Today was a generally warm, cloudy day with occasional light sprinkles and showers, quite a contrast with what was predicted only a few days ago. The high today at Sea-Tac was 69F with a trace a rain. More rain fell over the south Sound and the coast.
Lets take a look at the National Weather Service forecasts for Sea Tac and vicinity provided on various recent days around 4 PM:
Friday: MONDAY...MOSTLY SUNNY. HIGHS IN THE 70S
Saturday: MONDAY...MOSTLY SUNNY. HIGHS MAINLY IN THE 70S. LIGHT WIND. Sea-Tac: 79F, 10% chance of rain (they mean measurable rain--at least .01 inch)
Sunday:MONDAY...MOSTLY CLOUDY. HIGHS MAINLY IN THE 70S. LIGHT WIND. Sea-Tac 75F, 10%
and this morning (Monday) at 4 AM the NWS was going for clouds with 75F at Sea Tac and 10% chance of rain.
We ended up with a cloudy day, with some sprinkles over most of the area and light rain in some locations. Upper 60s and low 70s.
So what changed? As I will discuss below, this was a difficult forecast, with the region right on the edge of big rain gradient...and the forecast models had quite a bit of uncertainty.
Here is the 24-h precipitation forecast from the UW prediction system starting at 5 AM this morning (and thus ending 5 AM on Tuesday morning). Not too bad. Shows the heavier showers on the coast and over SW Washington. But this is a very short forecast and we expect it to be excellent.
Now here is the forecast for the same period (24 h ending 5 AM on Tuesday) that was run at 5 AM Saturday. Big difference, the precipitation was well off the coast.
And here is the forecast starting Friday morning: same thing.
Now what changed? The flow field aloft was just subtly shifted between these forecasts..and that made all the difference between sunny and mid to upper 70s and cloudy, lower 70s/upper 60s with sprinkles. Lets take a look.
Here is the forecast from Friday for the upper level (500 mb) flow. See that low right off our coast? Pretty unusual for this time of the year.
And here is the same upper level flow from this morning's forecast (which should be very close to reality). Can you see the difference? REALLY subtle.
This is an example of a very difficult forecast situation in which the outcome is very sensitive to small differences in the forecast flow field.
So how can we deal with such a uncertain forecast? Well, meteorologists now have a new tool--ensemble forecasts: running many forecasts each starting a little differently. Half of the forecast show rain, half not...50% chance of rain! Having a variety of different forecasts...all very reasonable..makes it more likely that the forecast system will capture an upcoming change. Here is an ensemble forecast starting Friday afternoon at 5 PM, showing the 12-h probablility of precipitation during the day on Monday:
This guidance gave a real heads up that there would be a significant chance of showers on Monday. This is the future. And forecasts will be more useful because of it.