ATL: INVEST 91L - Discussion (60/90)

[AutoPenalti]

AL, 91, 2025090412, , BEST, 0, 121N, 335W, 20, 1011, DB, 34, NEQ, 0, 0, 0, 0, 1014, 210, 90, 0, 0, L, 0, , 0, 0, INVEST, S, 0, , 0, 0, 0, 0, genesis-num, 021, SPAWNINVEST, al722025 to al912025,

https://ftp.nhc.noaa.gov/atcf/btk/bal912025.dat

Thread at Talking Tropics that was made for this area.

https://www.storm2k.org/phpbb2/viewtopi ... 1&t=124921

Oh boy, here we go.

[SFLcane]

Oh boy, here we go.

Euro being so far south has me a tad anxious. We shall see what happens but I think this one has someone’s name on it.

[Beef Stew]

Figured this would get tagged today. Definitely needs to be watched in the Lesser Antilles, and possibly the Greater Antilles and CONUS down the road...

[TomballEd]

About time.

The mid-upper low to the NW of 91L will likely pull this N. Of course, things aren't static, this could weaken or move W faster than 91L, and increase the threat to the NE Caribbean. This probably explains the 0Z Euro ensemble members hitting the Caribbean. All of this is short term, what happens N of 91L/future Gabrielle in the longer term I'll wait until the models converge a bit more.

[kevin]

For now, anything from the Caribbean to Newfoundland & Bermuda is still in play. Let's see what the ensembles show and whether the upcoming hurricane models have anything interesting.

[MEANINGLESS_NUMBERS]

Long tracking CV-type storm with models ranging from GOM to Newfoundland. This thread will be a banger.

[DunedinDave]

Long term this has Irma vibes. But I’m not sold yet. So far out…we are talking two weeks. And anything after Sept 15 from Africa usually recurves. Not saying this will…just saying I’m not hitting any panic alarms on the latest model trends bringing this into the East Coast or GOA. Let’s see where we’re at at this time next week.

[Woofde]

Most modeling keeps this kicking for a week+ after formation. This could be quite the ACE generator. I would not be surprised to see it follow in Erin's footsteps, ridging seems a bit stronger in the modeling though, we shall see.

[TomballEd]

Most modeling keeps this kicking for a week+ after formation. This could be quite the ACE generator. I would not be surprised to see it follow in Erin's footsteps, ridging seems a bit stronger in the modeling though, we shall see.

Erin trended W as modeling progressed, until the actual storm required watches and warnings in the Carolinas.

This seems to be already W of Erin, 91L could indeed be a CONUS threat. Besides a PR/USVI and Bahamas threat. Looking at the vis, this is a sharp wave in the ITCZ but it looks a whole lot better than it did yesterday.

[KeysRedWine]

Most modeling keeps this kicking for a week+ after formation. This could be quite the ACE generator. I would not be surprised to see it follow in Erin's footsteps, ridging seems a bit stronger in the modeling though, we shall see.

The lower initial position and the forecast 500mb ridging over the northeast in 1-2 weeks has me a bit more concerned than Erin. It'll be interesting to see how the ULL affect its intensity (and location).

[chaser1]

This particular disturbance strikes me to be more complex than most My initial impression was that 91L would be a fairly broad tropical cyclone from the onset but now I can see the opposite will hold true. Notice how the Invest is centered at (about) 12.5 N. While I do see that point for a broader East/West LLC, a more obvious circulation that I would associate with the MLC appears to be north of that point co-located with the more intense convection. That lead me to think that the disturbance was being impacted by a TUTT-like feature impacting southwesterly shear while also somewhat inhibiting vertical organization by impacting a northerly "tug" to the MLC. At least this was how I initially interpreted the satellite. This does not appear to be the case however.

Looking at the analysis, upper level shear is not a factor. It is not uncommon for us to see the LLC and MLC quite disjointed during the formative stages of development. Well, it appears that 91L is forecast to develop somewhat "unconventionally". What we have here is an axis where the stronger mid level energy is not only to the north of the weaker broad LLC, but one which the MLC appears with time to maintain its identity and slowly seperate from the LLC... as both features continue to move primarily westward in the near to mid term. Rather than the LLC to redevelop further north under the mid level feature, 91L is "at least forecast" to slowly organize its own vertical structure while remaining a few degrees further south.

If I'm interpreting 91L's forecasted evolution correctly, this would be why the strengthening process would be rather slow prior to reaching 50W (or even further west). All the while, mid level ridging over the Central Atlantic and North of 91L appears to expand westward and even strengthen from 594 to 597 height levels. I'm not really understanding how/why the LLC doesn't simply reorganize a few degrees further north under the more vigorous MLC feature but the models seem confident that it will develop between 12N - 13N.

I'll be especially eager to hear Levi's breakdown of the present environmental set-up and his take on this seperate mid level energy that will likely be forecast to remain a somewhat disruptive feature for a good part of next week. All of this simply adds to the typical murkyness of any long range track projections and in my opinion simply suggests that any and all long term model forecasts at this point are practically valueless.

[Deshaunrob17]

This is giving me Maria vibes. Models didn’t handle Maria well. Maria was initially expected to hit the islands as tropical storm or at most Cat 1. At the last minute, Maria models began going crazy with intensity ( mainly because Maria was a small hurricane). This system is likely to small due to dry air . Shear is low if stays on that southern track ( what can go wrong!).

[chaser1]

As a quick foot-note to my above post, the new 12Z GFS seems to confirm my suspicions. 91L will either
A) develop further north and more quickly then EURO & GFS is forecasting, or
B) slowly organize as a TD but remain weak, and appear to have an appearance of being "conjoined" with a twin MLC feature which will inhibit strengthening until somewhere between 50W - 60W.

[Fancy1002]

Is it just me or does it look like it’s heading into the gulf this run?

[Fancy1002]

What caused it to weaken on its approach to Florida?

[wzrgirl1]

60/90 now

Tropical Weather Outlook
NWS National Hurricane Center Miami FL
Issued by the NWS Weather Prediction Center College Park MD
200 PM EDT Thu Sep 4 2025

Tropical Atlantic (AL91):
A broad area of low pressure has formed along a tropical wave over
the eastern tropical Atlantic. Recent satellite imagery suggests the
system continues to slowly become better organized with shower and
thunderstorm activity along its eastern periphery. Environmental
conditions are favorable for additional development of the system
and a tropical depression is likely to form by this weekend as it
moves slowly westward at 5 to 10 mph across the eastern and central
tropical Atlantic. Interests in the Lesser Antilles and Leeward
Islands should monitor the progress of this system as it approaches
from the east by the middle of next week.
* Formation chance through 48 hours...medium...60 percent.
* Formation chance through 7 days...high...90 percent.

[BobHarlem]

Watching this (satellite) closely to see how the models are doing on intensity.

[TomballEd]

I can see the effects of dry air on satellite. Still attached to the monsoon trough evident E of the system, where a clear boundary exists between NE trades and SW flow. I don't know if 91L is extending the MT westward as it moves W. Nice outflow developing.

[DorkyMcDorkface]

It's not a given that #91L is going to rapidly intensify as it approaches the Caribbean Islands. The initial run of HAFS-B shows some mid-level shear undercutting the outflow and ventilating some dry air into the core of the system. 12z Euro shows the system weakening through the islands and dissipating in the Caribbean.

Model volatility is to be expected with what looks to be a small storm in a marginal environment. Could see some large swings in the forecast guidance until the structure is locked in. Thankfully still about a week away from the islands due to moving slowly, so lots of time to watch it.

 https://x.com/AndyHazelton/status/1963666408929378706

[Powellrm]

I wish unfavorable conditions such as dry air or sheer would “shred” developing hurricanes such as this one (as often in the past it might), but over the last few years I’ve seen that the warm SST act like steroids. And the generally unfavorable conditions seem like temporary hindrances to storms quickly ramping up. As soon as they have a break from the dry air, they quickly utilize that window for rapid intensification. It happens frequently but it’s always shocking nonetheless