GaWx

1. 0Z CMC Georgetown, SC hit with 975 mb H as not as strong Humberto (983 mb) doesn’t dominate: 2. 0Z GFS is safely OTS with Fujiwara from Humberto.

0Z UKMET: TD forms Mon night NW Bahamas, initially then drifts slowly NW followed by a stall and dissipation as the much stronger Humberto only ~500 miles east takes over: NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 102 HOURS FORECAST POSITION AT T+102 : 25.0N 77.5W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 1200UTC 29.09.2025 108 25.9N 78.2W 1008 27 0000UTC 30.09.2025 120 26.1N 78.3W 1007 23 1200UTC 30.09.2025 132 26.1N 78.1W 1007 20 0000UTC 01.10.2025 144 CEASED TRACKING

0Z Icon: Humberto pulls 94L ESE to keep it safely away:

Big news from Australia’s BoM: their model progs have just switched from 3.4 to relative 3.4 (akin to RONI)! Relative Niño indices Traditional Niño index values were used at the Bureau of Meteorology until September 2025. From September 2025, the Bureau uses Relative Niño indices, which measure sea surface temperature anomalies in the tropical Pacific Ocean in the Niño regions, but calculated relative to the global tropical region temperature anomaly. This is to relate the indices more closely to the localised processes associated with ENSO, rather than larger-scale tropical SST features such as global warming. Example: The Relative Niño3.4 index calculation: Relative Niño3.4 = S x [(Niño3.4obs – Niño3.4clim) – (Tropical Meanobs – Tropical Meanclim)] Here’s the latest BoM relative 3.4 model prog for Relative 3.4, which has a low of -0.7C for OND: http://www.bom.gov.au/climate/ocean/outlooks/index.shtml?index=nino34#region=NINO34 @snowman19

Update: -Season’s ACE is now up to 57.2. -Gabrielle has exceeded earlier progs and is now on its own progged to get ACE to ~60. -Humberto has been declared and is progged to add ACE of ~13 through Sept 30th though with lots of uncertainty, including upside to ~17-20. -That would get ACE to ~73-80. -But that’s not necessarily all as models are now significantly more bullish on still another TC by late this week from Invest 94L. A great level of uncertainty rests with 94L at this very early stage. The upside limit on that through Sept 30 is ~~10. -So, current rough projections are significantly higher for ACE as of Sept 30th. In my quoted prog from 3 days ago, I had the upside at 65-70. That upside limit has since risen to ~83-90. That compares to the 1991-2020 average of 94 through Sept. 30. Thus, if this upside range of 83-90 were to verify, Sept 30th ACE could rise all the way to only a little below the active 30 year average! -Oct-Dec 30 year avg is 28. So, if we were to get to 90 as of Sept 30th, getting to an end of season well over 100 would then be favored along with a reasonable upside of ~130-140 vs 1991-2020 avg of 122. That upside potential is extra realistic because of 1. mainly favorable MJO progged for next few weeks 2. weak La Niña on RONI basis is correlated most to above avg ACE late season 3. Euro Weeklies are active @snowman19@PhiEaglesfan712

Tropical Weather Outlook NWS National Hurricane Center Miami FL 800 PM EDT Wed Sep 24 2025 For the North Atlantic...Caribbean Sea and the Gulf of America: Eastern Caribbean Sea (AL94): Showers and thunderstorms have become better organized since yesterday in association with a tropical wave centered near Puerto Rico. This wave is expected to move west-northwestward around 10 to 15 mph, spreading heavy rainfall and gusty winds across Puerto Rico and the Dominican Republic through early tomorrow. The system is then expected to slow down and turn northwestward when it reaches the southwestern Atlantic. A tropical depression is likely to form when the disturbance is in the vicinity of the Bahamas late this week. Interests in Puerto Rico, the Dominican Republic, the Turks and Caicos Islands, and the Bahamas should monitor the progress of this system. * Formation chance through 48 hours...medium...40 percent. * Formation chance through 7 days...high...80 percent. Public Advisories on Humberto are issued under WMO header WTNT33 KNHC and under AWIPS header MIATCPAT3. Forecast/Advisories on Humberto are issued under WMO header WTNT23 KNHC and under AWIPS header MIATCMAT3. Forecaster Blake

I recommend folks who are curious follow the Invest 94L thread. If 94L isn’t pulled toward Humberto (via Fujiwhara), portions of the SE US may be in danger of a hit from 94L per the 12Z EPS, 18Z Icon, and others. In addition, the MJO is forecasted to be in phase 2 9/27-10/3. That has been the single most dangerous phase for the Conus overall since 1975 during Jul-Sept overall.

Example: 18Z Icon: no Fujiwhara and 94L gets stuck under the strengthening US Great Lakes ridge and subsequently turns NW toward C FL: Edit: And the GEFS/EPS are forecasting this to be during the potentially dangerous phase 2.

Followup: Upon closer inspection, 40% (20) of 12Z EPS members affect a portion of the SE with either a TC direct landfall, a skirting of the coast, or a weak remnant. Also, a clear majority of members that weren’t steered by a Fujiwhara from Humberto were in that 40%. So, whether or not Humberto pulls 94L into it might turn out to be the crucial factor for whether or not the SE US is threatened.

The 12Z EPS, like 0Z and 12Z of yesterday, actually has an uncomfortably high # of members (10+/20%+) hitting the SE US 9/28-10/4, which is earlier than the 10/5-7 12Z Euro freakish US hit. One thing that’s a bit concerning is the forecasted MJO phase 2 by both GEFS and EPS for 9/27-10/3. As I posted about last month, phase 2 has had (whether inside or outside the circle) by a wide margin the highest % (after taking into account % of days in each phase) of Conus Jul-Sept MH hits and along with phase 8 the highest % of H hits since 1975. GEFS EPS

12Z UK 94L: TD just NE of C Bahamas moving ESE NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 108 HOURS FORECAST POSITION AT T+108 : 24.5N 77.2W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 0000UTC 29.09.2025 108 24.5N 77.2W 1008 28 1200UTC 29.09.2025 120 25.0N 77.0W 1008 27 0000UTC 30.09.2025 132 25.0N 77.1W 1007 27 1200UTC 30.09.2025 144 24.4N 76.6W 1006 24 0000UTC 01.10.2025 156 23.9N 75.0W 1005 25 1200UTC 01.10.2025 168 CEASED TRACKING

12Z UK just W of Bermuda TROPICAL DEPRESSION 93L ANALYSED POSITION : 19.7N 53.9W ATCF IDENTIFIER : AL932025 LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 1200UTC 24.09.2025 0 19.7N 53.9W 1014 33 0000UTC 25.09.2025 12 20.6N 56.0W 1013 32 1200UTC 25.09.2025 24 21.3N 57.4W 1012 33 0000UTC 26.09.2025 36 21.7N 57.9W 1009 34 1200UTC 26.09.2025 48 21.7N 58.5W 1007 35 0000UTC 27.09.2025 60 22.0N 59.1W 1005 42 1200UTC 27.09.2025 72 22.0N 60.3W 1003 44 0000UTC 28.09.2025 84 22.2N 61.0W 1000 44 1200UTC 28.09.2025 96 22.8N 63.0W 998 47 0000UTC 29.09.2025 108 23.7N 64.5W 994 49 1200UTC 29.09.2025 120 24.5N 66.3W 992 52 0000UTC 30.09.2025 132 25.6N 67.8W 991 56 1200UTC 30.09.2025 144 27.3N 68.4W 991 54 0000UTC 01.10.2025 156 29.1N 67.8W 984 59 1200UTC 01.10.2025 168 31.9N 66.6W 972 61

12Z Euro: After forming in the Bahamas, it recurves away from the SE US and the center just misses Bermuda to the NW as it did for 93L two days earlier in this run. Then it turns back NW before ultimately turning back NE missing the NE US and barely missing Newfoundland.

94L 6Z Euro at 144 had a strengthening 998 TS moving NNW N of the Bahamas threatening the SE US. 6Z Icon at 120 was well E of Euro 6Z GFS had nothing from this, alone, although it appears to combine its vorticity with 93L and develop that well offshore —————- No UK run has had this as a TC AFAIK as of then. But 12Z UK: TD in NW Bahamas that drifts NE NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 138 HOURS FORECAST POSITION AT T+138 : 25.0N 77.1W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 1200UTC 29.09.2025 144 25.6N 76.9W 1010 30 0000UTC 30.09.2025 156 26.0N 76.2W 1009 24 1200UTC 30.09.2025 168 26.9N 74.2W 1008 27

0Z Euro has a 1006 TS hit NC 9/29

18Z Euro has a 1001 TS headed NNW in the direction of NC at 30N, 75W at 144

The 12Z slams Bermuda with this on 10/1 as a 954 MH.

The brand new (12Z) Euro has this on 9/29 hit NC as a TD.

12Z GEFS: another pretty active run

The 12Z UKMET threatens Bermuda: NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 90 HOURS FORECAST POSITION AT T+ 90 : 22.0N 61.0W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 1200UTC 26.09.2025 96 22.5N 61.5W 1010 35 0000UTC 27.09.2025 108 22.5N 62.5W 1008 37 1200UTC 27.09.2025 120 23.1N 63.5W 1007 38 0000UTC 28.09.2025 132 23.7N 64.1W 1005 45 1200UTC 28.09.2025 144 24.5N 66.1W 1003 41 0000UTC 29.09.2025 156 25.6N 66.8W 1000 43 1200UTC 29.09.2025 168 27.2N 66.4W 997 45

The unusual clockwise dominated MJO rotation that started Sept 1st is continuing and progged to keep going through at least Oct 6th per the images below. So, it went from phase 5 (inside) on 8/31 back to ph 2 9/2-8. It temporarily went forward to ph 3 for 3 days but then backtracked across 2 then 1 and then 8. It’s forecasted to continue backtracking through 7, 6, 5, 4, 3, 2, 1, and 8. I just looked at MJO diagrams for the 2nd half of each year back to 1975 and couldn’t find anything comparable:

Recon will shortly tell us more.

This was a TC on the admittedly untrustworthy GFS as far back as the 0Z 9/18 run and consistently on runs the last couple of days, including some ensemble members. Also, the Icon and Euro have at least hints that this may turn into something.

The AOI nearest to the Lesser Antilles may need its own thread due to a potential threat at least to the Bahamas late this week and possibly even the US E coast/Canada. This was a TC on the admittedly untrustworthy GFS as far back as the 0Z 9/18 run and consistently on runs the last couple of days, including some ensembles. Also, the Icon and Euro have at least hints that this may turn into something: East of the Leeward Islands: A tropical wave located about 400 miles east of the Leeward Islands continues to produce disorganized showers and thunderstorms. Environmental conditions appear marginally conducive for slow development over the next few days as the wave moves quickly westward to west-northwestward at 15 to 20 mph. By the latter part of this week, the system is expected to slow down and turn more northwestward, and a tropical depression could form late this week when the system is over the southwestern Atlantic Ocean or near the Bahamas. Regardless of development, gusty winds and showers are expected to affect portions of the Leeward Islands late tonight and Tuesday. * Formation chance through 48 hours...low...10 percent. * Formation chance through 7 days...medium...40 percent

BULLETIN Hurricane Gabrielle Special Advisory Number 22 NWS National Hurricane Center Miami FL AL072025 900 AM AST Mon Sep 22 2025 ...GABRIELLE RAPIDLY INTENSIFIES INTO A MAJOR HURRICANE SOUTHEAST OF BERMUDA... SUMMARY OF 900 AM AST...1300 UTC...INFORMATION ---------------------------------------------- LOCATION...30.6N 62.2W ABOUT 195 MI...310 KM SE OF BERMUDA MAXIMUM SUSTAINED WINDS...120 MPH...195 KM/H PRESENT MOVEMENT...N OR 10 DEGREES AT 10 MPH...17 KM/H MINIMUM CENTRAL PRESSURE...957 MB...28.26 INCHES Hurricane Gabrielle Special Discussion Number 22 NWS National Hurricane Center Miami FL AL072025 900 AM AST Mon Sep 22 2025 Satellite images indicate that Gabrielle has rapidly intensified into a major hurricane with a well-defined eye. The current intensity is set to 105 kt, a blend of constrained Dvorak estimates of about 100 kt with higher Data-T values of 115 kt. Further intensification is expected today, and the intensity forecast is raised for the first 24 hours, then kept the same afterwards. Small changes were made to the wind radii to account for the stronger hurricane. No changes were required to the track forecast. FORECAST POSITIONS AND MAX WINDS INIT 22/1300Z 30.6N 62.2W 105 KT 120 MPH 12H 22/1800Z 31.3N 62.1W 115 KT 130 MPH 24H 23/0600Z 32.9N 60.5W 110 KT 125 MPH 36H 23/1800Z 34.4N 57.4W 95 KT 110 MPH 48H 24/0600Z 35.4N 53.1W 90 KT 105 MPH 60H 24/1800Z 36.0N 47.6W 80 KT 90 MPH 72H 25/0600Z 36.7N 41.5W 75 KT 85 MPH 96H 26/0600Z 40.7N 28.9W 70 KT 80 MPH 120H 27/0600Z 44.2N 19.9W 55 KT 65 MPH...POST-TROP/EXTRATROP $$ Forecaster Blake