You've probably heard: the weather indicators are pointing towards rainy and unusually cold weather. Yet, our overall weather pattern hasn't changed much. We're still expecting a series of storms to move across the eastern Mediterranean this week (the first was last night and tonight (Monday-night), the third should be on Wednesday, a fourth on Friday, and another possibly early next week.
What's different, and importantly so, is that extremely cold air should settle in west of Greenland, over the northern European countries of Great Britain, Norway, Sweden, and Finland, and northern Russia. This is a long wave pattern that has been generally present for the last couple of weeks, but now is forecast to amplify as it gets colder. This is a change from last week's forecast for this upcoming time period.
Really, the most important center of cold air as far as we're concerned is the one forecast to be located over northern Europe, where our Friday's storm is suppose to originate. This is unusual because as noted at the Jerusalem Weather Forecast Facebook site (https://www.facebook.com/notes/jerusalem-weather-forecasts/a-detailed-summary-of-what-brings-major-snow-to-jerusalem/1957311591189265/) almost all of our snowstorms originate with cold air that plunges southward from Siberia through Turkey into the far eastern Mediterranean. One such storm was the December 2013 storm that buried Jerusalem in heavy, heavy snow over a three day period.
In this week's end of the week storm (Thursday night into Friday), the cold air should build southeastward from northern Europe. Some of the Global Ensemble Forecast System (GEFS) forecasts suggest that snow levels could be very low, as the mean temperature between the surface and 500 mb may be below freezing. The GEFS puts the changes of this type of cold at about 30%. The global model from the European Centre for Medium Range Weather Forecast (ECMWF) is actually forecasting snow in Jerusalem, even though this model's resolution is greater 10 km (so the topographical heights in the model are quite a bit lower than actual).
New forecasts are arriving as I write this, and I've been told that they are a bit warmer than today's earlier forecast. Right now, we can see with high certainty through Thursday, but Friday's storm remains at least as far as possible snow is concerned a mystery.