Thread by @althecat

The Euro @ECMWF model has been out of line with the @NOAA's as well as the CMC, KMA, UKMet wrt to the Middle East forecasts. I think I know why. And here are three animations that show this. The Euro model does not acknowledge the change related to moisture crossing the Sahara.

That said it is coming increasingly into line. The difference is most obvious in the EPS (Euro) ensemble model.

The GEFS is the corresponding GFS ensemble model for the same metric - 500hpa Height - which is a proxy for pressure - red/orange = higher.

Ensemble models put together lots of simulation model runs to try to average the outliers and increase confidence.

Some additional context.
1. The GFS model is the current gold standard global model. The ECMWF is its peer. The GFS is also longer (16 days vs 10 days).
2. The current models from both are currently like chalk and cheese. Latest runs below ECMWF (left) GFS (right)

[META:This explainer thread is going to be a fair bit longer than 3 animations. Sorry.]

& context 3. The ECMWF has started acknowledging transport of water across the Sahara due to a strong West African Monsoon, but it is not yet forecasting any Sahara rain.

Latest EPS vs CMC

[Correction: I am using 500hpa height anomaly here not height itself]

To continue....

The next two animations are the same measurement. "Geopotential Height 500hpa (a proxy for pressure)" first for the latest ECMWF and then in the next tweet for the GFS.

& GFS (note that this one is longer)

The difference is easy to miss so I have pulled out the relevant slides at 240 Hours. At the beginning they are roughly the same. But at 240 hours there is a massively important difference.

The greenblob [GFS - right image, top left corner previous tweet] is a low pressure proxy. It is the result of an new infusion of wet air which has transited the Sahara into the Middle East which ECMWF has not seen. And it is about to become a rain band across Iraq and Syria.

The images in (Previous Tweet PT) show précipitable water held in the atmosphere. Which is an ingredient for thunderstorms. The second ingredient is energy, which we will get to soon. The models are close but not quite the same at this point.

Here are wind forecasts at the same time - Middle East (top images) Europe/North Africa (bottom images) - and now you can see how much the models have diverged.

The 2nd ingredient for thunderstorms (1st being moisture) is Energy. Unfortunately I have no exact comparable metric to compare ECMWF/GFS. But instead we can look at animations of what we do have over the full period - K-Index in the case of ECMWF & CAPE in the case of GFS.

Now its pretty clear these are not the same thing in the context of the ME. K-Index is associated with moving wet air masses - Cape is more closely linked to the wet air masses over the Red Sea, Arabian Sea and the Gulf. However we can see the trajectory in each over time.

And here is the CAPE index from GFS.

My take out from this comparison is that K-Index readings reduce over time (indicating increasing stability) while CAPE is steadily increasing (indicating increasing instability).

In summary then:
- ECMWF simulations have not predicted the recent #ArabianStorms which were predicted by most other models.
- ECMWF forecasts include less water transport across the Sahara.
- ECMWF forecasts less moisture and less energy in ME = fewer storms.