Nice Weather Ahead, But Tough Temp Forecast

October 15, 2013

What an incredible day today.  I visited a school up in Battle Ground and while driving away it felt like spring.  Of course we’re in the middle of fall, but for some reason it felt a bit more like spring to me.  It might be due to all the rain last month; it’s greener than what we would typically expect this time of year?  I don’t know…anyway, a real nice day with temps up around 70 in the metro area.  Quite impressive considering we were down in the 30s this morning.  Easterly wind made it to about I-205, helping to warm us up a bit.

The Big Picture:  It’s going to stay sunny and warm for at least another week!

The devil is in the temperature forecast through the period.  The reason is that we are right on the cusp of “inversion season”; that’s when the decreasing sun angle gets so low that there isn’t enough midday/afternoon heating to get rid of the overnight inversion (cool air in valleys).  From November through late February I suppose one could say “the night rules the air mass”.  During that time easterly offshore wind doesn’t warm us up.  When we get a big ridge of high pressure and 60s/70s in the foothills, the warm air rides right over the cooler low-level air mass.   During the transition times (late October and early March), it’s really tough to tell how much the easterly wind will warm us up.  By the way, during this period, our “warm” weather then occurs with southerly winds, usually near/ahead of frontal systems.  All our record highs from mid-November through mid-late February are from some sort of southerly flow.

Typically a very good guide for short-term temp forecast is MOS (model output statistics) based off of models.  As opposed to just reading raw model output (usually not a good idea to read it literally), MOS uses algorithms that are based on what actually occurred in the past when certain atmospheric conditions were present.  For example, such and such 850mb temp plus a certain wind condition led to a certain temperature at a specific site.  Got that?  The problem the past few days is that MOS has been cooler than what actually occurred.  So it’s not real helpful for the next few days.  Actual model surface temps at Portland (NAM, GFS, ECMWF) warm a few degrees (4-6) from Wednesday through Friday.  So that would put us in the 70-75 range.   But easterly gradients just about go flat Wednesday, then turn breezy easterly again Thursday and Friday.  So we dropped tomorrow’s forecast down a notch to 68, then up a couple degrees Thursday and again Friday with the increasing easterly gradient.  Back to mostly flat flow Saturday/Sunday, so we’ll struggle to hit 70 again.  One more surge of easterly wind Monday-Tuesday next week with even warmer low level atmosphere should push us back up in the low 70s again.  One thing that can really mess us up this time of year is a wet air mass at the surface giving us fog through at least half the day.  I’m not worried about that for now since we’ve dried out quite a bit the last few days.  And I don’t see any low level moisture source the next few days either.   Add it all together and here’s what I get for a 7 Day forecast right now:



By that time it’ll be October 21st/22nd, definitely near the end of easterly flow helping with any warming, so the middle of next week might be the last of our warm/sunny weather.  Or at least the “warm” part of that.  Models are definitely sticking with dry theme through the next 7-10 days.  The 12z GFS broke the ridge down quickly just before Halloween, and the ECMWF seems to be trying to do that.  We’re probably going to stay dry through at least a week from Thursday or Friday, who knows what happens after that.  Notice the huge variation in solutions on the 12z ECMWF:


and the 18z GFS has a cold (but dry) trough dropping in on us for an early season cold spell right around Halloween:

tseries_850t_000-384_Portland (1)

Look at how different ensemble members want to bring in a cold trough at different times too.  Some nice variability in there!

Chief Meteorologist Mark Nelsen