Dry Weather Continues, But Cooler

October 23, 2013

Back in the “forecasting saddle” this evening and it feels good.  There’s nothing better than coming into the office, filling out the different forecast sheets, and checking out all the obs, maps, and numbers.  The big picture continues to be very easy at least.

1. Upper level ridging will hold strong near or just west of North America’s west coast for at least 7-9 more days.

2. A sharp (and chilly) upper-level trough will drop down the back side of the ridge Sunday, move through western Canada, and end up in southern Idaho by Monday.

3. Strong surface high pressure follows immediately Sunday night and Monday, bringing the first surge of cold/dry Canadian air so far this season into the Intermountain Region.

4. As a result, we’ll go from warm and sunny now to chilly and sunny Monday-Wednesday next week.

Today was our 3rd day at/above 70 degrees here in Portland, somewhat unusual (especially with all the sun) for the last week or so of October:


As I mentioned in a posting last week (before I was dispatched to Hawaii), temperature forecasting has been extremely difficult in this pattern.  Very warm air has been a constant in the mountains/foothills the whole time.  Larch Mountain, east of Battle Ground, has seen high temps between 70-85 degrees every day for the past week.  An amazing stretch of warm fall temps up there.  Meanwhile PDX has been between 63-73 degrees, averaging about 10 degrees cooler.  A stronger wind, likely either northerly or easterly, could have pushed the metro area up into the 70s to mid 80s any one of those days.  But once we lose the wind, the overnight inversion just doesn’t mix out this time of year.  An extreme example of that has been the persistent fog/clouds up in Puget Sound.  Poor Seattle has been stuck in the 50s for a week!  Yuck.  There’s plenty of time for that gray inversion stuff in November, December, January, and February;  glad we didn’t see the same thing happen here!

We do mostly lose the offshore (easterly) wind tomorrow and Friday, although it tries to make a comeback Saturday.  Then to strong onshore flow later Sunday.  As a result, even though upper level temps don’t change much through Sunday morning. our temps should drop here in the lowlands.  More fog and cloud cover is likely too.  I dropped Sunday’s high temp (down to 58) on our 7 Day Forecast assuming we will be stuck in fog/clouds most of the day.  We’ll see how that works out.

All models agree on the cold push of air and high pressure just to our east late Sunday through early Tuesday.  The only variation in solutions is how far west the trough digs and how cold.  The ECMWF at 12z was especially strong with offshore pressure gradients 15 millibars from Spokane to North Bend (OR) by Monday afternoon.  Here’s the map:


This is a setup for a gusty northeast wind all across the Pacific Northwest.   Dewpoints will plunge and we won’t have to worry about fog for a few days afterwards.  At first the wind will keep overnight temps up (Monday night), but then after that we should get our coldest nights of the season so far.

Take a look at the 12z ECMWF ensemble chart:


Then the 18z GFS chart:


Notice quite a disagreement on how cold the air will be as it heads south over us.  About 1/3rd of the ECMWF ensembles don’t bring any cold air south at all!  Or at least far enough to our east that 850mb temps only drop a few degrees!  The GFS on the other hand has much better consistency with itself, showing temps bottoming out around -2 on average.  In fahrenheit, that means 5,000′ elevations will drop from around 60 degrees Saturday to around 30 Monday.

No sign of rain through Halloween, so we’ll probably end up with .87″ rain for the month.  That still wont get us in the Top 5 though.

There are signs that the ridge breaks down around the END of next week, but no sign of wet westerly flow.  There will just be a better chance for some sort of light rain or showers towards the following weekend, the first weekend of November.

Chief Meteorologist Mark Nelsen