The latest on Invest 99L and a look toward its future

Good evening! During a quiet Tuesday afternoon weather-wise throughout a large majority of the Central and Eastern United States, attention has turned to the tropics. Tropical Storm Franklin continues to move towards the Yucatan and Invest 99L, a tropical wave well out in the Central Atlantic Ocean, continues a slow movement northwest. But how much of a threat do either of these systems pose to the U.S Mainland, specifically 99L?  Lets clear the air, discuss the synoptics, and shed any misconceptions about tropical threats over the next week or so.

First, Tropical Storm Franklin is about to reemerge over the Bay of Campeche. The storm has weakened over the past 12 hours, with convection diminishing around its center as the storm moved across the Yucatan. Despite the weakening intensity, the storm has maintained solid banding structure, low level inflow, and upper level outflow on most quadrants. Once the storm regains its inner structure over water, the atmospheric environment will be favorable for more strengthening.

There is a good chance that Franklin will become the first hurricane of the season, perhaps strengthening to a Category 2 storm. A strong ridge developing to the north of the storm will help Franklin maintain a westerly heading, on a track towards Central Mexico. This will limit the impacts for United States interests, even along much of the Gulf Coast states.

Invest 99L is a tropical wave which we have been watching more closely during the past week. Initially, this wave raised more eyebrows when the GFS model — for several cycles — showed more rapid development into major hurricane just east of the Northern Leeward Islands. However, increasing shear and dry air in the Central Atlantic has kept convection with this wave disorganized so far. This is likely to continue, until a TUTT (Tropical Upper Tropospheric Trough) lifts out and astrong upper-level ridge develops over more of the Western and Central Atlantic.

GFS ensembles showing more favorable environment with lower shear over north of Bahamas and Leeward Island by this weekend

GFS ensembles showing more favorable environment for tropical development with lower shear over north of Bahamas and Leeward Island by this weekend

The wave will be steered more northwest underneath this ridge to just east of the Bahamas by this weekend. The more reliable ECMWF models and its ensembles are showing the development of system into a tropical storm and a hurricane.. While model forecast skill with the intensity of tropical cyclones is typically poor, this solution has support from many of its ensemble members, which adds some confidence in development. Other operational models including the UKMET and GGEM support the storms development as well.

The GFS model and it’s ensembles, however, keep this storm weak while shifting it further through the Bahamas and Florida. Considering its poor performance in the tropics overall, especially so far this season, we are skeptical of its tropical solutions whether they be on the weaker or stronger side. Its solution must still be considered, especially with the potential for dry air to impede development, but we are leaning more towards the ECMWF suite at this time. .

ECMWF ensemble 500mb pattern forecast at 120hr favors Invest 99L to recurve into North Atlantic

ECMWF ensemble 500mb pattern forecast,  favors Invest 99L to track more northwest over several days, before likely it turns more northeast between the trough over the Eastern US and a ridge over Atlantic.

The pattern on along the East Coast will be have to be monitored over next several days. At this time, it appears the storm will continue to track northwestward until it reaches a break in the ridge just off the Eastern Seaboard. Meanwhile, a broad trough over the East Coast will slide towards the tropical system. This will act to sweep the storm system eastward towards the ridge, promoting a northward turn and eventually a northeast/east seaward turn away from the coast.

While the storm system currently appears likely to remain offshore, there are some indications that a PRE (Predecessor Rain Event) may develop over parts of Mid-Atlantic or Southern New England sometime Sunday through Tuesday. This occurs as tropical moisture streams northward, ahead of the center of the storm system itself and underneath the right-entrance region of the upper-level jet streak. Abundant lift and moisture could support some very heavy rainfall that could promote significant flooding hazards. Forecast models often struggle handling the placement and intensity of these features — so confidence is currently very low on if and when this will occur.

ECMWF models showing tropical mositure stream northward with two pwat maxium with "Gert?" and "PRE?" on Monday

ECMWF model showing tropical moisture stream northward with two areas of precipitable water maximums with possibly Gert and the PRE on Monday

We will continue to monitor Invest 99L over next several days. If this becomes a tropical storm, it will receive the next name on the Atlantic hurricane season list which is “Gert”. Again, while confidence remains low in regards to the systems actual track, current indications are that development of the storm system will occur — while the center of the storm will remain offshore and recurve off the Eastern Coast. Rainfall well ahead of the tropical system, however, may still impact some locations on the East Coast — and we will carefully monitor that potential over the coming days.

Have a great evening!

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