Weather It Is (Cold and Wintry/Snow)

Good Evening:

A "Warning" means that conditions are expected.

A "Watch" means that there is potential for, but no certainty.

Winter Storm Watch for the Jerusalem area (snow and cold) for Tuesday night into Wednesday morning.

Winter Storm Warning for the Hermon and Golan.

Winter Storm Watch for Sefad for 5 to 10 cm of snow/ice beginning Tuesday afternoon and lasting into Wednesday morning.

Gale Warning for coastal and mountain areas from Tuesday afternoon into Wednesday Morning.

Potential for flooding from Netanya to Haifa, and eastward toward Sefad, including the Jordan Valley on Tuesday and Tuesday night.

On Sunday night snow fell and accumulated (lightly) in the higher elevations of Gush Etzion, while  snow accumulated on cars elsewhere.  Heavy snow fell on the Golan.  The cold temperatures in the Jerusalem area were well forecast by our high resolution model, with these models correctly predicting a near iso-thermal layer from 850 mb to the surface.

However, the precipitation amounts were smaller than forecast.  As pointed by Yaakov Consor, winds were southwesterly ahead of the storm.  As in the past, this has caused a delay in the precipitation reaching the central area, and also reduced overall amounts compared to forecast amounts.  What is it about the southwesterly winds that causes an error in the forecast precipitation amounts?  Based on our research, the most likely reason is a lack of sensitivity to high concentrations of dust, which creates a lot of small drops, delaying the formation of large, precipitating "hydrometeors."

In Sunday night's storm, temperatures were very cold at 500 mb, and marginally cold enough at 700 and 850 mb for snow in the central mountains.  In the storm arriving tomorrow/Tuesday night, temperatures will be 1 to 2 degrees colder  at both pressure levels -- yet there is little talk about snow tomorrow night. Moreover, as pointed out by Mr. Consor, winds will be swinging around from the northwest, which provides greater confidence in the the stream of moisture and precipitation forecast over the central mountains.

So, why is there little talk of snow?  The first reason is that many people believe that that temperatures should be close to minus 30 C at 500 mb for there to be snow in Jerusalem, and until now temperatures were forecast to be warmer than minus 25 degrees Celsius.  Very cold temperatures at 500 mb indicate a potential for stronger precipitation and more mixing of cold air downward. Warmer temperatures aloft would slow the mixing of colder air towards the surface, delaying (or preventing ) any change over of rain to snow.

However, the latest forecast shows better phasing of the 500 mb trough with the lower level storm, which is associated now with a range of temperatures between -25 Celsius and -27 Celcius.  This could create heavier periods of precipitation, which would enhance mixing of colder air to the ground. Moreover, the strong westerly, upslope (the mountain) winds should enhance mixing of lower level cold, and the precipitation is suppose to last from early evening until the morning hours, providing time for colder air to seep downwards.

The very cold temperatures are expected to last until Friday morning, when there will be a sharp warm up to even spring-like temperatures that should last a few days, before winter and another storm return early next week.

Will there be a snow accumulation  in Jerusalem?  We know that the global forecast models are certain about the cold at the lower levels and middle levels, but less certain about the southern extent of the upper level trough and its associated cold air.  This lack of certainty combined with the difficulty of predicting lower level mixing lowers the confidence we have for an accumulating snow.   Still, the cold temperatures should create dangerous/icy travelling conditions at the higher elevations surrounding Jerusalem and in Gush Etzion, which will last into the morning hours.

Based on the high resolution model's forecast of precipitation amounts, and associated snow and ice amounts, the potential exists for 3 to 5 cm of snow and ice in Jerusalem, 5 to 10 cm in the higher areas outside of Jerusalem, and 10 to 15 cm in the highest areas of Gush Etzion.

Will we reach the potential? Very difficult to say with any certainty.

Barry Lynn