30 Hours of Rain; But Mostly Dry Weather Ahead

January 29, 2013

With just two days to go, January is going to end mild and mainly dry. 

Now as a whole the month was very chilly at the lower elevations due frequent inversions.  Portland will end up with its coldest January since 1993!  But much drier than average; probably just under 3.50″.   The mountains will end up about average, buoyed by the warm weather above the inversion for part of the month.

Take a look at the rainfall totals since midnight:

PLOT_Rain_Autoplot_Valley

Portland saw rain every hour between 5am Monday and 11am today (Tuesday)…about 1.5″ during that time.   Notice the strongly orographic effect to the rainfall.  Lots more rain the past two days eastside metro compared to westside.

Most of the rain lifts to the north the next 2 days as a warm upper level ridge builds in from the Pacific.  Snow levels have already risen above 4,000′ this evening and rise closer to 6,000′ tomorrow as the ridge gets closer.  By Friday, it’s close enough that cloud cover and all rain will be pushed well to the north.  At that time 5000′ temperatures are pushing 50 degrees.  Those warm temps at the higher elevations will continue through early next week. 

Last time this happened we saw very chilly temps in the valleys and lots of fog/frost.  This time it’ll be different for two reasons:

1. We start much warmer with the current mild surface airmass overhead.  Last time we started with a snow level almost at sea level.

2. The sun angle is getting higher and it’s a LITTLE easier to break some of the inversion.   Not a total breakout.  In fact the same atmosphere overhead in late February would push our high temps into the low-mid 60s!  It’s a few weeks too early for that.

Thus, I think we’re going to see our first 50-57 degree mostly sunny days west of the Cascades we’ve seen since early December.  We saw highs in the mid 50s earlier in January, but that was with rain and gusty south wind.

What about the big picture as we head into the first half of February?  I sure don’t see any pattern that would produce low elevation snow, extremely cold temps, or wild and wet storms.  Yes, for the weather geeks this slow winter weather will continue.  Here is a look at the 00z GFS ensemble forecast chart for the next 16 days.  You want to see 850mb temps down to at least -6 or -7 to get snow down in the lowlands.  Notice you don’t see that in the next two weeks:

tseries_850t_000-384_Portland

Chief Meteorologist Mark Nelsen