A Stormy Monday Night Ahead

December 16, 2018

7pm Sunday…

It’s almost Christmas, just 8 days away!  What does that have to do with stormy weather?  If you grew up here in the Pacific Northwest, then the upcoming week will finally look like Christmas seasons from the past…rainy & windy with mountain snow.

Six weeks into this “storm season” and it has been a real dud so far.  Very few storms and rain is running above 1/2 of normal since November 1st.  But we’re going to get another wind “event” Monday evening/night like we had this past Friday plus a real soaker Monday night.

A more typical strong westerly jet stream is developing for a few days and will be directly over us for the next 48 hours.  A system is developing out in the Eastern Pacific (you see it on the left side of this GOES-17 pic)

Capture

You can also see a long train of subtropical moisture embedded.  Models push all this moisture directly toward Oregon and it’s overhead tomorrow night, seen here on the “Integrated Water Vapor Transport” forecast

ivt.42.0000

That’s pretty juicy and models are showing a period of moderate-heavy rain tomorrow evening through Tuesday morning over and west of the Cascades.

RAIN

ECMWF gives us 1.5″ in the driest part of the lowland valleys to 3-6″ in the mountains Monday 4pm to Tuesday 4pm.

ecmwf_precip_24_portland_11

Both the RPM (The Weather Company’s model) and WRF-GFS from UW model are similar, although a bit wetter.  This is the latest RPM.  Notice 2″ or less in the valleys.  That’s not enough to cause widespread flooding after a dry start to the season.

RPM Precipitation Accumulation

This isn’t a “big flood” setup since the heavy rain spends less than 24 hours over us.  Yet rivers will rise quickly, a few creeks/streams may go over their banks, and Tuesday morning’s commute should be especially messy when the heaviest rain is moving through.  Expect some water on roads/highways in spots.  Only a few rivers should experience (minor) flooding.  The big rivers are fine since reservoirs are nearly empty after a dry fall and early winter.   The NWS has a Flood Watch out for our area for this reason.

Mark Warnings Flooding

WIND

We don’t have any strong area of low pressure to track with this storm, it’s just a relatively tight pressure gradient as the system comes through.  So no, I don’t see a big windstorm coming, but maybe speeds similar to what we saw on Friday inland.  That would be gusts 35-45 mph.  We’ll switch from east to southerly wind tomorrow afternoon and then it should be strongest along the I-5 corridor tomorrow night.

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Once we get past Tuesday midday or so it’s relatively quiet again, just some typical east Pacific cold fronts passing by.

Most of the precipitation will be as heavy rain in the Cascades Monday night and Tuesday; we’re going to lose some snow up there!  A little bit more rain Thursday, but then there are strong hints we’ll see colder systems Friday through Christmas Week.  Hopefully that will get more terrain open as Christmas Break starts for lots of kids this weekend, others are already out of class.

Chief Meteorologist Mark Nelsen