GaWx

Those diagrams suggest a transition from east based in OND to a cross between east based and central based DJF.

Though it still has no GOM TC, the 12Z UKMET continues to have E MDR TCG from a non-tagged AEW next week: NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 126 HOURS FORECAST POSITION AT T+126 : 14.5N 18.4W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 0000UTC 24.08.2023 132 14.7N 19.2W 1004 30 1200UTC 24.08.2023 144 15.8N 22.1W 1003 35 0000UTC 25.08.2023 156 17.0N 25.5W 1005 37 1200UTC 25.08.2023 168 17.6N 28.4W 1005 35

Per today's monthly summary of dynamic models: 1. Most recent CANSIPS is confirmed to be at +2.05 ONI peak OND vs +1.61 a month ago. 2. New UKMET now up to +2.00 NDJ. 3. The ONI peak from the average of all dynamic models has risen from +1.81 last month (OND) to +2.06 (NDJ). 4. DJF has risen from +1.50 to +1.91. 5. Based on the new UKMET also being super, I'm raising the chance for super ONI peak from 60% to 70%. It is still far from a lock largely because of OHC at last check only in the +0.80s though I expect it to rise substantially in the coming months. https://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/?enso_tab=enso-sst_table

8/18/23 like night and day vs 8/18/22 in MDR: 8/18/22 2AM EDT TWO: 8/18/23 2AM EDT TWO: SEVEN features have significant activity on the 0Z EPS just through day 10! That almost has to be near a record:

Indeed! Per official LV records that go back to 1937, the heaviest two consecutive calendar day rainfall on record is 2.59" that fell August 20-21, 1957. That was also during an oncoming strong El Niño, coincidence or not. Next was the 2.10" of 12/28-9/2004 (weak El Niño). The 3rd heaviest was August 21-22, 2012 (warm neutral ENSO), when 2.03" fell. So, 2 of the 3 heaviest were during August 20-22. The upcoming heavy rain threat looks to be centered on August 20-21!

Once again, the UKMET has no GOM TC. But for the 4th run in a row, it has TCG late in the run in the far E MDR: NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 150 HOURS FORECAST POSITION AT T+150 : 12.7N 17.2W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 0000UTC 24.08.2023 156 13.7N 18.3W 1004 32 1200UTC 24.08.2023 168 15.0N 21.3W 1005 30

12Z UKMET...This appears to be from 99L. It forms and dissipates within the Caribbean: NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 126 HOURS FORECAST POSITION AT T+126 : 15.3N 72.4W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 0000UTC 23.08.2023 132 15.7N 73.1W 1003 32 1200UTC 23.08.2023 144 16.6N 73.6W 1005 33 0000UTC 24.08.2023 156 17.1N 74.3W 1006 28 1200UTC 24.08.2023 168 CEASED TRACKING Edit: Upon further review, this may be from a feature west of 99L.

The 0Z EPS just through day 10 is very busy with many active members for 98L, 99L, the Gulf low, and two followup MDR AEWs. Also, late in the run there are ~half the members moving up from the Bay of Campeche with some members crossing over from the Pacific late in the run. So, that's a total of six different features of interest just through day 10. Thus if only half of these end up as a TC, that would be enough to yield three new TCs by 0Z on 8/27 (very busy vs climo averages for just a ten day period). That's like night and day vs what the models showed for the same part of August last year. Two of these six (the two in the Gulf) would be direct threats to the CONUS. I originally predicted 3 NS for August. I feel pretty good about that prediction.

Though the 0Z UKMET still has no GOM TC, it for the third time in a row has a TD form in the far E MDR at the end of its run: NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 168 HOURS FORECAST POSITION AT T+168 : 14.2N 17.6W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 0000UTC 24.08.2023 168 14.2N 17.6W 1004 34

Regarding the mid/MDR tangerine, which has become Invest 99L: TWO: now up to 40%/60% 0Z UKMET: yet again a TS in the Leewards, but this run then has it go NNW and later NNE track that aims well to the E of Bermuda: NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 84 HOURS FORECAST POSITION AT T+ 84 : 16.9N 58.9W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 1200UTC 20.08.2023 84 16.9N 58.9W 1006 28 0000UTC 21.08.2023 96 17.6N 60.3W 1005 34 1200UTC 21.08.2023 108 19.2N 60.9W 1005 42 0000UTC 22.08.2023 120 20.2N 61.4W 1003 39 1200UTC 22.08.2023 132 22.0N 61.8W 1003 44 0000UTC 23.08.2023 144 23.8N 62.3W 1004 35 1200UTC 23.08.2023 156 25.6N 61.8W 1005 31 0000UTC 24.08.2023 168 26.9N 60.9W 1004 27

Lol, I was just looking at today's updated SSTs so I could do an update. Yeah, CRW has consistently been the warmest. But even OISST 3.4 has just gone past +1.3 for the first time. That implies that ERSST is likely ~+1.2. OISST 3 is also at its warmest and is approaching +2.0.

Thanks. In Swain's article, he says: "climate change–which is likely increasing the duration and severity of droughts on the lee sides of the islands and the intensity of wet-dry cycling of precipitation (which favors extra vegetation growth, and then rapid drying of that extra growth during high risk periods)." Then you restated Swain's idea of wetter wet seasons and drier dry seasons. But then I read this from your last link about expected changes on Hawaii, itself, due to CC written by Kevin Hamilton: "We found that in the wet windward areas of Hawaii, rainfall is projected to increase substantially. That includes increasingly frequent extreme downpours. On the other hand, rainfall is predicted to decrease substantially over much of the rain shadow regions." ----------- So, to recap, Swain is suggesting wetter wet seasons and drier dry seasons. But Hamilton is instead suggesting wetter windward areas and drier leeward areas. So, Swain implies CC causes Lahaina to have both wetter winters (causing increased vegetation) and drier summers (causing more fire danger due to more fuel available that dries out in summer). But Hamilton is implying CC causes both winter and summer to be drier at Lahaina. So, per Hamilton, there actually isn't the increased fuel at Lahaina due to wetter winters. So, Swain (wetter) and Hamilton (drier) are literally on opposite sides on how CC affects Lahaina winter rainfall. Any thoughts about this? Doesn't it concern you? Thanks.

1. The 12Z UKMET is similar to the prior run with a new TC in the far E MDR at 168: NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 168 HOURS FORECAST POSITION AT T+168 : 12.1N 20.3W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 1200UTC 23.08.2023 168 12.1N 20.3W 1006 30 2. The UKMET still has no TC in the Gulf.

Regarding the central MDR lemon, the 12Z UKMET again develops this into a TS. It hits the Leewards to PR followed by a recurve E of the Bahamas to a position that could later threaten Bermuda: NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 90 HOURS FORECAST POSITION AT T+ 90 : 16.2N 59.1W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 1200UTC 20.08.2023 96 16.7N 60.2W 1005 34 0000UTC 21.08.2023 108 17.3N 62.1W 1005 35 1200UTC 21.08.2023 120 18.4N 64.4W 1004 36 0000UTC 22.08.2023 132 19.8N 65.4W 1004 40 1200UTC 22.08.2023 144 21.2N 67.5W 1004 45 0000UTC 23.08.2023 156 22.2N 67.5W 1003 41 1200UTC 23.08.2023 168 24.5N 68.2W 1004 40

What do you mean by "wet/dry season cycling" and how does that relate to increased CO2? Thank you in advance.

1. The overall magnitude of SAL is about to sharply decrease, which largely matches the typical decrease in mid August and is one of the main reasons that there is a climatologically sharp increase in activity in late August. 2. One of the typical effects of a moderate+ El Niño is overall increased Caribbean shear coming in from the west. However, to this point overall, that has yet to take place. Of course, that may start dominating at any point as El Niño strengthens.

The 0Z UKMET has a new TS form in the far E MDR in a week moving WNW: NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 156 HOURS FORECAST POSITION AT T+156 : 10.5N 16.9W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 1200UTC 22.08.2023 156 10.5N 16.9W 1006 28 0000UTC 23.08.2023 168 11.1N 19.4W 1006 35

For the central MDR lemon: 0Z UKMET NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 150 HOURS FORECAST POSITION AT T+150 : 20.5N 67.7W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 1200UTC 22.08.2023 156 21.0N 67.9W 1006 35 0000UTC 23.08.2023 168 21.7N 69.9W 1006 37 From 2AM EDT TWO: up to 20/40 from 10/30 1. Central Tropical Atlantic: Disorganized showers and thunderstorms located over the central tropical Atlantic are associated with an elongated trough of low pressure centered about 600 miles west-southwest of the Cabo Verde Islands. Environmental conditions appear conducive for gradual development of this system, and a tropical depression could form during the next several days while moving toward the west or west-northwest at about 10 mph across the central tropical Atlantic. * Formation chance through 48 hours...low...20 percent. * Formation chance through 7 days...medium...40 percent.

Central MDR lemon; 18Z Euro stronger than 12Z and chances increased from 0/10 to 10/30: Tropical Weather Outlook NWS National Hurricane Center Miami FL 800 PM EDT Tue Aug 15 2023 For the North Atlantic...Caribbean Sea and the Gulf of Mexico: 1. Central Tropical Atlantic: A tropical wave, interacting with a larger trough of low pressure, continues to produce disorganized showers and thunderstorms a few hundred miles west-southwest of the Cabo Verde Islands. Environmental conditions appear to be conducive for some gradual development of this system during the next several days as it moves to the west-northwestward or northwestward between 10-15 mph across the tropical Atlantic through the weekend. * Formation chance through 48 hours...low...10 percent. * Formation chance through 7 days...low...30 percent.

The SAL is forecasted to diminish substantially starting in ~5 days.

More on the central MDR lemon: The 12Z EPS still is active with this AEW with ~half the members having TCG though there are fewer H vs the prior run. While having fewer H, it actually is more threatening further west because the tracks are on average further SW with many more going across the LAs and then a good portion of those going well into the Caribbean followed by either dissipation or moving over Hisp/Cuba followed by Bahamas. Some though move NW toward Virgin Islands/PR followed mostly by E of the Bahamas. A few still recurve sharply before the Caribbean and then threaten Bermuda. Bottom line for 12Z EPS vs 0Z: not as many H but centered closer to the US over Bahamas vs 0Z's mainly E of the Bahamas to Bermuda.

What I mean by the 12Z Euro having just about nothing is that it has no surface circulation although it has trackable 850 mb vorticity further south. This even leads to a very weak low over FL at 240. The JMA has only a weak reflection at the surface (no circ), but it is a little stronger than the prior 12Z run and pretty much takes the UKMET track from the LAs to the SE Bahamas. You can tell from the widespread 2" of qpf in and near the LAs from this that it has some "oomph" so to speak. Ever since the August 4th EPS runs, this feature has more or less had some model support and it absolutely bears watching! Yet, the NHC still has TCG chances at only 10%. I'd raise them to 30%.

Yes, the 12Z GFS has no H. The last on the list with a H is the 6Z GFS. Meanwhile: Some pretty significant model news since the UKMET tends to be conservative: -The 12Z UKMET has TCG from the westernmost lemon in the W MDR on Fri (8/18) and it then hits the Leewards as a TS late on Saturday (8/19). -It then moves WNW to Hispaniola on Mon (8/21) still as a TS -It ends up near SE Bahamas as a slowly strengthening TS on Tue (8/22) moving slowly WNW and in a potentially threatening position to the Bahamas/SE US. NEW TROPICAL CYCLONE FORECAST TO DEVELOP AFTER 78 HOURS FORECAST POSITION AT T+ 78 : 14.2N 51.4W LEAD CENTRAL MAXIMUM WIND VERIFYING TIME TIME POSITION PRESSURE (MB) SPEED (KNOTS) -------------- ---- -------- ------------- ------------- 0000UTC 19.08.2023 84 14.7N 53.5W 1005 30 1200UTC 19.08.2023 96 15.4N 57.4W 1004 34 0000UTC 20.08.2023 108 16.3N 60.5W 1003 37 1200UTC 20.08.2023 120 17.5N 63.5W 1004 40 0000UTC 21.08.2023 132 17.8N 67.0W 1004 37 1200UTC 21.08.2023 144 19.4N 68.7W 1004 38 0000UTC 22.08.2023 156 20.3N 71.2W 1004 37 1200UTC 22.08.2023 168 21.0N 72.9W 1003 39 Edit: But the 12Z Euro like the GFS has just about nothing.

GFS 12 runs out of 63 since 7/31 with hurricane (7 of them over last 11 runs)(last 9 H within 8/25-30): -7/31 12Z: hit SC 8/16 -8/3 6Z: GOM hit MX/TX border 8/16 -8/6 0Z: MDR 8/18-21 -8/9 18Z: just off SC 8/25 -8/11 0Z: GOM just off LA 8/26 -8/13 0Z: GOM 8/26-8 -8/13 6Z: GOM 8/25-6 that hits MS 8/26-7. -8/13 18Z: GOM 8/25 hits S TX 8/26 -8/14 0Z : off SC/NC 8/26-29 -8/14 6Z: off NC 8/28-30 -8/14 12Z: GOM 8/27 hits LA 8/28 -8/15 6Z: GOM 8/26 hits FL Panhandle 8/27