February; Snow Wait, Not Winter This Time! Increasing Chances for Severe Wx!

This time, instead of posting about a snow or ice storm and class cancellations from slick roads, my post has a much more uncanny message to it for this time of year. This time we are talking about the very real and increasing potential for Supercell thunderstorms, hail greater than 1" in diameter, straight-line wind gusts of over 70 mph, heavy rains, and the possibility for an isolated tornado, or maybe a few. In this post I have broken down the setup in the Synoptic Scale, Mesoscale, and Microscale systems in order to help everyone better understand different atmospheric interactions that produce such awesomely powerful storms.

*At the bottom I have described expected types of weather which we could experience here and* *potential hazards to be aware of, and the timing of storms.*

Terminology

1. CAPE - Convective Available Potential Energy: A measure of instability in the atmosphere. This value represents the "ability" for an unstable air parcel to rise and condense, ultimately forming cumulus towers an creating a pressure anomaly in the atmosphere
2. LFC - Level of Free Convection: This term refers to the point at which an atmospheric parcel is completely buoyant and will rise freely until it reaches the atmospheric temperature again
3. EL - Equilibrium Level: This level in the atmosphere is that point which the parcel temperature again reaches atmospheric temperature and is no longer positively buoyant.
4. Wind Shear: Wind shear refers to the change in wind speed and direction. Wind shear can be calculated both horizontally and vertically and is used in a variety of ways including being able to visualize the rotation in the atmosphere.
5. Helicity: This parameter is used in severe weather forecasting. One of the variables which goes into calculating helicity is wind shear. It allows forecasters to compute atmospheric rotation.

Synoptic (Large) Scale: 100km - 1000km

A cold front has made its way down into the southeastern United States, meaning colder air temperatures. The area of interest with this particular front lies in the "warm sector" of the front, or the area ahead of the front associated with warmer temperatures. When a front passes, there is a shift in the wind and a change in the air mass. Cold fronts, because of the nature of cold air to forcefully shove warm air out of its way, are typically associated with severe weather as the temperatures warm up and moisture with that warmth helps to increase instability along the front. Instability is measured in a meteorological parameter called CAPE, or Convective Available Potential Energy. The value of CAPE is calculated by taking the area on a skew-t chart under the moist adiabat from the LFC (Level of Free Convection) to the EL (Equilibrium Level).

This frontal system has a strong cold wind behind it from a Canadian air mass, and a strong warm wind ahead of it, yet in almost the opposite direction, from the seasonally warming Gulf of Mexico. In addition, the sub-tropical jet stream, or the jet stream typically associated with changing weather here in the southeast, is racing eastward. This jet lies atop a southward-blowing wind behind the front, and a northward-blowing wind ahead of the front. This change in wind speed and direction both horizontally and vertically is referred to as "wind shear".

The severity of these storms depends on where the boundary is for the warm sector. That boundary between the colder air mass and the warm sector is where the most instability is to be found, and if sufficient moisture is present then it is possible for cumulus towers to form rapidly.

Mesoscale (Medium Scale): 1km - 100km

The wind shear I mentioned above really comes to play in the mesoscale. A supercell thunderstorm is basically a thunderstorm rotating around a centralized rotating updraft. Typically a supercell is associated with the production of hail, heavy rains, and tornadoes. There are exceptions to every rule, but for all intents and purposes a supercell is a severe, rotating thunderstorm.

The wind shear is essentially what spins up the thunderstorm in order to produce a "mesolow" or "mesocyclone", or an area at the base of the supercell which descends slightly from the base of the thunderstorm and typically results stronger counter-clockwise rotation. In addition, the presence of the jet stream aloft, cooling air from precipitation will not hinder the main updraft caused by instability. This off-set cooling actually increases the temperature gradient in the smaller scale and thus increases the storms ability to move air rapidly upward. As a result, these storms could last for hours.

In the main updraft of a supercell, near the mesocyclone but in-between the precipitation underneath the anvil and the precipitation-free mesocyclone is where the hail-core can be found in storms that produce hail. Why?  Well, updrafts are strong enough to carry a droplet up into the cloud where there are ice crystals and supercooled water droplets. Supercooled water then freezes to the ice crystals and the droplets grow. They fall into the updraft where they are carried back up and the process continues until it falls out of the updraft or becomes too heavy for the updraft to lift again.

Microscale (Small Scale): 1m - 1km

The microscale/mesoscale boundary is where a lot of the forecast difficulties take place. Within the microscale we look at the 0-1km helicity, or rotation in the atmosphere. Basically, if the helicity is strong enough, that means the surface rotation is strong enough that tornado production is possible.

Other importance in the microscale involves the Rear Inflow Jet, surface interactions between the ground moisture and the base of the mesocyclone, and circulation around the mesocyclone, but going into detail in these areas is not necessary for this post.

Thresholds:

In severe weather forecasting, we have various parameters and thresholds for those parameters which tip us off as to whether there is a significant threat or not.

1. CAPE - Varies, but anything more than 1000 Joules/kg is sufficient for strong instability, especially this time of year.
2. SCP (Supercell Composite Parameter) - If greater than 1 there is a good chance for supercell formation
3. STP (Significant Tornado Parameter) - If greater than 1 there is a good chance of tornado formation
4. 0-1 km Helicity - If greater than 200 m^2/s^2 there is a good possibility of strong rotation at the surface
5. 0-6 km Wind Shear - If greater than 40 m/s there is sufficient change in the wind direction in the atmosphere to produce a tall rotating column (main updraft in a supercell)

What we are looking at for tomorrow:

1. CAPE: 1300 J/kg
2. SCP: 7
3. STP: 6
4. 0-1 km Helicity: ~250
5. 0-6 km Wind Shear: ~80 m/s

Basically, all roads lead to Rome, and all parameters for tomorrow lead to a severe weather outbreak in Central-Eastern North Carolina.

Hazards:

There are many hazards to be aware of for this event:
1. Wind - Straight-line winds from heavy falling precipitation as well as rear inflow jets from pressure differences could allow for wind gusts tomorrow over 60 mph.
2. Flooding - Heavy rains associated with supercell thunderstorms or a squall line could cause flash flooding.
3. Lightning - With this type of system being made of mix-phased clouds, charge differences develop and lightning can be a dangerous aspect of the storms.
4. Hail - Because the updrafts are so strong, hail formation is definitely possible. How large that hail gets depends on each specific storm, but the National Weather Service has mentioned the possibility of hail up to 1" in diameter with the strongest storms.
5. Tornadoes - With these types of storms and the parameters we are looking at for tomorrow, isolated tornadoes are a real threat.

As a result of these hazards, power outages are also a threat. Quick, heavy rains can cause saturation in the surface dirt and heavy winds can blow over trees. In addition to recommending staying off the roads during these storms, I would recommend parking your cars under a cover for falling debris associated with strong winds.

Timing

For Raleigh it looks as though the most opportune time for storms to make their appearance would be between 1:00 p.m. and 4:00 p.m. tomorrow afternoon. Before that time frame the instability will likely not be high enough for supercell production, and after that the moisture in the atmosphere will have been precipitated out. Areas to the west could see activity earlier than lunch, and the further east you go the later the chance gets. Rain is likely throughout the morning, afternoon, and evening, however the big storms have their main shot from early afternoon to early evening.

Please stay tuned in to your local weather stations for the most updated information! Be safe! As always, go Wolfpack!