Weather It Is (Our Weather and Predicting Snow in Jerusalem)

Good Afternoon:

The "mid-week" storm we spoke about last week will arrive later today. It will arrive as light to moderate rain in most of the country. Temperatures aloft (at 500 mb) will be cooling off to comparatively cold levels by Thursday afternoon; however, the available moisture will be less. Hence, there is the possibility of a light wet snow at the higher elevations, but little or no accumulation is expected. Moreover, surface temperatures will probably be too warm in most locations (except the Hermon) for snow to accumulate as well, regardless of precipitation amounts. 

Another storm will arrive on Shabbat. It should produce more precipitation. Right now, there is only a 10% chance of snow with this storm (except on the Hermon, where snow chances are higher).

Late next week, there is the possibility for a more significant winter storm.

Barry Lynn

Here is our our latest thoughts on predicting snow in Jerusalem.  

Prepared by Dr. Barry Lynn: Weather It is, LTD, Efrat, Israel

Over the last several years, our weather has been unusually snowy. Using our high-resolution forecast model with a general picture of the synoptic (general) weather pattern has allowed us to achieve unparalleled success in forecasts of snowfall accumulation.  

Nevertheless, we over-predicted the snow amounts from the first January snowstorm of 2015 in the center of the country, where there was only about 30% of the snow minimally forecast.  Comparing our high resolution forecast from last year's December snow to this year's January snow provides us with enough data to be able to better utilize the high resolution forecast data going forward. That is, we expect to further improve our forecast predictions of snow amounts by better utilizing the high resolution data in the future.

Last year, the high resolution model data forecast a lot of snow in a very favorable snow environment.  This year, the high resolution forecast predicted relatively small snow accumulations compared to last year, even though the synoptic environment was again highly favorable for snow. The high resolution forecast became available about 12 hours before the start of the snow in the north (where our forecast was very good), and the following discussion might have mentioned that the latest forecast indicted reduced snowfall forecast amounts. 

Given the analysis below, it is now obvious that there was higher likelihood of less snow (even though there was significant amounts) in the neighborhoods of Jerusalem and Gush Etzion) than last winter's December snowstorm. 

The attached "pdf" shows our analysis of last year's and this year's snow events. For those of you are interested, I will summarize it here.

Figure 1: The forecast snow amounts in December 2013 are quite high compared to those in January 2015.  Only Neve Ativ and Sefad were forecast in this January storm to have snow amounts similar to what fell in last winter's storm in Jerusalem and Gush Etzion. This should have been the most obvious indication that snow amounts were to be less in the high elevations location in the  center of the country.

Figure 2: Almost 50% of the total precipitation in the Old City was suppose to accumulate as snow in last winter's storm.  Less than 20% of the total was to accumulate as snow in this January's storm, and there was less than half the total amount of precipitation in this year’s storm compared to last winter’s storm. Moreover, surface temperatures were forecast to be near or below freezing in the December Storm, but not in this year's January storm.

Figure 3: The hourly rate of precipitation in the Old City was much higher in the last winter’s December storm than in this year's January storm. Moreover, the snow rate was a much higher fraction of the total precipitation rate (assuming a 1 ml to 1 cm of precipitation to snow ratio).

Figure 4: This shows the same information as Figure 3, but for Efrat (Gush Etzion) where there was more snow in December's storm and in January's storm than in the Old City.  The hourly rates of precipitation were similar in both locations, but the fraction of snow to precipitation was higher. Our analysis showed that this was due to colder forecast surface temperatures in Gush Etzion than in Jerusalem's Old City.  The observations confirmed this analysis.

Figure 5: This shows precipitation and snow rates for Neve-Ativ and Sefad for this January's storm.  The precipitation rates were higher than in the center. Moreover, the snow rates were higher in each location than in the center. In Neve-Ativ, the ration of snow to precipitation was almost 1 to 1.

In summary: the high resolution forecasts appear to be a very useful tool for snowfall prediction-- even more useful than a general analysis of the synoptic situation – especially for the prediction of relative snow amounts in Jerusalem and location in the north from storm to storm. 

Further analysis showed that using the model to predict snow might lead to an error with a factor of 2 in the forecast snow amounts, while using the synoptic picture in combination with forecast precipitation amounts can lead to an error in order of magnitude (10 times).

Barry Lynn