For more on the heavy rain threat, see the previous post.
The latest high resolution modeling is in agreement with the more general modeling that the steadiest and heaviest rain will not be affecting the Garden State until Sunday Afternoon and Sunday Evening. I still also believe that the lowest rain amounts will be recorded around Cape May with the highest amounts in Northwestern New Jersey.
Recent heavy rainfall in February, especially north and west of Philadelphia, and all of the melting snow has caused our waterways to run above normal. Therefore, flash flood guidance is low across a good portion of the Northern and Central New Jersey. According to the updated guidance, one to two inches of rain in six hours or less is all it would take to cause flooding in the Raritan Basin region. For Southern New Jersey, three to four inches of rain would begin to cause flooding. Unfortunately, the latest model guidance has the heaviest rainfall axis where the guidance shows the lower values. This is the reason a flood watch remains posted for Northern and Central New Jersey. Two to three inches of rain with locally higher amounts are possible in Northern and Central New Jersey. One to two inches of rain is expected elsewhere in Southern New Jersey. Cape May could see amounts under an inch. We need to watch to see if any training or terrain effects result in a narrow corridor of 3-6 inches of rain.
For the New Jersey counties within the flood watch, there is a strong potential to see creeks and streams rise and exceed flood stage. The watch area could also have numerous road closures and washed out roadways if amounts exceed two inches. Outside of the watch area, while the road closures may not be as widespread, some street flooding and poor drainage flooding is quite possible. The entire forecast area will likely see hydroplaning conditions by nightfall. Ditches will also fill to capacity.
By evening, everyone should be into steady moderate rain with those pockets of heavier rain. The low developing on the front will be so strong that low-topped convection may develop in the entire region, despite a true lack of instability. In our eastern counties, the lack of rain initially during the day could allow for temperatures to spike up into the sixties and this may add marginal instability in those areas. Don’t be surprised to see some thunderstorms Sunday Evening and Sunday Night. The higher resolution modeling wants bring a line of convection through the area from 4:00 p.m. to 9:00 p.m. The low pressure center will allow for Garden State to be within the right quadrant and any unforeseen instability would be able to trigger potential severe thunderstorms. Sometimes these events could cause a wet microburst or a hard to detect, weak tornado embedded within the heavy rain.
Let’s know get the wind potential. The cold front will cause a strong southerly flow to develop in advance of it. The heavier rain could certainly transport stronger winds to the surface. The winds aloft do not look the most impressive, but there is enough of a kick aloft to be concerned about. With the low crossing our region, the gradient does tighten enough to suspect that a period, perhaps short-lived, of windy conditions could occur. With the saturated ground, the wind potential must be monitored. South winds could be sustained 20 to 25 MPH with gusts of up to 45 MPH. I could even see some areas, especially in the coastal counties; see wind gusts of near or in excess 50 MPH during the frontal and low pressure passage. The wind will be dependent on the exact low pressure center movement and there is low confidence about the wind threat.
Some of the model guidance wants to show this ending as wet snow and sleet Northwestern New Jersey. It will be dependent on whether the cold air catches up with the precipitation behind the cold front. It is not impossible to see this happen, but I have my doubts. I don’t see the best setup for this to occur. The primary impact in these areas will be the flooding and even mudslides in the higher terrain.
The rain will end rapidly on Monday and we will deal with lingering runoff causing flooding. The wind will be stiff with the departing low and incoming high. Tuesday looks seasonable. Wednesday will be mostly cloudy as a warm front passes through and this front perhaps could create some showers. By Thursday, another strong front with low pressure will bring a heavy rain threat. More thunderstorms and windy conditions are possible.
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