Weather It Is (The Aftermath (And Dynamic Cooling))

Good Afternoon:

There was a lot of excitement the last week, and we certainly should be excited about almost 5 consecutive days of rain, in which we received more than 75 mm of rain (in Gush Etzion).

Some were relieved that the conditions were not strong enough for a significant snowstorm, while others, though were disappointed.

In a nutshell, the forecast models (especially our high resolution forecast) did very well in predicting that at most there would be some wet snow, and/or a very light accumulation in the highest elevations.  Moreover, as predicted, there were temperatures below freezing last night, and even ice (again) on our swimming pool.

The next week or so looks to be milder, and spring may have sprung by the start of the following week (after some tropical showers).  Yet, there is a 10% chance that it will turn cold and rainy at the end of the month.

So, why the hype? What did we all get so excited for?  There is something called "Dynamic Cooling," which can happen when there is very strong convective storms:

https://forecasterjack.com/glossary/glossary-of-cold-season-terms/dynamic-cooling/

Dynamic cooling is the cooling of the lower atmosphere through two processes: i) the mixing of relatively warmer air at the surface with comparatively (unstable) cold air aloft, and ii) the melting of snow/ice as it descends (which requires energy), lowering the temperature enough such that further falling snow/ice can actually reach the surface.

It is important to note here that air that descends in a stable atmosphere (where the lapse rate is adiabatic) does not cool the lower atmosphere. The reason is that air warms as it descends because of an increase in pressure. The result is that air parcels oscillate up and down, but never too far in either direction. Moreover, if air were to relatively cool as it descended, there would be quite a wind coming from above (and it doesn't --  so you intuitively know this to be correct).

Another way the atmosphere can cool is when liquid drops evaporate on the way down. It cools to the wet bulb temperatures (see: https://www.quora.com/What-is-the-physical-significance-of-wet-bulb-temperature).  This often happens in the northeastern United States when a storm advances northward up  the coast, and high pressure from the north blows cold, dry air into the storm.  The evaporation of droplets as they fall into this relatively dry air will occur until the wet bulb temperature is reached (or saturation), and the temperature will be colder than before (and perhaps even below freezing).

I don't think we have the latter situation to often -- and cold air often arrives from over the sea so it is near saturation anyway. Instead,  we must rely on dynamic cooling and the advection (arrival of cold air from the north), and it was this type of process that led meteorologists to be hopeful for an accumulating snow on Thursday.

However, our highest resolution forecast (which has a resolution high enough to predict dynamic cooling in strong storms) indicated only a mixture of wet snow/ice that would accumulate only small amounts at best, and only in the highest areas of the central mountains.

Looking at the big picture (especially when the later global model forecasts were a bit colder than previous) did lend some excitement to our winter (and even I was hopeful), but the fine scale details indicated:  no -- there wouldn't be a big snow.

Well, we're going to have to wait until the end of the month, or possibly next winter.

Shabbat Shalom,

Barry Lynn