Why We Cannot Be More Specific as to Locations

A question I'm getting is why we cannot yet be more specific as to where Sandy's worst effects will be. It is a fair question and I'd like to answer it.

1. The storm is still more than 72 hours away. Should be self-explanatory.
2. In part, because of #1, the model guidance is inconsistent and will be for a while.
3. The models we use more than 24 hours out cannot replicate some of the small scale processes involved with Sandy.

Let's consider reason #2 for a moment.

Click to enlarge. Last two runs of U.S. GFS model

The area in red is roughly the area where winds may be high enough to cause power failures. It is huge. But, look more closely (click to enlarge). The GFS model run using data from 8am EDT this morning (left) has an area of wind gusts of more than 90 mph near Washington, DC (grayish brown color) but nothing of that magnitude near Boston. The run from 2am EDT this morning (right) has gusts to near 100 mph off the coast from Boston but nothing nearly that high near D.C.  

Why the difference? Even with computers that operate at 172 trillion calculations per second (!) that is not fast enough to make these worldwide forecasts on a fine enough scale to capture the thunderstorm-related complexes within Sandy that will cause locally stronger winds (reason #3). Plus, we do not sample the atmosphere at a fine enough scale.

When we get within 24 to 36 hours in advance, we switch to models with much better resolution (2.5 mile data input) that are "focused" on the area of interest (i.e., to greatly decrease computation time as opposed to making a forecast for the entire world). Those models attempt to replicate, on a limited scale, the processes that occur in thunderstorms. So, we should be able to give some advance notice of where the highest winds will occur and the peak speeds.

In the meantime, I'm forecasting gusts of 80 mph or higher as a general number so people can prepare. Will some areas have higher gusts? Likely so, but I can't forecast exactly where as yet.

A final thought: Based on some media coverage I'm seeing, the threat of flooding in coastal areas is being underplayed. Especially, if you live ten feet or lower above sea level, you need to be prepared to evacuate should the order be given.