After a week-long lull, the Atlantic basin has once again roared to life. Three regions of tropical activity now lurk across the Atlantic, including two named storms. Neither system is expected to remotely approach the potency of Hurricane Florence, however, impacts to the United States cannot be entirely discounted.
This is your Sunday Storm.
Subtropical Storm Leslie is the least threatening of the systems to the United States. Located roughly 1700 mi southeast of the North Carolina coast, Leslie had maximum sustained winds of 40 mph with minimum central pressure of 1002 mb as of 11 am Sunday. Caught in the open ocean without steering winds, the storm was traveling westward at a meager 3 mph, essentially idling in the open Atlantic. She only recently intensified to named status. Leslie is subtropical as opposed to fully tropical, since the storm’s circulation is engulfed inside a much broader upper-level low pressure system, with the strongest winds located away from the center. The storm has a marginal warm core, as convection in the center of the storm is weak. A weak low pressure system is expected to develop just to the north of Leslie by Wednesday. The two lows will merge and travel harmlessly eastward, remaining out-at-sea.
Tropical Storm Kirk, on the other hand, is a completely different enterprise. The infrared imagery above visually depicts the most stark physical difference between subtropical Leslie and fully-tropical Kirk. Higher cloud tops, denoted by warmer imagery, indicate regions of robust convection. The highest cloud tops, and therefore the strongest convection, associated with Leslie is split into two separate regions. One is confined to the south of the storm’s center, the other in feeder moisture bands drawn from the upper-level circulation well to the south and east of the center. Kirk’s convection is strongest towards the center and its northwestern quadrant, and it decreases radially in intensity. Overall, Kirk’s convection is much stronger than that of Leslie.
Will Kirk live long and prosper? Probably not, but possibly. As of 5pm Sunday, Kirk was a 40 mph Tropical Storm with minimum central pressure of 1006 mb located about 645 mi southwest of the Cabo Verde Islands. The storm was traveling west at 23 mph. An intensifying ridge to the north of Kirk should accelerate the westward propagation of Kirk to near warp speed, such that the storm should reach the Lesser Antilles by Thursday night. The ridge will also prevent Kirk from recurving to the north, limiting the storm’s potential threat to the US.
Over the next few days, Kirk should encounter a region of low wind shear and warmer sea surface temperatures, permitting intensification into a stronger Tropical Storm. The storm will only remain under these conditions for a short period of time. By Wednesday night, the ridge will weaken and Kirk will be driven into a region of very strong wind shear, as illustrated best by the 12 UTC September 23 HWRF graphic from NOAA’s environmental modeling site. The storm will slow down as it likely gets ripped apart by shear and dissipates somewhere in the southern Caribbean Sea.
It is unlikely, but not impossible, that Kirk escapes destruction. The storm is unlikely to achieve the strength required to recurve to the north and east. No hurricane model, global deterministic model, nor individual global ensemble member of the GEFS, CEPS or ECMWF forecast Kirk reaching hurricane status before making a pass as the Antilles. Some members of the GEFS develop a stonger Kirk and weaker ridge off the Southeast US coast, enabling the storm to narrowly escape to the north. But wind shear is expected to be strong to the north as well. Only if Kirk recurves to the north and survives the strong wind shear does the storm have a chance for survival. Given the storm’s Tropical Storm status upon reaching this juncture, the odds of survival look grim. Perhaps that’s a good thing, since the positioning of the ridge to the north would likely drive Kirk northwest, close to the US mainland.
Invest 98L is the third disturbance of interest. Last week, this system made waves across social media before it could do so in the Atlantic. Many on social media inaccurately touted that these poorly organized clusters of thunderstorms presently located between Bermuda and the Bahamas are the zombie remains of Florence. These storms made headlines in particular because the weak low pressure system associated with these storms is expected to recurve northward along or into the Southeast US coast, as if Florence were making a second swipe at the Carolinas. These assertions are simply untrue.
Technically, Invest 98L is associated with Florence’s remnant low, but it is most definitely not Florence’s remnant low. When Florence’s remnants exited the New England coast as a weak extra-tropical cyclone (like a Nor’Easter), the parent low had already been absorbed by an upper-level trough, which propagated out to sea. The absorption of a once-tropical cyclone by an upper-level trough is always the result of extra-tropical transition because cold-core extra-tropical cyclones are strongest aloft and require jet stream dynamics to intensify. At the surface, the cold front associated with the parent trough instigated cyclogenesis along the Mid-Atlantic coast where the temperature gradient was the strongest. The weak low-pressure system that formed as a result of this retrograded to the southeast and is now known as Invest 98L.
There is fairly unanimous agreement that what is now Invest 98L will make a swiping pass at or make landfall near the Carolina coast. This is per the Tropical Tidbits graphic above depicting a suite of deterministic and ensemble mean storm tracks. The same stubborn high most likely preventing Kirk from curving northward will direct Invest 98L clockwise around its center towards the Southeast coast. Despite its close proximity, the cluster of storms likely won’t pose much of a threat to the Southeast besides a few heavy downpours and rough surf. Upper-level winds will only remain light until the storm begins to approach the Southeast coast. The strong wind shear will inhibit tropical development, despite sea surface temperatures still exceeding the 26° C requirement for tropical cyclone development.
There is a chance that the low-wind shear environment will persist just long enough for Invest 98L to become Tropical Depression Twelve, but those chances are fairly small. Of the most commonly used model guidance, only the ECMWF ensemble (EPS) and the HMON, one of NOAA’s hurricane models, admit this possibility. The 12 UTC September 23 EPS in particular is quite bullish about the possibility of tropical cyclone development, per the following probability graphic below, courtesy of weathermodels.com.
That 85-90% of EPS members predict tropical cyclone development is quite misleading. Less than 50% of EPS members predict what could be Tropical Depression Twelve intensifying into a Tropical Storm before making a swipe at the Carolina coast. Afterwards, the low will likely interact with a cold front along the Southeast coast and possibly become extra-tropical or dissipate. Any rain that arises out of this potential system would be confined to coastal areas of North Carolina, and it likely won’t account to more than a few heavy downpours producing up to 1-1.5″ of rain. In other words, Invest 98L will likely not add to woes from Florence.
It’s only two weeks past the peak of hurricane season. There will likely be more tropical threats on the horizon. Stick with WeatherOptics as we provide further updates on this week’s threats and on future threats that soon emerge.
Also, keep an eye out for the 5 Things to Watch tomorrow morning.