Weather It Is (A Looming Storm)

Good Afternoon:

At the risk of anthropomorphizing the weather, one might take umbrage at the bit of irony exercised during our last two storms.  In the first storm, temperatures several hundred meters off the ground (at 850 mb) were just below zero, while in the second storm they were about 2 degrees Celsius below zero.  Yet, snow and ice accumulated in Gush Etzion during the first storm, but did not during the second.  Moreover, the first storm brought about 5 mm of liquid equivalent (melted) rain to Gush Etzion, while the second brought close to 35 mm.

The difference between the two storms was a strong temperature inversion above 700 mb in the second, and not the first, which limited evaporational cooling of the air layer between 850 mb and the surface several hundred meters below.  Just to note, it's not the mixing of air from the colder 850 mb layer (that just generally redistributes the air with height as sinking air warms and rising air cools), but the physical processes related to evaporation and condensation.  Evaporation of droplets cools the air, and the melting of snow can also cool the air.  So, while it was amply possible to freeze ice cubes on the Hermon (at 850 mb and higher), those below were left with just cold drinks. Further, they suffered the indignity of watching snowflakes melt either on the way down or when they reached the ground, and I can tell you it is not a pleasant sight to see or to hear (the screams of thousands of melting snowflakes) -- or maybe that was just me sighing.

I could also mention that actual temperatures were about 1 degree warmer at the surface than forecast.   Had the opposite occurred, the snow probably would have accumulated.

The problem with the above two sentences is that it is short on actionable information.  True actionable information is given through forecast probabilities. When the probability of a weather event is greater than the cost (protection against the event) to loss (losses occurring if no protection is taken) then preventive efforts should be made.  If not, then, no action should be taken. That could be as simple as leaving early or deciding to put snowplows on standby.

We actually run a high resolution ensemble forecast in order to more accurately assess forecast probabilities.  From these forecasts we can say how probable is it that rainfall will exceed 50 mm, for example, or winds will exceed 75 km /h, etc.  However, the grid spacing of these ensembles is not fine enough to see the probability of snow accumulation in the Jerusalem area in storms where surface temperatures are forecast to be very close to freezing.

Hence, we're left with saying: if our highest resolution (single) forecast is accurate, this will happen.  But, to offer guesses about what might happen if the temperature will be 1 degree Celsius colder is really just speculation (and I've decided to no longer spectulate!).

Anyway, next week offers the possibility that our ensemble may in fact be useful at forecasting snow amounts, since there is about a 30% chance that temperatures will be far enough below zero to imply that what will be important will be how much snow, not if it will snow.

But, first, it's going to warm up quite a bit through Shabbat, before temperatures fall sharply into early next week and rain returns with snow on the Hermon.  The storm following about a week from now is the one that could have meteorologists most excited.  It also has the greatest chance this year of bringing snow to Jerusalem.

Barry Lynn