Monsoon Moisture

As most of us know, summertime in the Western US is largely dry, while east of the Rockies it's rather wet and humid.  You can easily see this pattern in the normal rainfall for August.

But there is one notable exception to this pattern in the map above.  That's in the desert Southwest.   Once the Four Corners high pressure has become established in the summer, moisture from the south (i.e. Mexico) is drawn up into Arizona and New Mexico, as well as Colorado and southern Utah.  Afternoon thunderstorms are a daily occurrence there at this time of year.  It's their wettest time of the year.

Sometimes the moisture will make it farther north, into Burns, OR, Boise, ID and even Billings, MT.  But to get that moisture all the way up into the Inland Northwest is really tough.  Not only does it have to travel far to the north, it also has to move west, since Spokane is well west of Phoenix.  So Washington is the least likely western state to see monsoon moisture from the Southwest.

However, the upcoming pattern is one of those exceptions. The weather map below for Friday shows a strong area of high pressure over the Four Corners area (see the blue "H").  But the orientation of this high is towards the Northwest, giving us another scorching hot summer day with triple digit temperatures.  Dew points will also be high so that will make it feel even warmer. 

The orientation of this high allows moisture from Mexico to move up into California and Oregon, and eventually Washington and north Idaho.  This is aided by a low pressure system (red "L") off the coast of Baja California.

You can see this moisture moving northward in the 3 images below, from Thursday, Friday night, and then Sunday.

On Thursday, the moisture is largely over Arizona and southern California.

By Friday night, the moisture has moved into northern California, Oregon and is starting to enter Washington.

By Sunday, the moisture stretches from Seattle to the Dakotas.

The cloudiness is the reason why Sunday will be a few degrees cooler than Friday and Saturday.  Will we get any rain from these clouds?  The models are suggesting that the rain will mainly be confined to the mountains.  Here's the US model

And the European model.

You can see in both forecasts that there is some rainfall even in the Basin.  Again, don't get hung up about the exact details of where the computers say it will or won't rain.  The screaming message is that there is a chance of showers starting as early as Friday afternoon (mainly over the Cascades), and continuing into most of next week.  

There is low confidence of predicting who will get rain on what day in this pattern.  But there is much higher confidence that everyone will get a little rain now and then next week, I just can't tell you which day.  Thunderstorms are a good bet in this pattern, so there will be concerns about new wildfires.

As I said, temperatures will rapidly heat up from the mid-80s today (Wednesday), to lower 100s by Friday and Saturday.  Then we'll see 90s all next week.  There could be a cooler day in there somewhere if the clouds and showers are widespread enough.

Also notice that our dew points (black line above) will be around 50F, which is a little muggy for our area.  So we won't cool off as well at night (due to the clouds and humidity).  But if we do have a calm clear night, there could be a heavy dew from this pattern.