Friday, January 10, 2014

Types of winter storms that impact the Northeast.

I thought I would write a blog post on winter storms. The storms are given colorful names, which derive from where they originate and how they track.

Before I get into the types of winter storms; I will briefly explain about storms in general, and about something called cold air damming.  

Winter storms usually form when an air mass of cold, dry, Canadian air moves south and interacts with a warm, moist air mass moving north from the Gulf of Mexico. The point where these two air masses meet is called a front. If cold air advances and pushes away the warm air, it forms a cold front. When warm air advances, it rides up over the denser, cold air mass to form a warm front. If neither air mass advances, it forms a stationary front.

Winter storms operate on a contrast of warm and cold air. Because low pressure systems rotate in a counter clockwise direction in the northern hemisphere, they bring warmer air on the storms east (right) side, and pull colder air down on their west (left) side.

All storms pull air in from the surrounding area, and wrap it around the center of low pressure. As the circulating air lifts, clouds are produced and pushed away from the storm, clouds from which precipitation will fall. Winter storms can drop a variety of precipitation types: snow, sleet, freezing rain, or plain rain.

During the wintertime, the air ahead of an approaching storm can create a big problem.

Cold Air Damming (CAD) :

Cold air is dense and in most cases sinks. On the other hand, warm air is less dense, so it likes to rise.

CAD occurs when low pressure centers associated with large weather system track to the west of the Appalachians.  Generally, the northwest side of a low pressure system is the cold sector and the SE side is the warm sector.  When a low pressure tracks west of the Northeast, it pulls warm air from the South northward.

In the Northeast there are three types of CAD events; one has to look at the 850mb and 500mb, to know which to forecast for.  But basically CAD  is triggered by an area of strong high pressure to our north or west (over northern Maine/Canadian Maritimes, or over New York State).  Since high pressure has a clockwise spin in our neck of the woods, the high pressure pumps in cold air from the north / northeast.  As the cold air is pumped in from the north, it starts to sink to the south and west.  Eventually,  it runs into and starts to slide along the Appalachian Mountains.

This  topographically traps the cold air on the east side of the mountain chain, helping to enhance the cold air in place, but only at the surface. 

Now you can have warm air moving in from the other side of the Appalachian. or/and a onshore flow of warmer air.   Because the warm air is less dense it will rise up over the cold air (overrunning). This will allow  precipitation (rain) to fall through the cold air at the surface. The cold air will allow it to freeze either on the way down or on contact with the ground, trees, or power lines, in the form of freezing rain. This condition will persist until the cold air that is dammed up can be pushed out.

Here is a little about the types of winter storms we can get.

The storm tracks shown are for example, storm tracks can change a bit right or left, in actual events.

Coastal  Runner (Inland Runner):

The Coastal Runner is an area of low pressure that travels northward right along the coast, tracking northeast through metropolitan areas such as Washington DC, Philadelphia,  New York and Boston.  Coastal Runners typically bring heavy rains and flooding to the coastal areas, bringing  interior areas of New England and upstate NYS snow, sometimes even a blizzard.  


Great Lakes Cutter:

 As the name suggests, these are lows that go straight up into the lakes. A Cutter is a storm that develops out over the Plains states and tracks along a trough over the Western/Midwestern States  and moves northeast right over the Great lakes. This puts the Northeast and Mid Atlantic states on the warm side of the storm, we see southwest to south winds  funnel warm air up the east coast. Normally a Cutter will bring mostly rain to the Northeast, unless there is a CAD event.

Appalachian Runner (Apps Runner or Western Runner):

Appalachian Runners are another common storm that take place every winter. The Apps Runner as its name implies, is a low that cuts up the Appalachian Mountains. This spreads heavy snow into the Appalachian Mountains of extreme Northern South Carolina, North Carolina, and Virginia. For cities like Baltimore and Washington DC an Apps runner normally bring mostly rain with some mixed ice and sleet. in the winter they bring several inches of snow to the Ohio valley, Northwest PA, and maybe Western NYS, east of there they tend to give everyone rain, unless (like in a Cutter) there is CAD in-place in which case front-end snowfall or icing may occur. But they can bring snow into  Northern Pennsylvania and parts of Central and Eastern New York State. The snowfall in this type of storm is usually a high ratio /drier snow.


Alberta Clipper (Canadian Clipper): 

An Alberta clipper is generally an area of low pressure embedded in a strong jet stream flow, that develops and drops down from Alberta Canada moving south and east across the northern tier, during the winter months.  As their name implies a Clipper is a fast-moving system (like renowned fast moving clipper ships of the 19th century). Clippers are moisture-starved storms, typically bringing minor snow accumulation to the Northeast, but sometimes they can over produce bringing several inches of snow. Cold air normally follows in its wake, which can start up the lake effect snow machine across the Great Lakes snow belts.

There are two other types of Clipper systems.

Manitoba Mauler:

Manitoba Maulers as you can guess by its name, forms over the Provence of Manitoba Canada and moves SE. 

Saskatchewan Screamer:

A Saskatchewan Screamer, again named due to its place of origin.

Generally these systems will dig a little deeper than an Alberta clipper, and therefore, tap into some of the moisture from the Gulf of Mexico.  The result is that generally these systems can produce moderate amounts of snow.

Colorado Low:

The Colorado Low is an area of lower pressure that's a re-development of a previous system that entered the west coast and fell apart moving over the  Rocky Mountains; then reforms in the neighborhood of the state of Colorado (but sometimes it can develop over the Texas and Oklahoma Panhandles).  The area of low pressure will dip far enough south that a southerly flow will advect moisture into it from the Gulf of Mexico. A Colorado Low brings a wide range of weather types (snow, heavy rain, even tornado outbreaks), as it moves out of the Southern Plains and Midwest before moving into the Northeast. Usually this type of winter storm brings a narrow band of heavy precipitation on the western side of low. But, Colorado Lows can bring precipitation in the form of snow, sleet, or freezing rain into parts of the Northeast.


The Granddaddy of all  winter storms  is the Nor'easter. A Nor'easter is a powerful coastal low.  They get their name from the associated wind direction which comes from the northeast. The storm comes in two variants; the  first is called a  Miller A,  and the second type is called a Miller B. 


 Where the heavy snow falls and who it impacts depends primarily on the storm’s track, typically  swath of heavy snow sets up about 150-200 miles to the west and north of the Nor'easters  center.   Snowfall amounts  can vary greatly over a short distance, due to  small-scale bands of intense precipitation that can remain nearly stationary for several hours.  These bands develop in a region of stretched airflow called the “deformation zone”.

A deformation zone forms  when winds at the surface converge (blow toward the same region), and winds aloft above that same area diverge (blow in different directions).  When this occurs  an area of enhanced lifting sets up….leading to an concentrated band of heavy precipitation. 

High pressure over New England pumps cold air into the Nor’easter at low levels.  As the coastal storm tracks northward along the East Coast, it lays down a widespread swath of moderate to heavy snow along its cold, western side.


Miller A:


Miller A's may develop over the Gulf of Mexico,  along the Gulf Coast, or off the Southeast Coast, normally the North Carolina Outer Banks.  The warm Gulf Stream provides moisture and acts as an energy source farther intensifying the storm. As the storm moves up the Coast it also gathers  moisture from the Atlantic Ocean.  As the strong area of low pressure moves up the Eastern Seaboard, it will  phase, or combine, with other storm energy as it moves  north and east, deepening more and more as it heads for the Northeast. 
As the costal moves up the coast, high pressure will move in over  Northern areas of the Northeast or over southeastern Canada.  This area of high pressure pulls down plenty of cold Canadian air over the Northeast, giving the storm what it needs to make lots of snow.
When the conditions are just right, the central pressure of a Nor’easter can “bomb out” (rapidly deepen).  The heaviest snow typically falls while Nor’easters are rapidly deepening.
If the high pressure north of the Nor’easter is really strong and firmly anchored, it impedes the northward movement of the storm.   When this happens is can greatly slow or even stall the  Nor'easter, which can lead to  prolonged and very impressive snowfall.  

Miller B:

This type of Nor'easter forms when a strong area of low pressure approaches our region from the west, traversing the Ohio Valley.   As the primary system gets close to/moves over the Appalachians, it transfers its energy to the coast, creating a secondary low off shore, often off the Delmarva  peninsula  coast.  As the secondary low deepens and moves toward the Northeast.  Heavy snow starts to fall on the western (cold side) of the storm.
Miller B's are more difficult to forecast than Miller A's, you're watching two areas of low pressure,  everything depends on the timing and nature of the energy transfer, the location  of the transferred secondary storm center, time of year, temperatures, and a number of other factors.
Well that's about it, hope you enjoyed reading this post. My next post will cover the types of wintertime precipitation, including types of fog. It will also talk about up-sloping,  down- sloping, and a little about the  snow/ liquid equivalent ratio.


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