*******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...FRIDAY OCTOBER 20 2023 1:09 PM EDT...
The birdseye view chart below has been updated to include the surface and upper air analysis for 0600Z October 19 which were effective at the time the forecasts and discussions below were created.
Tropical Storm Tammy continues to approach the Lesser Antilles and was upgraded to a hurricane with 75 mph maximum sustained winds as of 10 AM EDT. The Tammy forecast discussion below did account for Tammy becoming a hurricane today.
...THURSDAY OCTOBER 19 2023 12:38 PM EDT...
Note the usual surface and upper air analysis are not included in the above chart to ensure a more timely release of this update... those parts of the chart will be added later today.
The broad central Atlantic tropical low pressure area has consolidated into Tropical Storm Tammy within the last 48 hours... with the northern half of the Lesser Antilles expected to see coastal storm surge... heavy rainfall... and winds up to hurricane force by late Friday and Saturday. See Tammy section below for more information.
Elsewhere noting the following disturbances in the Atlantic basin:
(1) The remnant surface trough of what was Tropical Storm Sean has passed north of the northeastern Caribbean Islands and is not expected to re-develop due to increasing shear... see area of interest #47 section below for more information.
(2) For the northwestern Caribbean Sea... the tail end of a surface front persists but has not evolved into a tropical disturbance. However enough tropical upper ridging and outflow may persist further south... to the east of Nicaragua and to the south of the current central US and northwestern Atlantic upper troughs... to make increased thunderstorm activity and a tropical disturbance. Should this in fact occur... will consider adding an area of interest in future updates.
(3) An eastern Atlantic tropical wave of low pressure has seen an increase in thunderstorms well to the south of the Republic of Cabo Verde Islands with the support of the outflow of tropical upper ridging expanding in the wake of the westward-retrograding eastern Atalntic upper vorticity lobe. Over the next five days... the current eastern Atlantic upper vorticity... the current northwest Atlantic upper trough... and portion of the current north Atlantic upper vorticity in the vicinity of Greenland is expected to descend southward toward the tropical wave and along the east side of the central Atlantic deep-layer ridge... particularly as the warm core deep-layer ridge gains dominance due to northward warm air transport ahead of the frontal system now approaching from the central US. Will add the tropical wave as an area of interest within the next five days should it slip westward and away from the approaching suppressing upper vorticity and attempt to develop. The tropical wave has its best chance to move away from the upper vorticity and become an area of interest in the 5 to 7 day window while moving toward the Lesser Antilles. In particular by day 7... Tammy becomes wrapped up into the upper trough now approaching from the central US... with southward cold air transport on Tammy's west side potentially amplifying the cold core upper trough southward toward the Lesser Antilles and this tropical wave. As such the southeastern divergence zone of the amplifying upper trough may also aid the tropical wave in becoming an area of interest in the vicinity of the Lesser Antilles in around 7 days.
TROPICAL STORM TAMMY... The broad central Atlantic tropical low pressure area has consolidated toward the west and became Tropical Storm Tammy as of 5 PM EDT October 18. It is possible that southerly flow on the east side of the neighboring upper vorticity lobe to the west enhanced the poleward outflow of this system to help in the consolidation of the surface low pressure field into Tammy. Even though Tammy has been moving largely westward under the influence of the central Atlantic deep-layer ridge... some northward angle in the track is expected soon due to an embedded ridge weakenss in association with a stalled surface front in the western Atlantic and northwestern Caribbean. A gradual turn to the north and then northeast is expected through day 5 as the surface ridge weakness expands with the arrival of the current central US surface frontal system and upper trough... with the eastward slant by day 5 as Tammy chases the ridge weakness. My updated track forecast is adjsuted based on the storm's initial position which is west of my previous forecast given that Tammy consolidated toward the west side of its incipient central Atlantic low pressure field. Regarding intensity... the western outflow of the storm appears blocked by the presence of the neighboring upper vorticity lobe as the thunderstorm activity is lopsided to the southeast of the center. Despite this the new tropical storm has already intensified to 60 mph maximum sustained winds per this morning's aircraft recon data. In addition over the next 24 hours the upper air pattern may improve as the upper vorticity lobe shifts west and further away from Tammy while attracted toward the approaching central US upper trough... and I forecast Tammy to slowly strengthen into a minimal category 1 hurricane by 24 hours. And while remaining in a low shear and upper outflow environment... I project a higher intensification rate into a top-end category 1 hurricane by 48 hours. Shear from the approaching upper trough increases after that time. However the effects of the shear may be mitigated as the storm begins accelerating northeastward more parallel to the upper winds and by day 5 the storm is aligned with the supportive divergence zone of the upper trough... thus I do not show Tammy weakening below a minimal category 1 hurricane through day 5.
Coastal surf is expected for all of the Lesser Antilles island chain... Puerto Rico... and the Virgin Islands within the next few days. The northern half of the Lesser Antilles (Martinique... Dominica... Guadeloupe... Antigua... Barbuda... Monserrat... Saint Kitts and Nevis... and Anguilla) are most likely to see more direct impacts such as coastal storm surge... heavy rainfall... and winds up to hurricane force by late Friday and Saturday. I recommend preparations in the northern half of the Lesser Antilles need to be completed no later than early tomorrow morning.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Oct 19)... 60 mph maximum sustained wind tropical storm centered in the central tropical Atlantic at 13.5N-55.1W
IOH 24 Hr Forecast (1200Z Oct 20)... 75 mph maximum sustained wind hurricane centered just east of the central Lesser Antilles at 15.5N-59.5W
IOH 48 Hr Forecast (1200Z Oct 21)... 95 mph maximum sustained wind hurricane centered just west of Antigua and Barbuda at 17.5N-62.2W
IOH 72 Hr Forecast (1200Z Oct 22)... 75 mph maximum sustained wind hurricane centered north of the northeastern Caribbean Islands at 21N-64W
IOH 96 Hr Forecast (1200Z Oct 23)... 75 mph maximum sustained wind hurricane centered in the western Atlantic at 25N-63W
IOH 120 Hr Forecast (1200Z Oct 24)... 75 mph maximum sustained wind hurricane centered in the central Atlantic at 28N-59.5W
******National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT***************************
Landfall (1200Z Oct 21)... 75 mph maximum sustained wind hurricane centered over the northern Lesser Antilles at 17.5N-62.1W
5-Day Position (1200Z Oct 24)... 85 mph maximum sustained wind hurricane centered in the central Atlantic at 28.4N-57.9W
AREA OF INTEREST #47 (REMNANTS OF SEAN)... The remnant surface trough of Sean has continued west-northwest around the southwest side of the current central Atlantic deep-layer ridge... allowing it to pass north of the northeastern Caribbean Isalnds. Even though it has gradually seen an increase in its thunderstorms due outflow produced by the upper layers of the ridge... they have become pushed to the east of the surface trough due to increased westerly shear of the upper trough now in the northwest Atlantic. Shear will increase further as the next upper trough approaches from its current central US position... and re-genesis of Sean into a tropical cyclone is not possible going forward. This is my final statement on this area of interest on this blog.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 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 19) CMC Model Run...
**For Tropical Storm Tammy... moves into the northern Lesser Antilles and Virgin Islands through 66 hours as a weak tropical cyclone... while curving north-northeast begins to intensify and reaches 25.5N-62.5W through 120 hours just below hurricane force
**For area of interest #47... no development shown
**Outflow of upper ridge cell produces tropical low east of Nicaragua and near 12.5N-81W through 102 hours... tropical low makes landfall on east coast of Nicaragua at 126 hours
0000Z (Oct 19) ECMWF Model Run...
**For Tropical Storm Tammy... moves into the northeastern-most Lesser Antilles through 54 hours while strengthening as a compact tropical cyclone... curves north then north-northeast while remaining a strong compact tropical cyclone and reaches 29N-57.5W at 120 hours
**For area of interest #47... no development shown
0600Z (Oct 19) GFS Model Run...
**For Tropical Storm Tammy... moves into the northeastern-most Lesser Antilles through 51 hours while strengthening as a compact tropical cyclone... curves north then north-northeast while remaining a strong compact tropical cyclone and reaches 25.5N-60.5W at 120 hours
**For area of interest #47... no development shown
0000Z (Oct 19) NAVGEM Model Run...
**For Tropical Storm Tammy... moves into the northeastern-most Lesser Antilles through 48 hours while strengthening into a high-end tropical storm... while rapidly accelerating north-northeast becomes a hurricane that reaches 29.5N-59W at 90 hours... continues northeast into the open north Atlantic through 120 hours while reaching 43.8N-40.5W where it transitions into a non-tropical frontal cyclone over cooler water
**For area of interest #47... no development shown