okx_skewT_0105.gifWhat can Gothamist say about the upcoming weather other than it sucks eggs? Frankly, we have no idea what "sucks eggs" means or where that phrase comes from, but it doesn't sound good and neither does our weather. Look for rain to mix with sleet and snow as the day progresses. If it is still raining after the ground freezes look out for freezing rain in the mix. Eventually all the rain will turn to snow. Expect snow to dominate the precipitation mix in the northern suburbs, where they are under a winter storm warning.

The conspirators in this mess are a warm front to our south, a low pressure system in the Ohio Valley, and an Arctic air mass coming down from Ontario. Here's the drill: Right now we have warm air at the surface, warmer air above that, and cold air above the warmer air. Snow is falling from the top cold layer of air. It melts in the warmer air below and reaches the ground as rain. As the Arctic air mass moves southward it is going to cool the lower layer of the atmosphere first. Precipitation will then start as snow, melt to rain, then re-freeze, producing sleet. Not all the precipitation will do this, however. Some will not not refreeze, giving us rain, some snow will not melt and will reach the ground as snow. Once the ground temperature falls below freezing and if it is still raining, the rain will freeze when it hits the ground. All the rain will turn to snow once the Arctic air pushes out the warm air.

For the weather geeks among us the graph above depicts temperature (red) and dew poitn (green) data from this morning's weather balloon launch from the National Weather Service office in Upton. Full diagram here.