*******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...SUNDAY OCTOBER 22 2023 1:52 AM EDT...
The birdseye view chart below has been updated to include the surface and upper air analysis for 1800Z October 21 which were effective at the time the forecasts and discussions below were created.
The southern Caribbean tropical low pressure area... tagged as area of interest #48 in the update below... has recently seen an increase in concentrated thunderstorms just west of its center of rotation. The NHC has recently increased odds of tropical cyclone formation to 30% in their tropical weather outlook.
...SUNDAY OCTOBER 22 2023 12:50 AM 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.
Hurricane Tammy has curved northward across the northeastern-most parts of the Lesser Antilles island chain... with weather conditions in the region to improve over the next 48 hours as the hurricane lifts northward and away. See Tammy section below for more information.
Elsewhere noting the following disturbances in the Atlantic basin:
(1) A quasi-stationary tropical low pressure continues over the southern Caribbean Sea and is expected to move into southern Central America over the next 48 hours... there is a small possibility of tropical cyclone formation here. See area of interest #48 section below for details.
(2) A tropical wave of low pressure moving quickly westward toward the Lesser Antilles behind Tammy may itself attempt to develop... see area of interest #49 section below for more details.
HURRICANE TAMMY... Tammy turned more sharply northward than the previous forecast which allowed its center to pass just east of the northern half of the Lesser Antilles island chain. The northward turn was aided by the combination of the eastern Caribbean upper vorticity and expanding western Atlantic surface ridge weakness associated with the frontal system now departing from the eastern US. As of 5 PM EDT the center passed just east of Antigua... and as of 9:15 PM EDT the center passed directly over Barbuda. While evolving into a more compact hurricane with a circular core of thunderstorms Tammy has been gradually intensifying to 85 mph maximum sustained winds. The updated short-term forecast track points are adjusted north and east to account for Tammy's recent sharp northward turn. The long-term forecast guidance however has changed... now indicating that the bulk of the surface frontal system and northern half of the associated upper trough will leave Tammy behind. By days 4 and 5 the western convergence zone of the northern upper trough fragment is now expected to build a surface ridge to the north of Tammy that would prevent the storm from continuing northeastward out to sea... with the storm also interacting with the southwestern upper trough fragment which evolves into a cut-off upper vortex with time. The updated long-range forecast track is slower to the northeast to reflect these changes... and has a more north and less east angle by day 5 due to the surface ridge building to the north and as the storm whirls into the northeast quadrant of the cut-off upper vortex. For intensity... Tammy is expected to strengthen further in the next 24 hours as the storm now has an organized thunderstorm core and will be compact enough to avoid the approaching shearing southwesterly winds associated with the upper trough. The effects of the shear may be mitigated as the storm begins accelerating northeastward more parallel to the upper winds and becomes aligned with the supportive divergence zone of the upper trough... however given the amount of shear shown through 72 hours I do project Tammy to dip below hurricane force. By 96 hours the shear over Tammy reduces while upper divergence simultaneously increases as the aforementioned southwestern upper trough fragment evolves into a cut-off upper vortex... and so I forecast Tammy to re-acquire hurricane strength by then. The combination of water temps below 26 deg C and ingestion of cooler drier air associated with the upper vortex should cause Tammy to begin losing tropical characteristics by 120 hours... and weakening is shown as the storm whirls into the northeastern part of the upper vortex where upper divergence is less. Should the storm completely whirl into the center of the upper vortex after 120 hours... the vortex is not forecast to be cold enough for instability and thunderstorms in the 24 deg C sea surface temperature environment... therefore a complete loss of tropical characteristics and continued gradual weakening in an environment lacking upper divergence should continue after 120 hours if the current forecast trends hold.
Regarding impacts to land areas:
(1) Coastal storm surge... heavy rainfall... and gusty winds affecting the northern parts of the Lesser Antilles island chain will reduce over the next 24 hours at the storm lifts northward and away. Coastal surf may linger through 48 hours until the storm moves further away.
(2) Coastal surf affecting the Virgin Islands and Puerto Rico should also gradually reduce over the next 48 hours as the storm lifts northward away from the region.
(3) Tammy may begin meandering east of Bermuda beneath an upper vortex by 5+ days as discussed above... which could result in a period of surf reaching the shores of Bermuda during that timeframe.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Oct 21)... 85 mph maximum sustained wind hurricane centered just east of Antigua and Barbuda at 17N-61.3W
IOH 24 Hr Forecast (1800Z Oct 22)... 95 mph maximum sustained wind hurricane centered north of the northern Lesser Antilles at 20N-62.5W
IOH 48 Hr Forecast (1800Z Oct 23)... 80 mph maximum sustained wind hurricane centered in the western Atlantic at 24N-61.5W
IOH 72 Hr Forecast (1800Z Oct 24)... 70 mph maximum sustained wind tropical storm centered in the central Atlantic at 26.5N-59.8W
IOH 96 Hr Forecast (1800Z Oct 25)... 75 mph maximum sustained wind hurricane centered in the central Atlantic at 30.5N-57.5W
IOH 120 Hr Forecast (1800Z Oct 26)... 70 mph maximum sustained wind tropical storm centered in the central Atlantic at 34N-57.5W
******National Hurricane Center (hurricanes.gov) official forecast as of 5 PM EDT***************************
Peak Strength (0600Z Oct 23)... 100 mph maximum sustained wind hurricane centered north of the northern Lesser Antilles at 20.9N-63.5W
5-Day Position (1800Z Oct 26)... 70 mph maximum sustained wind tropical storm centered in the western Atlantic well south-southeast of Bermuda at 28N-62.6W
AREA OF INTEREST #48... A quasi-stationary tropical low pressure continues on in the southern Caribbean Sea... in the vicinity of 11.2N-80W... with the support of outflow provided by a regional tropical upper ridge. The southwestern convergence zone of the current upper trough departing the eastern US is producing a surface ridge over northern Central America and the Gulf of Mexico that continues preventing the westward progress of the 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. An increasing westward drift of the tropical low into southern Central America... under the influence of the forecast eastern US surface ridge... is anticipated over the next 48 hours. In this update I am lowering my peak odds of tropical cyclone formation to 30% as this system has not made any progress in developing further... therefore the potential for cyclone formation before landfall is reducing.
Regardless of tropical cyclone formation or not... periods of heavy rainfall are possible for Costa Rica... Panama... and Nicaragua over the next few days.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Oct 22)... 15% chance of tropical cyclone formation (east of the Nicaragua/Costa Rica border near 11.2N-81W)
IOH 48 Hr Outlook (1800Z Oct 23)... 30% chance of tropical cyclone formation (southeast coast of Nicaragua near 11.2N-83.8W)
IOH 72 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... 20%
Formation chance through 7 days... 20%
AREA OF INTEREST #49... Satellite imagery over the last few days suggests a low-latitude tropical wave of low pressure has been moving quickly westward under the influence of the central Atlantic deep-layer ridge from the eastern tropical Atlantic to 10N-47.5W as of 0000Z tonight. The latest NHC TAFB surface analyses is marking the tropical wave as a surface trough. Although there are also a variety of suppressing upper vorticity features descending southward toward the tropical latitudes under the influence of the deep-layer ridge's east side... the tropical wave has outpaced those features and has maintained an expansive area of showers and thunderstorms and could soon take advantage of the same upper ridge cell (upper outflow region) that helped Tammy develop. Therefore I have added this tropical wave as an area of interest for further development... and is the forty-ninth tropical Atlantic area of interest I have tracked on this site this year.
Forecast track shows a gradual turn to the north in the vicnity of the Lesser Antilles island chain as this system moves around the southwest side of the central Atlantic deep-layer ridge and chases the surface ridge weakness associated with Tammy. Through 96 hours I gradually ramp up to a peak 20% odds of tropical cyclone formation... I have selected with odds above 10% even though there is a lack of computer model support showing development due to the favorable upper ridge cell that will be persisting over and just east of the Lesser Antilles island chain. By 120 hours I begin to lower development odds from the peak as this system encounters increased southwesterly shear on the southeast side of the upper vortex that Tammy is forecast to also become entangled with (more info on this upper vortex is in the above Tammy forecast discussion). Regardless of tropical cyclone formation or not... the Lesser Antilles may be exposed to heavy rains and gusty winds from this system not long after Tammy departs and by Tuesday.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z Oct 23)... 0% chance of tropical cyclone formation (central tropical Atlantic near 11N-52W)
IOH 48 Hr Outlook (0000Z Oct 24)... 5% chance of tropical cyclone formation (just east-southeast of Barbados near 12.5N-58.8W)
IOH 72 Hr Outlook (0000Z Oct 25)... 15% chance of tropical cyclone formation (just east of the northern Lesser Antilles near 16N-59.5W)
IOH 96 Hr Outlook (0000Z Oct 26)... 20% chance of tropical cyclone formation (east-northeast of the northern Lesser Antilles near 19N-59.5W)
IOH 120 Hr Outlook (0000Z Oct 27)... 15% chance of tropical cyclone formation (central Atlantic near 22.5N-59.5W)
*****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/).
1200Z (Oct 21) CMC Model Run...
**For Hurricane Tammy... moves across the northern Lesser Antilles through 18 hours... subsequently moves northwest and then northeast and reaches 30N-60.5W through 120 hours.
**For area of interest #48... no development shown
**For area of interest #49... no development shown
1200Z (Oct 21) ECMWF Model Run...
**For Hurricane Tammy... moves across the northern Lesser Antilles through 24 hours... south part of upper trough now departing from eastern US becomes a cut-off upper vortex with Tammy whirling cyclonically beneath the upper vortex and weakening to a remnant low near 20.5N-66.2W through 120 hours due to a lack of divergence at the vortex's core.
**For area of interest #48... no development shown
**For area of interest #49... no development shown
1200Z (Oct 21) GFS Model Run...
**For Hurricane Tammy... center passes over the northeastern-most Lesser Antilles through 21 hours... subsequently moves north-northwest and then northeastward and reaches 31.8N-57.5W through 120 hours.
**For area of interest #48... develops into a compact tropical cyclone making landfall over the southeast corner of Nicaragua at 54 hours... dissipates due to landfall and while over southwestern Nicaragua through 78 hours
**For area of interest #49... no development shown
1200Z (Oct 21) NAVGEM Model Run...
**For Hurricane Tammy... moves across the northern Lesser Antilles through 18 hours... subsequently moves north-northwest and then northeast and reaches 35N-53.5W at 120 hours.
**For area of interest #48... no development shown
**For area of interest #49... no development shown