*******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 11:38 PM EDT...
The birdseye view chart below has been updated to include the surface and upper air analysis for 1200Z October 20 which were effective at the time the forecasts and discussions below were created.
As of 11 PM EDT the latest aircraft recon data indicates that Tammy has strengthened further to 80 mph maximum sustained winds while continuing to move toward the Lesser Antilles island chain.
...FRIDAY OCTOBER 20 2023 10:16 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 within the next several hours.
Tammy in the central tropical Atlantic has become a hurricane with the northern half of the Lesser Antilles island chain expected to see coastal storm surge... heavy rainfall... and winds up to hurricane force by late tonight and into tomorrow. See Tammy section below for more information.
Elsewhere noting the following disturbances in the Atlantic basin:
(1) A tropical low pressure area has materialized in the southern Caribbean Sea and is expected to move into southern Central America within the next few days... tropical cyclone formation is a possiblity here. See area of interest #48 section below for details.
(2) Satellite pictures indicate a tropical wave of low pressure is in the vicinity of 7.5N-32.5W. Over the next couple of days the current upper vortex just north of the Republic of Cabo Verde Islands... the current north 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 latitudes 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 eastern US. However the tropical wave appears to be moving fast enough to the west such that it will likely outrun the suppressing upper vorticity that dives into the tropical latitudes... and as a result this tropical wave could become another area of interest with development potential as it moves toward the Lesser Antilles over the next five days.
HURRICANE TAMMY... While continuing to barrel mainly westward toward the Lesser Antilles... Tammy has gone on to gradually intensify into a category 1 hurricane as of 10 AM EDT. This is due to the gradually improving upper air pattern where the disrupting upper vorticity in the eastern Caribbean has drifted westward and away from Tammy while attracted toward the upper trough now over the eastern US. The gradual turn to the north has not begun yet and my updated forecast track below is adjusted for this. Previously the northward turn was anticipated from the combined effects of the eastern Caribbean upper vorticity and the expanding western Atlantic surface ridge weakness as the current eastern US frontal system is beginning to push offshore. The delay in the north turn so far is probably attributed to the above-noted westward retrogression of the eastern Caribbean upper vorticity and the steering central Atlantic deep-layer ridge gaining re-enforcement while merging with the northwest Atlantic surface ridge induced by the convergence zone of the current north Atlantic upper trough. The northward and then northeastward turn through day 5 is imminent as Tammy will be chasing the surface ridge weakness of the eastward-moving frontal system that will soon eject from the eastern US. Regarding intensity... Tammy has not established a well-organized inner core and eye yet on colorized infrared satellite pictures and so I continue to avoid forecasting a peak intensity above top-end category 1 status. The delayed north turn of Tammy means that the storm will take a little longer to reach the shearing upper southwesterly winds associated with the upper trough of the frontal system... and so I only show slight weakening from peak intensity between 24 and 48 hours. Beyond 48 hours the effects of the shear may be mitigated as the storm begins accelerating northeastward more parallel to the upper winds and by days 4 and 5 the storm should be aligned with the supportive divergence zone of the upper trough. However given the amount of shear shown in the lastest long-range forecast model fields I do show Tammy gradually weakening to just below hurricane force through day 5.
Coastal surf is expected for all of the Lesser Antilles island chain... Puerto Rico... and the Virgin Islands over the next couple of days. The northern half of the Lesser Antilles (Martinique... Dominica... Guadeloupe... Antigua... Barbuda... Monserrat... Saint Kitts and Nevis... and Anguilla) are likely to see more direct impacts such as coastal storm surge... heavy rainfall... and winds up to hurricane force by late tonight and tomorrow... preparations here should be rushed to completion now. Due to the delayed northward turn noted in the prior paragraph... islands further south (Barbados... Saint Vincent and the Grenadines... and Saint Lucia) are now in play for heavy rainfall.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Oct 20)... 75 mph maximum sustained wind hurricane centered just northeast of Barbados at 14.1N-58.9W
IOH 24 Hr Forecast (1800Z Oct 21)... 95 mph maximum sustained wind hurricane centered over Guadeloupe at 16.2N-61.2W
IOH 48 Hr Forecast (1800Z Oct 22)... 90 mph maximum sustained wind hurricane centered north of the northern Lesser Antilles at 19.5N-63W
IOH 72 Hr Forecast (1800Z Oct 23)... 75 mph maximum sustained wind hurricane centered in the western Atlantic at 23.5N-61.5W
IOH 96 Hr Forecast (1800Z Oct 24)... 70 mph maximum sustained wind tropical storm centered in the central Atlantic at 26N-59.8W
IOH 120 Hr Forecast (1800Z Oct 25)... 70 mph maximum sustained wind tropical storm centered in the central Atlantic at 32N-55W
******National Hurricane Center (hurricanes.gov) official forecast as of 5 PM EDT***************************
Landfall (0600Z Oct 22)... 85 mph maximum sustained wind hurricane centered over Barbuda at 18.1N-62.1W
Peak Strength (1800Z Oct 22)... 90 mph maximum sustained wind hurricane centered north of the northern Lesser Antilles at 19.5N-63W
5-Day Position (1800Z Oct 25)... 75 mph maximum sustained wind hurricane centered in the central Atlantic at 28.4N-57.7W
AREA OF INTEREST #48... Based on the latest satellite imagery a southern Caribbean tropical low pressure has become better defined near 11.2N-80W with the support of low shear and outflow beneath a regional tropical upper ridge. The associated curved thunderstorm banding primarily lies on the west and south sides of the center... with the southern-most bands overspreading Panama and far eastern Pacific waters. The NHC has added this disturbance into their tropical weather outlook... and this is the forty-eighth tropical Atlantic area of interest tracked on this site this year. The southwestern convergence zone of the current eastern US upper trough is producing a surface ridge over northern Central America and the Gulf of Mexico that is preventing the westward progress of the new tropical low pressure area. During the forecast period the southern part of the upper trough breaks off into fading upper vorticity over the northwestern Caribbean while the northern part continues on eastward in the mid-latitude westerlies... and the aforementioned blocking surface ridge as a result becomes replaced by a further-north surface ridge over the eastern US tied to the convergence zone of the northern part of the upper trough. Therefore an increasing westward drift of the tropical low into southern Central America... under the influence of the forecast eastern US surface ridge... is anticipated. I have assigned odds of tropical cyclone formation higher than today's NHC 2 PM EDT outlook due to the organization this system is already showing in satellite pictures... as well as the favorable upper winds outlook as regional tropical upper ridging is expected to persist while the aforementioned decaying northwestern Caribbean upper vorticity does not get too close to disrupt this system. However I keep peak odds of tropical cyclone formation below 50% as global models do not develop this system.
Regardless of tropical cyclone formation or not... a prolonged period of heavy rainfall is possible for Costa Rica... Panama... and Nicaragua over the next few days which could result in flooding. Should tropical cyclone formation occur... the most likely region for gusty winds and coastal surf would be northern Costa Rica and southern Nicaragua.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 21)... 20% chance of tropical cyclone formation (southern Caribbean Sea near 11.2N-80W)
IOH 48 Hr Outlook (1800Z Oct 22)... 30% chance of tropical cyclone formation (east of the Nicaragua/Costa Rica border near 11.2N-81W)
IOH 72 Hr Outlook (1800Z Oct 23)... 40% chance of tropical cyclone formation (southeast coast of Nicaragua near 11.2N-83.8W)
IOH 96 Hr Outlook (1800Z Oct 24)... 0% chance of tropical cyclone formation (southwestern Nicaragua near 11.2N-85.5W)
******National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT***************************
Formation chance through 48 hours... 10%
Formation chance through 7 days... 20%
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields(http://moe.met.fsu.edu/tcgengifs/).
1200Z (Oct 20) CMC Model Run...
**For Hurricane Tammy... moves across the northern Lesser Antilles and passes just northeast of the Virgin Islands through 42 hours... curves north then northeast and reaches 27.5N-60W at 120 hours.
**For area of interest #48... no development shown
0000Z (Oct 20) ECMWF Model Run...
**For Hurricane Tammy... moves into the northeastern-most Lesser Antilles through 42... curves north and then northeast and reaches 25N-59.5W at 120 hours
**For area of interest #48... no development shown
1200Z (Oct 20) GFS Model Run...
**For Hurricane Tammy... center passes just east of the northeastern-most Lesser Antilles through 33 hours... continues northwest and then northeast and reaches 31.5N-54.5W at 120 hours
**For area of interest #48... no development shown
1200Z (Oct 20) NAVGEM Model Run...
**For Hurricane Tammy... moves into the northeastern-most Lesser Antilles through 30 hours... curves northwest and then northeast 31.8N-54W through 120 hours.
**For area of interest #48... no development shown
**Tropical wave currently near 35W longitude becomes tropical cyclone near 15N-54W at 90 hours... curves north toward the ridge weakness associated with Tammy and reaches 18N-54W through 120 hours while strengthening