Thursday, September 26, 2013

Heavy Precipitation Update

We still have a very early heavy precipitation event ahead of us.  The latest model runs are now shifting the precipitation a bit south, so the southern Washington and Oregon Cascades will get hit harder.

Here is the 72h precipitation totals ending 5 AM on Monday from the UW WRF model.  A large proportion of the Cascades, Olympics, and coastal mountains will get 5-10 inches, with some peaks getting more (more than 10 inches!).  Moderate rain hits eastern Washington and substantial totals will fall on the Idaho Rockies.


An interesting aspect...look offshore.  Precipitation is far less.  The precipitation in our region is associated with a plume of moisture coming off the Pacific (see graphic below of water vapor content at 5 AM Saturday), but only when the moist air is forced to rise by the terrain does large amounts of moisture get released as rain or snow.

Three Pulses

The rain will hit in three pulses.  Today is the weak one...it will start raining this morning and end in the afternoon.   The break will last until early Saturday when a a far larger event will hit.  And then another on Sunday.

 Here is an infrared satellite picture for Friday AM around 6 AM.  You can see the first week system hitting the coast, but look offshore....WOW..   A huge, extensive very wet system in the Gulf of Alsska.    And it is heading our way!

I expect a number of daily records to fall.

Another point:  we will get a similar amount of rainfall as struck Colorado a few weeks ago, but the damage will be far less.  Why?   First, our rain will come more slowly and will be more uniform in time.  No flash flooding. Second, our terrain and drainages are far more accustomed to heavy precipitation than Colorado.

With all this precipitation, the rivers will respond.  The Northwest River Forecast Center is predicting some will hit flood stage (see graphic, red dots are river locations where flood stage conditions will occur).  I suspect they will add rivers farther south tomorrow.


This event is sufficiently unusual that several rivers are predicted to exceeding previous records for the upcoming dates.  For example, consider the Skagit River near Concrete,WA (below).  Not only is the river predicted to exceed flood levels (red line), but the river exceeds daily records (red triangles) for several days.  If you live near rivers or streams, you better be ready to protect low-lying assets.


And did I mention the strong winds?  I will save that for another blog...

 _______________________________________________________
Bellingham Talk on October 15th
I will be giving a public talk on "The Future of Weather Forecasting" in Bellingham on October 15th.    In this talk, I will discuss the development of weather prediction from folk sayings to numerical weather prediction, and describe what I think will happen over the next decades.  For more information, go here.


 Reading the Northwest Sky, Seattle, October 1
I am giving a five-lecture evening short course: "Reading the Northwest Sky: Understanding Our Weather and Climate"  
October 1, October 22, November 5, November 26, December 3  
Kane Hall: University of Washington  
Co-Presented by University of Washington Alumni Association   and Seattle Public Lectures.If anyone is interested, more information here.

1 comment:

redbird said...

How long are the sessions for that class? (I followed the link and looked around, but can't find that information.) Thanks.