NHC Unveils New Product with Potential Tropical Cyclone in Atlantic

The NOAA/NWS National Hurricane Center (NHC) issued its first-ever Potential Tropical Cyclone advisories Sunday afternoon, highlighting the threat from a strong tropical wave in the central Atlantic that is likely to affect the Windward Islands as a tropical storm. Another system in the northwest Caribbean remains on track to move into the Gulf of Mexico, where it could affect areas from Texas to Florida as a tropical storm later this week.

The new Potential Tropical Cyclone advisories will provide more detailed guidance on systems that are not yet at depression strength but that have a chance of intensifying and bringing tropical storm or hurricane conditions to land areas within 48 hours. In their announcement of this and other service changes for 2017, NHC said: “Under previous longstanding NWS policy, it has not been permitted to issue a hurricane or tropical storm watch or warning until after a tropical cyclone had formed.  Advances in forecasting over the past decade or so, however, now allow the confident prediction of tropical cyclone impacts while these systems are still in the developmental stage.  For these land-threatening ‘potential tropical cyclones’, NHC will now issue the full suite of text, graphical, and watch/warning products that previously has only been issued for ongoing tropical cyclones.”

Potential tropical cyclones will be assigned numbers as part of the same chronological list that includes tropical depressions. Thus, the current system is PTC 2, even though it is the first PTC ever to be classified, because it follows Subtropical Depression 1 (which later became Tropical Storm Arlene). A potential tropical cyclone will retain its PTC number should it intensify to TD strength.

As of 5:00 pm EDT Sunday, the ill-defined center of PTC 2 was about 800 miles east of the Windward Islands, moving west at a speedy 20 knots (23 mph). Even though the maximum sustained winds associated with PTC 2 are estimated at 35 mph, the system lacks the closed center of circulation that would allow it to be classified as a depression.

Figure 1. Enhanced infrared satellite image of Potential Tropical Cyclone 2 at 2215Z (6:15 pm EDT) Sunday, June 18, 2017. The coast of South America is faintly outlined in magenta at lower left, and the Windward Islands are off the image on the left-hand side. Image credit: NOAA/NESDIS.

The outlook for PTC 2

Showers and thunderstorms (convection) remained fairly sparse on Sunday with PTC 2, but a compact center of convection had begun to develop, with some rotation evident in satellite loops on Sunday afternoon. If this center continues to intensify with the usual nighttime peak in convection on Sunday night, we will likely have a depression or a tropical storm by Monday.

Over the next two days, PTC 2 will be traveling over quite warm waters (sea surface temperatures of 28 – 29°C or 82 – 84°F) with light to moderate wind shear (5 – 15 knots) and a very moist atmosphere (midlevel relative humidity around 80%). Given these favorable conditions, NHC is predicting PTC 2 to become a tropical storm by Monday afternoon. The next name on the Atlantic list for 2017 is Bret, followed by Cindy.

Models are in close agreement taking PTC 2 toward the west or west-northwest toward the Windward Islands by late Monday or Tuesday. The government of Barbados has issued a Tropical Storm Warning for Barbados, St. Vincent, and the Grenadines. NHC is predicting peak winds of 50 mph. With the highly favorable conditions ahead of PTC 2, I would not be shocked to see it intensify a bit more—especially if its eventual center of circulation ends up on the compact side. The 12Z Sunday run of the HWRF model, among our most reliable for predicting short-range intensity, brings PTC 2 to Category 1 hurricane strength as it passes near Tobago and Barbados.

Once it passes the islands, PTC 2 is very likely to wither in the face of increasing wind shear and low-level divergence—hallmarks of the eastern Caribbean hurricane “graveyard” that often afflicts early-season cyclones. None of our reliable operational models sustain PTC 2 beyond the central Caribbean.

Figure 2. Although dozens of tropical cyclones have formed during June in the Gulf of Mexico, and a few in the Northwest Atlantic, only four have been known to develop during June in the Main Development Region east of the Antilles in NOAA records dating back to 1851. Blue tracks denote tropical depression strength; green denote tropical cyclone strength; yellow denotes Category 1 hurricane strength. Image credit: NOAA Hurricane History.

June cyclones are few and far between in tropical Atlantic

In NOAA’s hurricane database extending back to 1851, there have been only two tropical-storm-strength systems east of the Antilles in the Main Development Region of the deep tropical Atlantic prior to July 1:                                                  

•  the 1933 Trinidad hurricane, which affected Trinidad and Venezuela on June 27-28 as a Category 2 before making landfall south of Brownsville, TX, as a Category 1.
•  1979’s Tropical Storm Ana, which developed in the central Atlantic on June 19 and struck St. Lucia on June 23 before decaying in the eastern Caribbean.

Two weaker systems have also formed in the MDR in June. Tropical Depression Two from 2000 developed on June 23 at a location that was extraordinarily far east—around 10°N, 20°W—in the eastern Atlantic, but it failed to reach tropical storm strength. TD 2 from 2003, which formed in the central Atlantic on June 11, lasted only a day as a tropical cyclone.

The current potential tropical cyclone is also noteworthy in its very low latitude of 7.5°N. If and when it develops into a depression, it should be far enough northwest to avoid breaking the record set by Hurricane Isidore in 1990. Isidore was classified as a depression at 7.2°N, making it the lowest-latitude tropical cyclone ever recorded in the North Atlantic.

Figure 3. Enhanced infrared satellite image of 93L at 2207Z (6:07 pm EDT) Sunday, June 18, 2017. GOES-16 data are preliminary and non-operational. Image credit: MSFC NASA Earth Science Office.

93L likely to become a tropical storm in Gulf of Mexico

Models agree that the system known as Invest 93L in the northwest Caribbean is very likely to become a tropical storm in the Gulf of Mexico and affect some part of the Gulf Coast later this week. There remains plenty of uncertainty in the outlook for 93L, which has yet to carve out a closed low-level center of circulation. The large Central American gyre surrounding 93L is roughly centered in the extreme southwest Caribbean north of Honduras, drifting toward the north-northwest.

A large area of strong convection broke out on Sunday well east of 93L, dumping torrential rain and producing gale-force winds. This complicates the forecast for 93L, because the mid-level spin (vorticity) associated with this convection might be enough to prod a center to develop beneath it. Barring that, models agree that a center should develop or intensify in the Gulf off the Yucatan Peninsula coast by Monday or Tuesday. In its Sunday afternoon tropical outlook, NHC gave 93L 70% odds of developing into at least a depression by Tuesday and 90% odds through Friday.

Although models agree that 93L will likely come ashore somewhere on the Gulf Coast as a depression or tropical storm, the outlook remains unclear as to just how strong a storm it might be and especially where it might make landfall. Sea surface temperatures are slightly below average over the central and northern Gulf of Mexico, around 27°C (81°F), and wind shear will be increasing toward the Gulf Coast. The shear and the less-than-torrid waters ought to be enough to keep a lid on any rapid intensification of 93L, and it appears highly unlikely 93L will reach hurricane strength. If the compact upper-level ridge now over 93L translates northwest with the system, it would raise the odds of strengthening. In any event, the big story is likely to be excessive rainfall and the potential for flooding.

Figure 4. The 18Z Sunday run of the GFS model, which brings 93L slowly onshore along the central Gulf Coast, predicts that rainfall this week could exceed 10” over several parts of the central Gulf Coast. The exact track of 93L and the resulting locations of heavy rain are still too soon to predict with confidence. Image credit: tropicaltidbits.com.

Track outlook for 93L

An upper-level trough will be moving across the eastern U.S., creating a weakness in the flow along the central Gulf Coast that may or may not be strong enough to urge 93L northward. A weak cut-off low in the Bay of Campeche adds another wrinkle, as it could be enough to help bring 93L westward. The European model has consistently brought 93L toward the Texas or Mexico coast, while the GFS has favored the central Gulf Coast. These model tendencies continued on Sunday, setting up a “Euro-vs-GFS” battle of the sort that’s become familiar in recent years. When averaged over a long period, the European model typically outperforms the GFS, but on any given Sunday (or Monday or Tuesday), the GFS can just as easily produce the better forecast, so we’ll have to wait and see which contender wins this battle. Sunday’s 12Z and 18Z runs brought the two models somewhat closer together, with the ECMWF trending toward the upper Texas coast and the GFS toward southeast Louisiana.

The weak steering currents may give 93L a period of slow, erratic motion. Should this occur near the coast, it would raise the odds of extremely heavy rain near and just rightward of its center. The GFS has consistently called for a pocket of 10” – 20” rain along the central Gulf Coast, assuming that 93L makes landfall in that vicinity. Rainfall with 93L would probably be somewhat less along the Texas coast, but still potentially very heavy, should the European track prove correct. If convection persists well east of 93L, then parts of the Florida Peninsula may also experience quite heavy rain.

Simultaneous Hurricane Hunter missions are possible on Monday

NHC has tentatively scheduled Hurricane Hunter flights for Monday, as needed, into PTC 2 and 93L. “2 recon planes scheduled to fly on 2 June systems simultaneously across 2 days!” tweeted Eric Blake on Sunday. “Believe that is unprecedented in modern times (for June)."