[alert type=”error”]Storm Prediction Center has placed parts of the area in a Moderate Risk for severe weather on Thursday. Details. [/alert]
A strong storm system traversing the Ohio Valley will strengthen and shift towards the Northeast US on Thursday, bringing the potential for several types of hazardous weather. Forecast models suggest the system may be anomalously strong for this time of year, with pressures dropping to near 990mb as it moves through the Mid Atlantic states. This is absolutely unprecedented for this time of year, as normally the jet stream is too weak this time of year to support such dynamic systems.
A warm front is forecast to lift northward from the Mid Atlantic states to a position near Central New Jersey on Thursday afternoon. The combination of building instability near and south of the warm front, as well as favorable wind fields and forcing for precipitation will lead to the potential for not only heavy rain and strong winds, but severe weather…including the potential for a few tornadoes.
If you have not already, I highly recommend you check out our previous technical discussion, as it will help to explain some concepts that I will talk about in detail for the rest of this post, and give you a pretext as to what is going on.
The exact position and timing of the surface low, cold front, and warm front will be crucial in determining where the tornado risk is, and where the northern boundary of the severe weather can be expected. Latitudes north of the surface low can expect heavy rain of 2-4″ and little in the way of severe weather, areas with the same latitude as the surface low can expect 1-2″ of rain with a chance of being hit by a squall line producing borderline severe winds, and latitudes south of the surface low can expect severe weather and a few tornadoes. The problem is, the exact position of the surface low is still a bit uncertain, but we’ll do our best to warn everyone of the possibilities.
Our first two images will illustrate the NAM’s boundaries and its movements throughout the day on Thursday. The NAM, at 2 p.m. on Thursday is forecasting a 990mb surface low in SW PA (top left panel). You can see two distinct temperature gradients — one to the south of the low, and one to the east of the low. I quickly highlighted the approximate warm front position in red and the cold front position in blue, at that time.
As I said in yesterday’s discussion, the cold front often triggers a linear convective mode. But look at how far east the warm front extends away from the front! Areas just south of the warm front and away from the cold front can expect a discrete supercellular convective mode. Thus, at 2 p.m., SE PA, SW NJ, DE, and MD could really be under the gun for a tornado threat, and perhaps severe hail as well.
A very key fact that one must understand for this threat: the slower the cold front advances, the longer the warm front will be the main boundary for thunderstorm initiation in the very favorable tornadic environment, thus discrete supercells. A faster moving cold front means that it would eventually be the dominating player in the favorable tornadic environment, potentially “wasting” the environment and turning the convective mode more linear, since it would more quickly intercept that environment. Linear squall lines can still provide embedded tornadoes, so a squall line does not mean there is no tornado threat, but it would be significantly reduced compared to that of more discrete supercells.
Also note the southeast surface winds that even exist to the south of the warm front in SE PA on the bottom left panel — when you combine that with the potent mid level winds from the WSW associated with this powerful storm, you get strong, deep-layer shear supportive for severe weather and even tornadoes. More will be discussed about the wind profiles later in this post.
At that point in time, the warm front is well south of the NYC Metro region, so we are not under the gun for severe weather. Will this change in time? Let’s look at the NAM valid for 5 p.m.
Once again, I have sketched approximate warm and cold front positions. What is important to note is that the warm front has shifted northward towards Central Jersey, which is concerning since that would theoretically put Central Jersey in the game for severe weather, especially once you get to Trenton and points southward. However, notice that the cold front has shifted eastward pretty quickly — so by this point, the convective mode should be linear for the DC area and Philly now becomes borderline. Still, though, just a bit to the northeast of Philly and points eastward, there is a great juxtaposition of being south of the warm front, relatively far from the cold front, and surface winds backing to the southeast. Thus, a tornado threat would still theoretically continue, and perhaps extend as far north as Trenton, NJ. Storms of more mixed and linear modes would be ongoing to the west, producing potentially severe straight line winds and perhaps an embedded weak tornado or two. The NYC area is still relatively safe, as they are north of the warm front.
Some other key factors to take away from this: if the warm front is able to slide northward to central Jersey while the cold front is still back west of Harrisburg, that could be enough of a distance to warrant a discrete supercellular mode for central Jersey. However, the warm front will still be pretty hard-pressed to inch north, while the cold front will most likely tend to move east a bit quickly. This is primarily because of the fact that ocean temperatures are still chilly this time of year. As you can see, the surface winds are southeasterly and even easterly north of the front. What that does is usher in colder air from the ocean, which will battle with and halter the warm front’s northward progression. This is why I think it will be quite hard for the front to approach NYC during daytime heating when the atmosphere would theoretically be unstable. Also, it makes the discrete supercellular window for central NJ be a lot more brief than that of SE PA and S NJ. This is because by the time the warm front does clear, a bit of daytime heating is lost and the cold front gains some steam. It would be irresponsible for me to not warn the central NJ area for tornadoes given the wind shear in place, but the biggest threat, to me, still appears to be south of central NJ.
Let’s look at some instability parameters to further illustrate the importance of the boundaries. The NAM, valid for 2 p.m., shows an instability gradient similar to that of the warm front — not a coincidence at all. Values of surface-based CAPE soar to 3,000 J/KG in the warm sector of SE PA, NE MD, DE, and extreme SW NJ. And of course, these areas were well located within the area of discrete convective mode, at that time.
The NAM, valid for 5 p.m., shows the strongest surface-based CAPE certainly inching north and east with the warm front, but staying to the south of central NJ, thus limiting the tornado threat. However, values of 1000 J/KG are found just SW of Trenton, NJ, which is a tad borderline for severe weather, but enough to be on alert. The values increase to 2000 J/KG as you head near Philly, which is conducive for severe weather. Of course, as alluded to earlier, the mode might be linearizing at that point, anyway. The NYC area still has essentially no surface-based instability.
Another important factor that I must reiterate: Any slight shift to the north of the warm frontal boundary could put central Jersey in the game for a tornado threat, which is why this situation bears watching so closely. It would also increase the NYC area’s severe weather potential, though tornadoes appear unlikely north of central Jersey.
Although I’m still skeptical regarding the NYC severe weather threat — what could happen is that towards the evening, a strong squall line that previously formed over a more favorable environment could be allowed to expand northward in coverage as the warm front continues to slowly push northward. However, at the same time, the line would gradually be weakening due to a loss of daytime heating. Another reason I’m skeptical of discrete supercells for the NYC Metro region is because as darkness approaches, we really can only obtain elevated instability, as opposed to surface-based instability — the warm front moving north will only provide modest surface-based instability post peak daytime heating. The elevated instability is co-located with only a little bit of speed shear, not directional shear, since the stronger directional shear is closer to the surface, whereas once you go above 800mb, the shear is really only speed shear and not directional shear. Without the surface-based instability in this setup, prominent, rotating updrafts may be hard to come by for the NYC Metro region.
So, NYC could still get clipped by the northern side of an expanding, but weakening squall-line as the evening approaches. This could still be something to watch just because the line could have peaked quite strongly, so a weakening line could still provide borderline severe winds. Of course, we will continue to keep you updated on the situation.
Let’s now take a quick look at the GFS’s surface-based CAPE. The first image is valid for 2 p.m. Notice how much higher the values are and how much further north its boundary is compared to the NAM. Also notice, however, that despite how much more latitude the boundary gains, that it still sinks rapidly southward once you head towards NYC, because of the colder ocean waters.
Let’s now take a look at the NAM’s forecast sounding for SE PA, valid for 2 p.m., and why the tornado threat does exist. Just like in yesterday’s sounding, the wind profile really stands out. You have SE winds at the surface veering and increasing in magnitude to 70 (!) knots at the 700mb level. That is an absolutely incredible magnitude of speed and directional shear, and even more than what we saw in yesterday’s sounding. I was initially skeptical of the threat for strong tornadoes (EF-3 or greater), and I still think that would be very few and far between, if any, but I honestly would not be surprised to see a strong tornado on Thursday.
There certainly are a couple of caveats to this sounding, such as the fact that the shear really decreases once you go above 700mb, and that the mid-level lapse rates are poor. This is what probably prevents this from being a historic tornado outbreak, to be honest. But when you couple the strong moisture in the low-levels of the atmosphere going “straight up” towards the mixed layer, with warm temperatures and very sufficient low-level lapse rates, with bulk shear potentially in excess of 60 knots perpendicular to the cold front in that exact layer where the lapse rates are as strong as they are…it would honestly surprise me if there were not a few tornadoes in the northern Mid-Atlantic on Thursday.
Eventually, the lack of directional shear above that layer in combination with the cold front moving eastward will turn the threat linear by later in the afternoon, but before that, many areas could really be under the gun. Fortunately, the weak mid-level lapse rates may be able to prevent updrafts from remaining healthy over a long period of time, so long-track tornadoes appear unlikely at this time. For our area specifically, the weak mid-level lapse rates may also prevent any squall line from truly maintaining its peak strength, especially once daytime heating decreases, which may save NYC and points east from a severe weather episode. However, the low pressure system and cold front may be potent enough and in close enough proximity to provide enough synoptic lift to force updrafts upward anyway — another reason why boundary positioning is so important!
We will strongly have to monitor the position of a pre-existing MCS that will move through the Mid-Atlantic on Wednesday night. This could have an effect on how much the atmosphere is truly able to destabilize, if it cannot move out of the way early enough. However, pre-existing MCSs also tend to keep the atmosphere more moist, helping to lower LCL heights, and they also create outflow boundaries, which can help create lift for thunderstorms well ahead of the cold front — an area where the mode is more discrete. Of course, “crapvection” will have to be monitored, given the weak capping and the strong synoptic ascent from the potent surface low. But as I said yesterday, warm fronts tend to be a bit more “selective” about their lift than cold fronts are, so I tend to think that “crapvection” may only create a bust — if any — on a local level.
Let’s shift gears back to the NYC Metro area specifically. Since the warm front has appeared to shift a bit to the north on the modeling guidance, that also shifts the heaviest rains to the north as well. This is because lift along a warm front tends to be a bit more horizontal as opposed to purely vertical, so the heaviest rains will be a bit displaced to the north of the warm front, as opposed to right on top of the front. I generally expect about 3/4″ to 1″ of rain in central NJ, about 1 to 2″ of rain for the immediate NYC Metro region, and 2-4″ of rain once you head north of Westchester counties, though this will be fine-tuned with time.
We will also have to hope that the GFS is not onto a trend regarding moving the warm front further north, as that would place more of our area under the gun for severe weather. As I said before, the colder ocean temperatures should almost definitely prevent a tornado outbreak from occurring in the NYC Metro area itself. However, the threat does exist for 1-2″ of rain, strong synoptic scale winds from the powerful storm, as well as being on the northern fringe of a borderline severe squall line. Not nearly as dangerous as a tornado threat or even a stronger line of storms like what the ceiling is for our friends to the south, but certainly a situation that will need to be monitored.