*******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...WEDNESDAY SEPTEMBER 27 2023 12:52 AM EDT...
The birdseye view chart below has been updated to now include the surface and upper-level analysis as of 0000Z September 27.
...TUESDAY SEPTEMBER 26 2023 11:55 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 within the next few hours. 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 #41 section below for information on a tropical wave of low pressure now in the western Caribbean which continues to remain active with abundant thunderstorm activity.
(4) See area of interest #42 section below for possible subtropical development offshore of the eastern United States and northwest of Bermuda by this weekend.
TROPICAL STORM PHILIPPE... Big changes with Philippe's track forecast as the sheared storm has turned more westward (less northward). This is an indication the sheared storm has weakened to a shallower tropical cyclone now decoupled from the shearing upper southwesterly winds... with the storm now purely being steered westward by the current Atlantic surface ridge. Models have also trended notably more south and west with Philippe's long-term position for two reasons... (1) the initial more west and less north track causes Philippe to miss the current central Atlantic surface ridge weakness being generated by the current northwest Atlantic upper trough and hence Philippe is guided more west in the long range by the current eastern Canada surface ridge as that ridge later descends southeast into the Atlantic... (2) area of interest (AOI) #39 to the southeast is doing much better and hence has a better chance to deflect Philippe's track. While pouring over recent model data... my updated track forecast in also reflects a more southwest long-term position of the storm even though I think Philippe will be far enough away from AOI #39 to not interact with it... because I expect the current Atlantic surface ridge and eastern Canada surface ridge that descends into the Atlantic will be close enough to the storm to keep Philippe moving fast enough through 72 hours to keep it away from AOI #39. And even though by 96 and 120 hours Philippe will slow down its westward progress and potentially bend more north toward a northwest Atlantic surface ridge weakness to later be generated by the current north-central US upper vortex (when that vortex's divergence zone moves toward the Atlantic)... AOI #39 is also likely to be held back and away from Philippe while interacting with a portion of the current Atlantic upper vorticity string as a strong/tall tropical cyclone.
Here is a breakdown of the finer details of my updated forecast track and intensity below:
(1) Through 48 hours... expecting some north angle in the track to resume as the shallow tropical cyclone becomes affected by the current central Atlantic surface ridge weakness mentioned above. During this period the north-central US upper vortex begins to produce a western Atlantic surface frontal system whose northward warm air transport amplifies the current Caribbean upper ridge cell northward into the western Atlantic. In turn the western Atlantic upper ridge cell begins to pull the southwest end of the current central Atlantic upper vorticity away from Philippe which may reduce the shear over the storm... indeed this process may have already begun as thunderstorm bursts have recently begun re-firing close to the center of circulation. However I do not increase the intensity of Philippe during this part of the forecast as the western upper outflow of AOI #39 may impart southerly shear over Philippe.
(2) Between 48 and 72 hours... expecting Philippe's track to bend more west as the eastern Canada surface ridge moves into the Atlantic. My intensity projection is lowered as Philippe moves directly under the central Atlantic upper vorticity axis where upper convergence is likely to suppress Philippe.
(3) From 72 to 120 hours... expecting Philippe's track to slow down and turn more northwest as what is now the current north-central US upper vortex begins moving offshore while its divergence zone develops a full-blown northwestern Atlantic surface frontal low pressure system that tries to attract Philippe. By this point the central Atlantic upper vorticity string will have likely fractured into a Caribbean upper vortex to the southwest and upper trough to the east interacting with AOI #39... leaving Philippe underneath the low shear and upper outflow environment beneath the western Atlantic upper ridge cell by 96 hours such that I show a recovery in Philippe's strength. At 120 hours the forecast track takes Philippe toward the north side of the western Atlantic upper ridge cell where upper westerlies are likely to begin shearing the storm again... thus I show no increase in strength from 96 to 120 hours.
Even though the long range forecast position for Philippe is notably more southwest and closer to the northeastern Caribbean Islands... the tropical cyclone is also likely to remain weak for the reasons discussed above... therfore there is only a medium instead of high chance of coastal surf generated by Philippe affecting the shores of these islands late this week and into the weekend.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
0 Hr Position (1800Z Sep 26)... 45 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 17.3N-50.3W
IOH 24 Hr Forecast (1800Z Sep 27)... 45 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 18.5N-54W
IOH 48 Hr Forecast (1800Z Sep 28)... 45 mph maximum sustained wind tropical storm centered northeast of the northern Lesser Antilles at 20N-59W
IOH 72 Hr Forecast (1800Z Sep 29)... 30 mph maximum sustained wind tropical depression centered north of the northern Lesser Antilles at 20.2N-63.5W
IOH 96 Hr Forecast (1800Z Sep 30)... 45 mph maximum sustained wind tropical storm centered north-northeast of Puerto Rico at 21.2N-65W
IOH 120 Hr Forecast (1800Z Oct 1)... 45 mph maximum sustained wind tropical storm centered well north of Puerto Rico and well east-northeast of the eastern Bahamas at 22.5N-67.5W
******National Hurricane Center (hurricanes.gov) official forecast as of 5 PM EDT***************************
Loss of tropical cyclone status (1800Z Sept 30)... 30 mph maximum sustained wind tropical low centered north of the Virgin Islands at 20.2N-64W
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 11N-36.5W... or in alignment with my previous track forecast. This system has become notably more robust while developing thunderstorms in all quadrants of its surface low pressure spin... and with this system to remain aligned with low shear and upper outflow beneath a central tropical Atlantic upper ridge cell in the short-term I am now expecting tropical cyclone formation. Therefore I have begun a specific tropical cyclone track and intensity forecast as outlined below.
Even though the previous forecast track has performed well so far... my updated one is adjusted in the long range by showing a more west and less north slant to reflect Philippe's shift in the forecast track as this system will be attracted toward Philippe's surface ridge weakness. Noting by 72+ hours the eastern part of the current central Atlantic upper vorticity string is expected to be hurtled toward this system by the current northwestern Atlantic upper trough and also by the expanding western Atlantic upper ridge cell noted in the above Philippe forecast discussion. Because this system is likely to briskly strengthen into a strong/tall tropical cyclone in the next 48 hours... expecting the westward speed to slow and northward angle in the track to increase by 72 hours while the interaction with the upper vorticity begins. By 96 to 120 hours this system will have likely whirled beneath the upper vorticity... however a continued northwestward track is shown while gravitated toward a large western Atlantic surface ridge weakness associated with Philippe and northwestern Atlantic surface frontal system to be generated by what is now the current north-central US upper vortex (when the divergence zone of that vortex later moves offshore). For intensity... I do not quiet show hurricane status as shear increases from the approaching upper vorticity by 72 hours. By 96 hours some of the model data suggests the latent heat relase of this system will have weakened the upper vorticity into a small upper vortex just northwest of this system's surface center... therefore I show only a little weakening. However with this system likely whirling directly beneath the upper vorticity by 120 hours where supressive upper convergence would be occurring... I show a larger margin of weakening. Noting that just after the 120 hour period the northwestward drifting track would take this system into a more favorable environment beneath the expanding western Atlantic upper ridge cell... this could mean the beginning of notable strengthening after day 5.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
0 Hr Position (1800Z Sep 26)... Tropical low centered in the eastern tropical Atlantic at 11N-36.5W
IOH 24 Hr Forecast (1800Z Sep 27)... 45 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 12.5N-41W
IOH 48 Hr Forecast (1800Z Sep 28)... 65 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 15N-46W
IOH 72 Hr Forecast (1800Z Sep 29)... 70 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 16.5N-48.5W
IOH 96 Hr Forecast (1800Z Sep 30)... 65 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 18N-50W
IOH 120 Hr Forecast (1800Z Oct 1)... 50 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 19.5N-52W
******National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT***************************
Formation chance through 48 hours... 90%
Formation chance through 7 days... 90%
AREA OF INTEREST #41... A tropical wave of low pressure from the central Caribbean Sea has spent the last day pushing into the western Caribbean while interacting with an expanding Caribbean upper ridge cell. The upper vortex that was over the Yucatan peninsula has weakened to a trough lifting northward across the Gulf of Mexico and toward the current north-central US upper vortex... which has allowed the Caribbean upper ridge cell to expand westward over the tropical wave. During this past morning and afternoon southerly shear on the west side of the upper ridge cell pulled a wave of thunderstorms northward across the Cayman Islands and wesstern Cuba... with the sheared-off moisture also interacting with the eastern divergence zone of the Gulf of Mexico upper trough to extend the wave of weather northward across Florida and parts of the southeastern United States. More recently a new pulse of thunderstorms has developed notably further south and near the wave's maximum spin offshore of northeastern Honduras near 16.2N-83W... with this pulse of activity also bringing rainfall to eastern Nicaragua and Honduras for a time this evening. With the thunderstorms more collocated with the wave's maximum spin... it appears the Caribbean upper ridge cell may have expanded enough over the wave to temporarily reduce the shear. Therefore still monitoring this wave for signs of development over the next 48 hours before it interacts with the Yucatan peninsula landmass and experiences re-increasing shear from a wave of upper vorticity currently en route to the western Gulf of Mexico from the southwestern US. Albeit the shear from the approaching upper vorticity already increases before the wave makes landfall with the Yucatan which is why I cap my peak odds of development at a low 20%.
The forecast track of the wave is slow to the west-northwest toward the Yucatan while steered around the southwest side of the current eastern Canada surface ridge... the slow speed a reflection of how far away the ridge is. The above-noted 1800Z position of the wave's maximum spin is a little southwest of my previous forecast therefore my updated one is shifted slightly in that direction. Note that this tropical wave may bring gusty winds and heavy rainfall to the Yucatan region of Mexico and Belize by 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 27)... 10% chance of tropical cyclone formation (western Caribbean Sea near 17.5N-85.5W)
IOH 48 Hr Outlook (1800Z Sep 28)... 20% chance of tropical cyclone formation (near east coast of the Yucatan peninsula and just north of the Belize/Mexico border near 19N-88W)
IOH 72 Hr Outlook (1800Z Sep 29)... 0% chance of tropical cyclone formation (western Yucatan peninsula near 20N-90W)
******National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT***************************
Not in the official outlook
AREA OF INTEREST #42... The current north-central United States upper vortex is expected to merge with upper vorticity currently over the southwestern United States... with this wave of merged upper vorticity then moving offshore by days 4 and 5 and triggering a northwest Atlantic surface frontal low with its eastern divergence zone. Even though the current eastern Canada surface ridge will tend to move offshore while following the convergence zone of the current northwestern Atlantic upper trough... a portion of the surface ridge will be maintained over eastern Canada by the western convergence zone of the wave of upper vorticity. In turn northward warm air transport on the west side of the persisting eastern Canada surface ridge will keep warm core upper ridging amplified over Canada such that the wave of upper vorticity has potential to consolidate into an offshore cut-off upper vortex by day 5. In this scenario the northwestern Atlantic surface frontal low would whirl beneath the cut-off upper vortex while still over warm enough waters to potentially acquire core thunderstorms and subtropical characteristics... and various model runs over the last few days have hinted at this idea. Therefore I have upgraded the current north-central United States upper vortex to a tropical area of interest for the northwestern Atlantic waters northwest of Bermuda and offshore of the eastern United States for the days ahead. This marks the forty-second tropical area of interest for the Atlantic I have tracked on this site this year. Assigning a low 20% odds of subtropical cyclone formation for day 5 in this update as models are not consistent on whether or not the wave of upper vorticity will consolidate into a cut-off upper vortex... if this does not occur wind shear could end up being too high for subtropical development.
Regardless of whether or not the above-mentioned forecast northwest Atlantic frontal low acquires subtropical characteristics or not... this system could produce coastal surf for parts of the United States east coast and/or Bermuda by the weekend. For parts of the United States east coast... this would be a prolongation of the long-lasting period of coastal surf that has already been occuring due to the fetch of onshore-pushing wind caused by the pressure gradient between the north side of ex-Ophelia and south side of the eastern Canada surface ridge.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 27)... 0% chance of subtropical cyclone formation (southern Indiana near 39N-86W)
IOH 48 Hr Outlook (1800Z Sep 28)... 0% chance of subtropical cyclone formation (West Virginia/ Maryland/ Pennsylvania border near 39.8N-79.5W)
IOH 72 Hr Outlook (1800Z Sep 29)... 0% chance of subtropical cyclone formation (just offshore of southeastern New Jersey near 38.8N-74W)
IOH 96 Hr Outlook (1800Z Sep 30)... 0% chance of subtropical cyclone formation (offshore of the eastern United States near 37N-70W)
IOH 120 Hr Outlook (1800Z Oct 1)... 20% chance of subtropical cyclone formation (offshore of the eastern United States near 37N-69.5W)
******National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT***************************
Not in the official outlook
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields(http://moe.met.fsu.edu/tcgengifs/).
0000Z (Sep 26) CMC Model Run...
**For Tropical Storm Philippe... moves west-northwest to 20.2N-55W through 54 hours while remaining a weak tropical cyclone... subsequently interaction with area of interest #39 and surface ridge departing from eastern Canada turns the track more westward with Philippe arriving to 21N-66W at 120 hours as a remnant tropical low
**For Area of Interest #39... tropical cyclone formation suggested near 13.8N-42.5W at 36 hours... moves west-northwest to 18N-57.5W through 120 hours while acquiring hurricane strength
**For Area of Interest #41... no development shown
**For Area of Interest #42... current north-central US upper vortex moves offshore from the northeastern US through 102 hours while its eastern divergence zone triggers a frontal low near 38N-72.8W... upper vortex and frontal low become vertically stacked offshore of the mid-Atlantic United States and near 36.5N-73.8W through 120 hours over warm enough waters where subtropical development is possible.
0000Z (Sep 26) ECMWF Model Run...
**For Tropical Storm Philippe... moves west-northwest to 21N-59W through 72 hours while remaining a weak tropical cyclone... subsequently interaction with area of interest #39 and surface ridge departing from eastern Canada turns the track more westward with Philippe arriving to 20.5N-67W at 120 hours as a remnant tropical wave
**For Area of Interest #39... tropical cyclone formation suggested near 15.2N-45W at 54 hours with tropical cyclone reaching 17N-52W at 120 hours as a tropical storm.
**For Area of Interest #41... no development shown
**For Area of Interest #42... no development shown
1200Z (Sep 26) GFS Model Run...
**For Tropical Storm Philippe... moves west-northwest to 21.2N-57.5W through 48 hours while remaining a weak tropical cyclone... subsequently interaction with area of interest #39 and surface ridge departing from eastern Canada turns the track more westward with Philippe arriving to 20.2N-66W at 120 hours as a remnant tropical low
**For Area of Interest #39... strengthening tropical low with multiple centers consolidates into a strong tropical storm near 14.8N-46W through 48 hours... reaches 18.5N-51.5W at 120 hours while approaching hurricane strength.
**For Area of Interest #41... no development shown
**For Area of Interest #42... current north-central US upper vortex moves offshore from the northeastern US through 90 hours as an amplified trough whose southeastern divergence zone triggers a surface frontal low offshore of North Carolina and near 33.8N-74W at 90 hours... frontal low strengthens into a cyclone with possible subtropical characteristics to the northwest of Bermuda and near 34.5N-67.5W by 132 hours.
1200Z (Sep 26) NAVGEM Model Run...
**For Tropical Storm Philippe... moves west-northwest to 20.5N-59W through 60 hours while remaining a tropical storm... while interacting with area of interest #39 to the east changes to a west-southwest heading into the northern Lesser Antilles through 120 hours while strengthening into a hurricane.
**For Area of Interest #39... tropical cyclone formation suggested near 15N-42.5W at 24 hours... moves west-northwest to 18.5N-49.5W through 120 hours while strengthening into a hurricane.
**For Area of Interest #41... tropical low becomes better defined offshore of the northeastern Yucatan and near 20N-85.5W at 12 hours... tropical low lifts north-northeast and makes landfall on the west-central Florida peninsula at 78 hours and dissipates over the northern Florida peninsula by 114 hours.
**For Area of Interest #42... no development shown