Weather It Is (Two Storms/One Snowy)

Good Afternoon:

There are two storms on the way: the first will bring liquid precipitation to all but the Hermon (where 30 to 50 cm of snow will fall). This will also bring strong winds. The second has a high probability to be the coldest storm in this decade, but it will be a fast mover, and snow amounts will depend on important details still to unfold.  It will also bring gale force winds.

If one follows the travails of the  Global Forecast System's model (GFS) , then one notes that it is a warm "outlier" among the GFS ensembles (the "GEFS").   WRF high resolution forecasts based on the GFS have forecast a light to moderate snow in Jerusalem (and more in Gush Etzion) to actually no snow in  Jerusalem during our upcoming storm.  However, both the GEFS and European model (the "ECMWF" ensemble) have been trending towards colder temperatures, which would bring more snow to the central mountains.

A positive (or cold) trend does not mean that the trend will continue until it snows (a lot), but rather that the initial conditions and their spread now encompass more cold and snowy forecasts.  This means that the more we know about the upcoming weather, the more it "looks like" snow (as more forecasts end up producing colder temperatures and plenty of precipitation) despite using initial conditions that deviate from what our observations tell us.


Right now, about 30% of the forecasts (6/20) show a brief period of moderate to heavy snow in the afternoon/evening, which could leave Jerusalem without anything more than slush.  Thirty-five percent of the forecasts (7/20) suggest a moderate accumulation (5 to 10 cm) in Jerusalem from Wednesday afternoon into Thursday night.  Another 35% of the forecasts suggest a heavier snow that will last into early Thursday morning (not to mention the one forecast that prolongs snow into Thursday afternoon -- a 5% chance).  Along with the heavier snowfalls are extended hours of colder surface temperatures below freezing.

So, the chance for accumulating snow in Jerusalem is 70% -- assuming that the probabilities truly represent the possible range of initial conditions valid for the observed set of conditions (measured with satellite, radiosondes, airplanes, profilers, and surface stations).

The snow and freezing temperatures are most likely to arrive in the central mountains late Wednesday morning and will probably be a mixture of ice-pellets and snow, which will likely cause a quick icing of the roads. Combined with the winds and snow, blizzard conditions are certainly possible, if not likely during periods of heavy snow

At this time, we should stress that there is still a 35% chance of just slush in Jerusalem.  The next 12 to 24 hours should provide us with enough information to know where we'll be in the distribution, or where snowflakes will fall and stick.

Barry Lynn