August 10, 2014

Heat Wave, Thunderstorms, and the Cool Down

You know what they say:  it is often darkest before the dawn.

And this week, we will be warmest before the cool down.

Today (Sunday) the temperatures away from the water will peak in the mid to upper 80s, well above normal.

The plot below shows the temps the last two weeks at Seattle-Tacoma Airport.   Really warm before 6 August, near normal for four days, and climbing above normal yesterday and today.

But wait until tomorrow (Monday).

Tonight easterly flow will develop over the Cascades and a thermal trough will move northward. The forecast map at 2 AM Monday at 850 hPa (around 5000ft) illustrates this:

And the forecast surface pressure and low level temperatures at 2 PM Monday show the impacts, with a thermal trough (area of low pressure) extending into western Washington.  Red and brown are the warmest temps.

UW Probcast is going for the lower 90s in much of western Washington away from the mountains.

The air will become increasingly unstable over the the Northwest on Monday and continue into Tuesday.  In fact, the latest WRF model forecasts indicate extremely high values of potential instability, know as CAPE, for Monday and Tuesday afternoons.  Getting over a few hundred is high for western Washington...those afternoons we jump above 1000!

And the approach of an upper trough will initiate thunderstorms on both days, as well as bring more general rain. Here is a upper level (500 hPa) forecast map for 11 PM on Monday.  You can see the upper level trough approaching.

Let's look at  the 24-h precipitation totals ending 5 AM Tuesday and Wednesday, and the 72 h total ending 5 PM Wednesday. Wow...we are talking serious precipitation.  We start with showers over the Cascades and adjacent areas on  Monday night/Tuesday morning, followed by much heavier stuff over the northern portion of the State on Tuesday/early Wed.  Tuesday should be the more severe thunderstorm/convection day.   And the lightning could start more fires in eastern WA.

The 72h totals show .5 to 1 inches over much of western Washington.  This is serious rain for August and the first real rain for us after we pass through our low-rain period of late July and early August.  The beginning of the end....

And with the rain will come a cooling period down into the 70s.


  1. Do you have any suggestions for a person who needs to know when there will be a dramatic barometric pressure change coming soon? I've got a disability which is impacted significantly by pressure changes and I've looked all over for any kind of affordable device which would alarm or notify in some way that a big change is coming so that I can take appropriate steps (have medication ready, get home rather than be driving, etc). I know that they make very expensive equipment for mariners but those are a bit more than I need and far out of my price range. So far I am cobbling together the UW public tools for pressure history and wxmaps for Seattle area weather predictions but I need something that shows me both the trend and the prediction all at once and beeps when a big change is coming. Do you have any suggestions perchance, o wise weather guru?

  2. You should post about the bioluminescent waves happening on the coast of Washington right now.

  3. The Wednesday AM rainfall amounts seem inconsistent with the probability forecast of 30% and 20% rain for Tuesday and Tuesday night. And the 10% probability for the period is only 0.18". Why does your map show such large amounts of rainfall when the forecast seems to be much less and with a less than 50/50 chance of any rain?

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  5. beginning of the end of summer? nope, i refuse to accept that. also, taking a look at nws, the rainfall totals look to be below 1/4". am i missing something?

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  7. FunChefChick:

    Search the web for wpow1. There is a three-day posting of pressure trends, granted, looking backwards, but one can see the downswings coming if you read the forecast discussions.

  8. Hi Cliff,

    Can you please settle an argument for me and my coworker? How often do we see thunderstorms in Seattle?



Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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