June 06, 2019

Thunderstorms on Friday

Western Washington and Oregon do not experience many thunderstorms each year (see the climatology below)--less than 10, in fact.  The key reason?  The cool Pacific Ocean to our west.

But Friday could be a day that brings a flurry of thunderstorms, associated with towering cumulonimbus clouds, lightning and thunder.

Let me show the latest forecast from the uber-high resolution (1.3 km grid spacing) UW WRF forecast from this morning.  These images show SIMULATED weather satellite pictures, based on the model forecasts.

Here is the forecast for 2 PM Friday.  You see the white circles and ovals?  Those are large cumulonimbus clouds, many of them thunderstorms.  Mainly from Seattle south.

5 PM Friday?  Even more!  An explosion of convective storms, particularly over the coast and southwest WA.  And plenty over the southern Cascades and southeast Washington/northeast Oregon.

Even at 8 PM, thunderstorms continue over southeast Washington.

The precipitation totals from these storms are impressive, with some locations on the western side of the Olympics and the Cascades receiving 1-2 inches.

Why the potential for thunderstorms tomorrow?  A large change of temperature in the vertical and upward motion from an upper level trough.

The sun is very strong now, since we are only about two weeks from the summer solstice.  But aloft, cooler than normal air is now overhead.  For example, at 850 hPa--around 5000 ft-- temperatures are predicted to be about freezing (0C) tomorrow morning at 8 AM (see below).

As a result of cool air aloft, but strong heating at the surface, there will be a large change of temperature with height (also known as the lapse rate).  Such a large lapse rate leads to convection, in which the atmosphere breaks down into upward and down movements, not unlike the bubbling oatmeal in a heated saucepan.

A measure of the instability aloft is called CAPE--Convective Available Potential Energy.  A forecast for CAPE at 2 PM Friday shows modest values (pink color)--wimpy for central U.S. standards, but high for us.

The release of such instability is aided by having regional upward motion that helps push air parcels upwards.  And we will have a mechanism to produce such upward motion--the arrival of an upper level trough of low pressure aloft.  To illustrate, here is a forecast of heights (you can think of this as pressure) and temperature at 500 hPa (about 18,000 ft) at 2 PM Friday.   A cold (blue colors) upper level trough is over us, with a smaller scale trough approaching from off the Pacific.  Such a feature produces upward motion over our region.  

So be ready for lightning and thunder tomorrow, particularly if you live in the southern half of Washington or northern Oregon.  And the potential for lightning over the Cascades and eastern WA/Oregon is concerning, considering the potential to start wildfires.


  1. Oh my God, what will Jay Inslee do now?! Declare a global warming flood emergency?

  2. Aside from the wildfire potential from lightning, I'll enjoy seeing some real t-storms out here, for once! (I grew up in N. Illinois, so a typical T-storm is no big deal to me.) Bring on the instability! :D

  3. Cliff,

    Winds blew some small garden structures down and around this afternoon. But, it was over quickly. I was surprised by the impact, but it wasn't anything serious.

    It just ruffled the clematis' structure and turned it over. There were some tall daisies blown down, etc.

  4. May 19th we had nearly continuous thunder for about 30 minutes along with about .25" during that period. Lightning was impressive as well. It was probably the most intense thunderstorm I've seen since leaving Arizona for Oregon over 40 years ago.

  5. There was a small lightning storm in Everett Thursday evening.

  6. Lots of hail at my house Thursday in Sequim outskirts.

  7. Here's the latest comment from the forecast discussion.

    "The cool and cloudy conditions brought during showers through the early morning hours will provide a hurdle for any instability to get going today. Should the aforementioned holes
    in the cloud cover grow to allow sunlight in...then yes...current model solutions indicate that enough instability and CAPE are present for some weak thunderstorm potential."

    Sounds like typical one clap wonders to me. I like big thunderstorms, but prefer they don't impact our juniors baseball game tonight, so hoping for less action today.

  8. As far as the discrepancy between NWS and Cliff's analysis, the key one that I get, especially from the 900am Friday Forecaster Discussion is the amount of surface heating. NWS says if we get some holes in the cloud cover and heating at ground level, then we can expect T-storms.

    But if the overcast remains largely solid, then not so much. They are betting on the overcast not opening up.

    The big difference, as I see it, is that Cliff and his peers focus on improving their understanding of atmospheric physics and the development and refinement of computer models.

    The NWS guys use all of that work, and then make adjustments based on the strengths and weaknesses of models (which vary according to season, conditions, location and model type) to use their judgement and observations to generate a forecast. They knoew that what happens can be influenced by real time events that the models didn't fully capture, like the case here regarding the degree of ground heating from gaps in the cloud cover. And they have to broad-brush a bit given the variations in micro-geography of the area.

    It's very interesting watching this machinery working, which in the end is basically statistical.

    Tomorrow we'll know whether the models or the models+forecaster experience made the closest call. And, as usual, one's personal conclusion often depends where you are standing in the micro-geography. Sequim versus Maple Valley could give you very different answers.

  9. Joseph - Here is my answer. If you think snow is hard to predict, thunder around here is doubly or triply so.

    Here's my forecast. There will be some thunder and lightning somewhere in W. Washington today. It probably will not be exactly where its predicted. Most of us will see zip. For still more it will be 1 to 2 flashes and then that will be it.

    Truly good thunderstorms are a rare feature. I have seen 2 in my lifetime here, the first in 1984 and the second in 1999. Near-constant lightning flashes. Pitch black skies. And thunder strong enough to set off car alarms. As this blog has shared some fascinating blogs on past windstorms and worst case scenarios for the future, I would love an article on why those storms were so severe (and LONG with hours of lightning) and what would be the IDEAL atmospheric setup for severe storms over the Puget Sound.

    [image src="https://static.seattletimes.com/wp-content/uploads/2015/02/02112015-GG_01-1020x814.jpg"/]

  10. Here's hoping we get some of the action in Bothell!

    Sulla- we did have a good- and rare- set of 3 East coast style thunderstorms on May 3, 2017.
    You see- it's so rare I can remember the date!


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