GaWx

 Today’s SOI was strongly positive and models suggest a strongly positive period overall for the next 7-10+ days. That should help La Niña SST anomalies to return in Nino 3.4 by the end of the month most likely.

 So, now the 12Z Euro fully gives in to the GFS and is no longer recurving it in the E Atlantic! Check out the huge difference vs 0Z:

0Z Euro still recurved it early:

The 12Z Euro says forget the early recurve, I'm going to do what the GFS has been doing from the start! On the 12Z run, it has it at 16N, 32W, vs 20N, 27W, on the 0Z run meaning 425 miles to the SW:

There is now within the big AEW now moving offshore Africa a tight ball of convection with probably a LLC of sorts just offshore near 14N, 18W moving seemingly WSW per the visible loops. Let’s see whether or not this area ends up being the focus of this AEW and, if so, whether or not it develops further. The Euro suite doesn’t develop this and instead delays development til late tonight in the same area moving WNW. This may be why there has been such disagreement between the Euro and GFS:

 IR:

 And for the first time now we have a substantial split on the 12Z EPS, the sharp recurvers and those left behind moving mainly westward just past 30W, which means lots of uncertainty as this is still evolving::

Just like the prior run, the 12Z Euro has a sharp far E Atlantic recurve of this wave though not as early as the 0Z, which went quickly back into Africa. This is still evolving as the Euro had it come off Africa way up at 20N a few days ago. Now it is down to 16N and that's still 5 days out. So, far from set in stone yet. It may still recurve sharply, but if it stays weak near Africa, it likely won't sharply recurve.

And now the 12Z CMC has this same 9/11 AEW well out at 17N, 44W, moving due west at 240 (9/16):

12Z GFS is the 5th run in a row taking this same 9/10-11 emerging AEW into the Caribbean ~9/18 as a result of it staying weak. This wave is what is causing the convection at the Lesser Antilles on the far right in this image:

Here we go regarding that same AEW moving off Africa ~9/11:

1. ICON has it stronger and further out than earlier runs as it moves west at the end of the run. It is already pretty far north at 20N, which would suggest a good chance at an early recurve, but keep in mind that the models are jumping around as they try to figure out how strong this AEW will be and where it will track as evidenced by lots of run to run changes

2. UKMET finally has it as it just moved to within its 144 hour range. This track, too, would mean a likely early recurve but note that this has it a TS right off Africa and a near H within 24 hours, both of which could be too strong:

 NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 120 HOURS
              FORECAST POSITION AT T+120 : 15.6N  15.7W

                        LEAD                 CENTRAL     MAXIMUM WIND
      VERIFYING TIME    TIME   POSITION   PRESSURE (MB)  SPEED (KNOTS)
      --------------    ----   --------   -------------  -------------
    1200UTC 11.09.2021  120  15.6N  15.7W      998            39
    0000UTC 12.09.2021  132  17.3N  18.7W      992            50
    1200UTC 12.09.2021  144  18.9N  20.6W      983            57

The 6Z GFS/GEFS and EPS are continuing the idea that the AEW moving offshore Africa 9/10 (GFS) or 9/11 (Euro) (is the same AEW) may very well eventually be something to contend with. The 6Z GFS is the 4th run in a row that gets it near FL ~9/20 as a strong wave. Also, this run suggests there may be another active wave a few days behind it. In addition, note the strong E US ridge being suggested for around that period.

The same strong AEW currently over central Africa that the models have for many days been having move off 9/10-11 and becoming a TC almost immediately on a number of Euro and Icon runs is the strongest on the new GFS run that I can recall. Even though it weakens, it is now the 3rd GFS run in a row with a trackable AEW getting into the Caribbean 9/17 and is the most energetic it has been when getting there (strong wave). This once again gets to Cuba/FL Straits/S FL late on 9/20 and tells me once again that this could conceivably threaten the Caribbean ~9/17-20 followed by the CONUS starting ~9/20.

Climo says that a scenario like this isn’t at all far-fetched for an AEW moving off ~9/10, especially in La Niña.

14 hours ago, GaWx said:

0Z EPS is still very active in the MDR late in the run/something  to watch for late Sept:

12Z EPS for same time is not nearly as active as 0Z run: 

12Z UKMET: 915 mb just 100 miles east of Bermuda!
 

HURRICANE LARRY ANALYSED POSITION : 19.0N 49.5W

ATCF IDENTIFIER : AL122021

LEAD CENTRAL MAXIMUM WIND
VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS)
-------------- ---- -------- ------------- -------------
1200UTC 05.09.2021 0 19.0N 49.5W 933 106
0000UTC 06.09.2021 12 20.5N 51.4W 942 95
1200UTC 06.09.2021 24 21.7N 53.2W 931 106
0000UTC 07.09.2021 36 22.8N 54.6W 929 110
1200UTC 07.09.2021 48 24.1N 56.3W 919 113
0000UTC 08.09.2021 60 25.8N 57.7W 916 116
1200UTC 08.09.2021 72 27.5N 59.7W 918 118
0000UTC 09.09.2021 84 29.5N 61.7W 918 114
1200UTC 09.09.2021 96 31.9N 63.3W 915 116
0000UTC 10.09.2021 108 35.7N 63.3W 925 105
1200UTC 10.09.2021 120 40.8N 61.2W 929 105
0000UTC 11.09.2021 132 46.5N 55.5W 943 85
1200UTC 11.09.2021 144 52.3N 48.3W 971 46

52 minutes ago, jconsor said:

The plot thickens... there is clearly an upper trough/weakness in the subtropical E Atlantic that has been trending stronger past few days, but only a stronger system would really feel the effects of this.  GFS has that trough as well and it's even deeper than the ECMWF, but a sounding clearly shows mainly easterly winds from 500 mb and below around the tropical wave as it comes off the African coast, with SE winds above 500 mb. (Note it comes off the coast about a day earlier then the ECMWF).

The ECMWF/EPS have a known significant bias to show "landcanes" that are too strong inside W Africa and then coming off at an unreasonably high latitude.  This bias was on display seen even with a relatively weaker wave like the one that is just moving off the African coast today.  See how ECMWF trends about 350 mi SW with the center of vorticity vs. runs three or four days ago.
 

Interesting thoughts!

 And just like that, the 12Z Euro doesn't recurve it. Instead it starts out like it will recurve, but then it turns back W, then WSW, and then SW while weakening. Also, it comes off about 12 hours earlier than the prior run and closer to the timing of the GFS/CMC though still slower than those. These run to run changes and model differences show me that it is still too early to make a definitive call on what this AEW moving off ~9/11 will or won't do. After all, it is still ~6 days away from the coast and a lot can change before then.

Old Euro run (0Z) is along the African coast moving back inland to the NE:

New Euro run (12) is 400 miles to the WSW about to turn SW and weaken:

Per the 12Z GFS and CMC, that strong AEW over Africa that especially the Euro but also the ICON have been developing on 9/11 just offshore never recurves but instead stays weak and moves WSW in the E Atlantic. You can actually trace the vorticity/precip associated with it on the 12Z GFS then move W all of the way to the Lesser Antilles on 9/17 followed by it moving WNW into the Caribbean but staying very weak.

 There’s no suggestion Sept will turn out quiet. First, Larry is looking to end up being a hurricane for 10+ days and a MH for most of that period. In terms of ACE, that’s strong activity by itself….lots of energy for the first 13+ days of the month! Second, if the EPS has the right idea, there will be only a couple of days between the demise of Larry and the next MDR system or two. Third, there’s the threat of Gulf activity over the next 10 days or so. That may produce 1-2 more TCs. Fourth, ensembles go out only through 9/20. That still leaves 1/3 of the month that can’t even be seen yet. Fifth, there usually are one or more TCs that aren’t even hinted at far in advance on models. Sixth, there’s still the very real chance of a far eastern MDR TC, sharp recurve or not, near 9/11-12 from the suggested very strong AEW still forecasted over Africa in a few days and then moving offshore ~9/11.

 So, there really is no indication of anything less than an active Sept based on objective measurements.

0Z EPS is still very active in the MDR late in the run/something  to watch for late Sept:

H5:

 The signal for genesis of an AEW in mid-Sep along with mainly W movement is still there on the 12Z EPS. However, regarding the strong AEW still progged by several models to come off Africa ~9/11, the trend is for a sharp recurve due to a weakness in the far E Atlantic, which has gotten more prominent on the Euro suites as the high is further west in the mid Atlantic Ocean.

 So, it appears that there will probably be a strong AEW moving off 9/11 probably followed by a sharp recurve followed by an AEW moving off near 9/14-5 that then likely moves W to WNW well out into the Atlantic. We’ll see though as that’s still 10 days out and much can change just like the changes with the 9/11 wave.

 Here’s the 12Z EPS still showing a strong signal for a W to WNW moving AEW just after midmonth, which would need to be watched for late month:

12Z EPS at 360: most of these originate from a followup wave to the 9/11 one and almost all of these are moving pretty rapidly due W or just N of due W: 

12Z EPS H5 at 360 has strong ridging to the north allowing for pretty rapid movement W or just N of due W:  (the danger is not that this one high would move it all of the way to the other side but rather there's usually a mid ocean weakness sometimes followed by a handoff to a strengthening WAR).

12 hours ago, GaWx said:

 Consistent with the above 12Z Euro and the run before that, the 0Z Euro still has a TS from an AEW coming off Africa on 9/11 with a strong high to its NNW:

 

 

End of run:

 

Consistent with the prior 3 Euro runs, the 12Z yet again has a TS just off Africa that originates from an AEW that comes off ~9/11. But this time it comes off quite far north (near 20N) and thus easily recurves just offshore:

12Z UKMET: strongest yet (920mb!!) 250 miles E of Bermuda:

HURRICANE LARRY      ANALYSED POSITION : 14.5N  40.1W

     ATCF IDENTIFIER : AL122021

                        LEAD                 CENTRAL     MAXIMUM WIND
      VERIFYING TIME    TIME   POSITION   PRESSURE (MB)  SPEED (KNOTS)
      --------------    ----   --------   -------------  -------------
    1200UTC 03.09.2021    0  14.5N  40.1W      982            55
    0000UTC 04.09.2021   12  15.5N  42.7W      981            58
    1200UTC 04.09.2021   24  16.7N  45.4W      982            55
    0000UTC 05.09.2021   36  17.8N  47.6W      980            59
    1200UTC 05.09.2021   48  19.4N  49.7W      976            63
    0000UTC 06.09.2021   60  21.1N  51.5W      970            67
    1200UTC 06.09.2021   72  22.4N  53.5W      960            73
    0000UTC 07.09.2021   84  23.6N  55.1W      949            88
    1200UTC 07.09.2021   96  25.0N  56.9W      946            94
    0000UTC 08.09.2021  108  26.6N  58.3W      928           105
    1200UTC 08.09.2021  120  28.8N  59.7W      927           105
    0000UTC 09.09.2021  132  31.4N  60.7W      920           110
    1200UTC 09.09.2021  144  34.4N  61.1W      922           107
 

12 hours ago, GaWx said:

12Z Euro, like the 0Z Euro, has a TS just off the coast of Africa 9/11. This is consistent with the EPS runs I’ve been posting. Note how strong the central Atlantic high is to the NNW of it. If that were to verify, this likely wouldn’t recurve early and may even soon after this start moving WSW for a period, which would be a warning signal as Yaakov ( @jconsor) and I have noted, especially with the La Nina(ish) ENSO:

 

 

 Consistent with the above 12Z Euro and the run before that, the 0Z Euro still has a TS from an AEW coming off Africa on 9/11 with a strong high to its NNW:

End of run:

 0Z UKMET is again down to 922-3 mb and passes only 150 miles east of Bermuda then:

HURRICANE LARRY ANALYSED POSITION : 13.9N 36.8W

ATCF IDENTIFIER : AL122021 LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- -------------

0000UTC 03.09.2021 0 13.9N 36.8W 988 50

1200UTC 03.09.2021 12 14.5N 40.3W 986 49

0000UTC 04.09.2021 24 15.5N 43.1W 983 54

1200UTC 04.09.2021 36 16.6N 45.8W 979 60

0000UTC 05.09.2021 48 18.0N 47.8W 972 64

1200UTC 05.09.2021 60 19.4N 49.9W 971 66

0000UTC 06.09.2021 72 20.8N 52.0W 961 72

1200UTC 06.09.2021 84 22.0N 53.8W 950 84

0000UTC 07.09.2021 96 23.3N 55.5W 936 100

1200UTC 07.09.2021 108 25.2N 57.6W 933 97

0000UTC 08.09.2021 120 27.2N 59.5W 927 103

1200UTC 08.09.2021 132 29.5N 61.4W 922 112

0000UTC 09.09.2021 144 32.5N 62.5W 923 111

12Z EPS continues with a lot of activity in the MDR during the 11-15 day period from various AEWs, which the US may have to monitor for after 9/20:

12Z EPS has many more members vs prior runs hitting or grazing E New England or Canada vs the 0Z: