Stop Me if You've Heard This One Before
In a normal spring, rain isn't hard to come by in the Inland Northwest. Looking for dry days can be the challenge. But in a dry spring, like we currently have, we're always on the lookout for rain. And the computers have teased us with the potential for rain for the past few weeks, only to disappoint us at this last moment with only a few hundredths of an inch.
A couple of reminders from previous blogs. One is that the computers had at least been consistent on keeping low pressure in the West into early May, and that still looks to be the case. Also, the main reason we were drier than normal is the El Nino pattern of storms going by to our south.
So now we have the next teaser from the computers, with a decent rain forecast for this upcoming weekend. Should we believe it? The storm looks rather impressive on the weather maps.
There's a couple of important things to note. First, the low is again forecast to move by to our south. But the center of the low is going into Oregon, not California; so not as far south as its predecessors.
Second, the low is what we call a "closed low", meaning that the pressure lines are actually closed circles in the center. Not all lows are like that. But it's very important and not uncommon at this time of year. What that means is that the storm is moving slow, which allows bands of rain to move around the north side of the low from the east. This can be a very wet pattern for us. These are the kinds of storms that make May the wettest month of the year in Montana.
But should we trust this forecast? Based on the past performance, I'm nervous about putting too much faith into this event. And I'm guessing I'm not alone.
Here's the plume diagram for the US model ensemble. I've shown this before for past storms. It shows the 30 forecasts of precipitation accumulation. You can see that for the upcoming storm, the rainfall totals range from a tenth of an inch to one really wet forecast of over 2". That's a big spread. There's several wet solutions of over 1", but there's also a lot of dry forecasts of less than 0.20". The thick black line is an average of all of the different forecasts, which comes out to about a half of an inch.
At this point, Sunday looks to be the wettest day. Saturday should be in the 60s and breezy. There is a small chance the rain could arrive Saturday afternoon. This is from the European ensemble, showing the chance of rain on Saturday afternoon.
As the low moves by to our south, bands of rain could rotate around the low, moving into our area from the southeast and east. This means a couple of things:
- There won't be a rain shadow from the Cascades. The Basin has just as good of a chance of getting rain as other locations.
- The highest likelihood of rain will be in the south, closer to the low. Folks near the US/Canada border will actually have the lowest odds of getting rain.
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