Tuesday, April 13, 2010
Quite a few of you have emailed me asking about the summer forecast. What does one expect in an El Nino year? Is there any useful guidance for planning that summer outing or party?
Let me begin by being honest: forecast skill for summer made this far out has only marginal skill. Now that won't stop me from telling you what the official and unofficial forecasts are, but please don't depend on them for anything critical.
And keep in mind that summer generally start around here a week or two after the July 4th weekend. The inside joke in the local weather community is that summer starts on July 12th, and this is often pretty close. It is true that we are in an El Nino pattern right now (warmer than normal water in the tropical Pacific), but it is only a moderate one and our forecasting tools suggest that it will weaken rapidly this summer. Besides, El Nino does not correlate that well with our summer weather.
The National Weather Service's Climate Prediction Center provided the following forecast for this summer (June, July, August) temperature and precipitation. To create the forecast, NWS forecasters use many tools: El Nino/La Nina, historical trends, an ensemble of long-range weather forecasts, and much more. You note that they are going for above-normal temperatures over the western U.S. (Canada is white because they don't forecast there!) and below normal precipitation over Oregon and Washington. Good tomato weather.
Just looking at the influence of El Nino one finds very little signal. Here are plots of temperature and precipitation anomalies (differences from climatology) for El Nino years. All white. Very little signal.
As noted earlier, long-range forecasts only have marginal skill. Want proof?
Here is the temperature forecast from the ensemble of computer forecasts for last summer's temperature made in early May. Reality was much warmer than normal over the NW, while the models missed it completely.Now there is always the Farmer's Almanac, found in better supermarkets everywhere. Here is their forecast for this summer:
Summer will be drier than normal, with below-normal temperatures, on average, in Washington and above-normal temperatures in California and Oregon. The hottest periods will occur in late June and mid-July.
Dr. Nick Bond of NOAA and UW JISAO studied the Farmer's Almanac's skill...it didn't have any.
Anyway, I wish I could give you better guidance for this summer, but I can't. Oh, I almost forgot. When is the absolutely best time to plan than special outdoor occasion? The driest period of the year? The last few days of July. I love having a "dry-sky" party on July 30th. Precipitation has only fallen on about 7% of the time on such days.
Posted by Cliff Mass at 7:26 PM