Illustration (Image credit: Wix)
It looks like the next few days will be quite hot, with temperatures in the upper 20s to low 30s C (70s to 80s F) until Sunday. The hot weather is a result of complementary weather systems. The first is a high pressure located over northern Africa, which is nosing its way northwards along the coast. The hot winds are blowing from southwest around this area of high pressure. The second is actually a series of weak low pressure areas moving along the southern Mediterranean coast. The winds associated with the low pressure systems are called sharav or hamsim winds.
Yet, no matter what you call them, they are hot winds. Moreover, for those participating in various races or other sporting events, the dry heat associated with them can be dangerous. So, it may interest you to know that I received a phone call asking about the intensity of the heat for the races in Gush Etzion this Friday.
While I would like to help, one has to be careful when providing an answer to such an important question. One can look at global model forecasts, but the grid-spacing is too large and the time intervals too coarse to provide a meaningful or useful answer — though this is done far too often. Yet, when I mentioned that I would need a bit of time to formulate an answer — meaning to run forecast simulations at the appropriate grid size-spacing and forecast output intervals — I was told that they had to decide and that they would make do with the information they had.
Of course, this approach is not an optimal use of forecasts. For instance, our web site shows that the temperature range during the race should be between 28 and 32 C (82 and 90F). It also shows that the sharav will be approaching the coast after the race, which implies that the peak temperatures will be during the race.
Considering the summary of heat stress effects at this U.S. National Weather Service link, one might be concerned that temperatures would be approaching levels that can lead to heat cramps and heat strokes. However, the forecast on our web site while providing a general idea of the weather — it will be hot — was not run on a high enough grid-resolution to provide the details the race organizers needed to make an informed decision. In addition, there is the issue of risk, which can only be determined by a probabilistic forecast, i.e., an ensemble forecast. The goal would be to calculate the chance that temperatures, for example, will exceed certain values during the race itself.
All of these approaches — using higher grid-resolution forecasts and making multiple (ensemble) forecasts — can be done given some 'heads-up' warning time and the will and the recognition of the necessity to obtain the best information available. The same models can be used to predict heavy rains that can lead to floods. In fact, after some showers on Friday, there may be showers on Sunday; these showers could be followed by a steadier rain as a storm that looks right out of winter drops down from the northwest. The steadier rain could also again lead to floods, but also help keep up our green spring.
Otherwise, we shouldn't just 'make-do' until tragedies happen. Stay cool and keep an eye on the sky.
Jerusalem forecast (click here for updated national forecasts):
Image credit: The Jerusalem Herald
Dr. Lynn is a lecturer at The Hebrew University of Jerusalem, Earth Sciences Department. He is also CEO of Weather It Is, LTD, a company that specializes in reducing weather risk.