Thread by @NickPBassill

So #Laura's current position has been repeatedly south of what models forecast. Let's examine why that might be the case.

Look at this GFS trend in upper-level PV (shaded) and 850 vorticity valid at 00Z. Note how #Marco gets stronger with each forecast, and Laura dips south.

As #Marco is (correctly) shown to be stronger, latent heat release increases, and contributes to the upper level ridge to the north of #Laura (east of Marco). You can see this by the steady increase in deep blues (low PV values) with a stronger Marco. (same loop here as above)

That could be a coincidence, but now let's use some amusingly poor #HWRF parent domain forecasts to diagnose this impact. As above, here's a model trend in the outer domain of #Laura's forecasts, which only model #Marco on the lower-res parent domain (and poorly, I might add).

Let's compare 2 of those runs side-by-side (newest at right). The first has almost no #Marco, and puts the upper-high north of Haiti, which imparts very zonal flow over #Laura. The second has a better Marco, a high now north of Cuba, and thus has more meridional flow over Laura.

5/5) Incidentally, stuff like this is why I said we should be cautious about interpreting hi-res forecasts if the nests aren't run concurrently. (Although in this case most models struggled with #Marco and #Laura)

Thanks as always to @TropicalTidbits

https://twitter.com/NickPBassill/status/1296896839426416647