Saturday Forecast Looks Drier Here: But Heavy Rain Up North

November 12, 2015

9pm Thursday…

An interesting turn of events for this weekend; all models are keeping just about all rain out of the Willamette Valley and Clark County through Saturday afternoon.  I’m REALLY glad I didn’t make a big deal out of Saturday’s rain…whew!  It will eventually rain Saturday evening, but nothing too unusual.  Here’s the rain forecast for the next 48 hours from the evening UW-WRF model:

00zwrf_48hrprecipsatpm

and the evening NAM-MM5 from the UW:

00zNAM_24hrSaturdaypm

Not much rain through Saturday afternoon here…

Why the change?  It’s somewhat related to my weekend plans; I have a long weekend planned in Southern California.  Over the last few days I’ve been watching the maps for down there pretty closely and each day models have been digging our system farther and farther south.  At one point it looked like 80 degrees where I’m heading Monday and Tuesday.  Not it looks to be around 65.  The point is the southern part of this system is showing more and more energy and now we see that stalls the northern portion late Friday-Saturday.  The weak split can be seen in the 500mb chart for Saturday afternoon:

gfs_500mb

This may (or may not) be the beginning of a splitty winter pattern we would expect in a strong El Nino year.  We’ve seen this happen a few times in the past few weeks (the splitting) and it really shows up again on tonight’s GFS run.  That said, it IS the GFS…

A surface low even develops off the southern Oregon coast with this splitting.  As a result, rain totals by Sunday afternoon look very heavy from around the mouth of the Columbia River up into SW Washington north of Longview.  Watch out on the Chehalis River!  The Portland NWS has just issued a Flood Watch for that area in yellow.

RPM_RAIN_NWOREGON (1)

Note nothing too excessive south of the Columbia inland…maybe an inch or so in Portland.

For skiers, the good news is the snow changing to rain right now will let up and it’ll be mainly dry up there through Saturday.  Then a decent dump Saturday night and Sunday.  I’m thinking 6-10″ is likely at Government Camp by Monday morning.  Higher up additional snow will fall Monday before snow levels take a huge jump Tuesday.

Looking farther ahead models are a bit of a mess.  ECMWF and earlier runs of the GFS were very wet Tuesday-Wednesday with a ton of snow in the Cascades Wednesday.  This evening’s GFS has much more ridging and casts doubt on that snow forecast…we’ll see.

And a sure sign winter is almost here!

The 18z GFS gave us the first model forecast of an arctic blast.  There will be many more teases like this through February.  It showed a massive plunge of cold air (some snow to sea level too) the day before Thanksgiving.  Temps probably 25 degrees in the metro area on Thanksgiving Day for a high.  But, the next run has nothing of the sort…easy come, easy go!  Time to get your tickets and board the WishCast Express…

WishcastingExpress