August 08, 2019

Thunderstorms on Friday Night and Saturday Morning

Get your lightning rods handy, because there is a good chance that a large area of thunderstorms will sweep northward over Washington on Friday night and Saturday morning.

The action is associated with the landfall of an upper low on the southern Northwest coast.

At 2 PM Friday it will be offshore of northern California.

Crossing the coast around 5 AM Saturday.

And then weakens and pushes northward into Washington by 11 PM Saturday.

This kind of upper level trough passage is a classic thunderstorm/lightning producer in our region.  The trough provides upward vertical motion that produces clouds and precipitation--it also can release instability that can initiate thunderstorms.

Let me shown you the simulated radar imagery produced by the NOAA/NWS HRRR model (yellow, orange and red signify heavier precipitation). Friday at 4 PM the model is predicting thunderstorms on the crest of the Cascades.

By 10 PM, as the trough begins to swing northeastward, an area of showers and thunderstorms surges northward into Washington, particularly the southern Cascades.

And these showers continue to move northward during the next five hours (3 AM Saturday shown).  Some thunderstorms will be embedded in the precipitation.

The predicted rainfall totals  through 5 AM Saturday are substantial in some areas, particularly the southern and central Cascades, where the HRRR model is going for as much as 1-2 inches (see below).  Good for Seattle and Tacoma's water supply.  But we do have to worry about lightning-initiated wildfires.


  1. The NWS discussion for today (Friday) and tomorrow (Saturday):

    .SHORT TERM /TODAY THROUGH SUNDAY/...Low level onshore flow
    with a deep marine layer will mean another mostly cloudy day again
    today. A little lift during the morning hours may support drizzle
    over the lowlands again this morning as well. The air mass above the
    marine layer will again become more unstable during the afternoon
    and evening hours with another round of showers and thunderstorms
    over the Cascades again today. The flow aloft is expected to become
    more southeasterly this afternoon which may allow showers and
    thunderstorms if they develop to drift to the west over the
    foothills generally east of the I-5 corridor. Highs temperatures
    will reach into the 60s and lower 70s.

    The offshore upper level low will shift inland into Northern
    California and Oregon Friday night. This will allow bands of showers
    rotating around the low to work northward into Western Washington
    later Friday night. The upper level low will open up and weaken as
    it lifts northeast into British Columbia Saturday morning. The air
    mass will remaining generally unstable a threat of showers over the
    area for the entire day. Afternoon and evening thunderstorms over
    the Cascades will be possible again Saturday.


    1. So I'm lost as usual. Seems Cliff starts off saying Friday night, then says Sat night?

  2. Just saw the latest ENSO forecast and am a little concerned about La Nada winters here on Whidbey Island. The 2012-14 La Nada years produced a storm each of those years that was damaging for us on South Whidbey. One took down 7 trees on our property. Both put 90 foot firs down on the front lawn, one missing a shed by less than a foot and the other finishing the job. A neighbor lost 25 trees.

  3. Over here in eastern Washington, we are getting a sense that fall will arrive early this year. It's not something which can be argued scientifically. An early fall seems to be 'in the air' for lack of a better explanation.

  4. Please if you see this could you add the total accumulation of precipitation thru the weekend from the Euro. The GFS is producing an extreme amount of rain in Eastern Washington that looks suspect. This could effectively put the kibosh on the entire fire season.

    Even this mornings weak wave is producing significant rain in central east side. Dew points in the 60s, sunny temperatures in the 90s, this could be big.

  5. Has me confused. NWS as you note is talking about tonight and early Sat. Cliff's opening paragraph talks the same but then the longer narrative talks about Saturday?

  6. I'm rooting for thunderstorms.

  7. There were some wind direction changes in Port Angeles over the last 24 hours:

  8. We'll see,model forecasts favor ENSO-neutral through autumn and winter, but with higher chances for El Niño than La Niña.

  9. A steady rain and some thunder overnight (early morning Sat.) here in Ashland, Oregon.

  10. Jay Inslee must be pulling his hair out by the roots. We live on the east side of the mountains, and got a whole lot of rain last night. Where's my "drought emergency" that Inslee promised?!


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