Weather It Is (Two Storms, Different Outcomes)

Good Evening:

The last time the Jewish people experienced a hail storm like Friday night's, they were in the desert of Egypt and they didn't worry about the power being knocked off in a burst of lightning. Perhaps, then, it is a bit ironic that this prelude to leaving Egypt, and coming to the "promised" land -- was simply another prelude for a hail and lightning show.

One can probably describe the two halves of our recent storm as "polar opposites." The first half brought heavy snow (19/20 of February), while the second half brought continuous hail (20/21 of February).  In the first, the snow fell like a whisper; in the second, the hail fell like a percussion band (and the thunder to go with it sounded like a bass drum).

The first half was associated with extremely cold upper air temperatures and below freezing surface temperatures.  The second was also associated with cold upper air temperatures, although not as cold as during the snow part of our storm.  Temperatures at the ground were also warmer in the second hail storm, and a relatively mild and moist middle layer meant two things: the atmosphere was very unstable (conducive to thunderstorms) and that there was lot of riming of snow particles to produce graupel (what we're loosely calling hail here).

Most importantly, the collision of graupel and ice/snow particles in the presence of liquid water produces lightning, and there was a lot of lightning in the second part of our storm.

In fact, the second storm was remarkable for the duration of the hail, numerous and repeating thunderstorms, and for the fact that the hail accumulated on the ground throughout the night, even though the temperature never fell below 2 C (here in Efrat).  Had surface temperatures been a bit colder, we might have awoken to 20 cm of hail (really graupel that looks like snow-ice cubes).  Instead, as the hail melted, it was continuously replenished.

The top figure below show Jerusalem forecast rain accumulations ("rain-accu"), snow accumulations, and hail accumulations from the snow storm, while the bottom figure shows the same from the hail storm. The rain-accu is the sum of any rain, snow, and/or hail that falls.  We also show the forecast snow height (what we usually refer to as snow amounts or accumulation).  The snow height is what we have after snow falls and some of it melts.

During the snow part of our storm, the snow height was quite high (surface temperatures were very near of below freezing).  Moreover, the snow accumulation was about four times the hail accumulation. In contrast, the second storm produced almost four times the amount of hail versus snow accumulation. Moreover, there was plenty of water "left-over" to cause lightning in the presence of snow and ice/graupel particles.

In our early blog on Friday, we mentioned that the second storm would accumulate only light amounts.  What we didn't mention was that this light accumulation would mask such dramatic atmospheric processes leading to tremendous hail and lightning.

Like we did after the January snow-storm, we have modified our forecast output to more clearly show the types of precipitation reaching the ground, so that we can better convey the message we all need to hear to improve weather preparation and safety.

Barry Lynn