Stormy Monday PM…What Happened?

April 5, 2010

4:00 pm:  Whew!  Impressive surface low is moving onto the North Oregon Coast right now.   It’s giving us much stronger wind than expected as well.  The location of the low is obviously south of Astoria since there is just a light easterly wind at that location.  As the low has approached and just ahead of line of showers is where we’ve seen the big gusts.  Stronger wind above is mixing down just ahead of the rain.  Note the dewpoints that dropped quickly with the wind.  The pressure gradient is all of 5 millibars EUG-OLM and 4 mb. EUG-PDX.  Normally that would produce gusts maybe 20-30 mph at best…a good sign of strong wind mixing down from above.

It has gusted over 70 mph at some spots on the Coast and 40-50 mph here in the Valleys.  I see Forest Grove even had a gust to 53 last hour.  Wind should die down to gusts 25-35 mph quickly after 6pm.

9:00 pm:  Alright, the wind is pretty much gone now…time for the blame game.  So how did one of our strongest April south wind storms in years (the 2nd one!) slip by?  I just spent some time looking at the WRF-GFS from the UW.  I compared the forecast for today with last Thursday’s storm that produced gusts of 40-50 mph here in the Metro area as well.  Check out the maps…that model nailed today’s situation very well!!!  The 4km version showed a small area of sustained wind at ground level over 30 kts (35 mph) in the north Willamette Valley last Thursday for that first storm.  Check it out here.  An easier one to read is the meteogram for UAO (Aurora).  It shows the surface wind forecast.  I like to use that one because it’s out in the open in the northern Willamette Valley, away from any terrain issues the model may have.  For today’s system it showed a slightly LARGER area of 30kt. wind in the north Valley between 2 & 5pm…that link here., and the corresponding Aurora meteogram.   So we should have forecast gusts in the 40-50 mph range (just like last week).  The low pressure placement, movement AND depth was forecast very well too.  Astoria (just north of the low as it moved onshore) hung right around 1003 millibars all afternoon, finally now rising as the low has moved off to the east.  So to summarize, I think this is a similar situation to December 29th in that the models handled things well, but the communication broke down a bit.  In this case it was forecaster error.  I didn’t look at any maps either yesterday evening or this morning because I wasn’t working, so I was totally clueless when I walked in the door at 1pm.  Of course this system didn’t leave people stuck on the freeways for 5 hours, so luckily it didn’t have nearly the effect as the last time, but I hate missed forecasts!

Chief Meteorologist Mark Nelsen