*******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.*********
...TUESDAY OCTOBER 17 2023 1:07 PM EDT...
For the tropical latitudes of the Atlantic Ocean to the east of the Lesser Antilles islands... the remnants of Sean are expected to soon pass north of the islands and into the open western Atlantic waters... see area of interest #47 section below for details. The larger concern for the islands continues to be the large tropical low pressure area now in the central Atlantic... this system will remain parked in favorable upper winds while moving toward the islands through Friday and this weekend and remains at high risk of tropical cyclone formation... see area of interest #45 section below for more information.
For the northwestern Caribbean Sea... the tail end of a surface cold front has arrived... however a tropical area of interest has not materialized so far as stabilizing cooler dry air behind the front has kept widespread shower and thunderstorm activity from developing. Over the next few days regional upper outflow... particularly toward the north... may be suppressed by the passage of the current eastern US upper trough and another similarly-positioned upper trough to follow behind shortly thereafter. Therefore the potential for the tail end of the front to evolve into a tropical disturbance appears low at this time.
AREA OF INTEREST #45... The vigorous central Atlantic tropical low pressure area has finished absorbing the nearby trailing tropical wave of low pressure to the east... which has resulted in a larger system without a well-defined center of rotation as of this writing. This system remains parked in a favorable low shear and upper outflow environment beneath an upper ridge cell whose existence is owed to the relatively higher pressures between upper vorticity lobes to the east and west. Going forward the deep-layer ridge to the north will push this system and pair of upper vorticity lobes westward in tandem... which also means this system will remain stacked below the favorable upper ridge cell over the next few days. Most global model runs also still develop this system into a tropical cyclone. Therefore I am continuing a tropical cyclone formation forecast with a specific track and intensity projection... however my intensity forecast has been lowered once again as this system has not yet become a tropical cyclone.
The short-term track forecast has a slight north angle in the westward heading due to an embedded ridge weakness in the deep-layer ridge associated with the tail end of the surface front now positioned in the western Atlantic and northwestern Caribbean Sea. Toward day 5... a turn to the north in the vicinity of the Lesser Antilles island chain is expected for two reasons... (1) the western lobe of upper vorticity will stop moving westward and settle over the eastern Caribbean once it reaches the western extent of the steering deep-layer ridge... allowing this system to catch up to the upper vorticity and perhaps be bent more northward by it should it indeed become a stronger/taller system more coupled with upper winds... (2) even if this system does not become stronger/taller a surface ridge weakness helping the northward turn arrives in association with an amplified upper trough and surface frontal system departing the eastern US. For intensity... because this system is still classified as a broad feature without a well-defined center of rotation I give it 48 hours before suggesting tropical cyclone formation. I then show a more gradual intensification rate as this system could then continue on as a broad tropical cyclone lacking a well-organized inner core... and by the end of the 5-day forecast period the western outflow of this system is likely to be blocked by the upper vorticity lobe that settles into the eastern Caribbean as well as the approach of the amplified upper trough from the eastern US.
Interests in the Lesser Antilles should remain aware of this system as it has potential to bring tropical cyclone impacts (gusty winds... heavy rain... and coastal surf) to parts of the island chain by Friday and this weekend. It is hard to pinpoint exactly which islands in the chain will be most affected as the forecast track may still need future adjustments depending on where exactly this broad system consolidates.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Oct 17)... Tropical low centered in the central tropical Atlantic at 10N-42W
IOH 24 Hr Forecast (1200Z Oct 18)... Tropical low centered in the central tropical Atlantic at 11N-47W
IOH 48 Hr Forecast (1200Z Oct 19)... 35 mph maximum sustained wind tropical depression centered in the central tropical Atlantic at 12N-52W
IOH 72 Hr Forecast (1200Z Oct 20)... 50 mph maximum sustained wind tropical storm centered east of the southern Lesser Antilles at 13.5N-57W
IOH 96 Hr Forecast (1200Z Oct 21)... 60 mph maximum sustained wind tropical storm centered just east of the central Lesser Antilles at 15.5N-59.5W
IOH 120 Hr Forecast (1200Z Oct 22)... 75 mph maximum sustained wind hurricane centered north-northeast of the northern Lesser Antilles at 20.5N-60.5W
******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM EDT***************************
Formation chance through 48 hours... 60%
Formation chance through 7 days... 80%
AREA OF INTEREST #47 (REMNANTS OF SEAN)... The remnants of what was Tropical Storm Sean are still visible in satellite pictures as a surface trough... with the curvature of the cloud bands suggesting a maximum rotation in the vicinity of 20N-57.5W as of 1200Z today. The CIMSS 850 mb vorticity product (https://tropic.ssec.wisc.edu/real-time/windmain.php?basin=atlantic&sat=wg8&prod=vor&zoom=&time=) also shows a concentrated mid-level spin in association with the remnant system... and with the forecast track taking this system beneath the west side of the central Atlantic deep-layer ridge where supportive low shear and upper outflow exists I have classified the remnant system as an Atlantic tropical area of interest... the forty-seventh such area of interest tracked on this site this year. Some northward angle in this system's track is expected due to the surface ridge weakness associated with the current upper trough now pushing into the western Atalntic from the eastern US. However a complete turn to the north is not expected over the next 72 hours as most of the upper trough lifts out to the northeast which allows the surface ridge supported by the back side of the upper trough to fill in to the north of ex-Sean... the resulting forecast track takes ex-Sean in the western Atlatnic waters between Bermuda and the northeastern Caribbean islands. I have peak odds of tropical cyclone formation at a low 10% as the global models do not call for ex-Sean to re-generate into a cyclone. I end the outlook at 72 hours with 0% development odds as this is when this system would encounter increased westerly shear in association with the next upper trough to approach from the eastern US.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Oct 18)... 0% chance of tropical cyclone formation (north of the northern Lesser Antilles near 21.5N-63W)
IOH 48 Hr Outlook (1200Z Oct 19)... 10% chance of tropical cyclone formation (western Atlantic near 25N-65W)
IOH 72 Hr Outlook (1200Z Oct 20)... 0% chance of tropical cyclone formation (western Atlantic near 27.5N-69W)
******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM 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 (Oct 17) CMC Model Run...
**For area of interest #45... tropical cyclone formation suggested near 14.5N-48.5W at 36 hours... while strengthening into a hurricane passes over the northeastern-most Lesser Antilles through 90 hours... hurricane curves north to 24.5N-63.5W through 120 hours.
**For area of interest #47... through 90 hours ex-Sean surface trough curves north to 29.5N-70W shortly after which time it becomes absorbed by frontal system departing the eastern US
0000Z (Oct 17) ECMWF Model Run...
**For area of interest #45... no development shown
**For area of interest #47... ex-Sean surface trough dissipates near 20N-60W at 30 hours
0600Z (Oct 17) GFS Model Run...
**For area of interest #45... tropical cyclone formation suggested near 11.5N-48.5W at 36 hours... tropical cyclone strengthens into a hurricane while moving northwestward to 20.5N-59.5W through 120 hours
**For area of interest #47... ex-Sean surface trough dissipates near 20N-64W just after 30 hours
0600Z (Oct 17) NAVGEM Model Run...
**For area of interesta #45... tropical cyclone formation suggested near 14N-47.5W at 24 hours... while strengthening into a large and intense hurricane the center passes just northeast of the northern Lesser Antilles through 84 hours... hurricane curves north and then north-northeast while passing just southeast of Bermuda through 120 hours.
**For area of interest #47... ex-Sean surface trough dissipates near 27.5N-70W at 78 hours