Evening Zones: Rain tapers off, but more rain threats ahead

Good evening! Unfortunately, today was a nasty day, filled with numerous bouts of moderate to heavy rain. There was some subsidence in northwest inland areas, where coverage of rain was much more scattered and totals were only 0.25 to 0.5″, with even lesser totals in NW NJ. Rainfall totals increased a bit in the immediate NYC area, with JFK receiving 0.65″ of rain, NYC 0.59″, EWR 0.70″, and LGA: 0.64″ — a solid bout of rain. But generally anywhere from Monmouth County and south, rainfall totals exceeded 1″, with some areas getting between 2-3″ of rain in Southern New Jersey. The ingredients were all certainly there for a prolific rainfall event, but due to last-minute trends for a bit flatter of a height pattern ahead of a ULL, the pattern became a bit more progressive, and the heaviest rain thus stayed south of NYC. If we had the previously forecast height pattern from two days ago, the entire area may have exceeded 2″ of rain. Nonetheless, it was another solid rain event in the midst of a very wet stretch of weather, and there will be more threats of rain over the next week or two.

Rain will continue to taper off this evening. 850mb moisture transport has shifted from due west to east, which has allowed more dry air in the mid-levels to work its way in from the west. Thus, we do not expect any more rain this evening. However, the low-levels will continue to be saturated for the next few hours, so we may see continued low clouds, fog, and drizzle. But later in the overnight, a cold front will move through the area, shifting winds more northerly, and allowing more dry air at the surface to move in and break up the clouds and fog as well. Temperatures will generally be in the mid to upper 50s for lows.

This afternoon's 18z GFS valid for Tuesday night shows a coastal low giving coastal areas heavy rain. This solution is an outlier, as most other guidance is offshore.

This afternoon’s 18z GFS valid for Tuesday night shows a coastal low giving coastal areas heavy rain. This solution is an outlier, as most other guidance is offshore.

Offshore flow and subsidence behind Monday’s storm will allow for a good amount of sunshine for most of the day on Tuesday with high temperatures rising into the low 70s. Another disturbance will be gathering to our south on the downstream side of a large, full-latitude trough and move up the coast. This is a lead wave ahead of the main disturbance much further southwest, which are generally weaker and more fickle, but can still be pretty good moisture-producers. But the fickle, weak nature of this lead wave combined with the fact that although the trough is full-latitude and amplified, it is quite broad which can lead to a more progressive tendency to already inherently progressive lead waves. Thus, we expect this wave to mostly remain offshore.

But the above image is from the most recent 18z GFS, which shows this storm tracking close enough to the coast to give coastal areas some heavy rain and even some moderate rain for NYC, so it still does need to be watched. The vorticity with this wave is pretty impressive, and the jet in the bottom right panel is pretty strong. We are still pretty skeptical of this solution, due to the aforementioned reasons explained and the fact that even on the 18z GFS, the jet does not appear meridional enough to support heavy rain making it up the coast to our latitude from a lead wave like this. Regardless, some light rain is possible during the overnight hours on Tuesday night/Wednesday morning for coastal areas.

Today's 18z GFS valid for Thursday shows very heavy rain for much of the region.

Today’s 18z GFS valid for Thursday shows very heavy rain for much of the region.

Once that storm moves away, Wednesday will most likely see partly sunny skies with high temperatures in the upper 60s to around 70. But there will again be another threat of rain on Thursday, as the actual full-latitude trough itself — not a lead wave — makes its approach. The dynamics with this storm are very strong, as evidenced by the 500mb low closing off in the bottom-left panel, and the very strong jet in the bottom-right panel. There may even be a secondary coastal low developing to our south, which may help to turn the low-level winds a bit more easterly, which would more easily advect moisture into the area from the Atlantic, as well as provide some coastal flooding and pretty strong winds along the coast.

If all of the ingredients come together, this could certainly be a 1-2″ rainfall event. But it appears that the vorticity with this storm is somewhat disjointed, as some may stream out ahead of the main 500mb trough, and that area of vorticity may end up being associated with the main area of rain. This means that there may be some subsidence when the main 500mb trough moves through, and also may mean that the areas of heavy rain are a bit scattered in nature and/or offshore when the initial vorticity moves through. Thus, we are not yet convinced of a heavy rainfall event — but regardless it does appear wet. Enough residual blocking may keep some bands of rain in the area into Friday as well.

Today's GFS model valid for Sunday night shows an MCS (Mesoscale Convective System) moving into the area.

Today’s GFS model valid for Sunday night shows an MCS (Mesoscale Convective System) moving into the area.

Once the storm system moves away, it appears that some large-scale subsidence will move in on Saturday, allowing for a very pleasant start to Memorial Day Weekend, with mostly sunny skies and temperatures rising well into the 70s. But this subsidence will be between two systems, and the second system will move into the area later in the weekend. The timing with this disturbance is a bit more uncertain. But there is a pretty high likelihood of something forming with a very amplified, but collapsing West Coast ridge combining with a decaying block in the North Atlantic to allow for another large trough to form in the Central US and Great Lakes. Some vorticity may sneak out ahead of the trough sometime on Sunday or Sunday night, and allow for a MCS to move towards the area, which could produce some heavy rain and lightning. Then after a break in the action, the main trough may produce some rain on Monday.

That being said, the track of an MCS is very hard to predict at this range, and it could still easily miss the area, which would allow all of Sunday to be dry. But the idea of some rain moving in sometime on Monday with the main trough has merit. Fortunately, even if the MCS does hit the area, neither Sunday nor Monday appear to be washouts at this time.