Transition

Last night the setting sun was blood red from the smoke.
Today there was no sun, just low clouds.

It was obvious last night that something was changing.   Most obviously it was the wind, which had picked up considerably during the evening, bringing in cooler Pacific air.

The visible satellite picture at  5 PM today said a lot (see below).  Low clouds over the Pacific and western Washington interior.   And several thunderstorms over the eastern Cascade slopes and northeast Washington.

Temperatures were very seasonal, with highs in the 60s (near the coast) and 70s (western interior), and only the 90s in much of eastern Washington.

Rain has return with the thunder storms:  below is the 24-total precipitation ending 7 PM Friday.  The thunderstorms produced several tenths of an inch in several locations, with some favored spots getting more than an inch.   The problem is that this rain was accompanied by quite a bit of lightning over the east of the Cascade crest.  New fires are a distinct possibility, although the cooling temperatures and increased humidity will work against it.

Why this profound change?   Because the big ridge over the eastern Pacific has been replaced by a strong trough.  Let me show you.

Here is the upper level (500 hPa, about 18,000 ft) for Tuesday at 5 PM.  Big ridge over the eastern Pacific west of the Pacific Northwest.   And a low off of California.  This flow pattern brought easterly flow aloft over our region, bring smoke westward over Washington.

Here is the forecast map at the same level for 11 AM Saturday.  See the difference?  The eastern Pacific ridge is replace by a trough of low pressure!  And the flow aloft is southwesterly, resulting in the smoke from the BC fires moving away from us.  Relatively clean Pacific air.

The lower atmosphere over our region has been radically changed, with strong southerly and southwesterly flow and cooling by around 15-20F.  The figure below  shows the temperature and winds above Seattle Tacoma Airport over the past 24h.  Time is on the x-axis (in GMT or UTC) and pressure is the vertical axis (700 is the pressure at 10,000 ft).  The red lines are temperature (°C) and blue wind barbs are shown.  Temps have cooled by 10C (around 18F) and winds are moderately strong out of the southwest.  A surge of marine air has pushed inland.

The cooler pattern should hold for at least 3-5 days.    This low pressure pattern will bring substantial shower activity over the Northwest, although little will fall on Seattle.  Here is the UW precipitation forecast for the next 72h.   Showers over the mountains and offshore.  Large amounts over eastern Oregon.  There will be loads of lightning and new wildfire starts.

In any case, our heat wave is over.  Finished.   And one thing everyone should be sensitive too, is that it was a localized heat wave over the Northwest.  Most of the country has been cooler than normal, something demonstrated by the temperature anomaly map  for the first week of the month provided by the NOAA Climate Prediction Center (see map below).  But we have been red hot (or in this case, brown hot).   Another reason to be very careful in saying that our warmth was caused by global warming, considering how localized it has been,