Favorable Patterns for NC Winter Weather
North Carolina winter weather evolves from complex meteorological patterns that are difficult to forecast, and which lead to considerable variability in weather conditions across the state. As a result, the frequency and intensity of winter weather events are highly variable and dependent upon large (synoptic) and small (mesoscale) scale atmospheric patterns.
Large-Scale (Synoptic Scale)
1. Jet Stream (Eastern U.S.)
- The jet stream is a narrow band of strong winds in the upper atmosphere. In winter, it separates cold, Arctic air masses from warmer, continental air masses. Certain regions of the jet are also favorable for storm development, while others are not.
- The orientation and location of the jet stream has large implications on the weather year-round, but even more so in winter because the jet stream is stronger due to a large temperature contrast from the north to south.
- Many factors (such as climate patterns and upper-level winds) can alter the path of the jet stream. When the jet is aligned in a more west to east orientation, NC usually experiences relatively mild winter weather, with temperatures near average. When the jet stream dives southward over the eastern U.S., cold air is allowed to flow all the way from Canada or points northward into the base of the trough, and if the base is south of N.C., then things get mighty chilly for our state!
2. Cold Air Outbreaks
- North Carolina is susceptible to large variations in temperature during the winter due to its location and geographical features. When the jet stream dives south in the winter, it can bring bitterly cold, Arctic air all the way into North Carolina. When this happens, temperatures may drop as much as 20 to 30°F below normal! Typically, these very cold Arctic outbreaks will ease in a few days once the air mass begins to moderate.
Notice how low the maximum temperatures drop during the cold air outbreak that occurred from January 15-17th, 2009.The coldest day was on the 16th when Boone only reaches 16°F, Raleigh only reaches 28°F, and even Wilmington only makes it to 30°F!
1. Cold Air Damming
- Cold air damming (CAD) occurs when a shallow layer of cold air becomes trapped at the surface, with warmer air residing just above it. The Appalachian Mountains provide a barrier that prevents the cold air from scouring out, and allows it to dam up against them. Combine that with the fact that cold air is denser than warm air, thus preventing it from rising, and the shallow cold air becomes trapped.
- When cold air damming works in tandem with a storm system which transports moisture into the cold air in place over NC, wintry precipitation occurs. Depending on how deep the layer of cold air is, the precipitation can fall as snow, sleet, freezing rain, or just a cold rain. Often, events may begin as snow or sleet (or a snow/sleet mixture) before the warmer, moist air begins to erode some of the cold air and changes the precipitation over to freezing rain or just a cold rain.
2. Storm Tracks
- The path a winter storm takes significantly effects the type and distribution of precipitation across the state.
A more inland path over NC typically results in:
- Greatest coverage of winter weather in the western piedmont and NC Mountains
- Southerly winds from the low pressure system
- Warmer temperatures moving well inland
- Rain or even severe weather over central and eastern NC
A slightly offshore path (50-100 miles) allows for:
- Northerly winds
- Colder temperatures
- Greater coverage of winter weather in the Piedmont and western Coastal Plain
- The precipitation shield not extending as far to the west, often excluding areas such as Greensboro and Winston-Salem altogether
If the storm system tracks farther offshore:
- Cold air will be pulled all the way to the coast
- The precipitation shield will mainly stay east of I-95
- Coastal regions will receive significant amounts of snow
- The Piedmont and western Coastal Plain will receive little to no precipitation
3. Coastal Fronts
- A coastal front is a mesoscale feature that develop off the coast in response to strong temperature contrasts between air over the warm Gulf Stream and cold air over the Carolinas, often associated with CAD events. These fronts can be a catalyst for generation and/or intensification of coastal storms just offshore of the Carolina coast. In winter when a coastal front moves onshore, temperatures can increase by 30-50°F! Obviously, this can have major implications on winter weather; coastal fronts can mark the dividing line between rain and snow, but can also serve to enhance precipitation rates!
- A coastal front can significantly increase or decrease the amount of wintry precipitation. This is dependent upon a given areas location relevant to the front; (if an area is close to the front, but on the wintry side then precipitation rates may be higher, thus increasing accumulations of frozen precipitation, but if the area is on the warm side of the front then precipitation is likely to change over to rain, thus reducing wintry precipitation.)
4. Snow Banding
- Often in NC snowstorms, the upper-level features become favorable for mesoscale banding of precipitation. When this occurs, bands of slow moving, heavy snow set up which can result in highly variable snowfall amounts across a small area. Although forecasters can usually identify when conditions are favorable for snow banding, it is very difficult to pinpoint where they will occur until the event is underway.
Types of Winter Storms
The type of winter storm that affects NC also plays a significant role in the thermal profile and precipitation distribution across the state. Identifying what type of storm will affect the region is crucial to correctly forecasting the outcome of the event.
A. “Simple” Storm
- Consists of one well-developed surface low pressure system offshore of the Carolinas
- Precipitation is typically either rain or snow
- Little mixed precipitation (sleet/freezing rain) due to the well-defined low pressure system, which creates a more uniform thermal profile with sharp boundaries between freezing and above freezing air
- The rain-snow transition zone is relatively narrow due to the storm’s structure. Also referred to as a Miller type A storm.
B. “Complex” Storm
- “Complex” storm formation features two distinct low pressure systems; one well inland and a developing coastal low
- The thermal profile contains multiple above freezing layers which results in various precipitation types not only across the region but also in a specific location (depends on physical processes as well as precipitation rate)
- Sleet, snow, freezing rain or a wintry mix of all precipitation types are common
- Also referred to as a Miller type B storm
- Named for the continuously strong northeasterly wind that blows in from the Atlantic Ocean as the storm traverses near the coastline of the eastern US
- Can produce significant weather for a large portion of North Carolina such as: heavy snow, rain, and large waves that crash onto beaches which can result in coastal erosion and property damage
- Wind gusts can reach hurricane force (74+ mph)
D. Northwest Flow Snow
- These snow events usually affect western NC and occur when the wind blows up and over the Appalachians, which creates lift and allows precipitation to form
- When the air descends from the top of the mountains it tends to dry out, which results in little to no accumulating snow for areas of NC east of the mountains
- Occasionally, if an unusually strong disturbance coming from the northwest can survive the trip over the mountains, then accumulating snow coming from a northwest flow can occur over larger portions of NC
- Characteristics of a northwest flow snow include: little melting because of cold temperatures, and a high snow to liquid equivalency (20:1 or higher) which results in a dry snow