Ask the Weather Team
Ross asks: Why is the heaviest precipitation to the west and strongest wind to the east of a storm track?
Pete Bouchard says: If you're talking tropical systems, this is almost always the case due to the fact that maximum sustained winds are added to the forward speed of a storm. In the case of Irene, the northward movement through New York City and into Vermont placed the strongest winds were over Central and Eastern Massachusetts. To get an idea what we're talking about, look at the image below from the Hurricane of '38. The maximum sustained winds from the storm were 115 mph. Adding the forward speed of the storm (an unheard of - and never before seen - 70mph) to 115 mph gives you maximum winds of 185 mph. To the west, however, the forward speed is subtracted from the maximum winds, yielding wind speeds of 45 mph - a staggering difference!
In addition, tropical systems are undergoing a transition to a more nor'easter-like storm when they move into our higher latitudes, pushing the heaviest rain to the west of the track. There are several dynamic processes going on too that aid in this transformation.
If you're talking about run-of-the-mill storms like nor'easters, this is sometimes the case, depending on track. Due to decrease in friction (no trees, buildings or mountains), wind speed over water is 10-15 mph faster than that over land. This contributes to the coastal flooding, beach erosion, large waves, and, of course, higher winds along the coast.
Rainfall is greater over the hills and mountains due to what is known as orographic uplift. Mountains force air to rise higher, squeezing out more moisture - and subsequently more precipitation. This affect is shown below, and is one of the major reasons points north and west of I-495, in the higher terrain, get more rain (and snow) during a nor'easter.
It goes without saying that the higher the mountains, the heavier precipitation. Note in the image that in higher mountain ranges (like those in Northern New England) there can be a "rain shadow" affect, where very little precipitation falls in the lee of the mountain range. This is one of the reasons places in Northern New England can get a foot of snow in one spot, and only a few miles away get close to nothing in another.
Kathy asks: Is it possible for a tornado to touch down in the Cambridge/Boston area?
Pete Bouchard says: It is entirely possible! Tornadoes can happen ANYWHERE, and just because we're in a city, it doesn't mean we're in the clear. Fact is, tornadoes can and do strike in cities. Oklahoma City, Miami and even New York have had tornadoes touch down within city limits. Although we're not in Tornado Alley, we have had several tornadoes touch down in the metro area since 1950:
Amy asks: Is this one of the top 10 snowiest years?
Pete Bouchard says: Not quite yet. We're knocking on the door though. So far this winter we've topped 60" in Boston (normal for the entire season is 41.8"), that is just shy of the 73.1" we need to make it in the top 10.
We've been mentioning the top ten snowiest Januarys, however, and this one ranks as the third highest so far.
1) Jan. 2005 43.1
2) Jan. 1996 39.8"
3) Jan. 2011 38.3"
Dave asks: Why does Norwood always seem to be the coldest spot on the temp. maps?
Pete Bouchard says: The temperature is taken at the Norwood airport, which is in a relatively low spot compared to the surrounding terrain. It is also in the shadow of the Blue Hills. Both of those topographic facts mean that the cold air likes to "puddle" there, creating much colder readings than in nearby towns and cities - so much so, it "throws" the temperature range in our nightly forecast!
Chris asks: What is the Greenland Block and how does it affect us if we're so far away?
Pete Bouchard says: There's been a lot of talk about this block over Greenland both in this winter and last. It's been responsible for hammering the Mid Atlantic with snow last year, and producing our near-blizzard this Christmas.
Best way to describe it is to use an analogy. Think of a big boulder in a a stream. Water is diverted around the boulder and eddies are formed. Same goes for the high altitude "river of air" that steers our storm systems. If there's a high pressure system that won't budge in the jetstream, the storms are diverted around it.
When this high pressure system (or block) over Greenland is situated just right, we are at a junction point for storms. They form, slow down, and are redirected right near us. We can get whacked by snow (or rain if the weather is warm enough) for a couple of days before the storm finds a way around the block.
Typically these blocks last for several weeks when they set up, depending on their strength and size. Over the last decade, they have at times been gargantuan - logjamming weather systems as far back as the Great Lakes. Meteorologists from Environment Canada have theorized that the reason these blocks are large and persistent is because the Arctic Ocean is melting and a cause and effect feedback has been established between warmer ocean temperatures and upper atmospheric high pressure. Whether or not that is a major part of the puzzle of climate change is still being debated, but one thing's for sure: we've got to start boning up on the forecasts for Greenland.
