Surprise Thunderstorms

April 5, 2006

Oops!

There was no indication of anything other than maybe a sprinkle today in the models, but we ended up with thunderstorms in the metro area this afternoon.   Maybe my short post yesterday should have been even shorter?

As one of you posters mentioned in this afternoon’s comments, these storms were set off in the change to westerly flow.  Remember that we’ve been in southerly or southeasterly mid-level flow the last few days, but now the latest California trough is moving northeast into Nevada and SE Idaho.  So the storms fired up about the time the westerly flow was beginning to take over.  Some sort of added lifting occurred at that time.  I notice the Lifted Index forecast from ETA & GFS was around +3.  It usually needs to be around zero or below for thunderstorms.  No wonder they weren’t in the forecast.

But let’s move on…strong onshore flow developing this evening with the OTH-PDX gradient over 7 millibars.  That’s a major marine push in summer.  I notice our highest rooftop wind gust today at the station (west portland) is 24 mph in the last 10 minutes.  That should allow some low clouds to fill in by morning, plus the next California-bound low is approaching.  This system sends ample cloud cover overhead tomorrow along with a few sprinkles.  Then as it dives into California Friday, the clouds overhead lift north and offshore as easterly flow kicks in again.  So 70 is still looking good.  The 12z MM5 really shows the change to strong easterly flow Friday morning.  Check this graph out: http://www.atmos.washington.edu/mm5rt/rt/load.cgi?latest+YYYYMMDDHH/images_d2/kpdx.th.gif+text+12%20km%20Portland,OR%2045.59N,122.59W