Lightning Overnight?

June 27, 2011

Starting to feel a bit humid out there this evening with dewpoints rising and the cloud cover.  Of course that would be “Portland Humid” with a dewpoint still below 60 degrees…it ain’t exactly the Deep South!  A deep upper low is centered several hundred miles west of Coos Bay.  It won’t move in over us, but drop south to right around the San Francisco Bay area by Wednesday morning.  A very juicy front is offshore, and it’s raining pretty much everywhere from the Coast Range west.  The band of rainfall may not make much more inland progress since the low is dropping to our south. 

Of most interest the next 8 hours is an upper-level shortwave (similar to a ripple in a stream) that swings north through Western Oregon overnight and ends up in Washington by daybreak.  Both the WRF-GFS and our RPM show rain spreading north at the same time.  Our RPM has a very clear southerly flow type thunderstorm signature (see image above), showing storms breaking out over the Central Oregon Cascades around 8pm and then spreading north during the night.  The flow is almost straight southerly, so not much chance of anything over us (according to this one model), but if our model is correct, we could see a nice light show to our east between 10pm-2am.  We’ll see how it plays out.  The WRF-GFS shows a more solid band of moderate to heavy rainfall spreading north overnight.  That WOULD seem to match up with the current radar imagery better.  They both imply a very mild night; as soon as rain falls it’ll get quite humid too. 

Beyond that, Wednesday we’re into a westerly upper-level flow, marine airmass, and the usual light showers.  Plenty of cloud cover too.  After that the only forecast issue is only low cloud cover and how warm temps get.  We have a nice spell of summer weather beginning Friday and continuing through the middle of next week.  At this point it sure doesn’t appear hot, just warm with a minimum of morning clouds. 

Looks like summer will arrive BEFORE the 4th of July this year.

Chief Meteorologist Mark Nelsen