we face, take a look at these two forecast model charts. The colored blobs are predicted rain showers, with the brighter the color, the more rain expected.
Here is Model A, which shows relatively dry conditions over Washington with some rain centered down in Oregon and the coast is clear (no pun intended)
And here is Model B, which shows a pretty organized front coming into Western Washington with widespread rain falling along the coast and moving into the I-5 corridor.
The catch? Model A is from Monday night, depicting Saturday evening at 8 p.m.
Model B? Is the exact same model showing Saturday evening at 8 p.m again, only it's the version that was computed Tuesday morning.
So, same model, same forecasted time frame, just run 12 hours apart, and showing two completely different scenarios.
You can actually see the past six runs using this link:
And this is just one example. The inconsistency continues across the other versions of weather models too.
So bottom line, at this point, we'll just have to hope this system doesn't toss a "cut-off low" into your weekend plans.
(OK, now I get it. "Monkey wrench" is easier to say :) )