Thursday, April 24, 2014
David Epstein has been a meteorologist for more than 25 years. He spent 16 years in Boston and currently freelances at WGME13 in Portland.
In 2006, David founded GrowingWisdom.com, a business producing educational and marketing videos for the green industry. He currently is a professor at Framingham State College, teaches Jan Plan at Colby College and owns Bloomscapes Inc., a landscape design business.
David authored "Gardens Of New England" and his work has been published in Grolier's Science Annual for 10 years. He lives in South Natick, Mass., and has a summer home in Harpswell.
Tonight meteorologists and weather enthusiasts are quite excited to watch the upcoming development of the big ocean storm which will form tomorrow. You may be wondering what the big deal is about a storm that is more of a miss than a hit and you are certainly justified in that thinking. However, let me put the size of the storm in perspective.
When we evaluate storms, one of the ways we note the strength of a low pressure center is by its pressure. Storms exist to balance the atmosphere. Their purpose is to mix cold and warm air and try to achieve equilibrium. When a large difference in temperature exists and energy from the jet stream rides over this contrast storms often develop.
Air rises in storms, and as the air leaves the planet the pressure lowers. Wind is the air rushing into the storm trying to fill the void being created by its formation. If the storm is really large the air can’t fill the void fast enough and the pressure continues to fall. The faster the pressure drops the more wind we experience as those winds are attempting to balance the rising air.
Normally, on an average day, the pressure of the air above you exerts about 1013 millibars of pressure. In a typical strong storm this pressure can fall to 980 millibars. At that level of pressure a storm would generate a considerable amount of wind and precipitation. The faster the pressure falls each hour during a storms development the greater the intensity.
Astronomical spring arrives today just before 1PM at 12:57, if you are being precise. The reason I write astronomical spring is because meteorological spring began 3 weeks ago. Meteorologists and astronomers see the seasons differently. For those of us in the weather business, spring is the 90 days between the coldest and warmest 90 days of the year. Autumn therefore becomes the 90 days of transition between the hottest and coldest 90 day periods heading in the other direction.
This might seems like semantics, but when you purely focus on temperature the seasons are all shifted back about three weeks. It’s actually quite logical because the first three weeks of March are likely warmer than the first three weeks of December.
At any rate, spring has arrived on both fronts and while we haven’t seen much prolonged spring weather this will change.
Precipitation type is an interesting thing. There are times you think it’s cold enough for snow and it rains and times you think it’s warm enough for rain and it snows. To understand they type of precipitation we see here at the ground think about the atmosphere like a layer cake.
In the clouds, where precipitation forms is analogous to the top of the cake and the ground where the precipitation reaches is the bottom or crust (I like pie). What happens in the middle is quite critical for determining what for the precipitation will take.
Notice in the diagram below the red color represents air above freezing. In the snow situation, there isn’t any air above freezing. As the snow falls from the clouds, it’s able to make it to the ground without melting. In the other three situations, the warm air melts the snow before it reaches earth.
There’s a term some meteorologists use called “model hugging.” Basically, this means buying those models you hear so much about hook, line and sinker. The thing about the models, they are guides to what will happen in the future, not gospel.
Any model is not 100 percent correct, just ask an economist. When I forecast a storm I certainly review all the models, but ultimately I have to make the call myself. When it's three or four days before a storm, the models often are still all over the place predicting how the atmosphere will come together.
The models are amazing and it’s a wonder to me we can forecast the formation of anything 10 days in advance. Yet, they often don't have the accuracy needed for a confident forecast more than 72 hours ahead of time. This is why you hear so many "ifs" in the long range.
On a planetary scale, knowing a storm will form is important, but to get the forecast right for an area that includes Bangor to Bridgton and south to Portland, the model has to predict the track of a storm within 50 to 100 miles - otherwise the forecast won’t be accurate. The model could show a storm's track correctly 10 days in advance, but the odds are much higher it will do so two days before an event.
Welcome back to January. If the spring weather on Tuesday made you think winter was over, the howling wind and snow showers today will make you think it’s not going to end.
The mild air made quite a push westward and most of this storm fell as rain south of a line from north of Sanford to Naples, east through Lewiston/Auburn and on to the midcoast. For a time last evening the rain pushed as far west as Bridgton and Fryeburg, but then it quickly retreated east again.
Ski areas will continue to see some accumulating snow much of the day with another 3 to 6 inches. Elsewhere, 1 to 3 inches of snow could fall by mid-afternoon today. I expect a few spots in the mountains to hit the 18-inch mark or maybe even a bit more if some of the heavier bands of snow move through. This is going to extend the ski season by a couple of weeks.
The cold air remains in place through the entire day and overnight. When you get up tomorrow it will be at or below zero inland and in the low single numbers along the coast. The record low for Portland tomorrow is 2 below zero. While we won’t break that record, a low of 5 or 6 above for Friday will still make March 14 this year one of the coldest in recent memory.