The last weekend of April is upon us, but so is a clipper system that will have enough cold air to work with to produce snow from southern MN to southern lower MI this afternoon into tonight. Several inches of accumulation are likely. But how is this possible? Isn't the ground to warm this time of year? Aren't highs both today and Sunday near 50 degrees? Yes, these are important risk factors, but one advantage is the snow is coming at night. It will melt quickly on Sunday, but not before providing a wintry scene briefly early Sunday morning.
A area of Low Pressure will strengthen as it moves into Iowa around mid-day before moving through northern Indiana and Ohio tonight. The Low may weaken slightly as it does so, but it will still be strong enough to deliver a period of moderate to heavy precipitation to portions of southern lower Michigan tonight.
Waking up this morning, lows will be the lower-30s. The morning will be sunny with steadily warming temperatures before high cloudiness begins to move in from the west around mid-day. The sooner and thicker these clouds build, the cooler highs will be and the quicker temperatures will cool down tonight, which means the sooner the transition to snow can occur. Highs in most areas will be in the upper-40s to around 50 with an overcast sky developing by mid-to-late afternoon.
Despite the clouds, the air near the surface will be quite dry. Rain will be trying to move in from the west by late afternoon and early evening, but likely initially evaporating before reaching the ground. At this point, temperatures are still in the 40s and the lower levels are too dry to support precipitation, so we need rapid cooling and moistening if its going to snow.
SUPPORT FOR ACCUMULATING SNOW:
During the evening, surface winds will turn to the north/northeast as the Low Pressure center moves closer, meanwhile winds aloft will continue to advect in moisture from the southwest. With sunset and winds out of the northeast, temperatures will be falling and rain will eventually begin making it to the ground. This is because precipitation is expected to be moderate to heavy at times, which will inevitably saturate the entire layer, allowing rain to reach the ground. As the precipitation continues to fall, the layer will also cool due to evaporational cooling. As a result, the surface cools and saturates with time. Additionally, wet-bulb temperatures are likely to be below freezing, either in the lower-30s or even upper-20s, which means the air will continue to cool with favorable mechanisms in place until reaching these values. As a result a transition over to all snow is expected and the sooner it happens, the higher the potential snowfall totals.
With the moderate to heavy precipitation forecast, the snow should be able to overcome the warm ground temperatures. Elevated surfaces, rooftops, bridges, and grassy surfaces will be most susceptible to accumulations. Roads will likely become slushy and at times snowy and slippery, especially where the heaviest snowfall rates occur. The snow may fall at 1" per hour at times, with the total duration of the snow between about 8pm (depending on exact timing of cooling and moistening near the surface) and 4am.
RISKS FOR ACCUMULATING SNOW:
If the lower levels do not cool and/or saturate quickly enough, the duration of falling snow will be shortened, cutting back on snowfall totals. Additionally, if the snow is too light it will not be able to effectively stick to the ground. This time of year, the snow needs to be heavy enough to overcome the warm ground, since light snow will melt on contact. I am in favor with the discussion in the previous section and reflect that in my forecast below. However, even a slight shift in track, or a slight temperature difference could mean the difference between rain cutting back snow totals or heavy snow dominating and pushing near 6 inches in some spots. This is an event where it is essential from the standpoint of a forecaster to communicate the uncertainty in the forecast. This is a setup that contains more uncertainty than usual. Therefore, I ask all of you to please understand the uncertainty exists and if anything is unclear, messaging me questions is encouraged. Below are my official thoughts on the forecast.
Along and north of M-46 (west-east road that runs through Alma) isolated drizzle or flurries are possible, otherwise, south of M-46, the forecast is as follows: Clouds will increase tomorrow and rain will develop by early evening. A transition to all wet snow is expected between 8pm and 11pm becoming heavy at times. The snow will end around 4am Sunday morning. Accumulations of 1 to 3 inches are expected north of I-96 and 2 to 5 inches south of I-96. Slippery roads are possible Sunday morning while temperatures are still near freezing. Winter Weather Advisories are in effect from 4PM this afternoon until 8AM Sunday. Snow will melt quickly Sunday morning and afternoon as the sun returns and highs climb into the low to mid-50s. The forecast maps are provided below:
Snow Forecast by City:
Mount Pleasant: Trace to None
Detroit: 1" of less
Grand Rapids: 1-4"