Forecasting models and POPs

chumley · Post by **chumley** » Sep 20 2010 9:15 am

OK, a little morning frustration today. How is it that forecasting computers (and their human counterparts) can be so wrong sometimes?

POPs (Probability of Precipitation) are measured in percentages. How on a day when the forecast calls for a 0% chance of precip can it be raining on my way in to work?

I think I get more frustrated with the lack of updates to a forecast ... it still says 0% ... even though there are clearly rain showers out there!

This happens during monsoon storms, and especially during winter storms, in the opposite way. After a winter storm clears out, and the cold front clears things out, the forecast still calls for a 30% chance of lingering showers or something like that. Anybody who has lived here for more than two winters knows that once the front passes there will be a 0% chance after that (at least in the valley). I don't care what the stupid computer says. Sometimes looking out the window is better than looking at a computer screen.

And that's my Monday morning rant. Happy autumnal equinox (Wed 8:09p) everybody!

PLC92084 · Post by **PLC92084** » Sep 20 2010 9:29 am

Who, besides "Weather People", can be wrong so often and still keep their jobs!?

chumley · Post by **chumley** » Sep 20 2010 9:35 am

Presidents, Senators, Congressmen, Cabinet members, Governors, Sheriffs, Judges, teachers with tenure, union employees ... need I go on?

joebartels · Post by **joebartels** » Sep 20 2010 9:42 am

Jim, I mean Todd :STP: , it's a forecast not an exactcast. You guys are looking for variables in nature to not be generated by a random figure.

Personally I'd trust a meteorologist over the majority of Presidents, Senators, Congressmen, Cabinet members, Governors, Sheriffs, Judges, teachers with tenure or unions.

PaleoRob · Post by **PaleoRob** » Sep 20 2010 10:13 am

joe bartels wrote:teachers with tenure

Don't have to worry about them any more - Az axed them with SB 2011. No tenure for all state teachers, no matter their actual years in service.

chumley · Post by **chumley** » Sep 20 2010 10:19 am

joe bartels wrote:Jim, I mean Todd :STP: ,

Funny, except I like cold weather, clouds and rain. And Flagstaff.

joebartels · Post by **joebartels** » Sep 20 2010 10:41 am

chumley wrote:And Flagstaff

we're gonna need a bigger laughing icon
Seriously though I hope he gets to live in a place he enjoys.

chumley wrote:except I like cold weather

you're just sick in the head

Jim · Post by **Jim** » Sep 20 2010 12:48 pm

Oh, you two. When I read this I thought, "whoa, complaining about a meterologuist's obvious mistake is my job".

I actually am loving, or liking Flag right now. Probably because I can see the end coming up. Plus, the weather since I got back from Utah has been perfect. Just about 80 and sunny everyday. Great! I get what you're complaining about. I really hate it when they still say 6 to 10 more inches of snow when the storm is obviously past.

Add police to that list, too.

paulhubbard · Post by **paulhubbard** » Sep 20 2010 2:15 pm

The way the precip percentage is calculated is really rather simple; During all the days past (since record keeping began) when the weather was exactly or very similar to what the weather will be today, it rained on XX% of those days.

So if the weather has been just like today 100 times in the past and it never rained before on those days, there is a 0% chance of it today.

Rocket Science it ain't. And how about those Major League ball players?? If they do their job 30% of the time on average (300 batting average), they're considered a super-star??

Alston_Neal · Post by **Alston_Neal** » Sep 20 2010 2:34 pm

Wait a sec, you're all missing the core problem here...

chumley wrote: How on a day when the forecast calls for a 0% chance of precip can it be raining on my way in to work?

It rained and someone didn't share.

paulhubbard · Post by **paulhubbard** » Sep 20 2010 2:38 pm

So if the records go back 1,000 days, the next time the weather is just like today the chance of precip will be .1%

BobP · Post by **BobP** » Sep 20 2010 2:42 pm

Don't piss on my shoes and tell me its raining ;) . my favorite nypd blue quote

chumley · Post by **chumley** » Sep 21 2010 10:37 am

So here's the forecast for tonight. "70% chance of rain. 1/2" to 3/4" possible."

Ok, so how is it that we're going to get at least 1/2 an inch of rain, when there's a 30% chance it doesn't rain at all?

Wouldn't the forecast HAVE to be for ZERO" to 3/4" of rain? I mean, if its actually 1/2" to 3/4" then there has to be a 100% chance of rain! How else do you get to 1/2"?

There's two pieces of data in the same sentence that don't logically agree with each other!

BobP · Post by **BobP** » Sep 21 2010 10:42 am

chumley wrote:1/2" to 3/4" possible

ya gotta ASSUME thats the high-end ;) but I'm guessing no more then .314159265 on an inch.

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