Weather It Is (The Weather "Cabal.")

Good Morning:

Highlights: Wind, Dust, Chill, and Occasional Rain.

I received an interesting e-mail question.  "I'm sure you've explained this before, but when you talk about some GFS ensemble members that are showing the possibility of X or Y, does that mean an actual person/organization that is putting out an actual forecast, or is it simply one of several hypothetical possibilities that X or Y could happen?

"I mean, who is this 'member'? 

"(And are they sitting in some place putting out forecasts that are consistently wrong?  If so, wouldn't they get fired by now?)"

It is true that different government and private organizations do put out forecasts, and one could consolidate these forecasts into a group, and then discuss which "member" of the group says one thing or another. We might refer to these forecasts as the "Elders of Weathzion" (not to be confused with the "Elders of Zion" which is a construct of the ignorant and antisemites).

However, this is not the way we do things around here (although, I do listen in on other forecasts for comparison purposes, and an occasional tidbit of information). 

Rather, we're speaking here about a single forecast model (the Global Forecast Model (GFS) or our own Weather Research and Forecasting Model (WRF))  that is "tweaked" to produce multiple forecasts for the next two weeks (or 36 hours in case of the WRF).  How do they tweak it?  Keep in mind that we never exactly know the weather at any time.  For instance, my weather station, which is located in the Dekel area of Efrat, might measure a different temperature (wind, etc) than someone else's in the Rimon neighborhood.  Even more so, my observations themselves have a margin of error.

So, the folks who run NCEP (The National Center for Environmental Prediction) in their wisdom systematically perturb their measurements and then see what impact this has on the forecast.  As time goes on (several days), the impact can be quite substantial (and even over a single day if the weather is "mesoscale" (small) rather than "synoptic" (large)  scale).  However, when all members of the ensemble predict nearly the same weather, even several days down the road, it means that the "signal" (or (significant or strong) wave development) is such to imply that the initial condition "error" is less  important than whatever weather system is on its way. Such was the case before the large winter snow storm of December 2013. Hence, we have greater confidence in the forecast when many or even all members of the ensemble agree that an event will happen.

So, perhaps some of the error that our writer mentions is a result of initial condition uncertainty, and not a systematic error in the forecasts.

Speaking of which, we've been speaking about rain and cold weather this week for quite a while. After a warm day today, we indeed expect much cooler (chilly) temperatures and a chance of rain.  However, the chance for significant rain event is quite small.

Still, the water vapor loop shows a moisture stream from off eastern Atlantic, off the African coast.

http://tropic.ssec.wisc.edu/real-time/europe/movies/m7wvbbm/m7wvbbmjava.html

In fact, the first batch of heavier rain showers will likely occur late tonight and tomorrow morning (extending southward to Beer-Sheva).  Moisture from the Atlantic stream will combine with mositure from the Mediterranean to produce these showers.  The cold air will follow, and produce a second batch that will be mostly confined to the north central areas, and will occur on Tuesday.  Intermittent shower will occur in the cold air until the end of the week.

The main highlight here is the wind and chill, with strong or even gale force winds on Monday and Tuesday. Ahead of the today's storm, there will be dust overspreading the country. Heavier dust concentrations in the south will also make for difficult breathing for sensitive people.

We should start to warm up on Shabbat, but there is a 25% chance of significant rain late next week, with showers and colder temperatures again likely -- just in time for Purim.

Shavua Tov,

Barry Lynn