Tuesday, January 26, 2016

"The Blizzard of 2016" and predictability: Part II: When is a prediction a good one? When is it good enough?

Weather forecasts require the prediction of many different parameter values.  These include temperature, wind at the ground and aloft (winds that steer storm systems, and where planes fly), humidity on the ground and in the air (that determines rain and snowfall), friction (related to tornadoes and thunderstorms), change over time and the track of these things across the surface with its own weather-affecting characteristics (like water, mountains, cities).  Forecasters have to model and predict all of these things.  In my day, we had to do it mainly with hand-drawn maps and ground observations--no satellites, basically no useful radar, only scattered ship reports over oceans, etc.), but of course now it's all computerized.

Other sciences are in the prediction business in various ways.  Genetic and other aspects of epidemiology are among them.  The widely made, now trendy promise of 'precision' medicine, or the predictions of what's good or bad for you, are clear daily examples.  But as with the weather, we need some criteria, or even some subjective sense of how good a prediction is.  Is it reliable enough to convince you to change how you live?

Yesterday, I discussed aspects of weather prediction and what people do in response, if anything.  Last weekend's big storm was predicted many days in advance, and it largely did what was being predicted.  But let's take a closer look and ask: How good is good enough for a prediction?  Did this one meet the standard?

Here are predicted patterns of snowfall depth, from the January 24th New York Times, the day after the storm, with data provided by the National Weather Service:

And now here are the measured results, as reported by various observers:

Are these well-forecast depths, or not?  How would you decide?  Clearly, the maximum snowfall reported (42") in the Washington area was a lot more than the '20+"' forecast, but is that nit-picking?  "20+" does leave a lot of leeway for additional snowfall, after all.  But, the prediction contour plot is very similar to the actual result. We are in State College, rather a weather capital because the Penn State Meteorology Department has long been a top-rated one and because Accuweather is located here as a result.  Our snowfall was somewhere between 7 and 10 inches.  The top prediction map shows us in the very light area, with somewhere between 1-5" and 7-10" expected, and the forecasts were for there to be a sharp boundary between virtually no snowfall, and a large dump.  A town only a few miles north of us had very few inches.

So was the forecast a good one, or a dud?

How good is a good forecast?
The answer to this fair question depends on the consequences.  No forecast can be perfect--not even in physics where deterministic mathematical theory seems to apply.  At the very least, there will always be measurement errors, meaning you can never tell exactly how good a prediction was.

As a lead-up to the storm's arrival in the east, I began checking a variety of commercial weather companies (AccuWeather, WeatherUnderground, the Weather Channel, WeatherBug) as well as the US National and the European Weather Services, interested in how similar they were.

This is an interesting question, because they all rely on a couple of major computer models of the weather, including an 'ensemble' of their forecasts. The local companies all use basically the same global data sources, and the same physical theory of fluid dynamics, and the same resulting numerical models.  They try to be original (that's the nature of the commercial outfits, of course, since they need to make sales, and even the government services want to show that they're in the public eye).

In the vast majority of cases, as in this one, the shared data from weather balloons, radar, ground reports, and satellite imagery, as well as the same physical theory, means that there really are only minor differences in the application of the theory to the computed models.  Data resources allow retrospective analysis to make corrections to the various models and see how each has been doing and adjust them.  For the curious, most of this is, rightly, freely available on the internet (thanks to its ultimately public nature).  Even the commercial services, as well as many universities, make data conveniently available.

In this case, the forecasts did vary. All more or less had us (State College) on a sharp edge of the advancing snow front.  Some forecasts had us getting almost no snow, others 1-3", others in the 5-8" range.  These varied within any given organization over time, as of course it should when better models become available.  But that's usually when D-day is closer and there is less extrapolation of the models, in that sense less accuracy or usefulness from a precision point of view.  At the same time, all made it clear that a big storm was coming and our location was near to the edge of real snowfall. They all also agreed about the big dump in the Washington area, but varied in terms of what they foresaw for New York and, especially, Boston.  Where most snow and disruption occurred, they gave plenty of notice, so in that sense the rest can be said to be details.  But if you expected 3" of snow and got a foot, you might not feel that way.

If you're in the forecasting business--be it for the weather or health risks based on, say, your genome or lifestyle exposures--you need to know how accurate forecasts are since they can lead to costly or even life-or-death consequences.  Crying wolf--and weather companies seem ever tempted to be melodramatic to retain viewers--is not good of course, but missing a major event could be worse, if people were not warned and didn't take precautions.  So it is important to have comparative predictions by various sources based on similar or even the same data, and for them to keep an eye on each other's reasons, and to adjust.

As far as accuracy and distance (time) is concerned, precision is a different sort of thing.  Here is the forecast by our local, excellent AccuWeather company for the next several days:

This and figure below from AccuWeather.com

And here is their forecast for the days after that.

How useful are these predictions, and how would you decide?  What minor or major decisions would you make, based on your answers?  Here nothing nasty is in the forecast, so if they blow the temperature or cloud over on the out-days of this span, you might grumble but you won't really care.

However, I'm writing this on Sunday, January 24.  The consensus of several online forecasts was all roughly like the above figures.  Basically smooth sailing for the week, with a southerly and hence warm but not very stormy air flow, and no significant weather.  But late yesterday, I saw one forecast for the possibility of another Big One like what we just had.  The forecaster outlined the similarities today with conditions ten days ago, and in a way played up the possibility of another one like it.  So I looked at the upper-air steering winds and found that they seem to be split between one that will steer cold arctic air down towards the southern and eastern US, and another branch that will sweep across the south including the most Gulf of Mexico and join up with the first branch in the eastern US, which is basically what happened last week!

Now, literally as I write, one online forecast outfit has changed its forecast for the coming week-end (just 5 days from now) to rain and possibly ice pellets.  Another site now asks "Could the eastern US face more snow later this week?" Another makes no such projection.  Go figure!

Now it's Monday.  One commercial site is forecasting basically nothing coming.  Another forecasts the probability of rain starting this weekend.  NOAA is forecasting basically nothing through Friday.

But here are screenshots from an AccuWeather video on Monday morning, discussing the coming week.  First, there is doubt as to whether the Low pressure system (associated with precipitation) will move up the east coast or farther out to sea.  The actual path taken, steered by upper-level winds, will make a big difference in the weather experienced in the east.

Source: AccuWeather.com

The difference in outcomes would essentially be because the relevant wind will be across the top of the Low, moving from east to west, that is, coming off the ocean onto land (air circulates as a counter-clockwise eddy around the center of the Low).  Rain or possibly snow will fall on land as the result.  How much, or how cold it will be depends on which path is taken.  This next shot shows a possible late-week scenario.

Source:  AccuWeather.com
The grey is the upper-level steering winds, but their actual path is not certain, as the prior figure showed, meaning that exactly where the Low will go is uncertain at present.  There just isn't enough data, and so there's too much uncertainty in the analysis, to be more precise at this stage.  The dry and colder air shown coming from the west would flow underneath the most air flowing in from offshore, pushing it up and causing precipitation.  If the flow is more eastward of the alternatives in the previous figure, the 'action' will mainly be out at sea.

Well, it's now Monday afternoon, and two sites I check are predicting little if anything as of the weekend....but another site is predicting several days in a row of rain.  And....(my last 'update'), a few hours later, the site is predicting 'chance of rain' for the same days.

To me, with my very rusty, and by now semi-amateur checking of various things, it looks as if there won't be anything dropping on us.  We'll see!

The point here is how much things change and how fast on little prior indication--and we are only talking about predicting a few days, not weeks, ahead.  The above AccuWeather video shows the uncertainty explicitly, so we're not being misled, just advised.

This level of uncertainty is relevant to biology, because meteorology is based on sophisticated, sound physics theory (hydrodynamics, etc.).  It lends itself to high-quality, very extensive and even exotic instrumentation and mathematical computer simulation modeling.  Most of the time, for most purposes, however, it is already an excellent system.  And yet, while major events like the Big Blizzard this January are predictable in general, if you want specific geographic details, things fall short.  It's a subjective judgment as to when one would say "short of perfection" rather than "short but basically right.".

With more instrumentation (satellites, radar, air-column monitoring techniques, and faster computers) it will get inevitably better.  Here's a reasonable case for Big Data.  However, because of measurement errors and minor fluctuations that can't be detected, inaccuracies accumulate (that is an early example of what is meant by 'chaotic' systems: the farther down the line you want to predict, the greater your errors.  Today, in meteorology, except in areas like deserts where things hardly change, I've been told by professional colleagues who are up to date, that a week ahead is about the limit.  After that, at least under conditions and locations where weather change is common, specific conditions today are no better than the climate average for that location and time of year.

The more dynamic a situation--changing seasons, rapidly altering air and moisture movement patterns, mountains or other local effects on air flow, the less predictable over more than a few days. You have to take such longer-range predictions with a huge grain of salt, understanding that they're the best theory and intuition and experience can do at present (and taking into account that it is better to be safe--warned--than sorry, and that companies need to promote their services with what we might charitably call energetic presentations).  The realities are that under all but rather stable conditions, such long-term predictions are misleading and probably shouldn't even be made: weather services should 'just say no' to offering them.

An important aspect of prediction these days, where 'precision' has recently become a widely canted promise, is in health.  Epidemiologists promise prediction based on lifestyle data.  Geneticists promise prediction based on genotypes.  How reliable or accurate are they now, or likely to become in the predictable future?  At what point does population average do as well as sophisticated models? We'll discuss that in tomorrow's installment.

1 comment:

Ken Weiss said...

Yes, well, it's now Tuesday lunchtime, and the evidence is that the offshore scenario is going to be correct. The point is that even just 5 or so days in advance, as we've been discussing, predictions can be dicey. So what about predictions of things to come decades from now? Who would be so foolish? See our discussion of biomedical prediction tomorrow....