AccuWeather Global Weather Center -- 8 January 2014 -- AccuWeather.com reports following a buildup of ice this week, rain accompanying a thaw this weekend will bring the risk of sporadic flooding and ice jams to parts of the Northeast and Midwest.
Rain spreading from west to east Friday night into Saturday night will last less than 24 hours in most areas, but enough can fall in a short period of time to cause rises on small streams and in urban, poor drainage areas.
The single-digit and subzero cold of late has the amount of ice on streams and rivers increasing and becoming thicker in northern areas of the Midwest and Northeast.
With rain and warmer conditions on the way, the greatest risk for minor, sporadic flooding exists from northwestern Pennsylvania and western New York, eastward to central New England, and from northern and central Indiana to northwestern Ohio according to National Weather Service hydrologists.
According to Ronald Horwood, senior hazard hydrologist with the New England River Forecast Center, "There is some potential for flooding and ice jams this weekend, especially on the streams and smaller rivers from near Buffalo, N.Y., to Boston, Mass."
These areas have been very cold with locally heavy snow recently and will have the warmest weather with significant rain this weekend.
Farther west, there is a substantial amount of snow on the ground in much of Indiana and northwestern Ohio to neighboring Michigan. A fair amount of minor flooding is expected in the region with this weekend's mild air and rain coming in.
According to Joe Heim, senior hydrologist with the Ohio Valley River Forecast Center, "There is always the potential for very localized flooding due to ice jams and a thaw following a spell of cold weather, but ice jam flooding is dependent on the thickness of the ice, temperatures and rainfall that occurs, as well as the characteristics of each stream and river."
Meanwhile, in the mid-Atlantic region, the ice buildup on portions of the Susquehanna and Delaware rivers is being monitored.
"Fortunately, the ice is not very thick and most main channels appear to be open at this time Wednesday on the major rivers," Senior Hydrologist Charles Ross of the Mid-Atlantic River Forecast Center said Wednesday morning.
Ross cautioned that conditions along streams and rivers could change rapidly when the ice begins to break up and starts to move.
Later on Wednesday, one area where the ice became bottled up was along the Delaware River near Trenton, N.J., to Philadelphia. A flash flood watch has been issued for areas long the Delaware in this region.
A general 0.50 to 1.00 inch of rain will fall with locally higher amounts this weekend.
If more rain falls than is currently expected, then the potential for widespread flooding and ice jams will increase.
Most areas do not have a significant amount of snow on the ground to add to the runoff. However, a lack of snow cover and a frozen ground can lead to a more rapid runoff and sporadic problems where ice gets jammed up.
The duration of the cold spells has been short enough to keep ice formation to a minimum over all but the upper reaches of the Midwest, northern New York state and northern New England.
"The areas farther north, near the Canada border, will likely not get enough rain and high enough temperatures to cause rapid melting and shifting of the ice with this weekend's event," Horwood said.
Ice May be Dangerous
While ice has rebounded after last week's thaw, it is likely not thick enough to be venturing on in most areas.
"In my opinion, it is too dangerous to be on the ice due to the changeable conditions so far this winter keeping the ice too thin in much of the mid-Atlantic," Ross added.
The same conditions exist over much of the Ohio Valley.
Additional Hazards This Weekend
In addition to the risk of urban, small stream and ice jam flooding this weekend, motorists should be on the lookout for potholes.
As the ground thaws and rain falls, traffic moving over weak areas of pavement can produce potholes.
A more serious concern for areas that received heavy lake-effect snow in recent days and were hit by the snowstorm in the Midwest last weekend is the potential for roof collapses.
Not enough rain may fall to wash away all of the snow on roofs. The added weight can stress some flat and shallow angle roofs to the point of failure, especially where there is a more than a foot of snow accumulation. Remove snow from your roof only if you can do so safely. If there is any doubt, consult a professional.
By Alex Sosnowski, Expert Senior Meteorologist for AccuWeather.com