Update on Sunday into Monday’s Rain Event

We hope you’re having a great weekend so far! This article is going to be a quick blurb on the latest regarding the upcoming rain event. It still appears that most of Sunday’s batch of rain will miss to the north, as most of a small shortwave will miss to the north and get shredded out when it tries to approach our area. The thermal gradient via a warm front will be quite impressive, which will lead to a lot of lift in the atmosphere. Thus, some heavy showers and thunderstorms are possible in our northern suburbs from northern Westchester County on northward — perhaps more focused on Orange and Dutchess Counties or even further north, where some heavy rain is possible.

The NAM model valid for Sunday afternoon shows some heavy rain in northern suburbs along a tight 850mb gradient.

The above image is valid for late Sunday afternoon into Sunday evening. The vorticity that streamed offshore of Massachusetts was responsible for the heavy rain in the northern suburbs, as it crossed through Upstate NY. Also notice the extremely strong 850mb gradient on the top right panel; that is in advance of a warm front, which greatly helps for additional lift in the atmosphere. That warm front will continue to lift northeastward and shift the best gradient further north, so even when that low pressure system in the Great Lakes swings through, it will “find” the best thermal gradient and miss to our north again, but again provide heavy rain for northern suburbs during the overnight hours.

That being said, some rain is possible from NYC and south on Sunday afternoon and evening as although the main lift with the front will miss to our north, we will still get some lift from the front. Thus, light to moderate rain could fall more so in NYC and Long Island’s latitude and north. Anywhere from a few hundredths of an inch to a quarter of an inch of rain is possible in Northern New Jersey, where some steadier rain is possible.

After that disturbance moves away, there will be a lull in the action on Monday morning, but on Monday afternoon and evening, the main 500mb low will be intensifying and drifting towards our area.

Saturday’s 18z GFS model valid for Monday afternoon shows a secondary low pressure, giving flash flooding rains for Central and South Jersey, and moderate to at times heavy rain up to NYC’s latitude.

The above images shows the main 500mb low being quite strong and also providing deep southwest flow aloft for moisture, and also plenty of dynamics aloft with fast winds via a potent jet streak. The 500mb low and the jet streak combined with some high-latitude blocking will yield a secondary low pressure system developing off the coast — you can see the primary 1003mb low in Lake Ontario with the secondary low reflection near Atlantic City on the top-left image. This secondary low pressure system could provide plenty of heavy rain, particularly for Central NJ on south.

The big question is how far north this system gets. A lot of the rain may end up being suppressed south of NYC due to the height orientation being relatively flat out ahead of the main 500mb low, as well as the fact that the system is a bit disjointed — there is a lot of forcing near the actual core of the 500mb low as well as right where the secondary low initially develops, which could create some subsidence in between.

The past six runs of the GFS model have all shows a northwest trend with the secondary low pressure. If this were to continue, heavy rain would become much more widespread over the area on Monday (Tropical Tidbits).

But there has been a noticeable trend in Saturday’s model runs. Most of them have been inching the secondary surface low a bit further northwest, as the 500mb height orientation has become a tad more amplified as the core of the low is oriented a bit further northwest. This trend has been enough for models to now show some moderate to occasionally heavy rain making it as far northwest as NYC — though with the core of the heaviest rain still remaining to the south. There’s still a sharp northwest cutoff — with NW NJ getting less than a quarter inch of rain, while NYC and south receives 0.75″ or more. The RGEM model also shifted northwest, as evidenced below.

Saturday afternoon’s RGEM mode also shows heavy rain making it north to NYC and even NE NJ on Monday afternoon (Meteocentre).

But given the disjointed nature of the system and the still relatively flat flow ahead of the 500mb low, we do not necessarily think this trend will continue to a large extent. But the placement and orientation of the jet aloft does favor some rain — perhaps heavy at times — for some of our area. We are in the right-entrance region of a large jet streak, which is ideal for upward vertical motion for precipitation and lowering surface pressures — so the low may “find” those lowering surface pressures and track further northwest.

Saturday afternoon’s GFS valid for Monday afternoon shows a large, powerful jet streak to our north, giving our area a lot of upper-level divergence and thus lift for precipitation.

This would bring moderate to heavy rain on Monday afternoon and evening — with perhaps some embedded thunderstorms. But there will still be a sharp cutoff somewhere, so NW NJ and the Hudson Valley may not see much rain at all on Monday. And given the disjointed nature of the system, there could be a large variance in rainfall totals in a short distance, as some areas get up to an inch of rain in an embedded thunderstorm, and other areas just get light to moderate rain and amounts only up to a quarter inch.

We still have low confidence, though, because even the further northwest models are showing plenty of the aforementioned disjointed nature to the storm, which means that the south/less rain risk is still there, and that the models may not be handling this system very well. Some rain may linger into Tuesday as well.

We will be sure to keep you updated!