Happy Monday! It's the final week of October (somehow) with Halloween being officially less than a week away. In the weather-sense of things, that means we are only heading deeper and deeper into Autumn and closer to full-on Winter. Maybe not coincidentally, we have our first significant nor'easter of the fall/winter season to track early this week. If you are not ready for winter weather, however, there is no need to worry. The overall environmental airmass simply won't be cold enough to support any type of frozen precipitation across S'rn New York/Long Island, New Jersey, or Connecticut. With that said, this post will focus on the upcoming nor'easter and its expected impacts. These types of systems are common during the cool seasonal period of October - April each year. They derive their name from the northeasterly winds that blow across the coastal region of the Northeast (in this case). Just a little fun tidbit. Anyway, I will be breaking this post up into a few sections. First, I'll discuss the evolution of the system without getting too technical of course. Next, I'll break down the expected impacts and timing. Let's get to it!
Coastal Low Development
Low pressure systems that develop off our coast fall into the meteorological subset of midlatitude cyclones. These systems are defined by their temperature gradients and frontal boundaries, and are cold-core (they strengthen with increasing height in the troposphere). They derive much of their energy and sometimes rapid development from the aforementioned temperature gradients (baroclinic instability); indicative of the relatively cold/cool air over the Continental U.S. (CONUS) and the warm waters of the ocean relative to the land; aided by the Gulf Stream. In the case of our particular system, which is set to begin impacting the region tomorrow (Tuesday, 26 Oct), we have an upper-level trough depicted over the Midwest States and Lake Michigan region. This can be seen in the below image, courtesy of UCAR.
At the surface, a low pressure system is analyzed over the Illinois-Indiana border with a warm front extending ENE'ward towards the N'rn Mid-Atlantic and Northeast. This is what's primarily responsible for today's showery and inclement weather. Often times with coastal development, we have a system to our west plus another low, sometimes weaker, off to our south along the Southeast Coast. This can also be seen in the below surface analysis valid as of 25 Oct/12z this morning (from NOAA/WPC).
As the upper-level trough progresses E'ward through tomorrow, it will help to direct the necessary "spin" and temperature gradients within the atmosphere towards the coastal plain. This will help to kickstart what is known as coastal cyclogenesis (development and intensification). This can be seen when analyzing the 200 mb winds (jet stream level). We can make out the incoming trough beginning to interact with the NE'ward moving low to our south. The Midwest low eventually weakens in this environment as the coastal low gets "captured" by the trough, which helps it to deepen and strengthen. The low then begins looping back offshore Southern New England, which will only prolong the duration of impacts. The below upper-level GIF is courtesy of TropicalTidbits.com.
Expected Impacts
Like I stated above, wintry weather is not expected from this system in terms of snow/ice, etc. It will be an all-rain event for everyone in the NYC region. The direct rain from this coastal system (not today's rain) will begin to move into S'rn and Central NJ late this evening and will reach the NYC metro overnight into Tuesday morning. The rain will become steady and potentially heavy at times as the coastal low begins to develop thanks to mid- and upper-level forcing. You'll notice the NW'rn rain shield will begin to pivot Tuesday afternoon as the low strengthens and continues NE'ward offshore. In other words, the rain shield will be moving SW->NE before sort of stalling and turning more W'ward as the low offshore continues to strengthen and potentially loops a bit. The rain will begin to taper overnight into Wednesday. As you can see, this will be a rather long-duration event that could last upwards of 24 hours or so. The rain will also extend well inland, which will be aided by a trough that will extend NW'ward from the coast and possibly a complex interaction with more than one center of pressure/height falls. The rain will fall at a good clip, especially during the morning and daytime hours Tuesday. As such a FLASH FLOOD WATCH has been issued for nearly the entire NYC metro, from Central/N'rn NJ, across NYC and Long Island, across the Lower Hudson Valley, and into CT. A good 1-3 of rain is expected from this system with locally higher amount possible of 4-5".
In terms of wind and coastal impacts, the greatest wind threat will occur over E'rn sections of the region, such as across E'rn Long Island and CT. A WIND ADVISORY is now in effect for E'rn Suffolk County from Tuesday afternoon through Wednesday morning for 20-30 mph winds with gusts to 50 mph. If the low is closer to the coast, stronger winds may move further west. Sometimes it's difficult to get the strongest winds from the low- to mid-levels to mix down to the surface, but it could happen as some model guidance is indicating, which would result in the potential for higher notable impacts. Also, an E'ly component to the wind could help promote coastal flooding and some beach erosion. The exact strength of the winds is not set in stone, but if you are in E'rn Long Island particularly, just be aware the wind threat does indeed exist and it would be recommended to secure any loose outdoor items if you live in E'rn LI. Again, rain begins to tape late Tuesday and overnight into Wednesday. Windy and brisk conditions will persist later in Wednesday across coastal locations.
Okay. If you made it this far, I thank you. This will be the first significant coastal storm of the season. I hope you found this post helpful and informative. As always, for decision-making purposes, please consult official products from the National Weather Service (NWS). Please do not hesitate to ask me anything on Twitter, or if you would like to discuss any aspect of this storm/weather in general. I will be more than happy to answer and talk. Thank you again for reading, and please be safe!
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