Tuesday, January 26, 2016

Weather It Is (Cold and Snow Warning: update)

Good Morning:

An additional note:

Radar shows a broad band of precipitation stretching from Jerusalem, Gush Etzion back to the sea coast.  Light snow is falling in Gush Etzion.

The GFS ensemble shows a possible range of outcomes for convective precipitation amounts, which is why I mentioned that snow could fall heavily enough to cause icy and snow covered conditions.  However, it might now -- keep the former in mind, though, when evaluating your travel plans.

Barry Lynn

An added note:

The latest GFS data shows that about 10 mm of total precipitation should fall in the Jerusalem area in the next six hours, followed by another 4 mm.

Since temperatures will be cold enough for snow, this would bring an accumulation of several centimetres.

Considering our past situation, scepticism is needed, but this is the only time the GFS has been consistently been predicting more significant amounts during this second half of the storm. As I wrote below, this is a convective situation and a quick burst (lasting an hour or so) could accumulate snow quickly.

Previous discussion...

Where there's cold there's snow -- something forecasters in our part of the world pretty much go by. Certainly temperatures are cold enough for snow from now until Wednesday afternoon, and they will actually get colder.

Moreover, since 1950 there were 15 periods with three days of cold in Jerusalem below 6 C, and all but two of them had at least 5 cm of snow.

In fact, intense cold should last into Thursday morning, although by Wednesday afternoon the cold aloft will be dissipating.

So, where's the snow? First, the forecast model has correctly predicted the heavier precipitation amounts from Gaza to Beer Sheva, and lighter amounts in Jerusalem.  However, it over predicted the duration and intensity in Gush Etzion.  Precipitation has been more intermittent than forecast, so snow amounts were less than forecast.

As I mentioned, this is an unusual situation.  There hasn't been a strong coupling between the southern and northern Jet Streams, meaning that we've got the northern cold without the southern moisture.  In fact, the cold were getting at lower levels is more reminiscent of very cold mornings with clear skies.

So, were overlaying periods of instability that arise because of the very cold temperatures (i.e., showers forming over the ocean) and move inland on westerly winds over a relatively dry lower layer associated with northeasterly winds in the lowest levels.

The influence of the mixture of different weather regimes on precipitation amounts has been difficult for the model to predict. (It's also the reason I felt very unsure about the forecast, even today, when it could snow more than predicted -- which is not very much.)

Such is the weather -- it's not simple nor easy to forecast.

Still, because of the cold and possible snow showers travel is not advised -- especially tonight.

Here's a picture of snow in Efrat this morning.

Barry Lynn

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