Thunderstorms and Rain Coming to the Northwest

For most summers, during July or August there is an intrusion of "monsoon" moisture from the southwest into our region, resulting in showers and thunderstorms...and this year will be no different.

Such thunderstorms can be both good and bad.  Good at providing needed rain and lessening fire risk, but bad if lightning starts fires.

The predicted totals through Friday morning (seen below) range from a few tenths to a half inch, with higher values along the coast.

The wet fun will begin later this afternoon and evening as convection and thunderstorms build near the crest of the Cascades (see forecast for 5 PM below).

Overnight, more serious thunderstorms/convective showers will move into the region, as suggested by the forecast weather radar image at 2 AM from the University of Washington forecast system:

The latest forecast of the NOAA HRRR model is quite similar (simulated radar at 1 AM shown below).

Why this wet bounty?

Two things are occurring together.   The upper-level pattern includes an approaching upper-level trough (500 hPa, about 18,000 ft shown for tomorrow morning).  Such troughs produce upward motion (which helps initiate cumulus clouds) and bring in moisture from the south.

Such "monsoon" moisture and unstable air from the southwest are evident in the forecast below.

Total water vapor content of the atmosphere this evening.  

Blue and red are higher values

Enjoy the show overnight!

Wildfire Smoke Fills the Skies Above The Northwest

 If you viewed the skies around sunrise and sunset during the last day, it was evident that a massive cloud of smoke had moved in overhead.

For example, here is a northwest view from the Seattle PanoCam just before sunrise this morning.   Stunning.  Lots of smoke in the sky.

And if you needed more evidence that our sky was full of smoke, below is the visible satellite image at 6:52 AM this morning (Monday, July 13).  

Wow.  Lots of smoke over our region.

Yet, with all this smoke, surface air quality is excellent, as shown by the latest observations from the Puget Sound Clean Air Agency (green means good air quality).

How could this be?

The answer to this conundrum is that the smoke is located many thousands of feet aloft, so high that it can not mix down to the surface.   

To prove this, here are observations from a laser-based ceilometer in Marysville, Washington.  This device shoots a laser vertically, allowing it to sense smoke and clouds above the surface.

This morning, the smoke was around 3000 meters (about 10,000 ft) above the surface.

Where did this smoke come from?   

Not from local fires, but from major fires in the Canadian arctic and sub-arctic regions, something shown by the visible satellite image two days ago (see visible satellite image from two days ago):

This smoke then swung around an upper-level low off our coast and into our region.   I can demonstrate this with the upper-level weather map (about 18,000 ft, 500 hPa pressure) for early Sunday morning (below).  The wind barbs show the wind speed and direction.  

Air from northwest Canada moved offshore over British Columbia and then swung around the low and into the Pacific Northwest!

The pattern is now shifting, and the smoke aloft will move out of our region today.