Derecho's worst expected to pass south of Connecticut

Update: 2:15 p.m. — The threats of the derecho to southern Connecticut have been downgraded Thursday.

An unusually strong low pressure system is forecast to continue moving east-southeast across Pennsylvania this afternoon, central New Jersey this evening, and just southeast of Long Island Thursday night, according to the National Weather Service.

That means Connecticut, remaining north of this forecast track, could just face potential freshwater flooding of poor drainage and low lying areas, streams and rivers.

There are now lower rainfall amounts expected here, although still a lot of rain: 1.5 to 2.5 inches in the Wilton area; with isolated amounts up to 4 inches.

The forecasted wind speeds are now also expected to be less: east winds 20-30 mph with gusts of 30-40 mph along the coast, including Long Island and southern Connecticut Thursday evening with lower speeds from New York City north and west.

Lower tidal departures of only around one foot with the approach of high tide will mean water levels below the minor flooding benchmarks.

11:06 a.m. — There are flood watches in effect for Wilton and all of southern New England through Saturday morning as a large, potentially dangerous storm, known as a derecho, hits southern Connecticut Thursday morning, June 13. This comes on the heels of storms last Friday and Monday that dropped a combined 5.5 inches on Wilton.

A derecho is a cluster of thunderstorms in a line that moves east or southeast, according to the National Weather Service. As it progresses, the winds get stronger, often causing severe damage in its path.

The wet side of the derecho is typically to the north side of an east-tracking storm and the wind and severe thunderstorm is typically to the south side, according to the state Department of Emergency Management and Homeland Security.

This storm system is forecast to move from the Ohio Valley Thursday morning to the New Jersey Coast by Thursday night putting most of New England on the wet side of the storm. The storm system is then forecast to slow down and move just South of New England Thursday night and Friday Morning, according to state emergency management officials.

To the north of the storm track, a heavy band of rain is forecast to stretch from central Pennsylvania through all of southern New England. Total rainfall within this band is forecast to average two to five inches with locally higher amounts possible.

Soil conditions are currently saturated from all the recent rainfall across the state, emergency officials noted. Flash flooding is expected to occur if more than 2.5 inches of rain falls in any six-hour period between Thursday at noon and Friday morning.

Urban flooding is expected if rainfall rates exceed one inch per hour during this time. There is also a threat from gusty winds in thunderstorms which may gust up to 45 mph inland and 50 mph along the coast. Given the saturated soils these wind gusts could down trees in a few towns.

The forecast as of Thursday morning, based on radar and computer modeling is as follows:

Thursday: Rain entered the state from west to east between 8-10 a.m. The rain is expected to fall moderately to occasionally heavy at times for a six-hour period through mid afternoon. Some urban flooding is expected from late morning into early this afternoon. A break in the rainfall may occur for several hours late this afternoon and early this evening.

Thursday night: Rain redeveloping and becoming heavy at times for six to 10 hours. Urban and small stream flooding can be expected tonight with the medium sized rivers also approaching and going into minor flooding overnight. Some gusty winds along the coast up to 40 mph at times may down a few tree limbs or whole trees resulting in a minor number of power outages. The rain is expected to end before daybreak on Friday.

Friday: Rain showers are forecast to end completely by noon. Urban flooding is expected to subside around daybreak. However the mid-sized rivers are forecast to continue to rise for a few hours Friday morning before cresting around noontime. The larger rivers are expected to continue rising through the day cresting either Friday night or during the day on Saturday.