2006_03_graupel.jpgFebruary review: Lots of temperature swings as the atmospheric circulation pattern shifted mid-month. Central Park temperatures wound up slightly above average for the month. Precipitation was slightly below normal but more than an average amount of snow fell. All in one storm.

March preview: In like a lamb today. Tomorrow will be a bear. Pussycat weather on Friday.

Today will be very pleasant, if cool. Tomorrow is looking ugly. You have waterproof boots, right? The Weather Service has issued a Winter Storm Watch for most of the day tomorrow. They are expecting 5-8 inches of snow, sleet and possibly freezing rain during the day. Looking at the techie forecast discussion shows that they may reduce the predicted snowfall. The Weather Channel is calling for warmer weather, which introduces rain into the mix and reduces the snowfall amounts to 1-3 inches. AccuWeather's forecast is remarkably free of major storm hyperbole and they make the good point that tomorrow's snow, unlike February's, is going to be wet and heavy.

Since we may have five different types of precipitation tomorrow we thought we would give a short review because they often get confused. Pretty much all precipitation starts out as snow, or at least tiny ice crystals. If the ice crystals stay frozen all the way down to the ground we call it snow. If the crystals melt on their descent they become rain. That's obvious. People often confuse sleet with freezing rain. When ice crystals fall through a warm layer of air, melt, and refreeze before hitting the ground they are known as sleet. Sleet is normally transparent and bounces. Freezing rain stays liquid until hitting a frozen surface, upon which it forms a layer of ice, making driving hazardous and ripping down tree limbs and power lines. One form of precipitation we may get tomorrow that nobody has mentioned is Gothamist's favorite: graupel. Graupel looks like tiny tiny snowballs and forms supercooled cloud droplets freeze onto snowflakes.

Microscopic image of graupel from the Department of Agriculture