Two to four inches of rain have fallen on the western sides of the Olympics and Cascades.
The National Weather Service forecast released at 3 PM on Friday said:
SEATTLE AND VICINITY-
INCLUDING THE CITIES OF...SEATTLE...SHORELINE...FEDERAL WAY...KENT 330 PM PDT FRI MAR 13 2015
.SATURDAY...BREEZY...RAIN. RAINFALL AMOUNTS A HALF INCH TO ONE INCH POSSIBLE. HIGHS IN THE MID TO UPPER 50S. SOUTH WIND 15 TO 25 MPH.
.SATURDAY NIGHT...RAIN AT TIMES IN THE EVENING...THEN MOSTLY CLOUDY WITH A CHANCE OF RAIN AFTER MIDNIGHT. RAINFALL AMOUNTS A TENTH TO A QUARTER OF AN INCH POSSIBLE. LOWS IN THE 40S. SOUTHWEST WIND 10 TO
20 MPH BECOMING NORTHEAST TO 10 MPH AFTER MIDNIGHT.
.SUNDAY...MOSTLY CLOUDY WITH A CHANCE OF RAIN. HIGHS IN THE LOWER TO MID 50S. NORTH WIND 10 TO 20 MPH.
And they were not alone, the WeatherChannel forecasts were similar as was the forecast I gave on KPLU.
Why did we get it wrong? In fact, this is a good example of one of the most problematic and difficult forecasts: when an atmospheric river is hanging around our coast for an extended period, and waves/disturbances on this river of moisture moves the precipitation around, often in poorly predicted ways.
Let me begin by showing you an amazing image: the water vapor image at 10 AM on Sunday (in this image the NOAA GOES satellite is looking at wavelength in which water vapor is a very good emitter of infrared radiation). The brighter areas indicated more water vapor. You can see a plume of water vapor from the deep tropics. This is not a pineapple express, it is a tropical express.
An analysis of the total water vapor in a column of air (below) shows the narrow plume of high moisture content reaching our region (5 AM Sunday).
Let me show you the forecasts of one modeling system (the University of Washington's), but they all had the same issues. In this images, I will show you the three-hour precipitation totals ending 11 AM Sunday.
The latest run (6hr forecast that began 5 AM Sunday) is quite good with precipitation over the region. You see the undulation in the precipitation offshore? That is the result of a low pressure system (or wave) on the atmospheric river.
Similarly, a 30-hr forecast began 5 AM Saturday has the right story.
But the forecast begun 5 AM Friday has the band of precipitation over Oregon.
The forecast begun 5AM Thursday has the rain south of Puget Sound.
We can get a better view of a relatively strong low center that is moving along the atmospheric river, causing the precipitation band to ripple and undulate. Here is the UW WRF model forecast pressure pattern (solid lines) and temperatures at 925 hPa (about 3000 ft above sea level) for 5 PM Sunday, based on a the model simulation started at 5 AM Sunday. A strong low pressure center right over Puget Sound country. This should be close to correct. Expect strong winds behind it!
The forecast started 5 AM Saturday also has the low, but farther south. This forecast follows a similar track to the 5 AM Sunday prediction, but the low is weaker and slower.
However, the forecast starting 5 AM Friday is very different, with the low being weak and it never moves into our area.
This was a situation of substantial inherent uncertainty, since small undulations on atmospheric rivers can produce big changes in local precipitation. And the inability of the models to lock into the position and intensity of the strong low center before Saturday morning added to this problem.
A tool that meteorologists use is ensemble forecasting, running models many times with slightly different initial states and model physics. Was that useful in this case?
The NOAA/NWS SREF ensemble system started on Saturday at 8 AM, showed that most of the model runs indicated substantial precipitation on Sunday at Seattle (see below, shows 3 hr precipitation, time in GMT/UTC, black line is the average of the ensemble of forecasts.)
But the story is very different for the ensembles started on Friday AM, 24 hr before. The first peak in the average was for Saturday and the mean (black line) was clearly dropping on Sunday. But there was some hints of possible disaster: a few of the runs were going for heavy precipitation on Sunday.