The Rains Return

May 21, 2012

Just like that! After 15 days without measurable rain here in Portland, it’s wet again and our monthly rainfall total is right back up to average.  Considering the amount of rain expected the next few days, it’s quite possible this May could end up with above average rainfall; very strange considering it didn’t even rain for half the month!

Well, the rain WAS needed this time around.  I spent parts of the past 6 days off working in the yard and it was very dry…similar to early July.  One night camping at Oxbow Park (the only campground I’ve ever heard of with no alcohol allowed) was bone dry.  How often can you camp in mid May west of the Cascades and not deal with mud getting tracked into the tent or RV?  Although now that I think about it, maybe the “no alcohol and no dogs” rule at Oxbow explains why it was blissfully quiet sleeping down there???

Moving on…one wet system today, as you see from the rain totals above, and another juicy looking one for Tuesday.  Tomorrow’s system is quite impressive with a low pressure system racing into western Washington (towards Olympia) later in the day.  We should get a nice burst of southerly wind during the afternoon and evening along with a soaking too.  Maybe  a quarter to half-inch in the Portland Metro area.

Wednesday and Thursday look quite dull with onshore flow, light showers, and mostly cloudy skies.  Oh, and chilly temps too.  850mb temps (the temp in celsius around 5,000′) are near zero both days.  So just a skiff of snow on the passes and down to camping lakes in the Mt. Hood National Forest.  Brrr!  Chilly start to Memorial Day camping if you’re going early!

Models seem to agree (somewhat) on a deep trough moving to our south Friday-Sunday, leaving us all or mainly dry.  Then weak systems move by to our north later Sunday through Tuesday.  For that reason we’ve left a lot of clouds in our 7 Day forecast for Sunday and Monday, but little or no rain and temps near average.

Here’s what I DO NOT see for the weekend:

1.  A sunny and warm holiday weekend
2. A very wet and very cold weekend
3. Snow in the mountains below the tree line (6,000′) 

As May wraps up, our weather appears to stay near or below normal temp-wise, check out the 12z ECMWF ensemble chart:

As Cliff Mass mentioned at his talk at OMSI a couple of weeks ago, ensemble predictions do quite well.  For example on this chart, you see the BLUE “operational” run of the ECMWF, and then the RED or “average” of all the runs.  Note how the operational goes a bit higher next Tuesday and Wednesday, but is quite close to the ensemble mean otherwise.  The green line is the climatological 850mb temp during the period. This is a forecast with reasonably high confidence.  Now notice the difference on the 18z GFS:

It shows a bit more variation both between individual runs and the actual swings between troughs and some ridging.  Note the lower and later cold trough for late Monday and Tuesday, and a much higher swing above average later next week compared to the ensemble mean. 

Chief Meteorologist Mark Nelsen