Wednesday, September 28, 2016

A Wet Setup for the Eastern U.S.

Much of the northeastern U.S. has been dealing with abnormally dry to extreme drought conditions this past summer and early fall. Some relief came in the past two weeks in the southern part of the region, but that last U.S. Drought Monitor still depicted large areas of severe to extreme drought in the Northeast, with dry conditions generally extending southwest long the Appalachians into northern Georgia and Alabama.

Showers and thunderstorms, some severe, are already rolling through western and central Virginia and central North Carolina. This precipitation is associated with a somewhat complex surface system over the upper Ohio Valley and western North Carolina.

Surface map for 7:00 p.m. CDT Wednesday, September 28, 2016.

The surface system in turn is associated with a large closed upper low centered over central Indiana. That low has brought very cool weather to the Great Lakes and central Midwest along with the wet weather. Typically close or cutoff lows like this move very slowly, since they are separated from the overall jet stream flow. This upper low is expected to move very little over the next 48 to 72 hours, drifting south to northern Kentucky, then back north to northern Indiana.

500 millibar map for 7:00 p.m. CDT Wednesday, September 28, 2016

This kind of setup means a prolonged period of wet weather for the central Appalachians northward into New England. The heaviest rainfall is expected to be northeast of the low center. Over the next seven days the highest amounts are expected from the eastern Great Lakes across southern Pennsylvania into northern Virginia as well as southern New England.

Quantitative Precipitation Forecast for the 7-day period ending 7:00 p..m. CDT Wednesday, October 5, 2016.

The fly in the ointment at this point is Tropical Storm Matthew. It is currently located just east of the Windward Islands and is moving west. The latest forecasts take Matthew into the eastern and central Caribbean Sea by the weekend. Beyond that the forecast track for Matthew, which should by then be a hurricane, is uncertain and there is low confidence in the projected path.