Friday is ‘Debby day’ for NJ: Latest storm timeline and impacts

The Bottom Line

It is about to feel more tropical across New Jersey as rain chances go up, humidity increases, and the flooding risk ramps up again too.

Tropical Storm Debby made its final landfall early Thursday morning, just north of Charleston, South Carolina. It will ride up the Appalachians through the upcoming weekend.

New Jersey will experience the first direct Debby rain bands later Thursday. And primary impacts are still coming up on Friday — which I have unofficially declared to be "Debby day".

As we have discussed, New Jersey's forecast has trended away from a "worst-case scenario". But there will be rain and wind. And we can not ignore the risk of another round of flooding and severe weather.

Let's jump into the latest full rundown of storm timing and impacts, from rain to wind to tornadoes to surf.

Get our free mobile app

Timeline

Thursday will be very similar to Wednesday: Unsettled, cloudy, and generally wet. Despite the damp and dreary weather, at least we are getting a welcome break from the constant summertime heat and humidity.

Several waves of showers and thunderstorms will cross New Jersey from south to north Thursday. There could be some localized downpours and/or stronger thunderstorm cells, but I do not expect anything crazy.

Temperatures on Thursday will climb from the 60s in the morning only to the mid 70s in the afternoon.

From Thursday night to Friday, we will see additional pockets of rain. Again, potentially heavy at times. There will be substantial breaks of dry weather along the way. Temperatures will actually rise through the 70s overnight. And Friday will be quite humid and warm, with highs pushing into the lower 80s.

Debby is still on-track to pass just west of New Jersey as a tropical depression late Friday. So that is still the time frame for the most significant impacts here. Let's say 4 p.m. to Midnight. One final push of heavy rain and elevated winds Friday evening will be the grand finale of this week's stormy, active weather.

Rainfall Totals / Flood Potential

Between Thursday and Friday, I think most of New Jersey will see an additional 1 to 2 inches of rain in the bucket. At least half of that will probably come late Friday during Debby's "brunt". Heaviest rainfall totals will be to the north and west, away from the coast. (Closest to the bullseye of heavy downpours over PA and NY.)

If it really pours, I could see some totals in NJ reaching 3" or even 4". (After all, it is important to never underestimate the rainfall potential of tropical moisture.) That is more than enough to ring flooding alarm bells once again — especially since our ground is very saturated from this week's previous rain storms.

We have a useful product called Flash Flood Guidance to help gauge impacts in these situations. The National Weather Service describes it as "an estimate of the amount of rainfall required over a given area during a given duration to cause small streams to flood." It is based on factors like total precipitation, soil moisture, and stream flow.

That map tells a story. For parts of South Jersey, it would take over 5 inches of rain in 6 hours for flooding to occur. That ain't gonna happen.

Meanwhile, vulnerable sections of North Jersey could flood with 0.75-1.00" of rain in 6 hours. That probably will happen.

The flooding potential all comes down to 1.) where downpours set up Thursday into Friday, and 2.) how long the grand finale rain burst lasts.

Heavy rain leading to flooding of roadways and low-lying areas is still our biggest concern as Debby passes by.

Wind

It will get windier for a time. But wind speeds will stay below tropical storm force (sustained 39+ mph).

Most of Friday will be breezy. And a gusty thunderstorm cell is possible at any time. (I'll cover that in the "Severe Weather" section below.)

As Debby's center passes just west of New Jersey, the wind will peak. I expect top gusts around 30 mph inland and up to 40 mph along the Jersey Shore.

Especially when combined with soggy soil, that is enough to bring down some trees and cause sporadic power outages. But massive widespread damage? Nah.

Severe Weather / Tornadoes

I think this storm impact ranks in second place, behind the rain and flooding threat.

Tropical cyclones tend to have a lot of "twisting" energy stored within them, making a tornadoes a real concern when they make landfall. Especially around the northeast quadrant of the storm, which is exactly where New Jersey will end up late Friday.

In addition, Friday is going to be a warm and humid day. There is even the chance for a peek of sun to "cook" the atmosphere in the early afternoon. That raises the risk of a strong thunderstorm developing.

The severe weather threat will kick in starting around 3 or 4 p.m. Friday, through the evening. The natural biggest threat — in addition to heavy rain — would be for gusty winds. And again, we could very well see a tornado or two around as well.

There's not much you can do to prepare for a potential severe weather day. Just make your plans for Friday carefully, and stay closely tuned to changing weather conditions.

Coastal Flooding / Storm Surge / Rough Surf

Because Debby's low pressure center is passing to our west — inland — there is not a huge concern for coastal impacts.

The only big concern here is for rough surf. A high risk of rip currents is posted for the Jersey Shore for both Thursday and Friday, with ocean waves swelling to about 5 feet. Despite the less-than-stellar beach weather, there will still be some dodos who insist on going in the ocean and need to be rescued.

Storm surge will be minimal, less than a foot. Probably not enough to cause much if any coastal flooding.

The Extended Forecast

I believe our weather will turn completely dry by daybreak Saturday, setting us up for a glorious summer weekend.

Skies should become mostly sunny by about lunchtime on Saturday. Humidity will drop too, making the air much more comfortable. It will be a warm day, with high temperatures around 80 to 85 degrees.

Sunday looks good too, with partly sunny skies. Morning low temperatures will be near 60. Afternoon highs should reach the lower 80s.

We will stay in a pleasant, quiet weather pattern for the next week or two. I do not see any widespread rain chances, extreme heat, or horrendous humidity on the horizon.

Dan Zarrow is Chief Meteorologist for Townsquare Media New Jersey. Follow him on Facebook for the latest forecast and realtime weather updates.