STRIKING diff. btw 06z GFS 1200z prediction and 12z sounding

[tallywx]

As you can see, the GFS 06z 6-hour prediction for 12z shows a much weaker ridge than the ETA initialized based on 12z upper-air sounding data this morning. Pay particular attention to the 588 height line, which on the GFS barely clips the SE coast, but on the actual 12z plot covers the entire SE U.S. This could have major track implications for a more w-ward motion than even the much-corrected GFS currently predicts.

06z GFS prediction:


Actual:

[PurdueWx80]

An Eta initialization isn't necessarily what the actual height lines look like. Check out actual obs overlaid w/ a RUC initiation:



Still, your point is quite valid, and the 12Z Eta does respond to a stronger ridge by moving Frances into south FL.

[tallywx]

The 12z RUC obs seem to match up fairly well with the 12z ETA. The only differences I see are less of a 588 "bulge" into Ohio. Otherwise, pretty good match-up.

I wonder more how a stronger ridge would affect a GFS output, because using an ETA forecast for a hurricane is not wise, since the ETA can't take into consideration the strength of the system. I was using it just to show the "initial" 12z conditions to contrast that with GFS prediction of the 500 mb evolution.