As well evidenced by the recent inclement weather, the northeast is under a very active weather pattern, and after a couple of days of general relaxation, the weather looks to turn unsettled again on Thursday.
A powerful storm system will quickly traverse the country, giving the Midwest and Ohio Valley a severe weather event on Wednesday. This will head eastward and approach our area for Thursday. At this point, all eyes will turn towards the northern Mid-Atlantic states: in particular, southeastern Pennsylvania, Maryland, Delaware, northeast Virginia, and perhaps central and southern New Jersey as well. This storm system will provide very strong winds — well in excess of 50 knots — at 500mb, coming out of the WSW; generally a very conducive environment for severe weather. In addition, the potent vorticity, among other factors that will be explained shortly, will also be conducive for lift, in order to generate thunderstorms.
What is often true about these powerful storm systems is that they possess strong boundaries, such as cold fronts and warm fronts. It is along these boundaries, and on the warm sides of them, where we pay particularly close attention, because the warm, moist air in the warm sector is lifted near the boundaries themselves, helping to initiate convection, and thus, thunderstorms. Usually, a cold front in itself is a boundary that is only conducive for squall lines, because they are expansive, they favor lots of lift over the entire front (as opposed to a dry-line, where the lift is more localized), and move quickly — so the best lift continually catches up with any storms that form, causing more and more storms to form in close proximity to them, thus quickly favoring congealing into a line. This is what is typical for northeast thunderstorm setups, and one reason why you tend to see large lines of storms, rather than the discrete supercells that you get in the Plains. However, in addition to this cold front, there will also be a warm front extending from west to east, well out ahead of the cold front.
Lift along a warm front tends to still be sufficient, but a little weaker than that of a cold front (less surface convergence), so you get less updraft and outflow boundary overlap, which tends to favor more isolated, discrete thunderstorms. Additionally, while cold fronts tend to follow and eventually catch up with the storms they trigger, warm fronts do not. A warm front will tend to move northeast, and into more stable air, away from any pre-existing storms. Warm fronts also favor stronger directional shear than cold fronts, which is favorable for supercells. Since the warm front extends well out to the east of the cold front, there will be a few hour window in the early-afternoon where storm mode will be mixed and even discrete, since the cold front at that point is still too far west to turn the mode linear in the favorable thermodynamic environment. Additionally, the initial storm motion and shear vectors will be close to perpendicular to the cold front, allowing discrete storms to potentially out-run the cold front, at least initially. However, by around 4 or 5 p.m., the cold front will rapidly be racing eastward, catching up with the discrete cells and forcing a linear mode. Or, at the very least, will be close enough to where the surface winds veer back to the southwest, reducing the directional shear. At this point, a strong squall line could be approaching S PA/S NJ/MD/DE/VA.
At 2 p.m. tomorrow (see image), the NAM shows a 992mb surface low (top left panel), with a cold front draped to its south and southwest, as evidenced by the temperature contours. Also, you can see there is a temperature gradient as you head from west to east — that is where the warm front is. So, just a bit to the south of that warm front; especially where you really see those temperatures spike, yet are still close to the warm front is where we will really be watching at that time (SE PA, Maryland, Delaware, parts of Virginia and S Jersey). Note the strong southeast surface winds located in that area as well. The southeast surface winds combined with the strong WSW 500mb winds will promote strong, deep-layer shear supportive of supercells.
Fortunately for the NYC Metro area, we are on the cold side of the warm front. Thus, there will be a decent amount of lift around, but not enough in the way of instability to generate much in the way of thunderstorm activity. However, considering the strength of the storm system, and the fact that the northern side is often where the steadier, synoptic rains are as opposed to the mesoscale convection to the south, more heavy rain amounts of 2″+ may be expected for our area, or just north of our area. We will keep you updated on this over the next couple of days.
Is there a chance that this boundary could shift north and put NYC in a danger zone? I honestly don’t foresee that, though there could be some rumbles of thunder embedded in our heavy rain. Southern portions of our area may have to watch out, however; perhaps central Jersey. The warm front will try to push northward as the day goes on, which would push the warm sector a bit further north. However, the cold front will be moving eastward a lot more quickly than the warm front is moving northward, which leads me to believe that by the time central Jersey is truly in the warm sector, the storm mode may become more linear, which would force a squall line instead of a discrete supercell. There is the small chance of a short window for a supercell, but at that point, it becomes less favorable. This is why I think SE PA, Maryland, Delaware, NE Virginia, and southern Jersey will have a higher tornado threat than areas further north. Of course, squall lines are still something to watch out for, as they can produce some strong winds. We will of course keep you updated over the next couple of days. The primary threat for our area will still very likely just be the heavy rain with occasional thunder, with an outside chance of being clipped by a squall-line, rather than truly severe weather and tornadoes.
For us mesoscale weather nerds, let’s analyze a point-and-click sounding for SE PA, valid for 2 p.m. on Thursday. This highlights the severe weather potential quite well, but also introduces a couple of caveats. One thing that really stands out is the tremendously favorable wind profile. There are SE winds at the surface that gradually veer and increase in magnitude as you increase in height — a great combination of speed and directional shear to favor supercells and tornadoes. The low-level moisture is also pretty rich, which is great for surface-based instability and low LCL heights.
One thing you could be nit-picky about is that the directional shear is a bit lackluster once you get above 800mb. However, this is not the Great Plains, so it’s hard to expect a perfect wind profile. Additionally, the speed shear is still great above 800mb, which will still help for storm outflow, and squall-line formation, which is one of the reasons why the window for discrete convection is only a few hours at most, and why a potent squall-line may be the main threat by the late afternoon and early evening. The speed and directional shear from the surface through 800mb, as well as the instability in that layer are quite significant; certainly supportive of a conditional tornado threat. We tend to look for weaker surface-based tornadoes in this region, anyway, rather than the monster EF-5 wedge tornadoes with terrific vertical structures, so the juxtaposition of speed shear, directional shear, and instability in the lower levels of the atmosphere certainly suffices for that, and may even be strong enough for a few EF-1 tornadoes and an isolated EF-2 tornado. I personally think this threat is too conditional to be throwing out the “strong” tornado word. Another reason for that is the fact that although the surface based instability is great, and the bulk shear looks great, the CAPE as a whole is kind of “skinny”. The mid-level lapse rates are pretty weak, so updrafts may have a tough time maintaining themselves for a very long period of time. Thus, above 700mb, where the lapse rates weaken and the wind speeds are really strong, the updrafts might lose a bit of their momentum and get sheared apart, to some extent.
So, although there are many favorable factors for severe weather, including tornadoes in the Mid-Atlantic on Thursday, the mid-level lapse rates being weak, and the transition to a linear convective mode as the day goes on could potentially be pretty big caveats and will be something to keep an eye on. And, as is usually a potential caveat to severe weather in the northeast — “crapvection”. With the strength of the surface low and the cold front, along with potentially weak capping, there may be lots of weaker storms that fire before the atmosphere can really destabilize, which would have a stabilizing effect on the atmosphere. However, since the warm front does extend pretty far east from the cold front, I’m pretty confident that there will be an area removed from the cold front where “crapvection” will be limited. This is true for reasons already stated and implied, and also since the warm frontal boundaries tend to not lift too much too quickly, given a little more in the way of a horizontal component to their lift (as opposed to vertical for a cold front). So, in those areas that destabilize, it could be a pretty active day — relative to Mid-Atlantic standards — for severe weather.