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

[MatthewCarman]

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.

 

[mike scantlin]

"every time the % is this high it busts more than verify."


thats completely untrue and sounds like my grandma.




lets be logical. counties are big. you are not. its the likelyhood of whatever happening in any part of that county. they probably dont even divide it up by county. more than likely, they do it by area. its meteorology. its not an exact thing. it suprises me to hear a "storm chaser" say something like that.

 

[MatthewCarman]

"every time the % is this high it busts more than verify."


thats completely untrue and sounds like my grandma.




lets be logical. counties are big. you are not. its the likelyhood of whatever happening in any part of that county. they probably dont even divide it up by county. more than likely, they do it by area. its meteorology. its not an exact thing. it suprises me to hear a "storm chaser" say something like that.

I am sure they have this up for a whole area or divide it up for big areas but many times I have seen a % that high bust for a entire area. I am just trying to understand what is needed for a % that high or how the % system works etc. I want a better understanding of why they do this.

I know my thinking is wrong but I just get so annoyed when I have a 100% chance for a severe storm and dont see anything. (This has nothing to do with yesterdays 50-60% chance that verified)

 

[Tony Perkins]

What is the web page where it says there a "xxx% chance of Severe Storms"? I don't recall having seen that one before.

 

[MatthewCarman]

What is the web page where it says there a "xxx% chance of Severe Storms"? I don't recall having seen that one before.

It is the local forecast page at the noaa site. I guess I should have checked there before asking here but I had a brain fart. I will check the NWS site but any links here would be apreciated.

 

[Chris Vagasky]

I believe he is talking about this.

http://forecast.weather.gov/MapClick.php?site=DMX&map.x=197&map.y=174

It says for tonight:

90% chance of precipitation, with the possibility for severe storms

 

[nickgrillo]

Well, it's best to learn how to forecast yourself than depend on an NWS percentage... I'd have to suggest this link over any other:

http://www.theweatherprediction.com/habyhints/

I'd have to think being a member of ST for over a year that one would learn more about severe weather meteorology... Certainly am not intending to write this as a pun, I'm just saying... It is pretty easy to learn the basics of meteorology, just crack a book.

 

[Andrew Pritchard]

90% chance for thunderstorms = 10% chance for NO thunderstorms.

10% > 0%

They don't do it by county. They create the forecasts by area by dragging and highlighting the areas that they want. It isn't even a 90% "chance" either, it means they are expecting an approx. 90% COVERAGE of thunderstorms or rain or whatever they are calling for. Even at that, nothing is given so being upset because you didn't get rain doesn't make sense. As others have said, I've never understood why people only rely on the NWS or SPC. Whenever I see "Well, the SPC says this, so I'll be chasing" or "Well, the SPC is negative on the new outlook so I think I will stay home" I cringe. Especially on a storm chasing forum, from a storm chaser's forecast. I understand not everyone is as gungho about forecasting as myself or others, but I get the feeling some people have no understanding of forecasting basics sometimes. That or today's storm chasers are just lazy and would rather have someone tell them where to go. Those are the people who get made at the SPC when they don't see a tornado on a high risk day. They all have access to the EXACT same models and current data that you do. Just because they are paid more to forecast doesn't mean they have fairies tell them what will happen, and that those fairies are wrong sometimes to make you mad. Go over the stuff for yourself and decide if YOU think you will get rain or not.

 

[MatthewCarman]

Thanks Nick I will check those links out.

To be honest I rely on the NWS and NOAA for all of my forecasts. I know I should use the models and graphs etc but that part is always confusing for me. I need things as simple as possible or I get a headache or just get confused.

Do you guys know any other good sites for forecasting or weather outlooks?

 

[nickgrillo]

Thanks Nick I will check those links out.

To be honest I rely on the NWS and NOAA for all of my forecasts. I know I should use the models and graphs etc but that part is always confusing for me. I need things as simple as possible or I get a headache or just get confused.

Do you guys know any other good sites for forecasting or weather outlooks?

Jeff Haby's Hints is pretty much all you truely need to start out with... Just start learning about the basics about thermodynamics and kinematics, and then take it from there. You'll learn that if you ever want to begin chasing, you'll need to learn how to forecast yourself, and not rely on other's forecasts to pick your target, particularly graphics that depict synoptic-scale risk zones (i.e. convective outlooks issued by SPC).

 

[Robert Dewey]

Take it from Nick... he went from knowing not much to being a met without a degree in under 12 months.

Seriously though, Nick is right. If you plan on storm chasing, you really need to forecast for yourself. NOAA is good for getting a general overview, but you really need to buckle down and pick your own targets or build your own forecast.

 

[Tony Lyza]

Jeff Haby's Hints is pretty much all you truely need to start out with... Just start learning about the basics about thermodynamics and kinematics, and then take it from there. You'll learn that if you ever want to begin chasing, you'll need to learn how to forecast yourself, and not rely on other's forecasts to pick your target, particularly graphics that depict synoptic-scale risk zones (i.e. convective outlooks issued by SPC).

This post is dead-on 100%.

Matt, it takes time. You're not going to learn it all in one day. Most of those who forecast, even amateurs, take years to get to where they are. And if you're chasing, every bit of both synoptic scale and mesoscale knowledge you have will be nothing short of essential. Take my experience on 6/7. With only having a year of experience under my belt, my teams' goal was to chase in IL, where the forests and hills are more managable than WI. Therefore, we had to decide where to chase. Looking at the Davenport 18z sounding, we decided to target from De Kalb on nwwrd because the cap was fairly strong farther south and likely wouldn't break until after dark. We got to De Kalb, gathered data, moved to Dixon, decided we were a bit too far south still, and moved to Freeport. From there, we were in PERFECT position to intercept two supercells that were crossing the river into Carroll and Jo Daviess Counties. We positioned on a bluff outside Cedarville and captured a great view of two supercells as they passed nearby. The first one produced a fairly well-defined wall cloud (by 6/7 standards), and we actually got right underneath the second one and observed a weak anticyclonic funnel at the edge of it. A very rewarding chase, considering how that day evolved with the higher-than-I-expected-which-shows-I-have-more-to-learn-about-boundary-layer-mixing LCLs helping to create cool and wet RFDs that severely hampered the tornado potential, that would have been impossible if not for a basic understanding of the atmosphere and its processes.

Just start by browsing through the Haby Hints topics. For some, it's easy to learn the ropes. For others, it takes some time. It's well worth the effort. Trust me.

 

[Doug Lee]

I agree with what Nick and Tony said, and to add to it some. Don't just study meteorology and forecasting. Get some foundational knowledge of math and physics. You don't have to get too involved with it, though. How much you need depends on what you already know, and how far you want to go.

In math, an understanding of basic algebra should be enough, especially an understanding of graphs and how to manipulate equations. Beyond that, you might want to learn some about statistics. Your original question actually deals with statistics because the percent chance of storms is found by statistical methods.

In physics, I'd recommend learning about motion and dynamics, gravity, electromagnetism, and thermodynamics. Check with your local community college to see if they offer a course in general physics or general physical science. That would also be a good place to check on math courses as well. Some community colleges even offer a Intro to Meteorolgy course.

Don't forget your local library. You can easily get a card and check out books and other materials about weather. I've heard that reading the printed page is easier on the eyes than reading a computer screen.

Remember the important thing is to keep learning. Isn't that one of the reasons to go storm chasing in the first place?

 

[Derek Weston]

My thoughts on the SPC, and the new/learning forecaster....

As I'm sure a lot of you know, many folks chief interest in storm chasing is actually seeing the storm, and not much else. For many, it's a hobby, and in-depth forecasting isn't their interest. They'll take whatever they can get; they'll take the path of least resistence where finding a storm is concerned. (yes, of course more forecasting knowledge would help them to better achieve their chasing goals)

The folks at the SPC do represent some rather quality free information offered up to the public at large. And depending on your forecasting ability, they can certainly point out a thing or two you might not have been seeing. To this extent, I do believe they are a good enabler for the 'casual' chaser. (we're probably getting folks out there that might not have been out before such information was public)

At this point in my 'chasing career' I've only been attempting to forecast -- and chase, for that matter -- since spring 07, and I certainly find the SPC forecasts and discussions to be quite useful. While I certainly hope to keep learning and developing my forecasting skills -- though, this is just a hobby for me -- I'm still an opportunist and will take whatever I can get to get me closer to 'the storm.'

Mathew, a few words from a fellow newbie to the game:
1) Stop being intimidated. You've got to dive in. You're on a mission to understand something now. . . view it as exciting journy of discovery. (ok -- that was cheesy, but, point stands)
2) Don't become overwhelmed thinking of all the information you must learn. Learn one area, one subject at a time. It's not too hard. You don't need any advanced degree in anything to learn basic forecasting. Just focus on understanding one thing at a time. (For example, if you're trying to understand a forecast model such as the GFS, learn what the different symbols and measurements stand for. Just basic stuff. Honest to god, I didn't know what a wind-barb was until half a year ago... but it's not that hard to understand once you look it up, right?)
3) Ask "why?"... think "why?" You did that in this thread. Now do that in your quest in better understand and forecasting severe weather. When you see folks talking about this forecast, or that storm, or this weather-scenario, don't just mentally nod along, but rather, try to understand the significance of it. For example: you read a couple chasers throwing around the term "helicity" when speaking of a possible upcoming severe event -- go google it! Find out what it means. After doing so, you'll have gained another piece of knowledge. When I started reading this board I couldn't understand half of the crap folks were discussing. . . Not that I'm anything but a novice yet, but I've made some nice gains in this chasing season thanks to just a bit of reading and research. It's really not too hard -- give it a shot, it's quite rewarding.

 

[nickgrillo]

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.