Rain for the End of the Month

We've gone through the month of July without a drop of rain (a few locations had a sprinkle).  But all of that appears to be coming to an end.  As I wrote in my last blog, there would be a chance of rain in the final days of July.  Now that it's close enough we have some details.

First, we'll talk about moisture.  Currently, our atmosphere isn't bone dry.  We often look at a parameter called "precipitable water".  The name might imply it's how much precipitation you could get, but don't think of it that way.  It's just a measure of the amount of moisture that the atmosphere is currently holding.  If it's really low, then rainfall will typically be light, since there's not that much moisture in the atmosphere to form rain drops.  Here's our current precipitable water map.  Values in north and east Washington are generally around two thirds of an inch.

As our storm approaches from the west, the southerly winds ahead of the storm will bring more moisture into our area from the south.  Here's what our precipitable water will look like by Monday afternoon.

By Monday afternoon, the atmosphere will be holding about twice as much moisture as it is on Sunday morning.  Values of 1.25" do indicate the potential for a decent rain.

The other thing to consider is what is the mechanism that will cause the rain.  You can have a really moist, humid atmosphere with sunny skies.  Just ask the folks back east.

In this case, the cause of the rain will actually be a cold front.  Now, during the summer, cold fronts tend to be very weak.  This one is a little stronger than normal, but still nothing near a winter-time front.  So despite the moist atmosphere, the rain-maker isn't all that strong.

There doesn't appear to be much atmospheric instability.  So the threat of thunderstorms is low.  Not zero, but low.  Thunderstorms tend to generate downpours.  We could see some heavier shower embedded in the front, but these shouldn't be widespread.

The timing of the rain is pretty consistent among the computer models, reaching the Spokane area by late Monday afternoon.  There are a few predictions that bring it here a few hours earlier or later than that.  Showers will continue into Tuesday.

Rainfall amounts vary widely.  Some folks won't see anything, or maybe just a sprinkle.  But there is a low chance of a decent rain shower embedded in the front.

Here the European model forecast.  In general up to tenth of an inch, with the mountains receiving upwards of 0.20" of rain.

The US GFS model is much more reserved with the rainfall, confining it to northern Washington and the Panhandle.  This forecast is in the minority.

A lot of the other US models show stripes of heavier rainfall.  These are from the heavier rain showers moving from west to east.  Impossible to say where these heavier showers will be, so don't get too hung up over the details of this forecast.  But the overall pattern is probably accurate.

I hope you've enjoyed your week of cooler weather.  After this rain event to close out the month of July, the heat returns.  The weather map for Monday will look like this.  A cool atmosphere over the Northwest.

By Saturday, we'll see the complete opposite.  The Four-Corners high is back in place, with hot weather in our area again.  Triple digits will be possible.

So get ready for more hot weather starting Friday and continuing through next week.  Our dew points will remain high after this rain event, so high morning humidity with even some dew is possible.