September 13, 2013

Low Cloud Forecast Failure

What is the most difficult forecast for a Northwest meteorologist?

(a) Big windstorm from an approaching Pacific cyclone?

(b) Strong winds in the Strait of Juan de Fuca?

(c) Downslope winds in Enumclaw?

(d) Predicting fog/stratus burn out over Puget Sound during the fall?

The correct answer is d.  And today was a sobering example of a bad forecast.  And I will tell you why.

We woke up this morning with low clouds over western Washington (see image). 

One sign that the cloud deck was relatively thick was that clouds extended deeply into the Cascade valleys. Another sign was that there was light drizzle at a number of locations.

The official National Weather Service forecast even at 9 AM this morning was for a burn off of the low clouds during the afternoon..


This was not an unreasonable forecast for mid-September: the sun is still fairly strong, and our numerical models indicated a burn out during the late morning.  I also thought we would see some sun in the afternoon for the same reasons....we were both wrong.

Take a look at the visible satellite picture for 5 PM.  The clouds did burn off over Portland and Willamette Valley, over Bellingham, and Victoria.   The clouds pulled back from the Cascades...but Puget Sound was still socked in.   Not good.

The high-resolution UW WRF forecast had the clouds right at 8AM

 But the 2 PM forecast the clouds wer nearly gone over Puget Sound.
 And my favorite National Weather Service High Resolution Rapid Refresh (HRRR) Model had virtually no clouds at 2 PM, based on a 6-hr forecast.  Disappointing.

So what went wrong?   This situation exposes one of the classic problems of our current modeling systems:  they tend towards excessive mixing in the lowest layers under stable conditions, and such excessive mixing can destroy shallow fog and stratus layers. Human forecasts know about this tendency, but clearly our ability to subjectively correct the forecasts is not perfect.

The observed temperature soundings at Seattle (NOAA Sand Point) showed a deep cool layer (where the clouds were), topped by an inversion with a base at around 600 meters above sea level. (graphic shows temps for 8am (15 UTC) to 2 PM (21 UTC).
The model sounding (red temperature, blue dewpoint) was not bad at 8 AM (15 UTC):  the air was saturated low down (blue and red lines on top of each other) and the model had an inversion above.

But by 18 UTC (11 AM) the model forecast was way wrong with the saturation gone (no clouds) and the inversion way too weak.
At 21 UTC (2 PM), we were in disaster land.  The model inversion pretty much gone in the model.  The lower layer got even drier.

Anyway, this kind of behavior is not unusual and it reflects a basic model weakness.  For the technically minded, this is probably a deficiency in our boundary layer parameterization schemes.

When I biked to work in light drizzle I knew my earlier forecast of an afternoon break out was in trouble. 

This profession keeps one humble.


  1. Fallibility is at the core of weather and climate forecasting, and it happens sorta frequently. Still, I often wonder why a blown forecast can't be corrected. Even by mid-afternoon all media sources I checked still predicted a 5 PM high of around 78, including Accuweather, NWS, and Weather Underground.

  2. The summer is nearing its end and the available weekends to reliably catch great weather are too. Thursday night I was contemplating a quick drive down to the Oregon Coast or perhaps several daytrips up to the mountains near Seattle over the weekend. I looked at the forecast for the coast and it indicated clouds early this weekend, and rain hitting there earlier than the Puget Sound later this weekend. So I nixed the idea of Oregon. Friday morning I looked online and saw that same satellite image you show in your top image. It looked like we were really socked in and I had a feeling it was going to be like that all day. I've lived here for almost 30 years (and prior to here, the SF Bay area which can have similar low cloud effects) and you learn this over time. But yesterday morning Rainier looked in the clear. So I headed up to Sunrise on Friday, rather late in the morning. Sure enough, it was cloudy until about Greenwater. Beyond that point you finally rose above the cloud layer.

    This photo taken about 7 PM from Burroughs Mountain, points towards the Puget Sound basin. It faintly shows the thick clouds still infiltrating the area. It was like that all afternoon. I'm not quite ready for the weeks-long (month-long) stretches of clouds we often get from November until April. Often when it is cloudy in the Puget Sound region, a short distace away, you can get above or beyond them if you wish. Fortunately, with experience and the availability of near real-time satellite images such as what you show, you can sometimes make adjustments to your plans and still find sun. It was quite warm at 7500 feet on Friday. Even in the Sunrise parking lot (el.6400 ft) in the dark at 8:30 PM it was still comfortably warm.

  3. Overall I have been impressed with the forecasts. Particularly the recent heavy rain event, which was forecast well in advance, and the 90 degree Wednesday, also forecast well in advance. Impressive.

    Living so close to cool Puget Sound, I realize the low clouds sometimes do not burn off as predicted.

  4. The inversion has been especially strong and warm. On Thursday morning, I flew from Boeing Field to Aurora, OR. I departed around 7:00 a.m. into a low overcast (300-400 ft). I broke out about 900 ft. Above the cloud deck, which extended well south of Portland, the temperatures were way above standard. At 7000 ft (the standard freezing level is about 7500 ft), the outside air temperature was near 20 degrees C!

  5. This was exactly the blog I thought you needed to post today. So there you got it perfect.

  6. Wondering if it will burn off today, then? Are models similar today?

  7. Twas lovely in Bellingham most of the day. Then driving to the Sounders game, we entered the murk in Chuckanuts and didn't emerge from it again!


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