Forecast Evaluation

Boy, blow one historic blizzard forecast and people get all cranky*.  Except, as H. Michael Mogil discusses, it was an almost perfect forecast.  For the specifics of that storm and its forecast, I refer you to Mogil's article.

I'm going to take up the more narrow topic of forecast evaluation.  (Disclosure: I do work for NOAA/NWS, but, as always, this blog presents my thoughts alone.  Not least here, because I agree more with Mogil than the head of the NWS, Louis Uccellinni, about this forecast.)  One school of forecast (or model) evaluation looks at computing large scale statistics.  The most famous one for global atmospheric models is the 5 day, 500 millibar (halfway up the atmosphere), wave number 1-20 (large scale patterns), anomaly correlation.  When people refer to the ECMWF model (or 'Euro') being better than the NWS's model (GFS), this is usually the number that is being compared.  But I don't live halfway up the atmosphere, nor do most of you.  We're somewhere near the bottom of the atmosphere.  And there is much more of interest than just average temperature through a layer of the atmosphere.  So there are many other scores (dozens of them) -- See http://www.emc.ncep.noaa.gov/gmb/STATS_vsdb/ for some examples and discussion of what the scores mean.

Most of those scores, though, don't get to my personal -- weather forecast consumer -- interest.  Namely, I'm trying to make a decision of some kind.  NYC, which heard a forecast of 24" (60 cm) but got 9" (22 cm), presumably made decisions that they wouldn't have if they'd heard the perfect forecast that hindsight now provides.  It's here, I think, that we get to the meat of forecast evaluation.  Had this same error been made over the ocean, rather than over the most populated city in the US, with the rest being as it happened, the NWS would be getting praised for their great forecast.  The important part was not difference between reality and forecast, but number of people who made the wrong (in hindsight) decisions.

So let's explore evaluating forecasts by way of our decisions.  I don't make decisions for major metropolitan areas, and not about street plowing and so forth, so will leave that aside.  One realm of weather-affected decisions is in my running.  Let's ignore summer decisions (I'd as soon avoid thinking about what summers are like here) and go with the path as temperatures drop.  Normal gear -- in pleasant weather conditions, is t-shirt and shorts.  Once it cools below 60 F (16 C), I pull on a pair of gloves for my run.

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Hurricane Control?

Reader Bayesian Bouffant raised the question of whether we might be able to take some of the strength out of hurricanes, if not to really control them -- by way of an article Massive offshore turbine arrays would help us harness hurricanes.  The article is more positive about chances for weather modification than I, but let's take a look at some details and issues.

By way of background, hurricanes are heat engines.  The heat source is partly warm oceans, but mostly the latent heat released when water vapor condenses in the atmosphere.  The source of that water vapor is, again, warm oceans -- but by way of evaporating water vapor from the ocean surface in to the atmosphere.  When the water condenses, it releases heat, which drives the circulation, and makes for clouds.  The rising air draws more air in, again along the ocean surface. 

Over the years, there have been many suggestions for trying to weaken hurricanes.  One of the least reasonable is to throw a nuclear bomb at the hurricane.  Somehow this is supposed to 'disrupt' the hurricane.  But, hurricanes are heat engines, and nuclear bombs supply heat.  They also don't really have much energy compared to a hurricane!  In one day, a hurricane releases about 52*10^18 Joules (see Christopher Landsea's estimation of the wattage of a hurricane).  The Hiroshima bomb released about 67*10^12 Joules.  Roundly speaking, 1 day of a hurricane is a million Hiroshima bombs.

Since frontal assault is pretty well doomed by the fact that hurricanes are vastly, overwhelmingly, more energetic than anything humans have to wield, any attempt to control, or affect in any meaningful way, has relied on more indirect means.

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Atlantic Hurricane Season opens in US

Today is the official opening of hurricane season in the US, at least for Atlantic hurricanes.  NOAA's seasonal outlook (note that the term is 'outlook', not 'forecast') is for more hurricanes than usual.  But do read down in to the discussion of the science behind the outlook.

Also, of course be prepared if you live anywhere in the Caribbean, Central America, Mexico (east or west coast), Gulf Coast, or US East Coast.  As folks got reminded last fall with Sandy, this means all the way up the coast, not just southerly places.

Do read through the full information there, and follow up for more information.  If you've got kids, perhaps ready.gov/kids will be helpful.

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In the weekend heat

Sometimes it's easy to take the weather personally.  Yesterday, DC set its record for consecutive days 95 F or above (35 C), with 9, and will likely obliterate it by going to 11 straight days, adding in today and tomorrow.  The record it breaks, 8, was first set in 1987 -- the first summer I was here.  It was tied twice more, first in 1993 -- my third summer.  At the time, the summer of 1987 set many of the all-time records for sustained heat.  Probably several of the summers since then would have as well, if 1987 hadn't beat them to the punch.  Rephrase: many of the past 25 years would have set sustained heat records if compared only to the observations from 1880-1980.  What used to be unheard of is now 'normal', or at least common.  I'll be pulling down the data once NCDC is working again.

In the midst of the heat, well, we're hot here, and thunderstorms, I'll remind folks of the fact that you shouldn't run in thunderstorms.  That also applies to biking and walking and other outdoor activities.  If it's only heat you're dealing with, remember to drink enough fluids.  And, one trick for keeping cool while exercising is to dump some cool/cold water on your head.  Preferably to get a hat wet (it soaks up more water than my ever-thinning hair).  For more extreme cases, some ice cubes under your hat.

While my power came back after about 20 hours, friends didn't regain it until Tuesday (~90 hours) or Thursday (~140 hours).  Again, this is for wealthy, and densely populated, areas of a wealthy country.  West Virginia has not been faring as 'well'.  See this also.  Noticed in passing (unfortunately I don't remember exact source, quite possibly a Capital Weather Gang tweet*)  was that US average was 214 minutes of power failure per year.  Unfortunately, even the better parts of this area are likely far in excess of that.  Certainly my 20 hours this time are not balanced off with 6 previous years of 0.  In other wealthy countries, it's order 20 minutes per year.  In other words, US average is 10 times worse than other countries, and the Capitol area is closer to 100 times.  This is not the first time we've lost power this year, and won't be the last.

* My apologies to the original source if it wasn't this.  In any case, Capital Weather Gang is well worth reading, and has a blog as well.

At the same time as we were obliterating our record for sustained extreme heat, a friend bragged that Phoenix, Arizona, had its coldest 4th of July in 100 years -- 76 F (25 C).  We were 20 F hotter.  A different friend commented that he'd escaped the DC heat by going to the Bahamas.  Summer in the Caribbean to escape the heat ...

Adapted to the weather?

I'm going to do some complaining about the recent power failures from Chicago through the Washington DC area, and there will be some relevance to talk about climate change adaptation.  But first, a few words about my background to be making complaints.  I'm not someone who has never been without electricity for an extended period before, nor, for that matter, without running water.  I've hand-pumped my water for some weeks.  And I carried it a couple hundred meters in buckets to water my grandmother's recent plantings in the hot and dry, by the standards then, summers of 1987-1988 in the midwest. While I like modern technologies, not least because it is why I reached age 10, I know how to live ok on a late 19th century level, and have done so.

In discussions about climate change, I hear that "Don't worry, we'll adapt to it." and
"Mitigating climate change means returning to 19th century technology."

I'll leave aside whether the recent derecho represents climate change.  And even more so the question of whether the change has a human fingerprint on it.  I live in the national Capitol area for what is supposed to be the richest and most technologically advanced country on the earth.  And many, large population, counties around me are among the wealthiest in the country.  If any area should be well-adapted to current weather, forget to climate change, it is this area.  Let's just consider events already in hand.

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25 years ago this summer

I realized today that 25 years ago was my first summer in Washington DC area.  At the time, that summer set many of the local records for hottest summer, longest streaks of hot days, and many variations on that.  I've asked a friend who is local and extremely in to weather observation* how recent summers would compare, and he said, with little hesitation, 'hotter'.  What used to be record-setting is now not very noticeable.

I made the challenge/request on my twitter feed (http://twitter.com/#!/rgrumbine), but for here: I'd be interested in seeing a widget+ of some kind that would track this year versus 1987 in DC and Chicago.  Plus, of course, any other cities you're interested in.  1987 was, at the time, an extremely hot year.  For another comparison, say summer 1981?  1982 was part of the major El Niño of 82-83, so perhaps would not be a good year to reference.

For keeping an eye on weather by way of twitter, I like the Capital Weather Gang for Washington DC, and Tom Skilling for Chicago.  I'm sure there are a bunch of other good sites even for those two cities.  I just have personal connections to those two.  I was secretary to the local AMS chapter some eons back when Skilling was the chapter president.

For a mailing list to discuss weather on, send an email with the phrase 'subscribe wx-talk' to LISTSERV@LISTSERV.ILLINOIS.EDU Mailing lists, I'll add, are not web pages, or web sites, or chat rooms.  Different thing, with different strengths.

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Three feet of global warming

Now that that idiotic line is out of the way, let's do some thinking about what we should expect from climate change. I've been calling it climate change, rather than 'global warming', since at least 1993. A major reason being that there is more to climate than temperature, and changing temperatures affect more than just how hot it gets.

One of the things that temperature affects is how much water can be in the atmosphere. The hotter it is, the more water vapor you can have in the air before it starts to form a cloud. So one very simple expectation that we could have on climate is that warmer = more humid (absolute humidity that is). Since there's more than temperature to climate, we don't really expect it'll work out that simply everywhere, all the time. But it tells us one line of research to take -- look to see what has been happening to atmospheric moisture content.

Jeff Masters has written this up at his blog on the Weather Underground, and I'm relying partly on his notes in my write up.

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Snowflakes

Even if you're not around here (Washington DC area), I gather several folks have heard about our weather.  Having been reminded by JG that not everybody has seen snow, I caught some video of the flakes in action.  And being reminded of Nakaya's work on ice crystal types, I have a still that attempts to catch a few snow crystals.

Snow crystals on my coat sleeve:

Be sure to click on that to see full size.  One thing to notice, aside from the fact that many have already melted, is that most are little needles.  Some of the needles have clumped together, like the one near the loose thread in the middle of the photo.  If you look carefully, you can see that the needles have little balls on their ends. The little balls are liquid water cloud droplets that froze instantly onto the needles as the needles fell through the cloud.  It's called riming, even as it happens to a single snow crystal. (Riming is a serious hazard for aircraft taking off or landing in near-freezing conditions.)

Morning snow (small snowflakes):


Afternoon snow (bigger snowflakes):


Enjoy.

Doing science, with sea ice

Every so often, I commit an act of science. Like most acts of science, you almost certainly never heard about it. Like many, however, life was eventually improved for some people somewhere. I'm rather pleased about that side of it.

What was at hand was, on one hand (it does help to have many hands if you're in science), a fairly straightforward piece of engineering. On the other hand, a bit of science. Remember that I think both engineering and science are good things, if different. Engineering is mainly aimed at 'apply what is known to achieve benefit for someone', while science is aimed at 'try to understand more about the universe'.

Back in 1993, I was at the National Meteorological Center (NMC), the part of the National Weather Service (in US -- NOAA) that develops the new weather forecast models or tries to make the old ones better. My area was sea ice. Now, one thing we sea ice, polar oceanography, polar meteorology people were entirely confident about was that sea ice mattered, a lot. For, well, everything, or at least enough. If we didn't think it mattered, we'd hardly be spending our time studying it. People outside our little community, including folks working on numerical weather prediction, didn't think sea ice mattered for much. And, if it did matter, surely it was only something that mattered for long time modeling -- climate scale forecasting. Surely the ice was already well enough represented to be good enough for weather prediction purposes.

Partisan as I was, and am, in favor of sea ice, I must confess that there were (and are) good reasons to believe that for short range forecasting, you didn't need very accurate representation of sea ice. It doesn't cover much of the surface area of the earth. And, while it might be very reflective, at the times that there is the most ice that is most reflective, there isn't much sun for the ice to reflect. I could have simply sat back in a wrangle with the weather folks, endlessly asserting that sea ice was important, and how much energy sea ice reflected was still important, and weather is chaotic so it had to matter, vs. endless repetitions of their counter-arguments. Perhaps you've seen that sort of thing happen a time or two on a blog or two.

Instead, time to do some science. Run the experiment and see what happens. This has the downsides that it requires my time, and I have to run the risk of the experiment showing that I was wrong -- that modest changes to how much of the sun's energy sea ice reflects really did not affect weather.

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What is the future of weather?

The future of weather is change. Easy enough to make that statement, but since the question brings people here periodically, let's think about it some more. I earlier mentioned the topic in Weather will still happen. Entirely true, but maybe not as helpful as it could be.

Let's go back and think about what we mean in talking about weather. Partly, it means 'not climate'. Itself also not the most helpful comment. But let's continue with both weather and climate in mind. My touchstone is "Climate is what you expect, weather is what you get." Whatever it is exactly that is happening around you right now, that's weather.

We can think a little differently and decide that our expectation -- climate -- is also part of what's happening. In that case, weather is the difference between our expectations and exactly what is going on. Since I live near Washington, DC, and it's the middle of July, I expect it to be hot. More precisely, from the Weather Underground's reports for Washington National Airport, I expect today's high to be 88 F (31 C). That's the climate for that station. If the actual high were to be 88, then as far as high temperature went, we were exactly on our climatology. Conversely, we could say that there was no 'weather' -- no difference between what we expected and what we got. It looks like the forecast for today is for the high to be 5 degrees below the climatology. So it seems more likely that we'll have 'weather' of 5 degrees cooler than normal for the high.

If we look day by day for here and other middle or high latitude locations, we'll find days that are 20-30 F warmer than usual (10-15 C), and days that are 10-15 C colder than usual. That gives us a sense of how large 'weather' is -- give or take 15 C from climatology. The figure depends on locations and seasons. In the tropics the weather variations, in terms of temperatures that is, are smaller than in the middle latitudes (if I remember correctly, 5 C, 10 F, is considered a big deviation from climatology in the tropics).

The difference between scale of weather (how many degrees away from climatology you get) in tropics and the middle or high latitudes helps us see what is happening to cause weather. In the tropics, the solar input is relatively constant day by day through the year. With similar solar inputs, you reach similar temperatures day by day, and year by year. The larger differences from climatology occur when you have some big system (large cloud bands, clusters of thunderstorms, and up to hurricanes) active. The thing which drives those big systems are temperature differences. The systems then try to flatten out the temperature differences. In low latitudes, they're more effective at this, so you see smaller variations due to weather.

Come to higher latitudes, where most people live, and you see some of those tropical systems coming up your way (hurricanes, typhoons, etc.) carrying that very warm, very moist air -- replacing the more moderate air 'native' to your location. Or, here in the mid-latitudes, wait a bit and get a wave of cold air coming down from the colder higher latitudes. On top of both, you have the fact that the amount of sun you get varies by a lot through the course of a year. If the only thing happening were the change in solar input, we could calculate the temperatures, and temperature changes, we'd expect (a climate estimate) using the simplest climate model.

Now let climate change enter the picture. Will it change the fact that solar input varies little in the tropics and tremendously at the poles? No. Will it change the fact that far more solar input is in the tropics than in middle latitudes? No. Will it change the fact that weather systems respond to temperature differences across the planet by trying to smooth out those differences? No.

Since the answers to all those (and a host of others that are related) is no, weather will still happen. A little more detailed:
we'll still see days/weeks/months, even years, where the temperatures run below normal.
we'll also still see temperatures run above normal.
In the mid-latitudes, those differences on a daily basis will still be 10-15 C (20-30 F)

Now an application of climate change to our daily observations. Let's say (to keep the numbers easy) that the climate change of the last century were a 2 F (1 C) warming at my location. Before that warming occurred, the expected high would have been 86 F. Our actual high of 83 F represents 'weather' of 3 degrees F below the former normal. Given the current, warmer, climate, it means today's weather is 5 F below normal. Anything odd about 5 F off normal for a day? Hardly. The record low is 15 F below the average low, the record high is 12 F above the average high. (Tamer numbers here than I quoted above because a) it's summer and the ranges are smaller and b) I'm more knowledgeable about the weather for Chicago, which is more variable than DC.)

Weather will still happen, and have similar magnitudes to the past. What changes, as climate changes, is the average and some more subtle figures. They'll be the subject of their own note later.

Running in Thunderstorms, longer

Last summer I posted a short note about running in thunderstorms (don't), but just a quickie. A little more length in case someone is thinking that I'm just chicken and a Real Runner would go out anyhow. As I mentioned last time, I've run in snow, rain, sleet, well below freezing temperatures, high heat and humidity, and so on. It is only the thunderstorms that stop me.

The reasoning of the folks who have argued that it's ok to run in thunderstorms is that 'well, very few people are hit by lightning, so I'll take that tiny risk'. They're banking on arithmetic like this: 300 people per year are hit by lightning in the US, and there are 300 million people in the US, so their 'odds' are 1 in a million of being hit. Unfortunately, this is numerology rather than sound statistics.

The problem with that approach is, almost none of those 300 million are in a place where they could be hit by lightning. Most people, most of the time, are indoors or inside a car -- places you should go to avoid getting hit by lightning. So, with no effort or attention, most people could not be hit even if there were a thunderstorm in the area.

Once there is a thunderstorm in the area, almost everybody heads for safety -- and everybody should. Go inside your house (and stay away from the pipes); if you are away from homes, go to your car; if that's too far or you didn't drive, head for low ground; if you're in Flatlandia (which is where I grew up) head away from anything tall -- away from trees, telephone poles, and especially away from metal towers. These steps are easy and almost everybody takes them.

The right statistical question is "Of the people who are in a position where they could be hit, what fraction do get hit." This figure is more like 1 in 3000, rather than 1 in 1,000,000. Almost everybody does, intentionally or accidentally, take suitable precautions. Be one of them.

Since most of us who run, do so for our health, I'll point out that being hit by lightning is not good for your health. A high fraction of people hit, are killed. Of those who do not die, a high fraction have permanent health problems.

Beyond the fact that it's bad for your health and easy to take appropriate precautions, is the fact that shifting your schedule a little will let you get your run in easily. You need to give a thunderstorm about a 30 minute clearance, wait for 30 minutes after the last thunder you hear. But, an individual thunderstorm is not a long-lived event. 90 minutes is long. Moving your run 2 hours only means go out at 7 instead of 5, or vice versa. If it's very important to get the run in, then moving it a couple of hours is easy enough, and enormously safer. If you're looking at a line of thunderstorms, or a supercell, it's a longer window and you may well have to miss the run. On the other hand, such systems also produce tornadoes and large hail and have severe winds such that it'd be hard to do a workout anyhow.

For more information about thunderstorm and lightning safety:
Red Cross on Thunderstorm and lightning preparedness
NWS Lightning Safety
NWS home page -- with current watch and warning areas

Added: from Hank Roberts' September 21, 2010 comment:

Weather will still happen

It looks like this is a surprise to some folks, so I guess it bears repeating: Weather will still happen in the future. This is true regardless of whether the current scientific understanding about climate change is more or less correct or not. There's still going to be weather. By that, I mean that there will still be periods when your back yard will be markedly warmer or colder than usual, you get more (or less) snow than usual, and so on.

What's bringing it to mind is that with a recent cold spell around here (Washington DC area), I've heard many more comments about how "There can't be global warming because it's cold here." (Or that it was cold when a rally was held, etc.) Now, if the people saying such things were honest (even if not correct), when we get to about 20 F warmer than usual this weekend (which is the forecast, vs. having been 20 F cooler than usual), they'd turn around and say just as loudly that there is global warming after all.

But this is one of the easier flags on whether you're dealing with an honest source. They ignore data that goes against what they want to conclude. Conversely, it's a good self-check when you start reading material that annoys you -- is it annoying because it presents evidence just as good as, or better than, you have for the conclusion you prefer? In that vein, if you think I'm ignoring important evidence, do bring it up to me.

As you know from earlier notes, if you've seen them, it's a silly thing to draw conclusions about global climate from a few days of local temperatures. Or even a few years of even global temperatures. If you haven't been here before, take a look at

• Results for deciding climate trends


• How to decide climate trends


• What is climate - 2

Start of Numerical Weather Prediction

I'm going to wind up with chaos and climate, but the route starts with numerical weather prediction. Numerical weather prediction itself starts much farther back than most people realize -- now about 90 years old. And it didn't start with the simplest possible weather prediction model. If you're not up on your history of science and technology, you didn't bat an eyelash at my mention of numerical weather prediction being 90 years old. Electronic computers are only 60-70. 90+ years ago, when the first numerical weather prediction (NWP) was done, 'computer' meant a person who, with pen and paper, slogged through the calculations.

The first NWP was performed by hand, by Lewis Frye Richardson. He did so in between ambulance runs during World War I. The reason he was in an ambulance, rather than the much safer trenches, is that he was a conscientious objector to warfare (Society of Friends (Quaker) by religion). Nevertheless, he did survive the war and completed his numerical prediction. It was finally published in 1922, with it having been essentially completed in 1919.

The model he used was what we now call a primitive equation model. It took the laws of conservation of mass, energy, and momentum in their full complexity and tried to solve them. The first successful numerical weather prediction was not made until about 1948, published in 1950: Charney, J. G, R. Fjortoft, and J. von Neumann, "Numerical Integration of the Barotropic Vorticity Equation", Tellus, 4, 237-254, 1950. It was done on a much simpler equation -- bearing much the same resemblance to the primitive equations as the simplest climate model I've mentioned before does to a full complexity climate model. This model (which Richardson could have done by hand more easily than what he took on) was run on one of the first electronic computers -- ENIAC.

The first model to implement something comparable to what Richardson tackled was not done for another 2 decades after the ENIAC model (the '6 layer PE' model Shuman, F. G., and J. B. Hovermale, An Operational Six Layer Primitive Equation Model", J. Applied Meteorology, 7, 525-547, 1968).

In any case, Richardson's forecast has often been called a glorious failure in the ensuing decades. The failure part being that the forecast was so drastically in error -- predicting a surface pressure change of over 100 mb, when only 6 or so would have been considered large. He did recognize the likely sources of his problem, but doing this kind of computation by hand was too expensive (in time) to do multiple trials to nail down exactly which was the source of his problem and which idea for repair would take care of his problem.

The glorious aspect is his enduring contribution to the field. For starters, he invented numerical weather prediction. Much of what is done today is still dependent on approaches he invented. He also foresaw massively parallel processing, and one of the central problems in that -- making sure that your different processing 'nodes' (people, in his case, computer processors in ours) remained synchronized.

For more on Richardson's forecast, see especially Weather Prediction by Numerical Process, by Lewis F. Richardson. Original publication in 1922, republished in 1965 by Dover Publications. This is the original, full, document. More recent consideration was made by G. W. Platzman, "Richardson's Weather Prediction", Bull. Amer. Meteor. Soc., 60, 302-312, 1968. Platzman includes discussion of the sources of Richardson's problems. There is a later note in BAMS by Platzman giving more considerations and ideas.

No running in thunderstorms

Blogging instead of running right now. I run through almost all weather, including rain, snow, sleet, -10 F temperature, -30 F wind chill, temperatures over 100 F, etc. The one type of weather I won't run in is thunderstorms. It isn't strong now, but I'm still hearing occasional thunder.

Thunder means lightning, and lightning is bad for my health. Plus, I can't outrun lightning.