A Warm and Sunny Weekend Ahead? Probably Not!

April 15, 2013

I was on forecast duty early this afternoon after barely looking at maps the past two days.  Some minor changes in the general weather pattern over our region means a major change to the extended outlook.  3 days ago I was thinking 70s were very likely this coming weekend.  Of course that wasn’t within the 7 Day forecast at that time so you never saw them  in a forecast graphic.

For those just tuning in for the basics:

  • More clouds than sun this week
  • Rain is most likely on Friday
  • Showers are possible again tomorrow afternoon, late Wednesday, or early Thursday.
  • This weekend’s forecast is very much up in the air, models are all over the place beyond Saturday.
  • I’d put a couple of big question marks for Sunday and Monday, but that doesn’t make for good TV.
  • In general, temperatures will be warming a bit the next 7-10 days.

The big change in the maps/models compared to what I saw Friday?  A building upper level ridge late this week is much flatter (heights are lower), and slightly farther offshore.  Sound familiar?  This is a similar setup to the pattern we have seen several times since early January.  One in which weak disturbances ride over the top of the ridge and then scoot across the Pacific Northwest.  They are quite weak, but northwest flow running into the Cascades is quite efficient at squeezing lots of moisture  out of the clouds and keeps us gray here in the lowlands.  That seems to be the case from late Wednesday through at least Saturday.  Interesting that a setup like that keeps reappearing.

Of course with an upper-high just offshore, slight variations in the position make for a huge variation in our weather.  If the ridge is a little closer we get very sunny and warm spring weather.   Position it farther offshore and we get onshore flow/showers with a surface high to our west.  Here’s a good example of the variation on the 12z ECMWF ensemble chart:

tseries_850t_000-360_Portland

Remember the blue line is the operational run that we all get maps/data for.  The red line is the average of all 51 ensemble members.   Good general agreement through Saturday the 20th (the line “21”).  Then they diverge wildly.  Just two days later, they are anywhere from a -4 at 850mb (similar cold temps that we’ve seen the last two days) to a +14 (high temps well into the 70s or even 80).  The ensemble mean and operational are both around +4 at that time with lots of cloud cover and showers just to our north.  The 12z GFS is similar:

tseries_850t_000-384_Portland

a wide spread after Saturday.  The 18z GFS was even cooler and wetter, although that run was nearly the coldest of its ensemble members from Sunday through most of next week.  Then it swings way up to the warmest beyond that point.

tseries_850t_000-384_Portland_2

This is a good example of how you can use ensembles to forecast…in this case it’s probably safest to go for a “middle ground” until models calm down a bit and settle on a reasonable solution.

One item hasn’t changed on the ensemble average on all three charts the past few days:   Upper level temps are generally average to above average starting Thursday through most/all of the extended period.  We’ll see if that holds in the coming days.

Chief Meteorologist Mark Nelsen