29 December 2012

Ten Things You Should Know About Weather Forecasts

Posted by Dan Satterfield

Among those of us who forecast the weather for a living, one of the most frustrating complaints is a call or email asking why you busted the forecast. Imagine how much more frustrating it is when you actually hit it spot on! Let me start by saying that if a viewer mistakenly believes that I blew the forecast, then a large part of the blame should fall on me! I’ve long heard my fellow forecasters complain about this, and I am guilty as well, but to my fellow forecasters, I say look in the mirror before you blame someone for not paying attention!

That said, there will always be someone who hears what they want to hear, or blames me for someone else’s two-day old forecast (they heard second or third hand)!  So, with that in mind, here are some tips to better understand a weather forecast (and to keep the forecaster sane):

1. Percentages: I don’t like to use them often, but when you see a 30% chance of precipitation, it only means that there is a 1 in 3 chance that your rain gauge will get .01 inches of rain or more in that period. It says nothing about how long the rain or snow will last or how heavy it will be. It also says nothing about what kind of weather happened on this day in the past. I’ve written about this in detail here.

Snowfall forecasts are very difficult. If your local weathercaster is forecasting amounts more than 2 days ahead of the storm then you should change channels. Responsible forecasters will never do this.

2. Sell By Date: We can usually give a fairly accurate temperature forecast for three days. Beyond that it gets worse each day farther out you go. If I am within 2 degrees on the forecast for tomorrow, then it was an acceptable forecast. If I am within 4 degrees on a forecast at day 5, then it was also a decent forecast. If you call me to complain that I was off by 5 degrees on day 7 of the forecast, you will not get much sympathy.

3. Snowfall: Forecasting snow fall is very difficult. One reason is that the rain to snow ratio varies from 4 to 1 on up to 25 to 1 (the 10 inches of snow to 1 inch of rain ratio you’ve heard of is only an average)! If I forecasted a tenth of an inch of rain and we saw three tenths you would probably think it was a decent forecast. If that much rain fell as snow then the amounts could range from one to six inches! There are a ton of variables in a snow forecast and I’ll make it the subject of a coming blog post.

The rain/snow line in online or on air radar images can be off by 30 miles or more. Take this data as an educated guess.

4. RADAR: You really do not understand the weather radar imagery we show. Sometimes it takes me awhile to figure out what I’m really seeing! Just because you see rain on the radar does not mean it is raining at the surface. Also, it has become popular to show rain falling as snow in a different color, but this is just an estimate and the dividing line may be off by 20 to 30 miles. These images are produced using an algorithm that takes into account such things as surface temperatures, elevation, and in many cases upper level temperatures. They can be easily fooled!

The National Severe Storms Lab. in Norman,OK has a project called PING to improve this. With dual polarimetric radar it should be possible to do a much better job and you can help by downloading a smart phone app! Click the image below to help science!

5. Two Week Forecasts: Some online websites give you hourly temperature forecasts out to 15 days. These are computer generated and are worthless.

6. Snowstorm Minerva? I don’t care what name a cable weather network has given to some storm, and I also could care less about some TV index of severe weather they are playing up. If you have a question about it, ask them! I also have no idea why their forecast is different from mine, and it’s not something I’m losing sleep over either.

7. Email or Facebook for more info: If you need detailed information for a certain time period and place, ask us! I know I speak for most forecasters when I say that Email or Facebook is by far better than calling. Oh, and do not email your local forecaster to ask if the 2-4 inches of snow for the area includes your street. It does. Also, I cannot give you a forecast for your daughter’s wedding three months from now. Don’t laugh, I get at least two of those a year (see number 5). What we CAN do is give you the climatalogical averages for the day in question, including the record highs/lows and sunrise and sunset.

8. Is a Front Coming? A cold front does not always mean rain and it can rain a lot with out a front nearby. A front is the leading edge of a colder or warmer airmass. Nothing more.

Dan's pic. What's left of a trailer that was once on the blocks in the background.

9. Get A NOAA Radio: If we have a tornado warning in the middle of the night, the only people who will likely see the warnings are insomniacs and those who have a NOAA Weather Radio. Get one. I recommend the Midland WR 300 and the best price I have seen on one is here. This model allows you to turn off every alert except tornado warnings for your area.

10. Mobile Homes Are Bad Places In Tornadoes: When we say to abandon mobile homes during a tornado warning, we really mean it. The odds of you being injured are at least 20 times higher than in a regular house.