Plaincast

What has changed

Flood Watch in Effect for the Lower Hudson Valley, Northeast NJ, NYC, portions of western LI, and southwest CT from Tuesday morning through Wednesday morning.

Messages

Scattered showers and thunderstorms into early this evening. An isolated severe thunderstorm is possible.

Heavy showers and thunderstorms will bring the potential for flash flooding, mainly Tuesday afternoon through Wednesday morning. There is the potential for life threatening flooding during this time.

Potential for minor coastal flooding mid to late week with a coastal low.

Discussion

.KEY MESSAGE 1. Widely to scattered showers and thunderstorms can be expected through early this evening.The environment is more pulse-like with weak shear and moderate to high storm energy. An isolated severe thunderstorm is possible. Life cycle of these storms will be short.

The activity is forecast to weaken quickly this evening with the loss of daytime heating. However, the CAMs do show the potential for scattered showers and thunderstorms late tonight into Tuesday morning along the coast as the airmass destabilizes and low-level convergence develops in a strengthening southerly flow.

.KEY MESSAGE 2. A highly anomalous summertime pattern will feature the formation of an upper low late tonight into Tuesday, as shortwave energy drops southeast across the Lower Great Lakes into upstate NY. The associated surface low occludes and falls below 1000 mb. For this time of year, both the 500 mb and 1000 mb geopotential heights are near all time lows (well below the tenth percentile using the NAEFS). The combination of large scale lift and precipitable water values increasing to around 2 inches is setting the stage for a significant rainfall event across the region. The heaviest axis at this time is forecast to be across NYC, NE NJ, and the Lower Hudson Valley, with 2 to 5 inches of rainfall. Localized higher amounts are possible. The 12Z HREF shows a high neighborhood probability of 70%+ for an inch plus/hourly rates by Tuesday evening and 5 inch storm totals in this corridor by Wednesday morning. It also shows a significant signal for 3 inches/3 hours at 10 to 30 percent. While this may seem low, these probabilities are a good signal for high hourly precipitation rates. The WPC Urban Rain Rate Dashboard (URRD) is also showing significant support for these rates with some members of the ensemble supporting hourly rates with Annual Return Intervals (ARI) of 50 to 100 years. Bottom line, the ingredients are there for a potentially significant rainfall event. However there are likely to be some adjustments to this axis over the next 24 hours.

The rainfall will likely come in south to north oriented bands as advertised by the CAMs as energy rounds the upper low along with good upper level divergence. For much of Tuesday, these bands of showers and thunderstorms will likely be confined to the watch area, weakening as they pivot east. However, as the upper support and low level jet work east Tuesday night, the heavy rainfall will likely work across LI and southeast CT. Guidance generally keeps total at 2 inches or less across these locations. However, there is the potential for higher totals and this area will have to be watched for possible expansion of the flood watch for Tuesday night into Wednesday.

The SPC marginal risk for severe weather on Tuesday gets just into southern portions of the NY/NJ metro. There is decent turning of the vertical wind profile with a 2 percent chance for tornadic development. LCLs are low and instability is marginal. However. lapse rates are poor and deep convection supporting severe weather is unlikely.

Areas of showers with embedded thunderstorms will likely continue Wednesday into Thursday with the upper low tracking slowly east across the Northeast. The deep moisture will shift to the east and hourly rates will be significantly lower.