A Very Dry March; Mild Weather Continues

March 25, 2013

9:45pm Monday…


What a difference one year makes!  Our temps this month are running slightly below average, but rainfall is WAY below normal.  And this weekend was great with excellent weather for getting outdoor projects done; so now my back is tired and I’m “relaxing” at work. 

Last year was our wettest ever March with 7.93″, constant rain and several bouts of wet snow to the lowest elevations.  This year no snow at the lowest elevations and very little even up around 1,500′.  Last year I had 20″ of snow at 1,000′ in March; this year just 1/2″ last Friday morning.  Right now we are at 1.46″ rainfall for the month.  Even if we somehow squeeze out another .50″ in the next 3-4 days (unlikely), that means we’ll be below 2″ for a monthly total.   That’s only happened twice in the past 20 years (2004 & 1994).  This March has been a very nice payback for the really wet March weather we’ve seen the past two years.  It’s especially pleasing that the eastern part of the USA is whining about how “winter won’t let go” as we finally get some nice spring weather for once.

I might be jumping the gun a bit, but a cursory glance at past springs shows that the under 2″ March totals are frequently  followed by a drier than average spring.  Interesting eh?

The next few days involve a real pain for forecasting.  Models are showing weak southwest flow over the Pacific Northwest as a new upper low develops in the Eastern Pacific to our southwest.   So we just have leftover moisture over us that afternoon heating work on.  Most likely we’ll see pop-up showers Wednesday-Thursday afternoons, possibly tomorrow or Friday too.  I’m not really seeing any low lifted index numbers or high CAPE; both would be an indication of nice spring convection.  But we could have some weak “action” those afternoons.  We’ll see what happens each day.

Beyond that, models all agree that the upper-low sits just off or on the California coastline over the weekend with upper-level heights rising off the SW BC coastline.  This is a nice block pattern with offshore surface flow and a warm atmosphere overhead.  The last few days of March we can get some very warm weather out of this.  850mb temps are forecast around +8 to +10 with abundant sunshine and some offshore flow.  That means high temps the warmest we’ve seen so far this spring; right around 70 degrees.  A +10 with perfect offshore wind flow and totally sunny skies has pushed the PDX high as high as 74 degrees in the past in the month of March (according to my chart).

So, it’s a risky forecast, not because of uncertainty, but because it’s a holiday weekend (Easter) and I REALLY don’t want to screw up the forecast.   Nothing’s worse than forecasting a nice weekend on a Monday and then having to back off on that later in the week.

Here are the two ensemble charts from today, the 12z GFS and the 12z ECMWF, click for a better view:

tseries_850t_000-360_Portland tseries_850t_000-384_Portland

Note the unusually high uncertainty beyond Sunday.  Models are all over the place for next week, although splitty flow seems to want to continue.  Split-flow is terrible in winter because it leads to boring weather and weak storms.  But split flow in the spring often means warmer than average weather and sometimes thunderstorm setups.  Example A:  a new wrinkle on the brand new 00z GFS, the upper-low to the south this weekend moves off to the east and ridging stays right over us on days 7-8 (MON-TUE) instead of a cold trough coming in from the north…very nice if you want mild spring weather to continue.

By the way, I’ll have very low tolerance for misbehavior or drama this week on the blog comments.  It’s been real mellow on here; could we keep it that way for a while longer?

Chief Meteorologist Mark Nelsen