Rainfall from two runs of the GFS model (in inches). The GFS model run from last night brings Irene ashore in South Carolina and tracks it northward over the mid-Atlantic resulting in 5 inches or more of rain. The GFS model run this morning tracks Irene from Cape Hatteras to Cape Cod with the South Carolina coast and interior mid-Atlantic just getting fringed by rain bands. (Graphics courtesy StormVistaWxModels.com)

The short answer is that it’s too early to say with much confidence. Why is that?

The image above should answer the question pretty clearly. Last night’s GFS model showed the entire metro region through the Northeast getting upwards of 5” of rain. Today it simulates just some showers, with the heavy rain pushed off to the east and north from the North Carolina Outer Banks to New England. The most likely time frame for heavy rain, should it materialize, is between late Saturday and early Monday.

Over at MarylandWeather.com, Frank Roylance quotes Jeffrey Halverson, associate professor of geography and environmental systems at UMBC, saying the following:

“For three days now, it [the GFS] has been portending a significant heavy rain event for the Mid Atlantic, and wind/high surf along the Eastern Shore. The track, heavy rain footprint and slow speed of the storm through the Mid Atlantic continues to look very Agnes (1972)-like.”

But the latest GFS simulation of Irene bears little resemblance to Agnes. And even in prior runs, Agnes had a very different evolution developing near the Yucatan Peninsula before tracking through the eastern Gulf of Mexico and up the East coast. We need to be very cautious bringing up “analog” storms to compare Irene to.

While everyone should be aware of the possibility of a heavy rain scenario this weekend, model guidance is likely to take another 36-48 hours before it locks in on the right track.

Those who were around for past storms may remember Hurricane Hugo in 1989 was forecast to bring heavy rain to the D.C. area but veered west. In 1985, there was all sorts of hype about Hurricane Gloria, but it went east, just grazing us.