Weather It Is (Disgruntled, and Rightfully So -- Yet...)

Good Afternoon:

Dust Storm Warning: Tuesday will see very high dust concentrations throughout the country.  Tuesday night, the high dust concentrations will be pushed southward as the cold front enters, and be confined to the areas south of Jerusalem on Wednesday.  In the areas south of the confluence zone, dust concentrations will remain very high (e.g., in Beer-Sheva, Arad, and southward).

This rest of this conversation concerns the forecast for Jerusalem.  There are no changes to forecasts elsewhere.

People are disgruntled.  At various times during the last week it appeared favourable not only for snow in the north, but for the center as well.  Then, yesterday, the highest resolution (single/deterministic) forecast indicated that the storm -- although fierce -- would only graze the Jerusalem center area.  Thus, it was necessary to  to downplay the potential for snow in the center.

Yet, the GFS ensemble shows that there is still a lot of variability in the forecast development of the 500 mb wave pattern.  Temperatures Wednesday evening on average are forecast to be near -27 C, but could be as cold as -30 C (or as warm as -22 C). In contrast, the 700 mb and 850 mb temperatures are pretty much holding within a narrow range.  Hence, the eventual details of the storm (in terms of precipitation amounts and snow elevations) depend on the evolution of the 500 mb wave pattern and are still subject to change.

Also, if you look at the forecast precipitation amounts for the center they remain quite high. Given that the GFS ensemble data is a grid size of 100 X 100 km, we don't expect the model to forecast sharp gradients in precipitation (like we wrote about earlier).  However, what this tells us is that the potential for heavier precipitation to occur in the center is still there -- it just depends on how much the storm deviates from its current forecast path -- something that can happen even a couple days in advance when the variability in the 500 mb pattern is so high.

In summary: this forecast is not yet written, or rather history is not yet written.

Friday will bring another chance for snow, but only a chance.

Barry Lynn