The slow thaw starts Sunday, but not without issues

Rain, wet snow, freezing rain and sleet all possible

The slow thaw starts Sunday, but not without issues
Snow at Seattle's Space Needle Saturday. (Morgan Palmer)

Finally the snowstorm is over. The remaining areas of snow across mainly King County north will continue to dwindle into the overnight. We will enter a relative lull in precipitation through mid-morning Sunday. However, it will remain very snowy and icy and any slushy spots will re-freeze tonight so travel will remain difficult into Sunday morning.

SUNDAY PRECIPITATION TYPE: Temperatures will rise on Sunday to above freezing in many spots by late afternoon, but to get there it will be messy at times through the day Sunday. As moisture moves in off the Pacific and temperatures warm aloft to above freezing, the surface temperatures will stay below freezing through the morning. This means we could see a variety of precipitation types from just some pockets of wet snow, to sleet to a cold rain or a combination of the three. There could even be some freezing rain, particularly across the far south in the morning into early afternoon. By later in the afternoon, more locations from Seattle south could see more rain showers than snow or sleet while across the north, it probably stays cold enough for snow for most of the day Sunday.

WHAT ABOUT SNOW? Moisture won’t be as rich with this next weak weather system so amounts won’t be too terribly great. Models are hinting at another inch or so of slush in many locations – give or take – through Sunday. Obviously while this doesn’t seriously exacerbate our travel woes, it doesn’t help matters much. The late-day hours above freezing will help melt the snow naturally but it’s a long thaw ahead.

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URBAN FLOODING/STRUCTURAL FAILURES FROM RAIN INTO SNOW: The biggest problem we will likely face is the rain that will fall into the snow we have on the ground. This problem will continue into Monday. We could see some structure collapses (from outdoor dining tents to carports and sheds to – hopefully not – roofs) as the snow is weighed down and traps rainwater. Also the water may not be able to find a way to open drainages causing standing/backing up water and urban flooding. This is a problem we always face after a heavy snowfall and we will deal with this Sunday (where it rains) and into Monday and Tuesday as snow will be latest to thaw sufficiently where it has piled up the most.

FREEZING RAIN EARLY MONDAY MORNING? : Another possible issue will be areas of freezing rain or sleet on Sunday night into Monday morning. Temperatures will continue to warm aloft, bringing rain but the question is where will temperatures cool sufficiently overnight Sunday night into Monday morning to bring the surface back to below freezing. Some models hint at some issues with freezing rain icing of tree limbs, power lines and bare pavement, particularly toward the Cascade foothills Monday morning and perhaps in scattered locations around Puget Sound. Other forecast models just promote a more typical changeover from snow to rain with little or no icy funny business. But this is something we’ll probably just have to be watching for as it happens – and it may very well not.  Very small temperature variations will make significant differences in precipitation type and are difficult to predict.

THAWING NEXT WEEK: After mid Monday morning, temperatures will be rising and snow levels will pop up to above 1,500 feet. This will end our winter weather threat for the foreseeable future. However, the melt will take a few days beyond that with highs in the 40s and lows in the 30s so the first half of the work week will continue to bring the above-mentioned issues of urban flooding and standing water.