One More Sunny Day, Then Cooler/Cloudier

July 5, 2011

What a perfect 4th of July weekend eh?  The forecast worked out amazingly well (thanks to good modeling) with one cooler, cloudier day sandwiched between two bright and sunny days.  Fireworks were a bit nippy yesterday evening, but that’s the way it goes around here where temps drop quickly after sunset.  It HAS been really nice to wake up to the sunshine and we should have one more cloudless day tomorrow.  Then we revert back to the cooler-than-average pattern with marine air dominating west of the Cascades for quite some time to come.

I rarely show the maps below, but they make a good point today.  Click on them for a much better view.  This is the Global Ensemble Forecast System or GEFS.  Ensembles in this case are 20 different runs of the same model (GFS) out to 16 days.  The initial conditions are set, or “perturbed” slightly for each member of the ensemble.  By the way, this is becoming one of the holy grails for forecasting…running many versions of each model to see what changes.  This can increase or decrease confidence in the “operational” or real high-resolution run of the model.    For example, if most of the ensembles are signficantly different from the operational run, then it casts doubt on that run (a real caution flag).  What you see with these maps is the 500 mb. height line (588 decameters), one for each ensemble member.  These are called “spaghetti maps” for good reason, you’ll see in a minute.  The first map is just the 24 hour forecast of that height line, that’s Wednesday AM.  A 582dm height line right over us generally means little marine influence and lots of warm sunshine across the Pacific Northwest; exactly what we’ll see again tomorrow.  Heights are quite high across all of North American, a midsummer pattern.  Note that all the ensemble members are very similar, that’s to be expected just 24 hours ahead:

Now take a look at the spaghetti chart for this coming Sunday:

Quite a change isn’t it?  The familiar pattern of an upper-level trough in the West and ridging in the Eastern USA has returned.  Heights are not extremely low, but the pattern is clear.  And I’m quite impressed that all the ensemble members are still very close together on a 5 day forecast!  That’s 120 hours out.  The confidence is quite high that we’ll be in an upper-level trough this weekend.  At the least we’ll see morning clouds and PM sun, at the most a lot of clouds both days.  Now look further ahead, 10 days out, and you see why we use the term “spaghetti”:

This is the 240 hour forecast, the end of NEXT week.  Often long-range maps aren’t even close to reality, but on a 10 day ensemble forecast you aren’t looking for details, you’re looking for a pattern.  And note that ALL ensemble members have SOME sort of upper-level troughing along the West Coast still and high heights east of the Rockies.  A few of those members show a thunderstorm pattern for Eastern Oregon (southerly upper-level flow), but the big picture says cooler than average the next 10 days.  And for those of you (not sure who) looking for a searing heat wave, it appears to be extremely unlikely through the middle of July.

I’ll have next week off working on some outdoor projects (duck pond), so its perfect weather for that.  But if you want hot weather to heat up your local lake or river, that’s not going to happen.

Chief Meteorologist Mark Nelsen