Weather It Is (A Weather Review and Forecast)

Good Morning:

Today's rainy weather should generally leave between 10 and 25 mm of rain along the coastal regions.  There will be occasional lightning during the day as well.  There will be lesser amounts in the center of the country and up north.

Looking head, we see the possibility of another system approaching towards the end of the week.  At this time, it is not clear (from the ensemble data) how much rain should fall with this system. There is the possibility for significant rain, but it is equally as likely that rain will be generally light. 

I'd like to bring a greater understanding with regard to the recent heavy rain/lightning event that impacted the Jerusalem area Friday evening. There was also heavy rain and lightning from Beer-Sheva to the Dead Sea Basin.  This event was not forecast using the GFS forecast data from the previous night (the 0 GMT data), but only with the 6 GMT data (and an ensemble model). If you'd like a better understanding about the predictability of weather, please read on.

While speaking about the potential for terrorist attacks against the United States, Donald Rumsfeld -- then the Secretary of Defense -- stated: "There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know."

This quote is very relevant because it describes the state of the art with regard to weather forecasting.  It also provides an opportunity for improvement. 

1) Things we know we know: these are synoptic Systems often seen as areas of low or high pressure on the weather map.  This is the type of situation we forecast quite well and well in advance with our high resolution model (1.3 km single ("deterministic") forecast) a couple of weeks back, where there was heavy rain (> 100 mm) from Ashkelon eastward to Jerusalem. 

2) There are things we know we don’t know: the location and intensity of rainfall in a previous synoptically driven convective rain were forecast more accurately with our 4 km ensemble forecast.  In this situation, the coastal cities received localized heavy rain amounts of > 25 mm in three hours.

3) There are things we don’t know we don’t know: these are weather systems that originate in small convective clouds, and are smaller than the resolvable synoptic grid.  That is, they won't appear in the forecast grid of the synoptic scale model, even though they can (with a "fine eye")  be seen by satellite or in a lightning detection network.   Yet, these convective events  nevertheless grow up-scale to become mesoscale convective storms.  Once they reach the mesoscale, they are often resolvable by the global models presently in use today.   This is the event of Friday evening. It became apparent only with the 6 GMT forecast information -- that is, it grew upscale (with large enough size) to be "seen: (resolved) on the GFS forecast grid. Our high resolution forecast of this significant rain event became available just prior to the event, and was forecast only with our high resolution 4 km ensemble.  The deterministic model did not forecast this event.

There are avenues available for forecast improvement.  The use of satellite information or a lightning network might allow for the forecast of these events several hours in advance. 

Barry Lynn