Weekend Washout Ahead: Some Flooding Possible

December 17, 2014

Do you need to build an Ark in the next 3 days?  No, but some local rivers COULD see flooding late in the weekend or early next week if models are correct.

Between now and Friday night not a whole lot to talk about, although Thursday night’s system will bring some snow to the Cascades down to pass elevations.

Then the jet stream will punch directly into Oregon and the southern half of Washington.  Take a look at the forecast for Sunday morning

gfs_300

…speeds approaching 200 mph surging northeast out of the tropics and aimed right at the Pacific Northwest.  The precipitable water forecast from the GFS model shows the “atmospheric river” of moisture very clearly.  Precipitable water is a measure of the total column of moisture through the atmosphere at any one location.

pcpwater_4aSun

You can see how an event like this got the name “atmospheric river”; it’s a narrow, but massive, transport of moisture-laden air from the tropics into the middle latitudes.  In this case it’s transported over us.  Pineapple Express or Java Jet works too because the source region is in the tropics.

That is only part of the reason we have a huge soaking ahead.  The other is our mountain ranges.  Both the Coast Range and Cascade Range are oriented directly perpendicular to this flow of air.  In the case of Saturday and Sunday, the flow of air will be strong from the west…around 70 mph at 5,000′.  That air suddenly has to rise, cooling the airmass.  When that happens it has to dump it’s load of moisture in the form of heavy rain.  You’d be amazed at how efficient this can be.  I’ve seen just a few sprinkles in the valley all day in an event yet a full inch falls in the mountains.

In the case of this weekend, our models are painting a very dramatic picture with unusually heavy rain over our mountains from Saturday through Tuesday morning.  Almost all of it is done by Monday morning.  The “drier” ECMWF shows maybe 4-8″ on the west slopes of the Cascades.  Note Oregon is under the gun and it isn’t very heavy north of St. Helens or Mt. Adams:

ecmwf_precip

The GFS is crazy wet, one of the wettest runs I’ve seen, 12-15″!  That’s serious flooding of rivers draining the Cascades (Sandy, Clackamas, Molalla, Santiam) if that were to occur:

gfs_tuesam

The WRF-GFS, which is based on the GFS, is showing slightly more tame numbers…but still over 10″ in some areas:

wrf_tuesam

Can you see the rain shadow in the western valleys too?  Only 2-3″ in the valleys but 3 times as much in the Cascades!  That’s perfectly normal in a strong westerly flow.  Also notice the 1/4″ for places like Maupin or Warm Springs.

So do we have a bunch of flooding coming?  We don’t know yet because models could easily shift the rain north or south in the next few runs.  But it’s definitely something we are keeping a very close eye on.

You probably already figured this out, but almost all of the precipitation will be in the form of rain in the Cascades; I think by Monday morning there will be nothing more than a patch or two of snow below 7,000′.

Chief Meteorologist Mark Nelsen