Wednesday, December 30, 2015

Weather It Is (Winter Weather)

Good Afternoon:

I am bit worn out from following this storm.  Here are the highlights.

Through Saturday 2:00 A.M

Rain Amounts:  25 -- 50 mm across the hills east of the coastal plain; 10 to 25 mm in the coastal cities themselves.   Central Mountain: general 25 -- 50 mm, with > 50 mm in the area from Jerusalem south along the mountain spine.  Light rain in the Arava, but between 10 to 25 mm in the area of Beer-Sheva. Northern Mountains and Golan: 50 to 100 mm, with > 100 mm on the Golan.

Snow Amounts: 10 to 25 cm on the Golan and far northern mountains;  50 -- 100 cm on Har Hermon.

Winds:  Thursday > 50 km/h, and Friday > 60 km/h, with higher gusts.

Regarding the possibility of snow in Jerusalem. The first chance will be with the onset of the heaviest precipitation after midnight, Thursday night.  Temperatures at 850 mb will be falling below 0 and below minus 10 Celsius at 700 mb.  They will be close to minus 25 Celsius at 500 mb.  The next chance will be in the late afternoon and evening as temperatures at 850 mb and 700 mb drop to close to minus 3 C and minus 13 Celsius at 700 mb.

However, because the 500 mb temperatures will not be exceptionally cold, the precipitation will be mostly stratiform or orographic-convective, meaning that precipitation rates will not be overly high (except, perhaps, Thursday night when the cold air makes its entrance).  This means that snow amounts will not accumulate very quickly, and are more likely in areas higher than Jerusalem (like Gush Etzion). In fact, a first look at high resolution forecast surface temperatures in Jerusalem suggests that snow will not accumulate in Jerusalem, until possibly Erev Shabbat.

Lastly, we don't usually see a storm that is so cold at the 850 mb and 700 mb, but not at 500 mb.  Perhaps we're seeing the result of a lack of winter elsewhere, where the cold air has built up at the poles and is now able to head far south without strong upper level support (and an even stronger and prolonged ridge over Europe).

Looking further ahead: Motzei Shabbat appears to bring another 500 mb cold trough, but temperatures will start warming at middle and lower levels, so rain is more likely than wet snow (or with a possible mix).  The chances for snow are higher in the north.

There is still plenty of cold air to our north, and this will help to create a stormy weather pattern for the following week, if not more.

Barry Lynn
P.S. A more exact snowfall forecast will be made on Thursday, when our highest resolution forecast will extend until mid-day Friday.

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