New England May Get a Record-Setting October Storm, With Help from 93L

A fast-evolving cyclone will sweep northward from Florida to New England in a mere 24 hours late this weekend, bringing torrential rain—perhaps enough to swamp one-day October records—and the potential for very high winds and coastal flooding. This storm has an unusual fingerprint for late October, in part because it will be entraining moisture and energy from a tropical disturbance dubbed 93L that’s still simmering in the Western Caribbean.

As of noon EDT Thursday, the broad center of 93L was located near the east coast of Nicaragua, having moved little since Wednesday. Winds near the center of 93L are light, generally 5 - 15 knots, and showers and thunderstorms around 93L remain widespread but disorganized. Overall, the environment around 93L is supportive of development, according to the 12Z Thursday run of the SHIPS model. Wind shear is moderate (10 – 15 knots), sea surface temperatures (SSTs) are very warm (30 - 31°C or 86 - 88°F), and the mid-level atmosphere is very moist (relative humidity around 75%). However, 93L has failed to generate the spin needed for it to strengthen as a tropical cyclone, in part because the center has remained so close to land. Competition from an area of disturbed weather in the Northeast Pacific, which will likely develop into a tropical depression this weekend or early next week, may also be helping to suppress 93L.

Figure 1.  GOES-16 image of the disturbed weather surrounding 93L to the north (associated with a surface front and weak upper-level trough) and to the southwest (in the Northeast Pacific) at 2 pm EDT Thursday, October 26, 2017. Image credit: NASA/MSFC Earth Science Branch.

Outlook for 93L and this weekend’s Northeast U.S. storm

The window for 93L to develop as a classic tropical cyclone is rapidly closing, but the next several days will give 93L a chance to develop as a subtropical or hybrid-type system that would be entrained by the late-weekend East Coast storm. A cold front is beginning to encroach on 93L from the north, with strong northeasterly winds of 15 – 25 mph behind it. The front is at the tail end of a weak upper-level trough moving across the eastern Gulf of Mexico. The most intense showers and thunderstorms near 93L are actually well north of the center, in a band associated with this front and trough. Convergence along the front could help provide enough low-level spin for 93L to begin developing over the next couple of days. In its Tropical Weather Outlook issued at 2 pm Thursday, the National Hurricane Center gave 93L a 30% chance of development by Saturday afternoon and a 40% chance through Tuesday.

A much stronger upper trough will be moving across the Mississippi Valley on Saturday and pivoting northeast across New England by late Sunday. Starting late Saturday, 93L will come under the influence of this stronger trough. Wind shear will be increasing rapidly, and 93L will begin accelerating toward the northeast. The strengthening upper flow is not favorable for classic tropical storm development, but it could support the formation of a subtropical or hybrid-type storm, especially since 93L will continue to traverse very warm waters. By Saturday night, 93L will likely pass over or near South Florida, bringing the potential for a quick 3 – 5” of rain.

From late Saturday into early Monday, conditions will unfold at a breakneck pace, and the forecast becomes much more complex. A few miles above ground level, a jet streak with top winds exceeding 140 knots (170 mph) will develop over the Southeast U.S. early Sunday and race along the east side of the upper trough toward New York later in the day (see Figure 2). On its heels, another strong jet streak is expected to take shape further south, near the base of the trough, and move near the mid-Atlantic coast late Sunday. In such a setup, there is the potential for coupled jet-streak dynamics in the middle, just off the mid-Atlantic coast, where the regions of upward motion near each of the two jet streaks coincide and trigger a rapid drop in surface pressure. This coupled process may occur near the Gulf Stream, where SSTs are 1-2°C above average.

Figure 2.  Winds at 250 mb (about 34,000 feet high) will be howling at speeds from 100 to 150 knots (115 – 170 mph) at 2 pm EDT Sunday, October 29, 2017, based on output from the 12Z Thursday run of the GFS model. An area of coupling between two jet streaks may enhance vertical motion and favor the deepening of a surface low. Image credit: tropicaltidbits.com.

In their 00Z Thursday runs, two of our top long-range models handled this volatile setup quite differently. Both the GFS and European models developed a primary surface low off the Southeast coast on Sunday that races northward, with a smaller low to its south that appears to be 93L. The key differences emerged on Sunday night, when the GFS consolidates energy into the main low and slams it into Cape Cod around midnight. The 00Z Thursday Euro, in contrast, keeps the main low weaker and further west and quickly intensifies 93L into the dominant surface low, reaching Cape Cod in the pre-dawn hours Monday. In their 12Z Thursday runs, the GFS and Euro trended toward a weaker reflection of 93L, located further to the southeast and remaining well offshore, with a primary surface low moving north across central or western New England.

Both the ECMWF and GFS bring surface pressure below 980 mb as the main surface low moves inland. Readings this low have never been observed in southern New England during October, although even lower values were recorded from the New York area southward during Hurricane Sandy (2012).

Figure 3.  Lowest sea-level air pressures ever recorded in October for the U.S. Northeast. Values are in millibars and tenths of millibars, with the preceding 9 omitted: e.g., "793" = 979.3 mb. Most of the records along the Northeast coast were set during Hurricane Sandy (October 2012) or during a storm in October 2006. Image credit: NOAA/NWS Weather Prediction Center.

Very heavy rains and high winds a good bet for New England—but exactly where?

It’s too soon to know just how this situation will unfold, especially given the very rapid and complex development we can expect. Here are two potential impacts that will bear close watching:

Torrential rains and possible flooding. Regardless of the status of 93L, the very strong southerly winds accompanying Sunday’s trough will pull vast amounts of moisture northward along a narrow corridor that may bear the hallmarks of an atmospheric river. The rich corridor of moisture, and the very strong vertical motions associated with the upper trough, will favor a south-to-north zone of extremely heavy rainfall from late Sunday into early Monday. The exact placement of this south-to-north zone will be crucial, since rains could be much less heavy on either side. Current model trends suggest the most likely location will be somewhere between extreme eastern New York and western Maine. Within this corridor, rainfall amounts of 2 – 6” could be widespread, with local amounts of 8” or more possible, especially along south-facing slopes and gorges. Significant flash flooding appears quite possible.

Very strong winds just east of the primary surface low. Both the ECMWF and GFS suggest that a corridor of surface winds in the 40 – 60 mph range, with higher gusts, may crash inland somewhere from Rhode Island to the Maine coast. Expect wind forecasts to increase in magnitude for some locations as this event draws closer and forecast confidence improves.