Fall Moves In Quickly!

October 4, 2011

Someone in here today mentioned that the change in season (from warm and sunny) seems to be quite abrupt this year.  Specifically the last few days.  September was the 2nd warmest on record here in Portland; now the first few days of October have been below average (the daytime highs).  That’s quite a change.  Add to this is the fact October’s temperature changes more quickly than any other month.  We go from an average high of 70 the beginning of the month to upper 50s at the end.  Take a look at the temperature graph for this entire year here in Portland:

The delayed summer really stands out.  Notice the average high usually peaks in early August, but this year you can see the warmest days were from late August to mid-September.  Look closely and see the first few days of October; a sudden drop.  Picture this continuing over the next week and that chart will have a dramatic dip.   Speaking of dips, I like that drop during the brief arctic blast in late February.  Remember that was the beginning of the cool period that continued all the way through July.

So what about the next week or so?  We are obviously in a cool pattern.  Check out NCEP’s 6-10 500mb height anomaly forecast:

A significant trough over or just west of the west coast of North America.  I notice the tendency for troughs to drop in on us when looking at the daily maps of several different models.   Here’s a look at the ECMWF ensemble (51 of them!) forecast of 850mb temp:

The blue line is the forecast temperature at about 5,000′ (in Celsius) on the main “operational” ECMWF run.  The green line is climatology through the next 16 days (the upper-atmosphere cools in October, no big surprise).  All the black lines are the different ensemble members and the red is the average of all of them.  The big message is that both the operational run and ensemble average continue mainly at or below average through mid-October.  Cooler than average weather will be the result.  Notice the -2 deg temp on October 14th is coolest of all the ensemble members.

For now I don’t see any stormy weather ahead, in fact I have no idea why there is a high wind warning out for the Coast.  The gradient is too easterly and only the windiest exposed spots have any hope of making it to the 58 mph gust criteria.  The main energy with tonight’s system is slamming into California as well.

The newsies in here are somewhat excited about the possibility for snow in the Cascades the next 24 hours.  It appears it’ll be cold enough for sticking snow down to about 5,000′ or slightly under tonight and tomorrow morning.  That is mainly Timberline Lodge.  Precipitation looks a bit weak though.  Maybe just 2-3″ max at Timberline.  I like how the flow turns westerly late tonight through all of tomorrow (always better for snow than southerly flow). 

The saying, from a former co-worker, is that “October snow comes and goes, but November snow stays and grows” is usually correct.  Unless we have near-record cold weather the 2nd half of October, the atmosphere is just a bit too warm to give us a lasting snow base down below 6,000′ in October.

Chief Meteorologist Mark Nelsen