A Busy Weather Day Ahead…In August

August 12, 2014

Here in Portland July through mid October is the deadest time of the year weatherwise, with September generally being the peak of the boring weather season.  That’s because we rarely get thunderstorms west of the Cascades.  But today is one of those rare times in which we could see some real action.

The reason is that we have a rare combination of lots of moisture all through the atmosphere, including here at the surface.  Plus a very warm atmosphere overhead.  The upper-level flow is southeasterly over us, which is bringing in more moisture from the southeast.  The atmosphere is unstable, which means parcels of air want to suddenly rise through the atmosphere and form those huge thunderheads.  And the final trigger for today is the upper-level low that was off the California coastline is lifting north through the state of Oregon this afternoon.  Those factors have all combined to put anywhere in Oregon over and east of the Cascades in the SLIGHT RISK category for severe thunderstorms.  The primary danger will be damaging outflow winds and hail.

But even west of the Cascades we’ve seen the action early.  Take a look at the hundreds of lightning strikes right after sunrise in the north Oregon Coast Range and Willapa Hills!  lightning_tue Those were all strikes in a 30 minute period!  I see that one RAWS station in the Coast Range had almost an inch of rain in a very short period of time.  With such a warm and humid airmass it can rain like it does on the Gulf Coast…buckets at a time.

Here is the latest lightning  image for western Oregon as of 10:00am:

LightningNWOregon

A bit quieter, but I think we’ll really see the action pick up this afternoon and evening.  At least the cloud cover will keep temperatures much cooler than yesterday.  Although it’s a humid day, high temps will stay well below 90 degrees.

To recap…

  • ANYONE could see a thunderstorm today
  • Everyone will not see one though
  • Any storm could include very heavy rain briefly
  • Gusty winds are possible within a few miles of thunderstorms, that’s because air rushes out of the bottom of storms and spreads out below
  • East of the Cascades the risk for damaging wind and large damaging hail exists

Tomorrow we’ll see our coolest day in quite awhile, highs only around 70 and showers/clouds.  More of a gloomy May (or late September) day.

Chief Meteorologist Mark Nelsen