A Mixed Bag of Precipitation – A Mixed Bag of Forecasting

Mixed precipBy Peter Collins Brown

The forecast for this Sunday into Tuesday (December 16-18) calls for a “wintry mix” of precipitation. This also means a messy forecast that is often plagued by the smallest of details and leads to many important questions. How cold is it going to be? Will it warm up enough to see only rain, or will we Bedfordites have to deal with a prolonged period of sleet, snow and freezing rain? We could see only one type of precipitation; then again, we could get all four types! Yet, most importantly for kids and parents alike is the issue of school on Monday and Tuesday. What exactly does this mixed bag of weather hold in store for Bedford?

Mixed precipitation—the mention of this term is enough to send shivers, literal and figurative, down any Bedfordite’s spine. The term “mixed” simply means that we have the chance of seeing rain, sleet, freezing rain and snow within the next five days. But, what is the difference between these precipitation types and their effect on Bedford? We all know what snow and rain are and their impacts on our daily lives. What about sleet and freezing rain? These are the two truly “mixed” types of precipitation—sleet, which is a mix of snow and ice; freezing rain, which is a mix of rain and ice.

Sleet begins its journey high in a winter stormas snow. As the snow falls to lower altitudes, it encounters a layer of above-freezing air. This freezing air layer in Bedford is caused by the nearby ocean, andair over the ocean in wintertime is typically warmer than air over land. This warmer air, carried by an easterly wind towards us, rises above the heavy, dense cold air at our surface. The snow becomes rain at an altitude of about 3,000 feet. But it takes a strong ocean wind of 25 to 35 mph to remove the cold air here. If the ocean wind is very light, a large layer of just-below-freezing air going up to about 2,000 feet stays in place; the rain refreezes into small balls of ice or sleet pellets. It’s too cold for any rain, but the cold air layer is not large enough to form snow again, so it forms sleet (which can be easily swept off of paths, sidewalks and roads).

Now, what about freezing rain? This is the true beast of any winter storm. Freezing rain begins life similarly to sleet. However, in this case, the ocean wind is a bit stronger than it is inthe sleet regime. There is a small cold layer high in the storm, and snow quickly falls into the warmer ocean air below. Due to thestronger ocean wind, most of the cold ground air is pushed out. Yet, in many inland towns like Bedford, if the air is below 30°F, the slightly stronger “seabreeze” is not enough to scour the cold air from ground-level to about 400 feet (A slight difference that can be extremely difficult for forecasters to pinpoint). The liquid rain falls into the sub-freezing air and solidifies on impact with the ground.

You can demonstrate the properties of freezing rain right in your own kitchen. Put a metal pan in your freezer for a half hour; then spray it lightly with cold water. The water droplets will freeze upon contact with the pan’s surface! Now imagine how slippery Bedford can get when a storm full of freezing rain comes its way.Freezing rain is a nightmare for anyone travelling in a car or trying to take off from an airport. Never mind that—In freezing rainwe can’t even keep our feet underus! Any small area of untreated surface will become treacherous. And It’s difficult  to sweep away or quickly melt a large area of solid ice.

So, what does our forecast look like for early next week? Will we be shoveling, sweeping, skating or just plain getting wet? It is going to be near freezing on Sunday when precipitation begins. A slight east wind will mean very little snow, with more in the way of sleet. The wind will shift to the southeast on Monday, bringing milder ocean air into Bedford. Since the ocean is still mild compared to mid-winter, the warmer air will win out, sparing us the effects of freezing rain. By Tuesday, it looks like plain drizzle will be with us, with temperatures in the high 30’s. Kids, school will be in session all of next week. Unfortunately, Bedford, Old Man Winter is here for now…

Keep our journalism strong! Support The Citizen Journalism Fund today. Contact The Bedford Citizen: editor@thebedfordcitizen.org or 781-325-8606

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