The details of next week's forecast remain obscure. The key, as mentioned previously, is the eventual positioning of a high pressure ridge over central Europe and the development of a low pressure trough over the eastern Mediterranean.
There is plenty of cold air available to produce a winter snowstorm. However, the possibility that the air will warm as it passes over the sea indicates that surface temperatures may be too warm for a heavy snow accumulation in the center and higher peaks of the Galilee. On the other hand, the storm will have strong upper air support (which means very cold temperatures), so graupel (sleet-like) and hail are a very strong possibility, as well as snow as the coldest arrives on Wednesday and Thursday. The snow may fall, but not accumulate except in typically cold areas surrounding Jerusalem.
The details pertaining to the arrival of the coldest air and its duration remain fuzzy. Two ensemble members are indicating a direct infusion of cold air, which would drop temperatures to unprecedented levels -- below both December 2013's snow storm and the storm the previous January (as cold as -40 C at 500mb, -20C at 700 mb, and nearly -10 C at 850 mb). In general, though, we are expecting temperatures at 850 mb below freezing from sometime Wednesday into Friday or Shabbat, with temperatures at -12 C at 700 mb and -30 C at 500 mb. Caution: to bring a large accumulating snow the 850 mb temperatures should be close to -3 C or lower, and we don't see the details yet regarding these temperatures.
Here's what we know:
1) Strong winds on Monday and colder temperatures. Possibly some rain.
2) Precipitation amounts will likely exceed 50 ml in most locations, and possibly 100 ml by the storms end. There will be lightning, with frequent lightning late Tuesday and Wednesday.
3) Rain with gale force winds (and possibly storm force gusts) on Tuesday.
4) Rain and possibly snow in the higher elevations on Wednesday. Gale force winds with storm force gusts again possible.
5) Rain and snow possible on Thursday.
6) More than 1 m of snow on the Hermon.
We are completing a long period of relatively tranquil weather. While it was rather warm and pleasant here, it was very stormy in most of Europe as a trough of low pressure remained anchored over northern Europe. This pattern has now shifted to the east and there appears to be a steady supply of of unseasonably cold air. to maintain this pattern. Hence, the extended forecast beyond Thursday shows that the unusual cold will persist into the following week (almost two weeks from now), and depending again on the positioning of the ridge and trough could bring more rain and snow.