*******Note that forecast and outlooks in this post are NOT the official forecast from the National Hurricane Center (NHC). They are my own detailed views on the Atlantic tropics based on current observations and latest computer model runs. As such do not make decisions based on my posts...consult news media...watches and warnings from your local weather office...and any evacuation orders issued by local governments to make the most informed and best decisions. Visit the NHC website hurricanes.gov (hurricanes dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.**********
...UPDATE...TUESDAY SEPTEMBER 26 2023 11:59 AM EDT...
The birdseye view chart below has been updated to now include the surface and upper-level analysis as of 0000Z September 26.
Also noting the tropical wave of low pressure to the southeast of Philippe... tagged as area of interest #39 in the update below... has become notably better organized in the overnight hours and through this morning while taking advantage of its low shear environment while Philippe is less organized due to shear. Odds of tropical cyclone formation for area of interest #39 are now higher than what I showed in the update below... and if current trends continue it is increasingly possible that area of interest #39 could overtake Philippe as the dominant system in the central Atlantic region in the long range.
...MONDAY SEPTEMBER 25 2023 9:40 PM EDT...
Note the usual surface analysis and upper air charts for the above birdseye view chart are still being assembled and will be released by tomorrow morning. This update is released now without those parts of the chart to ensure a timely release of this update.
Even though Tropical Storm Philippe in the open central Atlantic is the only active Atlantic tropical cyclone… the Atlantic tropics are still active with multiple areas of interest also being monitored as follows:
(1) See Philippe section below for more information on Tropical Storm Philippe.
(2) See Area of Interest #39 section below for more information on the eastern Atlantic tropical wave of low pressure southeast of Philippe being monitored for signs of development over the course of the next few days.
(3) See area of interest #40 section below for an update on the southern Gulf of Mexico surface trough of low pressure.
(4) See area of interest #41 section below for information on a tropical wave of low pressure in the central Caribbean which has recently seen an increase in thunderstorm activity.
Elsewhere... noting the following other disturbances not being considered areas of interest at this time:
(1) The surface trough of low pressure that formed near 25N-60W has continued northwest toward Bermuda. Further development is not expected as westerly shear increases with the approach of a northern fracture of the current eastern North America upper trough.
(2) Warm core upper ridging over Canada is expected to continue over the next few days due to warm southerly flow on the west side of the current eastern Canada surface ridge. This upper ridging will cause the northern half of the current eastern North America upper trough to enter the northwest Atlantic as an amplified wave…. and then the current north-central US upper vortex also enters the northwest Atlantic as an amplified wave as well. The eastern divergence zones of these waves will keep surface pressures in the northwest Atlantic and to the northwest of Philippe low… and the amplified nature of these waves could keep shear low enough for the development of an organizing subtropical disturbance within the surface low pressure field. However not adding an area of interest for the northwest Atlantic at this time as models have not been consistent in showing the development of a northwest Atlantic subtropical disturbance at this time.
TROPICAL STORM PHILIPPE... Philippe continues to get raked by westerly shear being imparted by the current central Atlantic upper vorticity string. As such the thunderstorm canopy of the tropical storm remains lopsided toward the east side of the surface center. The current position of Philippe is once again a little east of the previous forecast... and my updated one is also once adjusted accordingly. The more east position of Philippe could be an indication that Philippe's surface center continues trying to consolidate toward surface pressure falls caused by the warm core upper ridging and outflow generated by the thunderstorm canopy's latent heat release. The track forecast calls for an increase in the north angle of Philippe's westward track under the influence of the central Atlantic upper vorticity... followed by the development of a surface ridge weakness in the open central Atlantic triggered by the eastern divergence zone of what will be the northern fracture of the current eastern North America upper trough as that fracture moves into the northwest Atlantic. The steering picture around 120 hours becomes murky as some recent model runs indicate Philippe could be bent more westward in track from a fujiwhara interaction with Area of Interest (AOI) #39 to the southeast... which then tosses Philippe toward the current strong eastern Canada surface ridge as that ridge moves offshore (the ridge would then continue pushing Philippe more westward). Because AOI #39 is currently not organized and not in a hurry to develop... at this time I give low credence to the fujiwhara interaction scenario. Instead I have Philippe curve north-northeastward between 96 and 120 hours while dragged by upper westerlies and gravitated toward the eastward-shifting surface ridge weakness (the surface ridge weakness moves east as the above-mentioned northwest Atlantic upper trough generating the weakness moves east).
For intensity... I call for weakening in the next 24 hours as the shear worsens when Philippe gets closer to the central Atlantic upper vorticity. From 48 to 96 hours the westerly shear across Philippe may let up as the central Atlantic upper vorticity becomes tilted southwest-to-northeast under the influence of a strengthening Caribbean upper ridge cell... changing the upper winds over Philippe from westerly to southwesterly. Thus I show some recovery in strength for that timeframe... but not much as the southwesterly shear may still be strong. By 120 hours Philippe will likely be raked by worsening westerly shear as the aforementioned northwest Atlantic upper trough passes by just to the storm's north... thus I call for a lower intensity for that timeframe.
Update as of 5 PM EDT… Philippe has weakened to 45 mph maximum sustained winds… no change to the intensity forecast below as weakening in the short-term was already factored into the forecast.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
0 Hr Position (1200Z Sep 25)... 50 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 17.4N-43.9W
IOH 24 Hr Forecast (1200Z Sep 26)... 40 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 18.5N-49W
IOH 48 Hr Forecast (1200Z Sep 27)... 50 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 20.5N-51.5W
IOH 72 Hr Forecast (1200Z Sep 28)... 50 mph maximum sustained wind tropical storm centered in the central Atlantic at 22.5N-52.5W
IOH 96 Hr Forecast (1200Z Sep 29)... 50 mph maximum sustained wind tropical storm centered in the central Atlantic at 25N-53.5W
IOH 120 Hr Forecast (1200Z Sep 30)... 40 mph maximum sustained wind tropical storm centered in the central Atlantic at 27.5N-51W
******National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT***************************
5-Day Position (1200Z Sep 28)... 50 mph maximum sustained wind tropical storm centered in the central Atlantic at 24.6N-57.9W
AREA OF INTEREST #39... The eastern Atlantic tropical wave of low pressure to the southeast of Tropical Storm Philippe as of 1800Z was in the vicnity of 10N-32W... or on track with my previous forecast. Forecast track in the outlook below curves increasingly northward with time while gravitated toward the surface ridge weakness associated with Philippe. Even though the forecast track keeps this system in a good upper outflow environment away from the suppressing eastern Atlantic upper vorticity to the northeast... and even though most global models as of today are on board with developing this system into a tropical cylone... I continue to cap my peak odds of tropical cyclone formation at 50% as this system has actually become less organized since yesterday while featuring a linear northwest-to-southeast area of thunderstorms instead of curved bands associated with a developing tropical system. In addition by 4+ days the favorable upper wind envrionment is likely to come to an end as this system begins to encounter northerly shear imparted by the outflow of Tropical Storm Philippe. By 5+ days this system is at risk of being sheared apart and absorbed by relatively stronger Philippe if this system has not become a tropical cyclone by then... thus my odds of tropical cyclone formation at day 5 are lowered from the 50% peak.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 26)... 0% chance of tropical cyclone formation (eastern tropical Atlantic near 11N-37W)
IOH 48 Hr Outlook (1800Z Sep 27)... 15% chance of tropical cyclone formation (central tropical Atlantic near 12.8N-41W)
IOH 72 Hr Outlook (1800Z Sep 28)... 30% chance of tropical cyclone formation (central tropical Atlantic near 15.5N-44W)
IOH 96 Hr Outlook (1800Z Sep 29)... 50% chance of tropical cyclone formation (central tropical Atlantic near 18.5N-46.5W)
IOH 120 Hr Outlook (1800Z Sep 30)... 25% chance of tropical cyclone formation (central tropical Atlantic near 22.5N-47.5W)
******National Hurricane Center (hurricanes.gov) official outlook as of 2 PM EDT***************************
Formation chance through 48 hours... 40%
Formation chance through 7 days... 80%
AREA OF INTEREST #40... A surface trough of low pressure in the southern Gulf of Mexico has continued to drift westward under the influence of the eastern Canada surface ridge... because the ridge remains far away the westward progress of this feature has been slow. As of 1800Z the surface trough was located near 22.5N-88W. As discussed in the previous update... the surface trough has failed to develop while suppressed by upper convergence occuring between the northwest side of the upper vortex in the region and southeast side of an upper ridge cell persisting over Mexico. This is my final statement on this area of interest on this blog as the NHC has lowered development odds to 0% in their 2 PM EDT outlook... which likely means this area of interest will be cancelled from the NHC outlook going forward.
Update as of 8 PM EDT… this area of interest has officially been cancelled from the NHC outlook.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 26)... 0% chance of tropical cyclone formation (southern Gulf of Mexico near 22.5N-90.5W)
******National Hurricane Center (hurricanes.gov) official outlook as of 2 PM EDT***************************
Formation chance through 48 hours... 0%
Formation chance through 7 days... 0%
AREA OF INTEREST #41... A tropical wave of low pressure in the central Caribbean Sea has seen an increase in thunderstorms in the vicinity of 15N-79W through 1800Z today. The wave is being enhanced by divergence between the west side of the Caribbean upper ridge cell and east side side of an upper vortex over the Yucatan. The favorable Caribbean upper ridge cell has potential to expand westward and over this wave as the Yucatan upper vortex lifts northward and away while eventually pulled by the current north-central US upper vortex… therefore adding this tropical wave as an area of interest for possible development in the days ahead… the forty-first such tropical Atlantic area of interest tracked on this site this year.
The forecast track of the wave is slow to the west-northwest around the southwest side of the current eastern Canada surface ridge… the slow speed a reflection of how far away the ridge is. Even though the Caribbean upper ridge cell makes an attempt to expand over this wave as discussed above… southerly wind shear will never be eliminated as upper vorticity currently offshore of California and Baja California pivots eastward into the western Gulf of Mexico and limits how much the Caribbean upper ridge can expand. Therefore I cap peak odds of development at 20% through 72 hours. After that time the shear worsens as the western Gulf upper vorticity shifts east closer to this system while pulled around the current north-central US upper vortex… and land interaction with the Yucatan along the forecast track will not help with development. Therefore I end the outlook by 96 hours with 0% development odds. Note that this tropical wave may bring gusty winds and heavy rainfall to the Yucatan region of Mexico and Belize on Thursday regardless of tropical cyclone formation or not.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 26)... 0% chance of tropical cyclone formation (western Caribbean Sea near 16.5N-82W)
IOH 48 Hr Outlook (1800Z Sep 27)… 10% chance of tropical cyclone formation (west-southwest of the Cayman Islands near 18N-84.5W)
IOH 72 Hr Outlook (1800Z Sep 28)… 20% chance of tropical cyclone formation (east coast of the Yucatan peninsula near 19.5N-87.5W)
IOH 96 Hr Outlook (1800Z Sep 29)… 0% chance of tropical cyclone formation (northwestern Yucatan peninsula near 20.5N-89.5W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields(http://moe.met.fsu.edu/tcgengifs/).
0000Z (Sep 25) CMC Model Run...
**For Tropical Storm Philippe... reaches 24.5N-60.5W at 120 hours still as a tropical storm
**For Area of Interest #39... tropical cyclone formation suggested near 13.8N-42W at 60 hours... reaches 19.5N-50W at 120 hours as a tropical storm
**For Area of Interest #40... no development shown
**For Area of Interest #41… no development shown
0000Z (Sep 25) ECMWF Model Run...
**For Tropical Storm Philippe... weakens to a remnant trough near 17.5N-53.5W at 36 hours... remnant trough passes north of the northeastern Caribbean Islands and reaches 21.2N-65W at 120 hours.
**For Area of Interest #39... organizes into a tropical low near 11.5N-41W at 54 hours... tropical cyclone formation suggested near 16N-53W at 96 hours... strengthening tropical cyclone located near 16.5N-50W at 120 hours
**For Area of Interest #40... no development shown
**For Area of Interest #41… no development shown
0600Z (Sep 25) GFS Model Run...
**For Tropical Storm Philippe... moves northwest to 25.5N-54.5W through 96 hours as a strong tropical storm approaching hurricane strength... subsequently stalls at this location through 120 hours as a strong eastern Canada surface ridge moves in from the northwest and AOI #39 to the southeast tugs on Philippe
**For Area of Interest #39... tropical cyclone formation suggested near 12N-41W at 57 hours... through 120 hours turns northwest toward Philippe and stays a weak tropical cyclone due to shear from Philippe's outflow while arriving to 20.2N-48W
**For Area of Interest #40... no development shown
**For Area of Interest #41… tropical wave currently in the central Caribbean Sea organizes into a compact tropical low offshore of northern Belize at 63 hours... compact tropical low subsequently moves very slowly northwest and makes landfall just north of the Belize/Mexico border through 114 hours... tropical low located over the east-central Yucatan peninsula near 20N-89W at 126 hours
0000Z (Sep 25) NAVGEM Model Run...
**For Tropical Storm Philippe... curves north and reaches 27.5N-51.5W at 120 hours as an intense hurricane
**For Area of Interest #39... tropical cyclone formation suggested near 16N-48.5W at 96 hours... tropical cyclone moves north-northwest toward Philippe and reaches 18.8N-50W at 120 hours.
**For Area of Interest #40... no development shown
**For Area of Interest #41… no development shown