How does the NWS decide on the % for storms for each county?

[Alex Lamers]

There is a 90% chance of severe storms across central Iowa and a 90% chance for my county (Jasper). Does that mean I will get severe storms? I have seen a 100% chance of severe storms bust and every time the % is this high it busts more than verify.

How does the NWS decide if this county gets a 90% chance for severe storms or if that county gets a 10% chance etc? I see it bust more than verify when the % is this high for Iowa. I dont understand how this works.

The best severe weather days seem to be with low chances. May 7 of 2006 Eastern Iowa had around 17 tornadoes reported and the area had a 30% chance of storms, not severe just storms.

Well it has sort of been hashed out but I'll throw my 2 cents in here and hope I don't add to the confusion. When you see probabilities expressed as percentages on a local NWS forecast page those are merely the probabilities for measurable precipitation (defined as 1/100th of an inch of liquid) falling at that location for a specified period of time. Here is the exact definition I found by googling on weather.gov:

Is the likelihood, expressed as a percent, of a measurable precipitationevent (1/100th of an inch) at a grid point during the indicated valid period.

On a local forecast page and in zone forecasts (ZFP) these are almost always for a 12-hour period (i.e. Monday Night). The difference is when you click on the map, you are receiving the forecast for a small gridbox in the forecast area. The old ZFP, which still plays over NOAA Weather Radio IIRC, is the forecast for an entire county.

So if you saw a forecast for tonight, and there was a 30% chance of rain, then that would mean the forecaster is expecting there is about a 30% probability that you will receive measurable precipitation at your house.

As far as severe weather wording goes...if severe weather verbiage is added to the forecast, the little graphics at the top of the local forecast page will indeed say severe t'storms or something similar. However, the actual text displayed below is key. It should say something like Thunderstorms likely...some may be severe. Chance of thunderstorms 60 percent. Thus, the probabilities merely indicate the percentage chance of measurable precipitation. The forecaster is indicating that this precipitation will likely arrive in the form of thunderstorms, some of which may be severe.

The bottom line is that when you look at a local NWS forecast and the PoPs, you are just being informed on what they think the chance of your driveway getting wet is (more or less).

If you want a realistic look at the chance of severe weather occurring at your location, you would want to look at an SPC convective outlook. They aren't going to give you a probability of a severe thunderstorm at your house, but the risk levels should give you some idea. You can also check your local NWS office's Hazardous Weather Outlook (HWO), which usually details the severe weather risk for your local area.

And, like the others said, best way to learn is to dive in, make mistakes and ask lots of questions. It may seem overwhelming at first but eventually you'll get the hang of it! That Haby Hints page is quite the gem for beginners and those still learning.

AJL

 

[Tony Lyza]

See, learning in-depth relies on the dedication of the chaser... If one never plans on actually chasing, and would rather post on weather forums for other reasons, the dedication of learning how to forecast accurately by one's self is usually pretty low, at least from my expierence of being in the "chase world" for the past 4-5 years. I've seen people on the forum that have been registered for 1-3 years now, and have still yet to chase or learn anything about meteorology.

In 2005, I planned on spending $3,000 chasing for 4-8 weeks (ended up taking two seperate long-term trips that accrued a total of about six weeks). I had been significantly underaged chasing in MI for the prior couple of years, and had to break out of here in a big way. In 2005, I had went from the knowing the absolute basics of severe weather meteorology to being enthused with learning advanced areas of meteorology within months. I had pushed myself to the fullest in order to accurately forecast, since it was my own hard-earned money that was being thrown on the table to chase with. If one doesn't have that ambition to successfully chase, then they likely won't bother learning anything about meteorology. They'd rather polute the TA with fluff that us chasers have to sift through with a 1-bar WiFi connection at some hotel we caught a connection with in the middle of Kansas. We know the SPC just issued a MDT risk for the area, and that GRLevelIII indicates 2.25" inch hail, and that a tornado warning was just issued... Learn something, and add something useful...

It's easy to learn... Just push yourself.

Here here.

Having to fund your own chasing really makes you need to understand what is going on so that you are throwing money down the drain. For instance, my team and I started chasing two years ago. We fund ourselves, and we really don't have the money to go out on the plains right now (plus, with drive a '91 Plymouth Acclaim that has given us good reason to be nervous taking it on long trips). However, we have taken it into IL on several occasions (great country when photogenic storms come about, which hasn't been much this year ). Last year, we had little success aside from a fairly hair-raising experience on 9/22 between KAAA and KSPI. After understanding what we did wrong, learning how to correct it, and performing further study, I can say we have been much more successful in two chases this year than we were in six chases last year. Our greater understanding of what is going on has immensely helped us with choosing our locations and getting into position for the best shots we could get (and, trust me, we got about as good as anyone could have gotten in NW IL on 6/7 ).

Matt, don't hate learning. Learn to love it. Learning opens doors to opprotunities you never knew existed. It's alright to be nervous. I'm a little nervous about the likely prospect of me going to OU in Fall '08 and being faced with all the math and physics. But I'm also incredibly enthused and excited because I know that I will learn the most I possibly can while I'm there. Be excited to learn. If you really want to chase and forecast, you'll learn how if you give it the time and effort.

 

[MatthewCarman]

Thanks everyone I realy do need to learn more. First I made a forcast for thursday and assumed NE would not get anything based on early forecasts byt the SPC outlook and NWS. I screwed up. I never looked for myself to see what could happen I just assumed that was not the place to be. While only a few tornadoes reported I was not expcting any and made myself look bad in the forecasting forum. So yeah I do need to learn to forecast for myself instead of relying mostly on NWS/SPC and Stormtrack. I do have a basic understanding of forecasting and what to look for. I just dont know how to look for it. I will learn because I want to and need to know these things before I say something like that again.

As for % go thanks Alex for explaining that to me. It just seems like storms are always so intense over Des Moines and then I get the "leftovers". We get mabey one good severe storm to move through my county each year. Every slight risk so far this year for me has busted besides Thursday and I can only confirm a few hail stones made it to penny size. No branches snapped so winds were only in the 50 MPH range. It is sad and dissapointing.

To answer someone calling me a storm chaser no I am not. Let me be clear and honest with you all now. I have never chased before in my life. I plan to in a few years when I have a more stable job but at the moment I dont chase. I am far from a profesional chaser or someone who is very experiencced or knowledgeable. I knore more than the average person and have several books on weather but mostly for the basics or extreme weather. I got into Stormtrack through a crack. I sent in my info about me 3 times (Yes I did fill out the form and told the staff about myself) and only became a member when they were letting new people join for a short period of time. I joined because like all of you I LOVE the weather and cant get enough of it. When severe weather strikes I will be here reading the NOW threads. I do not make weather my life. I enjoy doing other things and try to balance weather into my life. I have been into weather my whole life. I would love to chase but I dont feel the urge to chase every slight risk. I am not like crazy over storm chasing but at the same time when severe weather is ongoing I cant take my eyes off the radar. I watch Twister a dozen times a year to lol So I feel I have a right to be here. Plus I donated to the Stormsof2006 DvD so I have contributed in my own way being a member. Let's just say I am still learning and have a long ways to go. Math is a horrible subject for me and I am a slow learner. Not the smartest guy on the block.

I do know what to look for in a storm and where to be when I am on the storm thanks to being a member of this site. I know my first post in this topic was a rant. I felt like venting. I was mad the storms producing destructive 60-70 MPH winds crapped out and like always I got nothing more than rain. I get stressed out if I dont find some way to vent and letting my feelings out is the best way of doing this.

I do apreciate the links Nick Grillo and others have posted and will check them out because I want to. Nick that is asome that you pushed yourself to the limit and learned alot about weather in a year. Mabey I need to do that.

Thanks for all of the reply's. Your friend MatthewCarman.