Another strong storm system will be affecting our weather in the near term. The storm is expected to bring a variety of weather to our region. Travel will become hazardous, if not dangerous in some cases with the potential for strong winds and flooding. The precipitation may even end as a wintry mix in our northern counties.
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 Philadelphia area. According to the updated guidance, one to two inches of rain in six hours or less is all it would take to cause flooding from Philadelphia and Delaware Counties and points north and west. For Southern New Jersey and Delaware, 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. These are the reasons Philadelphia and points north and west are under a flood watch for Sunday into Monday. I think many areas north and west of the Delaware River will see two to three inches of rain. Numerous creeks and streams in the flood watch area are vulnerable to flooding. South and east, one to two inches of rain is expected. Areas further to the south around Cape May could see less than an inch of rain. So, everyone in the flood watch has 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. 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.
The Delaware, Schuylkill, and Raritan Rivers could see significant rises. For the Delaware and Schuylkill Rivers, snow melting in New York and Northeastern Pennsylvania will send a lot of water downstream. In addition, there is likely to be a heavy axis of rain right in Northeastern Pennsylvania, Northwestern New Jersey, into Central New York. All that runoff will also flow downstream. With the heavy runoff expected in our area, one should monitor the potential for flooding along our area rivers…particularly if you live along them. I have seen numerous flood events that have turned out to be much worse than anticipated along our Delaware River. One thing that concerns me is that the models may be underestimating the terrain effects. If this was the case, four to six inches of rain could fall in a pocket which would lead to significant flooding. Without knowing how much rain will fall exactly, it is rather difficult to say with confidence there will be or not be river flooding.
There will be showers on Sunday Morning with a steady rain in the western fringes of our region. The rain will slowly spread east throughout the day. It could take several hours for the steady rain to enter our eastern counties. 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 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 low pressure center will allow for the eastern areas to be within the right quadrant and any unforeseen instability would be able to trigger potential severe thunderstorms.
Let’s now 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 New Jersey; 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 in the Poconos and 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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