Moisture from Bill will enhance rainfall across part of the Midwest by Friday and some of the Northeast on Saturday, leading to localized flood concerns.
The clockwise flow around high pressure anchored in the Southeast states will guide tropical moisture and the rainfall from Bill through the middle Mississippi and Ohio valleys on Friday into Saturday.
During Saturday and Sunday, the moisture will spread over parts of the Northeast.
At the minimum, enough rain will fall to cause flooding of urban and poor drainage areas with the likelihood of travel delays and disruptions to daily activities.
There is the potential for small stream flooding and perhaps rises on some of the major rivers, where the rain persists.
In a significant part of the Ohio Valley, 3-6 inches of rain is forecast with locally higher amounts.
In parts of the Northeast, 1-3 inches of rain is forecast but amounts can also be higher at the local level.
How strong the high remains will determine the north/south extent of Bill's heaviest rain and greatest risk for flooding over the Ohio Valley, the mid-Atlantic and New England.
Forward speed of Bill's rain is also another variable with the potential for the system to linger in Texas through Wednesday.
Tropical systems of this nature are prone to interacting with non-tropical features and can result in a narrow zone of intense rainfall and flooding or sometimes a pattern of more isolated drenching showers and thunderstorms.
In this case, it could be a combination of both.
According to AccuWeather Chief Meteorologist Elliot Abrams, "Since the system is still days away, we don't want to spray a huge area of the Midwest and Northeast with flooding, when in fact there is uncertainty about the rainfall pattern this far out."
A front dropping in across the Upper Midwest and southern Canada may stay totally separate from Bill's moisture or join with it and enhance the rainfall.
"In the Northeast, Bill's rain is most likely to take a path similar to that of the showers and thunderstorms since the past weekend," Abrams said.
This area encompasses a swath mainly from Ohio to Pennsylvania, New York state and New Jersey but could shift north or south depending on the strength of the southern high and position of the front farther north.
"As the storm begins to roll out of the southern Plains, we will narrow down the zone with the greatest potential for flooding in the Midwest and Northeast and adjust timing accordingly," Abrams said.
Because of the uncertainty of the timing and location of the heaviest rain, people should not change outdoor plans, but be prepared by having an alternative in place.