More than 600 lightning strikes in an hour were tallied over the Puget Sound Friday morning as peals of thunder rolled across Capitol Hill accompanied by some freakish hail and extremely heavy rain in a week that has been warm and sunny. Why the thunderstorms? Cliff Mass blames instability:

Why the thunderstorms?  We start with fairly unstable air above the surface.  In fact, you could see it destablizing during the evening last night with the formation of altocumulus castellanus clouds (castellanus indicating “castles” in the sky!)

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The National Weather Service says you can expect continued thunderstorms into the weekend:

SHORT TERM...THERE ARE SOME CHANGES TO THE FORECAST TODAY AS
  SHOWERS AND THUNDERSTORMS HAVE FIRED UP ACROSS PORTIONS OF WRN WA.
  THE 500 MB CLOSED UPPER LOW IS CENTERED OVER OLYMPIA THIS MORNING
  PROVIDING UNSTABLE CONDITIONS ABOVE THE MARINE LAYER. SAMPLE
  TIME-HEIGHT SECTIONS INDICATE LAPSE RATES OVER 8 C/KM ABOVE 850 MB
  AND A -17C COLD POOL AT 500 MB. BEST DYNAMICS ARE UNDER THE
  DIFFLUENT REGION N OF THE UPPER LOW. ELY FLOW WILL ALSO ALLOW ANY
  THUNDERSTORMS THAT DEVELOP OVER THE CASCADES TO EASILY MOVE OFF OVER
  THE LOWLAND AREAS. THE CURRENT RADAR LOOP SHOW SCATTERED
  SHOWER/THUNDERSTORM ACTIVITY NORTH OF A LINE FROM ROUGHLY
  HOQUIAM-SEATTLE. MODELS INDICATE LITTLE MOVEMENT OF THE LOW THROUGH
  MIDDAY THEN VERY GRADUAL MOVEMENT SWD WHICH WILL ALLOW SHOWERS AND
  THUNDERSTORMS TO DEVELOP FURTHER SOUTH TODAY...JUST ABOUT ANYWHERE
  ACROSS WRN WA. A FORECAST UPDATE WAS ISSUED TO INCREASE THUNDERSTORM
  COVERAGE AND ALSO INDICATE MORE CLOUD COVER. 

For those looking further ahead, the forecast for next weekend’s Capitol Hill Block Party looks quite lovely.