Major Nor’easter Could Get an Assist from 93L

Showers and thunderstorms continue to bubble around a large area of disturbed weather in the Western Caribbean. Designated as Invest 93L, the system had a broad center located on Tuesday morning just off the east coast of Nicaragua, near 13°N, 82°W. Data from the ASCAT scatterometer shows that winds are very weak near this center (only around 10-15 knots). Stronger easterly winds (15 – 20 knots) prevail well to the north of the center, and southerly winds up to 25 knots are flowing across the Northeast Pacific toward 93L.

Some of the fiercest Atlantic hurricanes on record have developed in the Western Caribbean during the latter part of the season, so it behooves us to keep an eye on this region. Wind shear is moderate across 93L (around 15 knots), the mid-level atmosphere is very moist, and sea-surface temperatures are extremely warm (30-31°C or 86-88°F). Fortunately, given its proximity to land and its weak circulation, it would take a while for 93L to evolve into a tropical cyclone, and our leading computer models continue to lean toward the idea that 93L will not organize at all. About a quarter of the 50 members of the European model ensemble run from 00Z Wednesday, and about a quarter of the 20 members of the GFS ensemble run, bring 93L to tropical storm strength. These numbers are up only slightly from the 00Z Tuesday runs we examined on Tuesday. None of the 70 ensemble members develop a hurricane out of 93L; most of them keep 93L bottled up in the western Caribbean, perhaps drifting into Central America later this week or over the weekend.

Figure 1.  Precipitation forecast for the 7-day period from 8 am EDT Wednesday, October 25, 2017, through 8 am EDT Wednesday, November 1. Much of the rain depicted over eastern New England is associated with a system now moving across the area. Image credit: NOAA/NWS/WPC.

93L likely to contribute to big rains in Florida, New England

A much more likely scenario than classic hurricane development is that moisture and energy from 93L will shoot north-northeastward and help contribute to very heavy rains in Florida on Saturday and in the Northeast U.S. from Sunday into Monday. All three of our top operational track models—the GFS, European, and UKMET—incorporate 93L into an elongated surface low this weekend just east of South Florida, where 3 – 6” rains are possible. The low will be associated with a strong upper-level trough diving into the eastern U.S. over the weekend, then pulling northeast early next week. Models predict the surface low will sharpen into an intense nor’easter-style storm system that will race northward across New England late Sunday or early Monday. The 06Z run of the GFS model deepens this low from 1002 mb just east of Miami at 2 am EDT Sunday, October 29, to 973 mb just south of Cape Cod at 2 am EDT Monday. Such a pressure drop would more than qualify as “bombogenesis,” which is defined as an extratropical low deepening by at least 24 mb in 24 hours.

A few hours of 40 – 60 mph winds appear possible just east of this low as it screams northward; the most likely location appears to be Cape Cod, based on current track forecasts. To the west of the low center, a quick shot of very heavy rain could fall on Sunday and Monday. The 06Z GFS suggests that 2 – 4”, with pockets of 4 – 8”, could fall along a north-south corridor that might be aligned with the Hudson Valley, between New York and Montreal. This comes on the heels of 2 – 5” observed in the 24 hours through Wednesday morning across parts of western Connecticut and Massachusetts. Heavy rain was streaming across eastern New England on Wednesday, with a flash flood watch in effect for Rhode Island and eastern Massachusetts.

In theory, the upcoming East Coast low could have some subtropical characteristics if 93L were to develop more fully over the next couple of days before its components were swept north. Right now, it doesn’t appear likely this will happen. In its tropical weather outlook issued at 8 am EDT Wednesday, NHC gave 93L a 20% chance of tropical development by Friday and a 40% chance by Monday (down from the 50% odds given on Tuesday).