Philippe Papin

8am EDT Oct 5th -- NHC is monitoring an area of low pressure associated with a tropical wave (#AL95) which now has a high chance (70% 🔴) of formation over the next week. It is forecast to approach the Leeward Islands by late this week. Late... https://x.com/NHC_Atlantic/status/1974805335572693115

I forgot to mention, all the images above are from @pivotalweather.com.

12z ECWMF 66h Forecast

12z GFS 66h Forecast

12z GEM 66h Forecast

ICON 66h forecast.

One of these models does not look like the others.

This is a 66h forecast, and while precip patterns look reasonably realistic in the #GFS, #GEM (Canadian), & #ICON, the linear patterns seen on the #ECMWF look out of place in the MDR tropical wave.

We'll see what verification looks like.

Translation - One of the weather computer models (Euro) is doing something that is not realistic & that could affect its ability to correctly forecast tropical system development out in the Atlantic Ocean.

There is now an MDR tropical wave in the outlook that has a medium chance 🟠 of TCG in the next 7-days.

The 00z Op ECMWF feels like deja vu, gravity waves in wave axis dispersing energy & preventing convective consolidation.

Seems unrealistic, but check back in 5 days to see how it performs. 🌀

Yeah using mp4s in general is a little harsh on image quality. Slowing down the animation can help, but it can certainly be lower quality than using .gifs though it also typically means smaller file sizes.

I'm not sure how I missed when they made the change, but .gifs now seem to upload & automacially be converted into .mp4's on @bsky.app.

Finally I don't have to convert them myself! I'm probably the last person to find this out, but hurrah! 👍

Swells and high surf from Imelda are expected to produce dangerous marine conditions and rip currents along much of the East Coast of the United States during the next several days.

This is list that includes tropical storm proximity -- not when only 2 #hurricanes where in close proximity. Still, we are likely moving up the ranking lists of closest interaction on that list too.

And with the 15z advisories -- #Hurricane #Imdelda & #Humberto are just 479 miles apart.

This might be the closest two simultaneous hurricanes have ever been in the Atlantic basin in the satellite era (since ~1975).

Close enough for their outer sfc circulations to interact as well.

Yes it's pretty darn unusual to see two #hurricanes in the North Atlantic basin this close. #Imelda & #Humberto.

It has happened recently w/ weaker TCs (e.g., #Philippe & #Rina in 2023), but unsure the last time both systems involved in binary interaction were both ≥65 kt in the NATL.

🌀🔄🌀

Nice thread by Tomer here. Agree it requires nuance when looking at ensemble means to determine how skillful that dataset is for TC track forecasting.

In addition — skill vs spread plots are helpful to see if the ensemble dataset is capturing realistic spread, or if it’s over or under dispersive.

But maybe diff here b/w #91L vs #Gabrielle, #Humberto, & #Imelda is that instability in late Sep is higher than early Sep & every little bit helps when trying to aggregate convection to get TC genesis.

I still think @btangywx.bsky.social’s argument that mid-lat synoptic variability matters too.

Gotcha… so your argument is an earlier CCKW helped the waves convectively enough to get them over the line to genesis before more neg large scale conditions took back over?

It’s funny how sometimes it works, & other times like #91L it can’t quite get to the point where WISHE can sustain convection.

Velocity Potential anomalies at 200 mb where +CHI (proxy for downward large scale vertical motion) shows up as brown colors and -CHI (proxy for upward large-scale vertical motion) shows up as green colors.

We are certainly experiencing a later than typical peak, but my confusion was that the sub-seasonal signals suggest conditions are not very favorable *now* in the Atlantic.

Yet we are having our most active period to date. Maybe climo increase in instability helps despite the unfavorable +200 VP'?

I sense some good research coming down the pipeline in an attempt to explain the mercurial nature of the 1st vs 2nd halves of September 2025 😃

Thanks for chiming in! It is still perplexing to see lack of activity in Caribbean/Gulf to date (save Barry). Obviously things can change in Oct/Nov but point 3) resonates since I think seasonal predictions can underestimate how much synoptic variability ultimately drives key TC activity periods.

Just posing the question -- I don't have any solid answers & was hoping to generate a larger discussion on why activity has suddenly picked back up without a good large-scale signal for it to do so in the NATL basin.

A provocative question: *Why* is activity in the NATL basin now? NATL is under ⬇️ motion via +VP' at 200 mb.
Maybe near peak annual instability helps? 🤷

@webberweather.bsky.social @ericblake12.bsky.social @drkimwood.bsky.social @tropicaltidbits.bsky.social @btangywx.bsky.social @bmcnoldy.bsky.social

Zooming out and looking at the GOES-19 Full Disk View, the western hemisphere is finally acting like September for #TC activity.

While ET #Gabrielle is existing stage right, #Hurricane #Humberto (Cat 4 125 kt) & #TD9 (30 kt) have taken its place.
Also can't forget Hurricane #Narda in the EPAC. 🌀

Tropical depression Nine is expected to strengthen and bring tropical storm conditions to portions of the Bahamas through Sunday night. Tropical storm conditions are also possible along portions of the east coast of central Florida beginning Monday, where a Tropical Storm Watch has been issued.

Advisories on Potential Tropical Cyclone Nine (formerly AL94) will start at 5pm ET, meaning that while it is not a tropical cyclone yet, there is high confidence of it developing and bringing significant wind or storm surge impacts to land within the next 72hrs.

Check hurricanes.gov for the latest.

NHC will initiate advisories on Potential Tropical Cyclone Nine (formerly AL94), located near eastern Cuba, at 500 PM EDT (2100 UTC). https://x.com/NHC_Atlantic/status/1971670145887084834

We continue to monitor Hurricane Humberto and AL94 in the Atlantic.

Make sure to check hurricanes.gov for the latest forecast.

Genesis remains a very tricky thing for models to forecast in the medium range -- but there have been large strides made in the last 10-20 years.

Have to remember 20 years ago they didn't have much skill at predicting genesis 2 days in advance, nevermind the 7 day lead time we strive for today.

I think you have the order correct -- I remember the full superensemble viewer you had going last year (the one that lets you toggle on/off all the model options & ellipses) and it was amazing. Please bring that back ASAP (of course when you have time).

Key Messages for the Western Atlantic Disturbance AL94 Thursday, September 25, 2025 - 1:00 PM EDT 1. AL94 is likely to become a tropical depression in a day or two near the Bahamas and interests there should monitor the progress of this system. Heavy rainfall is likely across portions of the Virgin Islands, Puerto Rico, the Dominican Republic, Haiti, the Turks and Caicos, and the southeastern Bahamas over the next day or two. 2. Interests in the central and northwestern Bahamas and the southeastern United States should monitor the progress of this system, as it is expected to pass near the central and northwestern Bahamas this weekend, then turn northward near or just east of Florida and be near the southeast coast of the U.S. early next week. While there is significant uncertainty in the future track and intensity of the system, the chances of wind, rainfall, and storm surge impacts for a portion of the southeast U.S. coast are increasing. 3. NOAA and Air Force Reserve Hurricane Hunters are scheduled to collect data near the system during the next few days, and additional upper-air balloon launches are occurring across portions of the eastern U.S. to provide additional data for weather forecast models. Stay tuned for a live update from the National Hurricane Center at 2:30 pm on Facebook & youtube.com/users/nwsnhc Hurricanes.gov

AL94 is likely to become a tropical depression in a day or two near the Bahamas and interests there should monitor the progress of this system. Heavy rainfall is likely across portions of the Virgin Islands, Puerto Rico, the Dominican Republic, Haiti, the Turks and Caicos, & the southern Bahamas. 👇

Now that #Humberto is a #TC, want to discuss an issue ECMWF exhibits in tropics, unrealistically strong gravity wave dispersion affecting convection/vorticity.

A d(prog)/dt analysis of EC shows #TCG may have been poorly forecasted by dispersion.

Graphics: @burgwx.bsky.social @ polarwx.com/models/

Seven-Day Graphical Tropical Weather Outlook Map of the Atlantic Ocean highlighting the three tropical disturbances - including Humberto, Gabrielle, and another area of interest.

8 am EDT - In addition to #Humberto and #Gabrielle, NHC continues to monitor #94L located near the Dominican Republic. This system has a chance of becoming a tropical depression in the vicinity of the central and northwest Bahamas during the next couple of days. hurricanes.gov

2 PM Thursday Sep 25 Update: Here are Key Messages associated with the disturbance (#AL94) located over the western Atlantic. The system is likely to become a tropical depression in a day or two near the Bahamas. Interests in the Bahamas and a... https://x.com/NHC_Atlantic/status/1971267883520098788