Author Archives: Shane D.

Southeast Ridge

The southeast ridge can be a semi permanent fixture within the pattern across the southeastern part of the US. While the picture below gives you one example of the orientation of the ridge it can be shifted more off short of the SE coastline or farther west toward Texas. Regardless of where its located it generally spells above normal temperatures and a lack of precipitation. The image below shows you heights at 500mb and what they would typically look like during a SE ridge event.

Miller A vs Miller B

Miller A:

Miller A storms are typically a single low pressure that develops in the gulf and heads toward the NE. The 93 Super storm is an example of a Miller A low pressure. These storms tend to have a narrow ptype transition zone and generally are rain or snow versus having freezing rain/sleet.

Miller B:

Miller B low pressures are the more complex of the 2. They feature a primary low that moves into the Tn Valley and begins to occlude over this area. This can also happen as a clipper systems moves into this area. As the primary low occludes the secondary low starts to take shape along the coastal front on the Carolina coast. This surface low becomes the primary low as it heads northeast. Miller B systems often lead toward messy precipitation transition zones as warm noses occur aloft.

Overrunning

Overrunning is characterized by a warmer air mass being lifted over a colder/more dense air mass. The most common way to see this produce winter weather in the southeast is an arctic front is pushed toward the gulf coast while the subtropical branch of the jet stream becomes active. This sub tropical jet leads to warm air moving over the cold dome and helps generate precipitation. Embedded within the southwest flow aloft we will see a few weak impulses and surface lows that will help generate areas of enhanced precipitation. 

 

Wedging/ Cold Air Damming

Wedging and cold air damming are a common occurrence across parts of the Carolinas and Georgia during the cool seasons. This wedge of cold air can lead to cloudy cool weather in the fall and spring and freezing/frozen precipitation in the winter. So what causes this?

The first step is for a strong surface high pressure to move across the northern US or southern Canada and into the Northeast. This surface high then builds southward on the east side of the Appalachian mountains. You can often see the progression of the wedge on current surface maps by the isobars, dew points and current temps.  The wedge air mass will be characterized by much lower dew point air and cooler temperatures than areas south, southeast, and southwest of the wedge front. The wedge air mass is also generally quite shallow and confined to the lowest few thousand feet of the atmosphere.

As the low level cold builds southward we often see warmer moist air forced over top of the wedge and we see precipitation break out. Since we are only looking at a shallow layer of cold air many times in the wedge areas this precipitation is in the form of freezing rain as you head into the deepest portions of the cold air mass there may be a change to sleet or even snow.

With the colder drier air deposited in the wedge zones and precipitation falling temperatures will fall to the wet bulb temperature and depending on wedge strength can slowly continue to fall as the precipitation falls. What happens as the event continues is one of 3 scenarios:

#1 Classic Damming: Classic cold air damming is where the surface high pressure locks into the northeast United States . This continual dry, cold feed will sustain the cold air damming regime the longest and can push the wedge into Alabama.

#2 Hybrid: In a hybrid damming scenario the high pressure is generally in the classic spot to start the event however it is either A. too weak to provide a strong cold air feed or B. kicks out of the NE during the event cutting off the dry cold feed. These events usually see a change to rain for a large area but can continue to see freezing rain along the foothills.

#3 Insitu: Insitu damming is created when dry cold air has been left behind in the damming regions and its overridden by a warm moist flow. Most times in the insitu damming scenario the cold high pressure over the NE is not present. These events are self limiting as rain falls and freezes on contact latent heat is released into the atmosphere and eventually

 

Once the event is over the wedge air mass itself is quite hard to break as low level moisture remains. Many times computer models will forecast the wedge breaking and temperatures warming while in reality the wedge stays in place and temperatures are significantly cooler.