Saturday, September 5, 2009

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.

21 comments:

Bham_Guy said...

We didn't get much rain up here in B'ham from this morning's front; however, we have been getting some decent wind gusts. The 10:53am report from BLI showed gusts to 37 MPH. Not bad for early September.

Josh said...

"It would be far more honest to communicate the inherent uncertainties in weather prediction, providing ranges and probabilities of potential outcomes"

Mass, Cliff. 2008 The Weather of the Pacific Northwest

Thursdays post could have had some of those potential outcomes.

Must read blogs said...

i looked out my window and saw the rain on the other side of i-5 east of chehalis and it never crossed i-5 like it normally would, it just stayed there, like there was some sort of pressure gradiant steering it away from here. Cliff, why is that?

Paul said...

The front passed Copalis Beach at 0745 marked by a sharp upswing in barometric pressure (3 mb in an hour) and a wind gust of 27 kts. Wind shifted from the East to the South. Rain started about 0100 and was intermittant through the night(accululation 0.14 in)

Lots of blue sky now (1430) and a brisk wind out of the south. (10 kts)

Paul Middents

CVM said...

It was a heck of a lot warmer than it was predicted to be in Bham. I left the house at 11:00 and it was already 66 degrees. In fact, as of 11:30 am, many were walking around in t-shirts at Boulevard Park. Windy as heck, but pleasant.

Computer models are getting better, but don't count on it to get everything 100% right.

bhd said...

Very frustrated by weather reports that don't apply to the Quimper Peninsula up here in Port Townsend. Can you cite a decent source?

Gary in Olympia said...

BHD,

A Canadian astronomy fan has a site which uses cloud/weather maps from Canada's Met system to develop hourly forecasts over a 48-hour period. Here is the Port Twnsd site:

http://cleardarksky.com/c/PrtTwnsdWAkey.html?1

I have found it a very good tool; particularly for scheduling outside garden work. Early Friday morning, he had the SAT PM clear sky on the forecast. Early SAT, he had moved up the timing and stretched the duration.

There are about 100 locations in WA State alone. 2500 & some in North America.

All of Cliff's blog readers should be able to find many nearby sites. Start with the Map links in the upper left corner.

Gary

Fleetwood said...

Seems like an error. Saturday the forecast was Rain but for the most part it was nice day. And today it is supposed to be Showers but we are getting dumped on in a driving continuous downpour.

SeaGary said...

Amazing downpours in Copalis Beach this early Sunday afternoon. Quite a ferocity of rain and wind for early September. Periods of thunder and lightning overnight and into today. It seems to me the models you posted on Sept 3rd, at least for this part of the coast were accurate.

Must read blogs said...

it has been pouring here in chehalis off and on all day. this am though its was an absolute downpour and i heard one rumbel of thunder.

natchrl8r said...

Still no rain or storms in Bellingham. I wish I'd planned more outdoor activity because there's been quite a bit of sun between yesterday and today and the moon was quite exposed last night. The wind has done a job on the trees though and the driveway is littered with ripe plums. Canning season is here!

athos said...

Man I really hate the rain shadow. I can see it in action: out of the west side of my house, sunshine. Out the east side, darkness, thunder, lightning, and heavy rain.

Tom said...

Scared off by the forecast. Wish I took my sailboat out yesterday!

32.5 East said...

Forecast error?

Well, I guess it depends on one's definitions of "forecast" and "error".

C.P.O. said...

I appreciate Cliff's openness regarding the issues of error and timing. The fact is that predicting this stuff is always going to involve some potential for error and you just have to roll with the punches and be willing to make quick adjustments when the forecasts don't work out.

mainstreeter said...

1.54 inches of rain at Olympia airport, Sunday. New record.

Gary in Olympia said...

The Community Collaborative for Rain, Snow, & Hail at CO SU produces maps and tables of precip daily.

There are already 180+ reports for WA up to 10AM. You can view the state and county rain reports at:

http://www.cocorahs.org/Maps/ViewMap.aspx

You can see on the WA map that there was a heavy precip band from Thurston Cty into SW King. The timing was different though as the 1.55" near Lakeland in King reported the heaviest shower at 06:30PM. Some areas of Thurston County had their heaviest shower around 10:30AM. Oly AP got 0.35" between 10 & 11 AM.

natchrl8r said...

Bellingham precipitation, .64 inches in the last 72 hours. Definitely a false alarm. Seattle gets all the thunder. :(

Must read blogs said...

nwcn reports that enumclaw got the tornado,

Robby said...

waiting for weather is my life to start popping off...

Entome0 said...

Hmmmm,
Did Cancel my hike in the Cascades, but went Camping on Whidbey. The weather turned out better than predicted and we had a wonderful time.
Here is the link to the only Rain that I saw on Saturday:

http://picasaweb.google.com/lh/photo/pzBbro7TpP_DIfI4Jz3sbA?authkey=Gv1sRgCOup89jjk_jngAE&feat=directlink