Dry Early Week; Then LOTS of Rain

December 15, 2019

7pm Sunday…

Today sure felt like December…we only reached 44 in Portland under mainly cloudy (but dry!) skies.  That’s about normal for this time of year but the coolest day in two weeks.

These next three weeks are (on average) the coldest of the year in Portland.  The average high temperature is just 45 degrees from December 14th through January 5th.  It goes uphill after that time; reaching 49 by the end of January.  Our “winter” occurs earlier than other parts of the USA.  The darkest/coldest time of the year is right around the solstice.  I’ve always found it interesting that in our climate we go from summer to winter much more quickly than the reverse.  Springs are LONG here but we cool down quickly from late summer to December.

What’s ahead?  Two dry days for most of the region Monday and Tuesday; that’s due to high pressure temporarily overhead

ecm_tuesdaypm

But then for about three days we’ll be under the influence of a strong jet stream coming at us from the southwest.  This chart shows windspeed up around 18,000′ late Thursday night.  Notice the strong jet aimed right at the Pacific Northwest.

gfs_500_friam

And an “atmospheric river” is transporting a tremendous surge of moisture from the tropics into our area.  Precipitable water map for Thursday evening shows values up around 1.00″ or so.  I’ve seen higher, but this is the “juiciest” setup so far this wet season.

gfs_precipwater_Thursdayevening

Models are all showing several surges of rain from Wednesday night through Saturday.  Each is a bit different, but notice the operational versions of the GFS/GEM/ECMWF all give a huge soaking to the northern half of western Oregon and southern part of western Washington.  Click on each for a closer view of rain forecast now through next Sunday.

So the million dollar question is whether this will just be a “very wet period” or will this be a flooding setup for some of us late in the week?  We don’t know yet, but we’re watching it closely.  Check a couple of model “ensemble forecasts”.  Both say that SOME PART of the region will get a lot of rain.  The ECMWF ensembles show widespread 2-4″ in valleys and more in the Coast Range by midday Saturday.

ecmwf_ensemble_precip_saturday

The GEFS ensembles are interesting because they are running as a separate model now from the operational model GFS-FV3.  It was upgraded this summer, but the ensembles continue to use the old model.  In this case they are both showing the same heavy rainfall.

gfs-ensemble-all-precipsaturday

To summarize, confidence is high that we’ll make up a lot of our November/December rainfall deficit in just 3-4 days later this week. 

We could easily see 2-4″ rain in the valleys; luckily reservoirs are at the lowest of winter and creeks/rivers are very low.

I’ll keep a close eye on it this week; otherwise enjoy the dry weather these next two days.

WHAT ABOUT CHRISTMAS SNOW?

I’m still seeing nothing for the lowlands the next 7-10 days.  We dry out around Sunday and then turn cool & drier heading deeper into Christmas Week.  It’s the same splitty upper-level flow we’ve been seeing since around Halloween.  As of now I don’t see a single model showing anything close to lowland snow through Christmas Eve or Day.  Check out the 51 ECMWF ensemble members.  Only 3 of 51 produce some sort of lowland snow between now and Sunday the 29th.   Sorry kids!

ecmwf-ensemble-KPDX-indiv_snow_24-6411200