Weather It Is (Things We Know: Or will it Snow?)

Good Afternoon:

I am sure that many readers appreciate the heavy rainfall, but more it must be satisfying to know that winter in Israel can still feel like winter's past!

To understand what may be in store for us next 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."

Mr. Rumsfeld was far more prescient than he possibly knew.  He might just have been speaking about the weather, if not the weather we "expect' next week.

Here are the things we know:  We know that the global weather forecasts are showing the possibility of the coldest weather in many years to arrive next 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 equally 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 (see: https://en.wikipedia.org/wiki/Snow_in_Israel).  

However, there are many known unknowns.  For example, there is something referred to as the North Atlantic Oscillation.  It has no significance by itself, but a negative NAO index indicates a weather pattern that can produce strong storms, but 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 next 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 will determine not only how much cold air arrives earlier versus later in the week, but just how much of our precipitation falls as snow.

We also have unknown, unknowns: we often assume that when the forecast synoptic pattern of the global forecast indicates certainty (> 90%) then 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 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.

Barry Lynn