Forecast Error?

Sometimes forecasts go wrong and sometimes the communication of details is inadequate...and today is a mixture of the two.

Significant showers are mostly over for the western lowlands and bright sun is now features over the San Juans and even a sliver down Puget Sound. (see satellite image). There is almost nothing immediately offshore. And the radar shows the rain mainly over the mountains now.

Forecast failure? Well, not exactly. The computer models yesterday showed a wet frontal system coming in this morning and that it would move through mid-afternoon, resulting in showers and sun breaks late in the day..and in fact the 3:30 AM NWS forecast tells this story:

330 AM PDT SAT SEP 5 2009
TODAY...RAIN AT TIMES THROUGH MID AFTERNOON. THEN MOSTLY CLOUDY WITH A CHANCE OF SHOWERS LATE IN THE AFTERNOON. HIGHS IN THE 60S. SOUTH WIND 5 TO 15 MPH.

However, there was a timing error...and the frontal moved through roughly 3-4 hours earlier than predicted. The result is that we have gotten into the post-frontal situation early...with sun breaks and only a few showers. The southern San Juans are in bright sunshine now and sun is blazing here in Seattle. But not for long. There is often enhanced clearing right behind the front, with some showers/clouds behind that clearing...and you will notice another band to our west. So don't expect a clear day--some of this cloudiness will move in during the next few hours. But the serious rain is over, sunbreaks will occur today, and I am heading outside to pressure wash part of my house in preparation for some painting. The NWS has even modified their forecast:

1030 AM PDT SAT SEP 5 2009

REST OF TODAY...SUN BREAKS AND SCATTERED SHOWERS. HIGHS IN THE 60S. SOUTH WIND 10 TO 15 MPH.

The lesson of all this? Timing errors can occur and you can enhance your ability to do things outside by watching the weather carefully. And my profession needs to communicate timing issues better--I suspect most people didn't realize the near certainty of a break this afternoon.

And I hope the new coastal weather radar will fix this kind of timing failure...providing information upstream that would have allowed a corrected forecast last night. I have a graduate student, Reid Wolcott, working on just this issue--developing approaches for using an upstream radar in numerical weather prediction. If his research pans out and the new radar gets installed, weather predictions may get a lot better around here.

The Weekend of Wet

Cancel your hike on Saturday. Get a good book (hint...a suggestion is on the upper right of this page). Don't water your garden tomorrow. The weekend of wetness is coming. And the end of any serious threat of wildfires over the Northwest.


But not tomorrow. A weak front is approaching right now (see image) and could bring some light rain to the coast, but only some sprinkles will hit over the interior.

But wait until Saturday, when a moderate front..with plenty of moisture... will reach us.
The rain from the front will approach the coast in the early morning hours and the western lowlands between 7 and 9 AM. The three hour rainfall ending 11 AM is shown below. The front will move through during the day, bringing rain to the Cascades and even some showers to the east of the crest. Take a look at the 24-h rainfall ending 5 PM below. Perhaps a 1/4 to 1/2 inch over the lowlands. Not a good day for a hike in the Olympics. You could get away with a hike on the lower eastern Cascade slopes if you go early.
But wait. The fun doesn't stop there. The front moves through Saturday evening, followed by a short break in the action. Then an upper trough moves in bringing more showers to the region on Sunday. Take a look at the precipitation for the 24-h ending 5 PM on Sunday below. Plenty of rain over the entire state..with particularly heavy rain on the SW side of the Olympics...roughly two more inches. And eastern Washington gets enough to wet things down.
And did I mention the winds? A surface low will accompany Sunday's trough...bringing windy conditions over the ocean and along the coast (see graphic). And 25-35 kt southeasterly winds will develop over NW Washington.
Time to find your rain jackets that have been buried in your closet. You will need it. And one more thing...watch the driving. It has been fairly dry and there is lots of oil on the road. Add water and it will be slippery.