Most of the Rain Will Stay South; Some Locally Heavy Rain Still Possible

Good afternoon, everyone! We hope you are enjoying your Friday. Some isolated thunderstorms have developed across the area — particularly across Long Island — where a seabreeze converged with prevailing northeast winds, thus leading to convergence and lift. Some heavy downpours were had with these storms. But with very little wind shear, they rained themselves out and weakened. A few more isolated showers and thunderstorms are still possible across the region for the rest of the afternoon and tonight with subtle boundaries and sheared out vorticity ahead of the main shortwave in the Mid Atlantic, but nothing is expected to be particularly noteworthy.

Currently, the Mid-Atlantic is seeing flooding rains, with frontal forcing, very high dewpoints, training thunderstorm cells, moisture convergence, and a dynamic shortwave all congealing in that area. Rainfall amounts have been 3″+ in one hour in many locations, as a truly unique combination of synoptic forcing and deep, moist convection hits the area. If this shortwave were able to turn northward, a lot of this activity could have hit NYC as well, as some model guidance as recently as yesterday was showing 5″+ of rain for NYC.

But there was simply not much room for the shortwave to turn northward, as a Canadian Vortex became misaligned with our main shortwave, causing a very positive, sheared out northeast tilt, as opposed to a negative, northwest tilt.

500mb analysis valid for Friday evening shows a very potent, but also very positively tilted shortwave being compressed by a Canadian Vortex. This prevents it from gaining latitude. (Tropical Tidbits).

500mb analysis valid for Friday evening shows a very potent, but also very positively tilted shortwave being compressed by a Canadian Vortex. This prevents it from gaining latitude. (Tropical Tidbits).

This positive tilt prevents the system from really ramping up and amplifying, thus leaving it no room to make a northeast turn, and instead it will sit to our southwest and gradually rot. The Canadian Vortex is also well east of the main shortwave, causing compressing flow out ahead of it, rather than phasing into the storm. If the vortex were further west and thus aligned with the shortwave, there would have been partial phasing and thus the vortex would have been a mechanism to help turn the storm northward. But instead, with it being east of the main shortwave, it hurts the storm’s northward progression.

Now that being said, it’s still a strong storm in a moist environment with some subtle convergence boundaries from the seabreeze, and the low pressure’s southeast flow ahead of it additionally converging with some northeast winds to the north from higher pressures. This will still allow some rain to make it to our latitude, and perhaps provide some isolated thunderstorms with heavy rain as well tonight, as shown in the earlier panels of the below 19z HRRR animation.

The radar animation of Friday's 19z HRRR model run shows isolated thunderstorms this evening, followed by some banding of rain making it to our latitude, but at a weaker state than in the Mid-Atlantic. (Tropical Tidbits).

The radar animation of Friday’s 19z HRRR model run shows isolated thunderstorms this evening, followed by some banding of rain making it to our latitude, but at a weaker state than in the Mid-Atlantic. (Tropical Tidbits).

There is also still a nice jet streak to our north, which may help some rain — perhaps briefly moderate to heavy — to reach our area at times, as evidenced by this HRRR gif — particularly for Central NJ where the surface convergence may be maximized. Thus, it is still possible for some isolated pockets of 1-2″ of rain from this system. And parts of Central NJ may even be just far enough south to get in on some of the main banding from the storm itself. But the over-arching picture is that dry air and subsidence will prevent widespread rain from making it up to NYC, as the inability for the main 500mb shortwave to turn northward allows most of the positive vorticity advection and cold-conveyor belt banding to be focused in the Mid-Atlantic and Southern New Jersey, where 2-4″ and locally double that of rainfall is possible.

 

Friday afternoon's NAM model valid for 2:00am Saturday morning shows subsidence via dry air at 700mb in NYC, parts of N NJ, and LI. But lift/convergence on either side of that subsidence could still lead to locally heavy rain elsewhere (NEXLAB).

Friday afternoon’s NAM model valid for 2:00am Saturday morning shows subsidence via dry air at 700mb in NYC, parts of N NJ, and LI. But lift/convergence on either side of that subsidence could still lead to locally heavy rain elsewhere (NEXLAB).

Another way this is illustrated is via 700mb relative humidity — a level we often look at for saturation and thus banding of rain. Notice the dry pockets of air that are very evident in NYC’s latitude through Long Island and parts of Central NJ. But just south of that towards Monmouth and Southern Middlesex and south, as well as even just north of NYC are more saturated — thanks to localized convergence as well as lift actually being enhanced adjacent to the dry air — a what goes up must go down analogy. It will be a nail-biter figuring out where exactly this subsidence and dry air at 700mb sets up, but regardless it’s a big enough patch of dry air to prevent widespread heavy rains from reaching the area, and explains the localized heavy patches of rain accumulation in the midst of relatively rainfall totals, such as the below HRRR model:

Friday's 19z HRRR model's total rain accumulation through 9:00am Saturday shows relatively low rainfall totals, but also multiple localized 1-2" batches of rain where localized convergence and banding may occur. (Tropical Tidbits).

Friday’s 19z HRRR model’s total rain accumulation through 9:00am Saturday shows relatively low rainfall totals, but also multiple localized 1-2″ batches of rain where localized convergence and banding may occur. (Tropical Tidbits).

Regardless, some banding of rain should make it to our area early Saturday morning. But on Saturday afternoon as the focus of precipitation shifts towards the surface low and pivots into cold conveyor belt banding behind the storm and in the midst of the 500mb low in the Mid-Atlantic, we will lose our convergence boundaries and just see totally dry northerly flow. Thus, Saturday afternoon should mostly be rain-free. The storm will get shunted far enough away by Sunday to make it a very nice day, helping to salvage a decent weekend.