Weather It Is (Snow Advisory/Extreme Cold)

Good afternoon:

Extremely cold weather is on the way for the next couple of days.  Moreover, next week should see continued unusually cold weather with a good chance of accumulating snow.

Global forecasts show temperatures will soon drop below freezing at 850 mb.  Temperatures will also be unusually cold tonight at 700 mb.  However, relatively warm 500 mb temperatures precludes cooling of the surface layer in the mountain areas through mixing and evaporative cooling, which is usually the mechanism for how temperatures reach freezing or below during snow events.  Hence, during this possible snow event,  any change over to snow will occur simply because the lower atmosphere is chilling to unusually cold levels.

Our latest high resolution WRF ensemble forecasts show that precipitation amounts should be highest over the mountain areas of Jerusalem during the late afternoon and nighttime hour.  This is due to overrunning of moist sea air over cold lower atmospheric air.  This should produce good snowflake growth.  Total amounts of precipitation at the ground are  likely exceeding 25 centimeters.  Some of this precipitation is forecast to (not melt and)  fall as snow, but how much is hard to tell.

These WRF forecasts are based on the Global Ensemble Forecast System, and these are actually running a bit warmer than the European forecast model.  Hence, our forecasts may be underplaying the potential for an accumulating snow tonight.

Based on precipitation amounts, one might expect 10 cm generally to accumulate.  However, surface cooling is the key and this depends on which of the global forecasts verifies. A degree warmer, and just mixed rain and snow.  A degree or so colder, and several centimeters of accumulation in the higher elevations in and around Jerusalem. 

The probability for precipitation is high and the probability for extreme cold is very high.  Will they come together to produce accumulating snow?  Like many things in life, no one really knows, not even your weatherman.

Be well,

Barry Lynn