WEATHER: Just What Don’t We Know About The Snow?

Snow in Jerusalem (Image Credit: N. Sher)

To understand what may be in store for us this week, we need to review a bit of history. After the attack of 9/11, former United States Secretary of Defense Donald Rumsfeld stated:

Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.

Mr. Rumsfeld was far more prescient than he possibly knew. He might just have been speaking about the weather — in fact the weather we "expect” this week.

What we know we know

We know that the global weather forecasts are showing the possibility of the coldest weather in many years to arrive this week. We also know that there will be prolonged and heavy precipitation over several days. Lastly, we know that there is no "flip-side" to the forecasts. We're not seeing an equal number of unusually warm vs. unusually cold forecasts within the global ensemble. It's either winter as usual or winter as we've not known it for many years perhaps going back to the time when snow fell in Tel-Aviv in 1950.

What we know we don’t know

There is something referred to as the North Atlantic Oscillation (NAO). It has no significance by itself, but a negative NAO index indicates a weather pattern that can produce strong storms that drop down west of Cyprus and then move across the sea. This means that these storms warm up in their lower levels too much for any heavy snow in the Jerusalem area. While half our forecasts are indicating abnormally cold air towards the end of the week, the other half are not. We just don't know how strong a "ridge" of warm air will build over Greenland (a negative NAO) — which would determine not only how much cold air arrives earlier versus later in the week, but just how much of our precipitation falls as snow.

What we don’t know we don’t know

We often assume that when the forecast synoptic pattern of the global forecast indicates a certainty of greater than 90%, we can confidently use a higher resolution forecast model to forecast the details of the upcoming weather event. However, there are unknown errors and non-linear interactions that can grow to even create variability in an otherwise "certain" synoptic forecast pattern. This can turn what looks like a sure snow event into a missed forecast, and even our higher resolution forecasts can't correct for that, as they depend on the information they receive from the global model.

So, we are left with uncertainty, but "hope" too if you like snow. In any case, I can state with near certainty that a lot of people will be following the weather over the next several days and hoping for Jerusalem snow. There are still plenty of things we don't know we don't know, but the things we know (or don't know) are becoming better known.

For instance, we see that our next storm — to arrive immediately after the relatively weak (but still) wet) storm affecting us now — will bring cold temperatures aloft, but nothing too chilly at lower levels. This means periods of rain mixed with graupel/hail. This storm will be arriving from our west as higher heights (NAO) build over Greenland; meaning that initially the ridge trough couplet (west to east, respectively) will be located far enough to the west so that the lower levels in the next storm will be moderated by the warm sea.

In contrast, we now have indications that the storm that should follow Shabbat or early next week will arrive from the north as Siberian air makes its way southward. Our current forecasts show a 60/40 split in favor of extremely cold temperatures aloft. This means that the forecasts are clustering around either a cold, snowy "solution" to the equations that describe our future weather or a warmer solution with a lean toward the colder forecasts. However, there is less certainty about lower level temperatures. Still, many ensemble members show temperatures below freezing at least at some point during the storm.

So, we know that we can be more positive about the possibility of our first snow in the Jerusalem area early next week, but there are still a lot of unknowns, such as just how directly the storm will arrive from the north. Besides the potential model errors, there are even unknowns that we don't know we don’t know — such as how many people want snow or don't want snow — which can sometimes tip the balance.

Jerusalem forecast (click here for updated national forecasts):

Image credit: The Jerusalem Herald

Dr. Lynn is a lecturer at The Hebrew University of Jerusalem, Earth Sciences Department. He is also CEO of Weather It Is, LTD, a company that specializes in reducing weather risk. Click here to read more of this writer’s work in The Jerusalem Herald.


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