The thunderstorm season has been pretty active so far throughout the Northeast, but this has not necessarily translated all the way to the coastal areas and the major I-95 cities. This is about to change, come Tuesday, as one of the better setups for severe weather we have seen in some time looks to take shape.
The synoptic weather pattern evolving has similarities to a lot of the better Northeast severe weather events dating back to the early 2000s. There is a large heat ridge in the Desert Southwest which extends east through the Southern Plains and Missouri Valley. To its north, there will be a rigorous shortwave diving down Southern Canada and heading east and southeast towards the St. Lawrence Valley. Over the top of the ridge and south of the shortwave, mid-level westerlies get accelerated rapidly. This helps with two things:
1) It leads to strong deep-layer wind shear with a westerly mid-level flow that is typically required for our area to have its best thunderstorms.
2) It helps to advect an Elevated Mixed Layer (EML) into the area, which has roots from the Desert Southwest. The fast westerlies push it into our area.
On the south side of the shortwave and north side of the ridge is where a boundary in temperatures will setup, and thus a cold front, and we will be on the warm side of it. Because of the strong ridging, the cold front will not rapidly sink southward. This gives the atmosphere plenty of time to heat up as well as time for thunderstorms to organize. In fact, many areas should hit the low to mid 90s tomorrow, and dewpoints are forecast to be around 70 degrees. This is very ripe for thunderstorms. Hot and humid air has the tendency to rise.
In fact, the instability associated with CAPE (Convective Available Potential Energy) is forecast to reach 3,000 J/KG, and even higher in some spots. This means that any updraft that can rise high enough to form into a cloud has the strong potential to explode into a thunderstorm.
While some recent model runs have been a bit less impressive with the EML, there are still hints of it and it definitely has a major impact. What an Elevated Mixed Layer does is that it pushes an area of very warm and dry air over the top of the surface. This very warm air cools off very quickly with height, which provides plenty of instability in the mid and upper levels of the atmosphere, something the Northeast often lacks. This warm air also dries out very quickly. When the mid-levels of the atmosphere are dry, it means that any evaporation can make mid-level temperatures even colder, which provides even more instability, lowering the freezing level which allows easier hail formation, and also promotes quicker downward motion to transport very strong wind gusts to the surface.
What it also does is provide a cap just above the surface. When the warm air pushes over the top, there is a brief area at the lower-levels of the atmosphere where temperatures are getting warmer with height, rather than colder. This stops upward vertical motion, which suppresses thunderstorm formation. This is good for severe weather, because if the atmosphere were to try and form thunderstorms too early, they would not be able to tap into the strong instability and surface heating. Instead, only the strongest, most organized updrafts can break that cap, assuring that all convection is significant. Additionally, that cap separates the very warm, moist air near the surface and the dry air aloft, assuring that the best severe weather ingredients in the atmosphere as a whole can be maintained.
The above image is a forecast sounding from the high-resolution NAM model valid for Newark at noon tomorrow, the ideal time to have a nice “setup” sounding for severe weather, rather than one where the cap is already broken. The red line is temperature and the green line is the dewpoint. The lines of constant temperature are the dotted blue lines, skewed slightly to the right. This means that the further left the temperature line is sloping, the faster the temperature is decreasing with height, which means more instability.
The surface temperature is very hot, but it gets colder with height very quickly. Also notice how the dewpoint line runs almost parallel to the dotted blue line, indicating that plenty of low-level moisture is getting trapped and pooled. This is ideal for severe weather, as this means that air at these levels is very buoyant.
Right above this layer, we notice the temperature briefly gets warmer with height and this is where the LCL is. The LCL is the Lifted Condensation Level, which where a lifted air parcel begins to condensate into a cloud. It’s pretty low, which assures that a cloud would begin to form early and have plenty of room theoretically to grow vertically tall.
But now let’s get back to that inversion; the fact that it’s right at the LCL means that when clouds try to form, they have a hard time doing so. This means that only the strongest updrafts will turn into clouds, since only strong updrafts can bust through that cap. This feature is part of the classic “loaded gun” sounding.
Now, above the LCL, we also notice that the air gets dry very quickly, as the dewpoint line quickly veers to the left. Additionally, the temperature continues to decrease with height very quickly. This is the EML. The CCL is the Convective Condensation Level, which is similar to the LCL, except that it only involves convection, rather than forced lifting. The CCL is only relevant when the surface reaches its convective temperature, which is when air has to automatically rise. We will not be reaching our convective temperature, so this is irrelevant.
The LFC is the Level of Free Convection. This is where air can rise freely, and is where the CAPE region starts. This is where the rising air’s temperature is warmer than the environmental region’s temperature, which is why it rises so freely. The CAPE region is drawn with the yellow line, and once air hits the LFC, it rises all the way to the top of the CAPE region. The fact that the environmental temperature decreases so quickly with height (thanks to the EML) is why the CAPE region is so large in area. Large CAPE is also good for lots of lightning, as this assures tall, vertical clouds where plenty ice particles can be found to lead to necessary charge differences to form lightning.
Clearly, the atmosphere is very ripe for organized lifting and thunderstorms. What is also important for thunderstorm organization is wind shear, which is the change in wind speed and direction with height. The winds are at the right, in knots, labeled by the “sticks”. Notice how at the surface, the winds are relatively light at the surface from the southwest, but quickly grow to 56 knots at 500mb, from the west-north-west. This leads to over 50 knots of wind shear from the surface to 500mb, which is significant. This means that updrafts can easily rotate (since the winds rotate and increase in speed with height), which makes supercell thunderstorms possible, and also increases the odds of very strong wind gusts to reach the surface.
Since we’re not going to reach our convective temperatures, we need some forced lifting in order for air to reach the LCL and take advantage of the high CAPE, high shear environment. The above image is the NAM model valid for 8:00pm — the bottom left panel is at 500mb, where there is a rigorous disturbance to the north. This sends embedded areas of vorticity southward towards our area, helping with lift. This, along with the cold front, should be enough to trigger thunderstorms and eventually break the cap. The top-left image shows clusters of precipitation have already formed, which is an area of thunderstorms.
There are still a couple of things we need to watch before we can be assured of a severe weather outbreak. For one thing, there will be an existing complex of showers and thunderstorms moving into New England tonight and early tomorrow morning. If this impedes the EML, then tomorrow’s thunderstorms won’t be as strong. Additionally, the strongest forcing looks to be north of our area, and if the EML isn’t strong enough to cap cloud coverage, then anvil blowoff from storms that form to the north can reach our area and limit our instability. These anvils are possible in the first place because of all the large CAPE that goes all the way to the top of the troposphere, which forces clouds to spread out.
Regardless, the combination of strong CAPE, shear, as well as a trigger for lifting and an EML mean that clusters of strong and severe thunderstorms are certainly possible. Wind gusts reaching 50-60mph, hail, and dangerous lightning are all possible. While a tornado threat is non-zero, the best chance of tornadoes appears to be to our north, where the strongest l0w-level turning of the winds is found.