Forecasting on the Edge

The Norman area sits on the edge of a possible heavy snow and/or sleet and/or freezing rain event for Tuesday. Which is it and how much, categorically? My answer as of midnight Sunday night/Monday morning: Still too soon to say! Anyone who tries to nail any spot down to a specific amount, or a narrow range (like, say, 5-6 inches) this soon is full of BS, and should be trusted no more than a used-car dealer in Vegas. Neither the human forecasters nor the models are that good yet.

While I have looked at some more recent forecasts, and have a decent grasp of the general scenario, I’ll first post and link to some SREF (short-range ensemble forecast) panels that I had a chance to grab yesterday afternoon from the morning’s run. They illustrate the difficulty faced by winter-weather forecasters really well!

[For the uninitiated, the SREF is a 21-member package of various numerical models. I don’t have room to explain it in detail here; but this site has a good summary of SREF and a big variety of forecast charts.]

The above forecast is the maximum value (basically, at any spot on the map) for the total liquid (melted) equivalent precipitation during 6Z-18Z (midnight-noon) Tuesday. Take every one of the 21 models, find its max precip, and plot that value, and basically that’s what this is–a heaviest-case scenario. Notice that most of the heaviest precip forecasts are near (but mainly south and east) of Norman. Similar forecasts ending later during the day are not quite so large around Norman. Again, this is liquid amount–not the equivalent of snow. For snow…

This forecast is for the average snowfall in inches at any given spot on the map for the 12 hours ending later that day (21Z or 3 p.m.). The times are offset some because of an expected change to snow in Norman sometime during that block of time, and as forecaster, I think this time selection probably will capture most (not necessarily all) of the snow event. Notice Norman is on the opposite edge of the heaviest snow belt from where it was with respect to the heaviest total precip accumulation ending a few hours before. Hmmm…so if we’re on one edge of melted equivalent and another of snow, where and when is the transition?

This forecast shows the average position of the freezing line southeast of Norman by 12Z (6 a.m.), but quite a bit of spread off that mean in the extreme positions (dotted and dashed lines). That freezing line matters hugely for what kind of precip we’ll have! The most likely precip type (out of all probabilities) shows rain east, a mix of sleet and freezing rain overhead and nearby, and snow just to the NW. At 12Z, we’re on the edge of a lot of things that only need to be a little bit wrong to trash the hell out of any forecast that’s too specific! Assuming the dominant forecast is accurate, this trend shifts east over us during the day to render all snow…

Now, by 21Z (3 p.m.), we’re pretty confident that it’s snow, being deep into the blue area, with all the models’ freezing lines past us, and (if you’ve also looked at forecast soundings, which I’m not showing) an understanding that it’s too cold aloft for sleet or freezing rain. But the critical issue is: when does that freezing line go past us, and how far behind it is the air above the surface warm enough to yield freezing rain or sleet before the change-over? On a national scale, it looks puny. Make a spot forecast, and just a few hours one way or another makes a huge difference.

Now let’s look at what we call “plume diagrams” — because they often look like plumes from a chimney. They’re actually spot forecasts of accumulated totals for the time period, in this case precip amounts for Norman, generated by the very same set (ensemble) of models. Each line represents one model in the set (colored by numbers at the top, so you can follow your favorite models for the situation, if you have some). The average forecast is black, with dots every 6 hours. The timeline goes from earlier at left to later at right…

This is the melted precip total, regardless of the form. We are in a drought, and this is what matters most in a hydrologic sense anyway, so we’ll look at precip totals first. Notice how the models generally agree well on when we’ll get the most precip–the steep ramp-ups between 6Z (midnight) and 21Z (3 p.m.) Tuesday. But they disagree vastly on how much (from less than a quarter inch to almost an inch and a half). Even if this were all rain, we would have one hell of a time forecasting how much…and we haven’t even looked at the precip type yet. Let’s do that!

This is the ensemble of models for accumulated freezing rain (ice). They’re all over the place too, and many of the same models that are heavier with liquid rain are lighter with ice. The green models set isn’t even there; none of those are forecasting freezing rain. That says there’s a lot of disagreement on when the transition happens with respect to how much is rain versus ice, if there’s any ice! Again, this wide variation is just in one spot (Norman)…not even considering the potential for much bigger or smaller amounts just E or W of here. Are you sure you even want to see sleet forecasts? Well, if you don’t, stop now, because that’s what’s next…

Oh, joy. Sleet forecasts are all over the place too. Not only that, a few models have high accumulations of rain, ice and sleet, a few are low on them all, and the rest vary greatly between which will be dominant, and by how much. Only the green members, which are all fairly dry across the board, seem consistent. It’s enough to make a forecaster with little patience for uncertainty throw his hands up and walk away in abject frustration. But wait! There’s more…namely, the one precip type that it seems everybody demands to know down to the inch: snow…

This model set says Norman will get anywhere from nothing to a foot, but with a low average of just above 2 inches. If those two blue models are onto something that the others are missing, millions of dollars in snow-plowing and salting expenses might be justified. If any of the models are right in forecasting a heavy snow band of, say, 16 inches someplace else, but are wrong on the location, we could get a lot more in Norman than the highest model predicts. Or, we hype it up, get nothing, all that road salt was laid down for naught, and the local governments are quite upset. Lots at stake here for the public, emergency managers, school systems, law enforcement, media, and your credibility as a forecaster…after a wildly uncertain period of rain and/or sleet and/or ice!

We’ve got quite a forecasting dilemma here. Can you see? It’s tempting to go with the averages as a hedge against a huge forecast error; but what if the extreme-upper model turns out to be right in Norman itself? Again, history tells us that sometimes, especially in the middle of a narrow and badly forecast snow band, even the extreme solution wasn’t extreme enough. Other times, the lowest solution wasn’t low enough, because we get barely a dusting, while either Slaughterville or Del City, each 15 miles away, gets over a foot. These things have happened before, and the forecaster needs to keep historical similarities in mind too.

Based on all that information, about the best we can say is that cold rain probably will change to snow, with a period of assorted freezing rain and sleet possible (but not for sure) sandwiched in between. How much? Too uncertain to call. That’s the honest answer.

At least the strong consensus is that snow, if any, starts after the rain and sleet or freezing rain. That’s not much consolation if Aunt Matilda is pestering you for an exact snow amount and when it will happen; and you just can’t explain these crazy uncertainties to her without sounding like a waffling, blathering know-nothing whose parents wasted money on your meteorology degree.

Uh oh…what’s this? After all that weeping and gnashing of teeth, let’s pretend a new SREF package has come in and the forecasts of many of the individual models have flip-flopped around like a fish out of water. Trends are up with some, down with others, earlier with some, later with others. As your local teenager might type in a text message, “OMG WTF!!!”.

If you are a forecaster, what do you do now? With all those mixed signals, and a historical precedent for everything from nothing to 15+ inches, this scenario can drive you to the brink of incoherently blubbering lunacy, and beyond.

One way to keep sane is give up, stop thinking about it, and forecast some average default, which of course a machine can do without your help. Don’t come crying to me, then, when you lose your job to that machine!

Another way to become more certain and confident in a forecast, as well as maintain sanity, is through strong physical understanding of the situation, which comes from a combination of education (school and self), training, experience, analytic skill, understanding the models, and the continuing motivation to keep up with it all. This “learned path” is not foolproof, but it’s an insurance policy against consistent failure, and one that sometimes pays off big in correctly forecasting an extreme event that lazy, “model-hugging” forecasters will miss. [About five years ago, I discussed the future of human forecasting in detail here. ]

In short, the forecaster must first diagnose what’s going on now, both to judge how the models are performing right from the start, and more importantly, to form a 4-dimensional conceptual model of the ongoing atmosphere in his own head. This means analysis–including hand analysis of surface and upper air charts–which takes time to do with due accuracy and attention to detail. A forecaster who tries to predict the future without a thorough understanding of the present is negligent, and prone to spectacular failure.

Then comes assorted model guidance. The SREF package of 21 models has far more ways to display output from them than I’ve shown here. For a truer appreciation, go to the SREF website and look in detail at everything it contains. The exercise, done carefully, will take an hour or more. Then comes operational models outside SREF, of which there are several. The variety of guidance available to the forecaster these days is dizzying. Truly I declare, there’s hardly time to look at a substantial fraction of it, much less all. It really is informational overload.

The ability to sift out the irrelevant and distill the pertinent in weather prediction is an uncommon skill, one gained mainly through experience. Even then, in the face of inflexible time deadlines, it’s easy for even the best and sharpest forecasters to overlook a potentially important but small detail anywhere along the way. Make it 4 a.m. for a rotating-shift worker, and the potential for human error rises (unless, like me, you are a bonafide night-owl). It’s also possible for some forecasters–such as the model-huggers I mentioned above–to have years and years of experience doing it in a poor way (in which case 20 years of “experience” is actually 1 year of experience repeated 20 times over). Every forecaster takes at least a slightly different approach from every other.

Given all these factors, no wonder one forecaster can differ so much from the next, and one forecaster’s own predictions can vary from one day to the next.

If you are not a meteorologist, have mercy on your friendly neighborhood forecaster. Don’t get upset with the meteorological prognosticators for being unsure, or changing their minds often, or giving you a wide range or possibilities, or differing a lot. And when your local forecasters get a winter-storm prediction right–or even close–heap laud profusely upon them, for they have accomplished an extraordinarily difficult feat. Remember: uncertainty is part of the deal. It is unavoidable, if a forecaster is being honest with himself and with you. The forecasters who are hedging or talking in probabilities are because it’s the smartest approach, the right approach.



Comments

One Response to “Forecasting on the Edge”

  1. tornado on February 3rd, 2011 2:09 am

    POST-EVENT VERIFICATION SUMMARY

    How did the numerical forecasts for Norman verify? The precip got here a few hours before all but one or two of the outlier SREF forecasts (see the plume diagrams above), around 03Z, in the form of thunderstorms that very briefly dropped a freezing rain/sleet mix before it became almost all sleet. The sleet dump continued intermittently until after 08Z, with about an hour of change to snow. From about 09Z on, it was all snow–this part being consistent with the plume diagrams for snow alone. The freezing line was much closer to the southern outlier at 12Z than to the mean. In this event, the outliers of the ensemble did better with timing, and the mean snowfall solution was about 1/3 of what we had (average depth 6 inches, more than all but 4 of the 21 member models). I measured .75 inch of sleet, about the same as the uppermost outlier of the model set, and close to the value of 3 others that were barely less. Event-total precip (melted equivalent) was 1.07 inches, solidly above the ensemble mean but still less than 6 of the individual members. The majority of members (and the strong ensemble consensus) did do well with one thing; timing of the end of the precip, around 18Z.

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