September 14, 2013

Super-Inversion and the Return of Thunderstorms

The low clouds did not burn off on Friday over Puget Sound and on Saturday they took their time....with blue skies only appearing mid-afternoon.  A major cause of this persistent low clouds has been the development of a very strong inversion in the lower atmosphere.   An inversion is when temperature increases with height (an "inversion" of the normal situation in which temperature decreases with height).   Here are plots of temperature with height on Saturday afternoon.

Wow... there is a cool layer about 600 m (around 2000 ft) thick, topped by an inversion of around 12 C (about 22F).  Yes, we are talking about warming by 22F in about 200 m (650 ft).   Some folks had some interesting hikes today I bet. 

Why so strong an inversion? Turns out we had some strong southeasterly flow aloft that not only brought in some warm air from the interior, but had some downslope warming as well. Warming aloft strengthens the inversion.

  Here is a time-height cross section of winds above Seattle during the past day (time increases to the left).  You can see northerly winds at low level (in the cold air) and easterly winds above.

Although inversions are fun, thunderstorms really get folks excited, and lightning and thunder will be back tomorrow (Sunday)...and particularly after 6 PM.

Later tomorrow, a sharp upper trough will be moving into our area from the southwest (see graphic)

At the same time, there will be substantial potential instability in the lower atmosphere (high values of CAPE:  Convective Available Potential Energy)....see graphic.
 With this combination, our models are predicting the development of convective showers (see WRF model forecast for 11 PM Sunday evening).  The prediction is for the heaviest showers to be over the Cascades and the eastern side of the Cascades, but don't be surprised if some of the action extends to the west.

Tomorrow, we can look at the forecasts of the HRRR:  High Resolution Rapid Refresh model to get a better short-term forecast.


  1. Interesting weather at Anderson Island right now. Dense, dense fog - and thunder! I've never seen that combination before.

  2. Weird in north Tacoma right now. Very still with fog from the inversion, but regular thunder from up above. Cant see the lightning due to the thickness of the cloud layer down low. Never seen (or heard) anything like this combination of conditions.

  3. We drove west over Stevens Pass yesterday and hit a thick cloud layer around Index or so. It cleared up again past Monroe. Now we know why. East of the mountains was gorgeous with sun and highs in the 90s. Seemed rare for this time of year but locals were nonplussed.

  4. This was a weird mix of events. In North Tacoma this morning, we had inversion conditions with fog, and it was very very still. But we had regular thunder claps around us for about an hour starting at around 8:30 A.M. It was rolling from every direction, but we never got rain or any mixing, and then it passed. You could see it on the radar, but there was a disconnect between the eye and the ear. Strange.

  5. Can you please give recommendations on where to hike this weekend without getting rained on (too much)? Thanks!


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