Oct 7: Coastal Storm Developing

Oct 7: Coastal Storm Developing

Written on 10/08/2019
Jonathan Carr

Discussion: October is traditionally known for coastal storms and nor’easters. The NJ-latitude Sea Surface Temperatures (SSTs) are about 10 degrees cooler than they were just 4-6 weeks ago and therefore are harder to sustain warmer tropical cores. It’s not impossible for a hurricane landfall in October but it would likely have to come in from the due S. Even Sandy transitioned from a tropical warm core hurricane to an extra-tropical cyclone prior to landfall. It couldn’t keep the warm core together over the 60-degree water.  Instead, for NJ in October, we typically now deal with mid-latitude cyclones having larger wind fields and colder rains. When they stall off the coast, and sometimes even retrograde towards land, they become nor’easters.

Model guidance is fairly locked on to the idea of an offshore storm system forming off the SE US overnight tonight. I’m already seeing some formation around the 30N 75W area. A massive departing area of high pressure will provide northward steering current and should take this offshore system up the E coast (a few hundred miles offshore). By Wednesday morning the system should run into a flexing upper-level ridge in SE Canada (high pressure at the surface to the system’s due N). This should be a “though shall not pass” moment for the offshore low. The system should then stall in place for Wednesday and Thursday before hopefully pulling away by Friday morning.

How far the system stalls offshore, of NJ, will have great influence over NJ impacts. The Euro and GFS are in disagreement about this. The Euro brings the system closer to NJ. The GFS keeps the system further out to sea. The main difference between the solutions, at the surface, is a prolonged weak coastal low vs a prolonged stronger nor’easter.

Regardless of how close the system gets to the SE of NJ there will be a stubborn high to the N of it. The two features will squeeze down the pressure gradient between them and ultimately produce strong onshore flow for ENJ. Therefore I’d say the forecast for minor-to-moderate coastal flooding, large surf and beach erosion is much more confident than expected rain amounts and wind values. We need more consistency in the models with tighter ensemble member clusters before we can lock down how strong the rain and wind will be.

Onshore flow should begin light for ECNJ/SENJ tomorrow, gradually ramp up Tuesday night into Wednesday and peak Thursday. The Euro has the system lingering until Saturday morning. The GFS has it out of here by Friday morning. Again, some model inconsistency that needs to tighten before committing to a forecast. I think we’ll be in much better position to gauge rainfall amounts and wind values tomorrow evening.

In English: An offshore storm system has the capability to bring rainfall, gusty winds and coastal flooding between Tuesday night and Saturday morning. The prolonged duration of the event could justify nor’easter status but let’s not freak out just yet. Areas of highest impact should be ECNJ/SENJ mainly due to gusty winds, coastal flooding and beach erosion. The rest of NJ could still see rain and wind but not as intense as the ocean-facing coastal areas. Onshore flow should start Tuesday and gradually ramp up into Wednesday. Wednesday PM through Friday morning should be peak intensity. Conditions should then gradually improve heading into the weekend. By tomorrow evening I should have a better handle on both rain and wind intensities. For now expect at least raw and damp conditions possibly some heavier rainfall. It’s that time of year everyone. Be safe! JC

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