Monday, January 20, 2014

Snowbands & Why They Love Long Island

If you've ever experienced a coastal winter storm while living on Long Island or the surrounding Metro area then you've likely seen the following evolution: A forecast comes out calling for a little bit of snow in the next few days which incites little panic among the public. The day before the snowstorm the forecast changes because there had been a lot of model uncertainty and suddenly the forecast snow totals are a lot higher and panic ensues! (Cue bread and milk chaos!) The storm causes numerous traffic problems including airport and even highway closures throughout the Tri-state area. The snowstorm ends and it turns out that your buddy a few towns over got a ton more snow than you did. What gives? The forecast snow totals were correct, but for your buddy and not for you. Some examples include the February Blizzard of 2013, "Snowmageddon" of December 26-27 2010, among many others. This scenario rings some bells, right? Let's get to the bottom of it.

Long Island is jutting right out into the storm track that is very active during the cool season months, i.e. October - March where coastal lows tend to develop after upper-atmospheric energy is transferred from the west over the Appalachian Mountains and interacts with the existing temperature gradient (baroclinic zone) that is in place at the coast sandwiched between the cold, continental air over the Eastern U.S. and the warm, moist air over the Western Atlantic Ocean fed by the Gulf Stream.

North American Storm Tracks (Source: Understanding Weather and Climate, 6th ed. by Aguado & Burt)
Once a coastal low forms, it usually strengthens and moves northeastward along the coast up towards New England. The broad area of precipiation that is usually light in nature can organize itself into narrow areas of intense snowfall, known as a snowband. A snowband can be a few miles wide and a few tens to about a hundred miles long-- and like most things they come in a lot of different shapes and sizes. When a coastal low forms, there is typically warm air to the southeast of it and cold air to the northwest of its center. Air moves in a cyclonic or counterclockwise direction which acts to create fronts, or boundaries between the distinct air masses. A snowband that is set to impact Long Island typically forms north of the warm front where warm, moist air is ascending over a wall of colder, more dense air. As the low strengthens, this warm air is vigorously churned counterclockwise towards the west and the snowband pivots to the northwest of the surface cyclone center. As the low matures the snowband can sometimes be found to the west of the surface cyclone center. You can see the positions of the snowband relative to the surface low in the following schematic:

The recipe for a very strong snowband includes strong winds clashing together to the northwest of a low pressure center that extends a few miles up into the atmosphere, sufficient moisture so that there is plenty of water vapor to condense into clouds and provide a lot of precipitation, and just like with summertime thunderstorms there should be a little bit of instability or a region of air in which a pocket of air will be less dense and able to rise rapidly. A lot of research has been completed investigating these snowbands and a Stony Brook University alumnus, David Novak, published many peer-reviewed papers about those that occur in the Northeast. He looked at many storms and determined the general ingredients needed to cook up a great snowband as summarized in the following figure.

The ingredients for a mesoscale snowband modified from Novak et al. 2004.
The two locations highlighted by the stars indicate the preferred locations for snowbands. In the red shading are regions of frontogenesis, or the creation (i.e. genesis) of fronts or the clashing of two air masses. The region enveloped in a scalloped line shows a region of deformation or where the winds have a component that are converging which enhances frontogenesis. While there may be snow all around the surface low for miles and miles it shouldn't be as heavy as within the regions of snowbands indicated by those stars-- all thanks to frontogenesis. Let's talk more about frontogenesis and not just because it's a really fun word to say.

Frontogenesis, or the formation of fronts, means that the atmosphere is unbalanced. The fact that there is a temperature gradient (that's what a front is) means that the atmosphere is a bit unstable and wants to even out that temperature gradient. How can it create a uniform temperature distribution if it has to fight against the really strong horizontal winds that keep making the temperature gradient stronger? To combat the imbalance, the atmosphere initiates a vertical wind circulation because when you can't go out you go up, right? The atmosphere induces a frontogenetical circulation that causes warm air to rise and cool on the warm side of the front (cooling down the warmer air) and cold air to descend and warm on the cold side of the front (warming up the colder air). This warm, rising air is usually incredibly moist so there's plenty of water vapor to create deep clouds that produce heavy snow and with that the snowband is formed. This circulation is usually very small because the temperature gradient at the front itself is very narrow in width so the snowband is very narrow as well. A summarizing schematic is provided below that shows a horizontal view of the snowband (yellow star) to the northwest of the surface low with the counterclockwise winds in the upper-right corner above a cross section showing the vertical circulation that causes a lot more snow to fall on your buddy's house (yellow star) versus your house (cozy cottage to the west or mansion to the east depending on your preference).

So why doesn't the weather forecaster know exactly where the snowband is going to set up and instead calls for a large amount of snow for the whole region? As you are now aware of, snowbands are very sensitive to particular ingredients with the most tricky being the location of the surface low pressure center. If the low travels a bit farther off of the coast then so will the snowband and the East End of Long Island may get hit a lot worse than NYC. If the low tracks closer to NYC then perhaps NJ will get hit harder. See the pattern? The following image shows the different weather model simulated locations of the surface low with the orange markers corresponding to the position of the low at 7 PM on Tuesday, January 20th from the Weather Prediction Center. There's quite a spread in locations, huh?

Low tracks from the Weather Prediction Center. Source
 Another issue is the fact that we've discussed that snowbands are a relatively small-scale phenomena. If weather models have really coarse resolution, or they can only predict larger systems better because of the size of their grid systems, then small-scale snowbands are really a challenge for them. Thankfully there is a whole suite of weather models made just for the purpose of looking at small-scale weather phenomena known as mesoscale models. Stony Brook University actually runs a two-member model ensemble in real-time and the model output can be accessed from our webpage. So what do our models say about the storm? They each tell a different story which is interesting but evident that this storm is presenting a real forecasting challenge. Here is a snapshot of simulated reflectivity from the 7 AM model-run valid for 1 AM on Wednesday, January 22nd.

SBU-WRF comparison simulated reflectivity for 1 AM EST January 22, 2014.

The model on the right has a snowband over central Long Island (indicated by the star but please don't expect stars to appear on actual radar data) whereas the model on the left doesn't have that feature. The model on the right also has the low pressure center farther northwestward towards the coast which is helpful with getting snowbands closer to the Island. In looking at the model more in-depth, the ingredients are there for snowband formation-- a well-defined low-to-mid-level circulation, deformation north and west of that circulation center and sufficient moisture and instability at times (especially ~11 PM EST). The fact that temperatures will remain quite cold throughout the most important layers of the atmosphere means that the snow will be dry and fluffy and this fact could lead to large accumulations versus if it was wet (see the NWS infographic on snow ratios).

As of 11 PM EST on Monday, January 20th the NWS has issued a Winter Storm Warning for most of the Tri-state area from noon on Tuesday to 6 AM on Wednesday. The heaviest snowfall should occur after 5 PM EST and the amounts can total as much as 14 inches with the snowband expected to affect the eastern portions of Long Island. You can pay attention to the regional doppler radar for narrow swaths of larger values of reflectivity that indicate a snowband and see where one sets up tomorrow night. Remember to stay safe and stay warm but most of all, enjoy the snow!

- For more information about mesoscale snowbands see this webpage:

- For forecasts and warnings please see the local New York Forecast Office of the National Weather Service (and check out their Facebook Page here!):

- For information about the formation of snowflakes and how they'll be observed at the ground please see this previous post:

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