Hot Day Ahead

May 29, 2007

SnapshotA nice little warmup today.  Some spots in the metro area jumped more than 40 degrees from sunrise until this afternoon.  The east wind always does it in the warm season, especially after a cool period (such as this weekend).  Notice how the east wind was strong enough to "cap off" temperatures just a bit.  I expected that, so I didn’t go for 90 today…tomorrow will be a different story. 
At 11pm we have a thermal trough just inland from the coastline.  It is created when easterly wind higher up in the atmosphere (4,000′ or higher) blows over the Cascades and/or Coast Range.  That lowers pressure in the lee (west side) of the mountains, creating an area of low pressure we call a thermal trough or "heat low".  It moves into the Willamette Valley throughout the entire day tomorrow.  So the sequence of weather events is for easterly flow to continue through the early morning hours, then we go calm in the valleys through tomorrow evening.  Then after 5pm or so cooler marine air pushes into the interior.  This is a perfect setup for record heat tomorrow.  A very similar event occurred two weeks ago on May 14th-15th.  Temperatures ended up much warmer than "expected".  The June version of my little "Mark Nelsen magic chart" still says 91-93 tomorrow, so no reason to stray from the 92 degree forecast.  That will be a record for the date.  All models show a shallow marine push tomorrow night, continuing into Friday.  Saturday should be just a bit warmer as a strong trough offshore develops brief ridging again over eastern Oregon/Washington.  So you can count on a summery Saturday.  Sunday is up in the air since some models bring that trough inland by Sunday afternoon…others don’t.  I assume slower is the way to go for now.  A sharp cooling on the way for Monday either way though…Mark

Just for fun, here is a little bit of "inside" gossip about TV stations and the competitive pressures of forecasting nowdays in Long Range Forecasts.