A Very Unstable Atmosphere

 The view looking northward over Lake Washington early this evening was dramatic and included towering cumulonimbus towers, some reaching 20-30 thousand feet (see my picture below).

A sure sign of a very unstable atmosphere.

Monet would have loved the scene

An unstable atmosphere has a lot of vertical mixing, with rising air associated with cumulus clouds and sinking air in between them, marked by clear skies.  An unstable atmosphere is associated with large decline of temperature with height, with warmer temperatures near the surface and cooler temperatures aloft.

And that is what we had this afternoon.    The temperatures of the eastern Pacific are relatively warm, with offshore weather temperatures around 60F.  Not Hawaii, but warm for us, following summer and early fall solar heating.

At the same time, a slug of cold air moved in aloft associated with an upper-level trough moving in off the Pacific (see temperature map for around 5000 ft for 11 PM today).  The colors show the difference in the temperatures from normal, with light green being the coldest.

With a relatively warm surface and cold air aloft, there was a large decrease in temperature with height, which resulted in the lower atmosphere convecting, with upward and downward motion.  The upward motions produced clouds, such as cumulus, cumulus congestus, and cumulonimbus, the latter including rain.

You could see the instability clouds this afternoon on the visible satellite imagery (see below, the star is over Seattle).  A HUGE field of instability clouds over western WA and the eastern Pacific.    Best so far this year.

Radar imagery picked up on the shower activity associated with the instability (see radar image at 5:45 PM below)

The instability was so great that the resulting thunderstorms produced substantial lightning (see below, x indicates a lighting stroke).

The instability and showers should decline tomorrow.

A (Typical) Soggy Future Ahead

In the Northwest, October is the transition month to the wet, winter regime that extends through mid-February.   And the next few weeks should reflect the typical changes.

But before I show you the latest forecasts, let's look at the typical precipitation of climatology at Seattle.  

The figure below shows the climatological probability of getting at least 0.01 inch of precipitation in a day at SeaTac.  After the dry minimum around August 1 (about 8% probability), there is a plateau (around 30%) in September, and then the big move upward in October to around 55% (October is delineated by the vertical lines).  

For more meaningful rain (a quarter inch in a day), there is a substantial increase in October, but the heaviest rain period is still a month away, centered in November.

This week will be a good one to keep your umbrella handy.  

There will be a slow, steady accumulation of precipitation, as shown by the accumulated rain at Seattle (below) through October 22.   This plot shows the precipitation accumulating from many model runs, something called an ensemble.

The map of the total accumulation through 5 AM next Wednesday shows rain over the entire region, with particularly heavy totals (as much as ten inches) over the western side of the terrain in BC.  The Olympics and North Cascades get a piece of it.

The official NOAA 8-14 day outlook is for cool and wet over our region (see below).

No complaints will be allowed.  We have been drier than normal for the past few months and soils are dry.    This rain is good news and will be modest.