Feet upon feet of lake-effect snow paralyzed parts of western New York late last week. Parts of the Lake Michigan and Lake Superior snowbelts have also been digging out from as much as 30 inches of snow.

(MORE: Snow Recap|Latest Impacts|Photos)

A temporary change in the weather pattern brought additional concern for stressed roofs and low-lying areas in the snow-weary Great Lakes.

Pattern Change

The pattern responsible for the relentless, record-breaking cold, not to mention the heavy lake-effect snow, finally backed off.

This allowed warmer air to return to the Upper Midwest and Great Lakes, finally boosting temperatures well above freezing in most of the Great Lakes. Buffalo, New York, recorded a high of 65 degrees Monday, just 3 degrees shy of a record for the date.

Rain accompanied the advance of warm air spreading north. The heaviest rain fell over Michigan, prompting flood advisories and a few flood warnings for some rivers in western Lower Michigan. Most of the flooding was minor.

Rain swept through New York fairly quickly and was somewhat scattered. Buffalo's official rain total was 0.13 inch on Monday.

Water content of snowpack in the eastern Great Lakes on November 21, 2014. Purple/pink shading over the Buffalo southtowns (circled) suggest from 3.9 to 9.8 inches of water contained in the snowpack.

(NOAA/NOHRSC)

Snow sags off the roof of a home in Lackawanna, New York on Nov. 18, 2014.

(JeanetLackawanna via weather.com/photos)

Adding Stress and Snowmelt

The first image at right is an analysis from NOAA's National Operational Hydrologic Remote Sensing Center from early Friday morning, estimating the water content in the snowpack. Circled in white, NOHRSC estimates anywhere from 3.9 to almost 10 inches of liquid water equivalent locked up in the heaviest snow-blitzed areas of western New York.

The National Weather Service office in Buffalo measured this directly on Friday by simply sticking a hollow cylinder into the snowpack, extracting a core of snow, and melting it down. They found that the hardest hit areas contain between four and six inches of water in the snowpack, with a few locations in Erie and Genessee counties having more than six inches of water.

Why does this matter?

With temperatures having risen above freezing for several days (and nights), and the rain that has fallen, there was a risk of flooding on creeks and rivers that drain Great Lakes snowbelt areas recently hit by heavy snow, particularly in the Lake Erie snowbelt.

Also, there's the stress the heavy snowpack in parts of the Great Lakes has been exerting on rooftops. There have already been more than 30 major roof collapses reported in western New York alone.

Severe weather expert Dr. Greg Forbes calculated a 12-inch snow cover on a 1000 square-foot flat roof, using a snow/water ratio of 10 inches of snow for every inch of water, amounts to a weight of 6240 pounds.

Normally, rain will run off a snowless roof through downspouts and gutters. However, a rooftop snowpack will absorb falling rain, adding weight to the now heavy, slushy snow.

Forbes says if an inch of rain were to somehow remain on the roof, say due to clogged gutters/downspouts, that would weigh 5200 pounds on that same 1000 square-foot flat roof. That 12-inch snowpack after a one-inch rain on a flat roof would weigh as much as 11,440 pounds, or over 5.7 tons!

And, yes, that's just a 12-inch snowpack. As you can see from the photo above, some homes in western New York had much more snow on their roofs.

The National Weather Service in Buffalo, New York, passed along the following tips:

- Snow removal from roofs is dangerous and should be done only by those qualified to do so following proper safety procedures.

- If you see sagging tiles or boards, or hear popping, cracking or creaking, your home or building is at risk of a roof collapse. Evacuate it immediately!

- Keep high-efficiency furnace pipes clear of snow, to avoid carbon monoxide buildup in your home. Make sure carbon monoxide detectors are working in your home.

Flooding

The sudden thaw, coming after a long period of unseasonably cold temperatures and heavy snow, resulted in the threat of flooding in western New York.

Ice jams were reported on Cazenovia Creek in the Buffalo area Sunday afternoon. Within a few hours, the ice jams appeared to have broken up; however, in any area where ice clogs a river or stream, water could back up behind it and cause flooding.

Flooding affected several roads in the Buffalo area Monday. Minor flooding is expected on several creeks and streams draining the areas with the heaviest snowfall.

Flood advisories and warnings were posted Monday for several rivers in western Lower Michigan.

MORE ON WEATHER.COM: Lake-Effect Snowstorm - Nov. 2014