*******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...THURSDAY OCTOBER 26 2023 9:59 PM EDT...
The birdseye view chart below has been updated to include the surface and upper air analysis for 0600Z October 25 which were effective at the time the forecasts and discussions below were created.
Regarding Hurricane Tammy... located in the waters east of Bermuda... from 11 AM EDT October 25 to 5 AM EDT October 26 the hurricane weakened from its peak of 105 mph maximum sustained winds to 85 mph maximum sustained winds while transitioning into a non-tropical frontal cyclone. The transition occurred as the hurricane lost its eye and warm core of thunderstorms while ingesting cooler drier air associated with the adjacent upper vortex to the west and also while the hurricane moved into cooler waters below 26 deg C. Shortly after the transition occurred... the remnant cyclone of Tammy has already gone on to re-develop thunderstorms near the center likely due to the amount of upper divergence in the system's environment. As such the National Hurricane Center (NHC) is already monitoring the remnant cyclone for transition into a subtropical cyclone or back into a fully-tropical cyclone... and as of the NHC 8 PM EDT outlook the odds of subtropical or tropical cyclone formation have already been raised to 50%. Regardless of the cyclone's status (non-tropical... tropical... etc.)... interests in Bermuda should be aware that it will swing westward toward the area through this weekend as discussed in the Tammy section below... bringing with it increased surf and gusty winds.
...WEDNESDAY OCTOBER 25 2023 11:35 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 abruptly strengthened into a category 2 hurricane this morning and is a concern for Bermuda as the forecast track hooks this system toward the area by this weekend... see Tammy section below for more information.
Elsewhere noting the following disturbances:
(1) A currently weak tropical wave of low pressure over the southern Lesser Antilles could take advantage of better upper wind conditions over the next few days... potentially brining periods of heavy rainfall across the northeastern Caribbean islands. See area of interest #49 section below for more information.
(2) The remnants of tropical depression twenty-one have re-organized over the far eastern Pacific after crossing southern Central America and have a high chance of becoming an eastern Pacific tropical cyclone. Because this site is focused on Atlantic tropical activity... refer to the NHC site hurricanes.gov (hurricanes dot gov) for the latest on this system and other eastern Pacific activity. However will carry statments regarding impacts to land areas from the eastern Pacific remnants of the depression on the home page bulletins of this site. There are some long-range model runs that show the remnants of the depression re-curving northeastward toward the direction of the southwestern Gulf of Mexico or western Caribbean Sea by day 7 under the influence of a frontal system moving across eastern North America. The bulk of the frontal system's surface layer will be focused across the Great Lakes and eastern Canada in 72+ hour window rather than the tropical latitudes... therefore I currently assess there is a low probability of the remnants of the depression coming back into the Atlantic basin as an area of interest. Instead the more likely scenario is the remnants continue westward across the eastern Pacific as a tropical cyclone while steered by a strong surface ridge that builds over western North America in the wake of the frontal system.
HURRICANE TAMMY... Tammy has continued to lift northeastward into the central Atlantic from the eastern Atlantic while chasing the surface ridge weakness associated with the current north Atlantic frontal cyclone and its trailing surface front. In the upper-levels the northeastward push is also being provided by the current western Atlantic upper trough. It was previously speculated that Tammy would weaken from southwesterly shear imparted by the upper trough... instead the thunderstorm latent heat release has maintained warm core upper anticyclonic outflow that continues to shield the hurricane from the shear. In addition the southerly jet on the east side of the upper trough and north of Tammy has helped enhance Tammy's poleward (northward) outflow. All of the above has allowed the hurricane to abruptly intensify into a category 2 with 100 mph maximum sustained winds as of this morning. The previous forecast track has done well and so the updated one below is similar and is based on the following events happening... (1) the current north Atlantic frontal cyclone and north half of the western Atlantic upper trough are now leaving Tammy behind... (2) the western convergence zone of the northern upper trough fragment followed by the convergence zone of upper vorticity that ejects from the current north Canada upper vortex is expected to build a surface ridge to the north of Tammy that will be preventing the storm from continuing northeastward out to sea... (3) a westward turn toward Bermuda is expected through 72 hours due to the combined effects of the surface ridge and southwestern upper trough fragment which evolves into a cut-off upper vortex with time. For intensity... the forecast is notably increased due to Tammy's recent bout of strengthening. Over the next 24 hours Tammy will be in an environment of reduced shear potential and increased divergence as the western Atlantic upper trough splits into its northern and southwestern halves... with the neighboring southwestern half of the trough becoming a cut-off upper vortex. While I do call for additional strengthening during this timeframe... I do not bring Tammy above a top-end category 2 as the forecast track brings Tammy toward the marginally favorable 26 deg C sea surface temperature isotherm. After 24 hours the combination of tracking into 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.
Regarding what will be the non-tropical remnant cyclone of Tammy... the latest model guidance continues trending with a less clean split of the western Atlantic upper trough... with some of the northern half of the upper trough lingering to the north of Tammy and the upper vortex. This could allow for the forecast remnant cyclone to be embedded in split flow upper divergence between the north side of the upper vortex and southwesterlies streaming into the lingering upper trough instead of Tammy being suppressed by a lack of divergence in the north side of the upper vortex. This means the remnant cyclone has more potential to be vigorous and could hang at or near hurricane force for some time. The weakening shown between 24 and 48 hours is associated with Tammy losing its thunderstorms and warm core outflow... but is gradual to reflect the now-forecast split flow upper divergence environment. The lingering upper trough in the remnant cyclone's environment also has implications on the forecast track. Instead of the remnant cyclone continuing westward on the south side of the surface ridge and north side of the upper vortex... the models show the warm sector of a frontal system pivoting through the North American Great Lakes region amplifying the current warm core Gulf of Mexico upper ridging into the northwest Atlantic which pushes the lingering upper trough southeastward... and in turn this could mean the remnant cyclone after 72 hours instead reverses southeastward away from Bermuda and into the open central Atlantic while pulled by the lingering upper trough. In this scenario models insist that the upper trough will move faster than the remnant cyclone which puts the cyclone in suppressive convergence between the northwest side of the upper trough and southeast side of the amplifying upper ridging... causing the remnant cyclone to weaken from 72 to 120 hours. Beyond 120 hours the remnant cyclone... if it survives the weakening phase... could make an attempt at coming back as a tropical system while the southeastward drift puts the system back into 26 deg C waters and as the amplifying upper ridging overspreads this system with lower shear and upper outflow. Long story short... the remnant cyclone of Tammy could have a long life.
Coastal surf will likely begin for Bermuda over the next 24 hours. If the current forecast holds... interests in Bermuda should be aware the remnant cyclone of Tammy would be bringing a coastal storm surge and gusty winds with damage potential by this weekend.
Update as of 11 AM EDT... Tammy has strengthened further to 105 mph maximum sustained winds.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0600Z Oct 25)... 100 mph maximum sustained wind hurricane centered in the western Atlantic at 25.6N-60.2W
IOH 24 Hr Forecast (0600Z Oct 26)... 110 mph maximum sustained wind hurricane centered in the central Atlantic at 30.2N-58.5W
IOH 48 Hr Forecast (0600Z Oct 27)... 90 mph maximum sustained wind hurricane centered east-northeast of Bermuda at 33N-60W
IOH 72 Hr Forecast (0600Z Oct 28)... Remnant non-tropical cyclone centered just east-northeast of Bermuda at 33N-63W
******National Hurricane Center (hurricanes.gov) official forecast as of 5 AM EDT***************************
Peak Strength (1800Z Oct 25)... 105 mph maximum sustained wind hurricane centered in the central Atlantic at 27N-59.4W
Loss of Tropical Cyclone Status (0600Z Oct 26)... 85 mph maximum sustained wind non-tropical cyclone centered in the central Atlantic at 28.9N-59.3W
AREA OF INTEREST #49... The tropical wave of low pressure in the vicinity of the southern Lesser Antilles continues to have a low-level signature... albeit a weak one as of late... on the CIMSS 850 mb vorticity product (https://tropic.ssec.wisc.edu/real-time/windmain.php?basin=atlantic&sat=wg8&prod=vor&zoom=&time=). On satellite pictures the wave continues to lack thunderstorm activity. Over the next 72 hours the wave is expected to generally shift northwestward due to the surface ridge weakness associated with Tammy and while transitioning into a surface trough supported by the southeastern divergence zone of the elongated upper vortex that will be interacting with Tammy (more info on the upper vortex is in the above Tammy forecast discussion). It is possible this system re-develops thunderstorm activity and becomes a bonadfide tropical disturbance while taking advantage of the aforementioned upper divergence zone... therefore I am continuing to mark the currently weak disturbance as an area of interest but assign a low 10% peak odds of tropical cyclone formation for the 5-day forecast period.
After 72 hours the disturbance is likely to bend more westward in track while coming under the influence of a surface ridge supported by the western convergence zone of the elongated upper vortex. Due to prolonged isolation from high-latitude cold air... the cold core upper vortex is expected to weaken during this time and become replaced by an eastern Caribbean upper anticyclone providing low shear and upper outflow... and this is when I assign the peak odds of tropical cyclone formation. Because this system has not yet moved north... the updated forecast track points are shifted southwestward and takes it cues from recent GFS model runs which insist on showing some development of this system. The updated forecast track could mean periods of heavy rainfall for the northeastern Caribbean islands (Lesser Antilles... Virgin Islands... Puerto Rico... the Dominican Republic... and Haiti) within the next few days... particularly if this system goes on to re-develop widespread thunderstorm activity.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0600Z Oct 26)... 0% chance of tropical cyclone formation (eastern Caribbean just west of the Lesser Antilles near 13.5N-62W)
IOH 48 Hr Outlook (0600Z Oct 27)... 0% chance of tropical cyclone formation (eastern Caribbean near 14N-64W)
IOH 72 Hr Outlook (0600Z Oct 28)... 0% chance of tropical cyclone formation (south-southwest of Puerto Rico near 15.5N-67.5W)
IOH 96 Hr Outlook (0600Z Oct 29)... 10% chance of tropical cyclone formation (south of the Dominican Republic near 16.2N-69W)
IOH 120 Hr Outlook (0600Z Oct 30)... 10% chance of tropical cyclone formation (south-southwest of the Haiti/ Dominican Republic border near 16.2N-72.5W)
*****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 25) CMC Model Run...
**For Hurricane Tammy... moves north-northeast and reaches 27N-59.5W at 18 hours... subsequently turns northwest and west and passes just south of Bermuda at 60 hours and reaches 31.2N-70W at 120 hours while quickly weakening.
**For area of interest #49... no development shown
**Remnants of Tropical Depression Twenty-One redevelop as an eastern Pacific tropical cyclone near 11.5N-89W at 60 hours... tropical cyclone drifts northwest into the Guatemala/El Salvador border through 120 hours... remnant low of the tropical cyclone moves north-northeast into the far western Caribbean waters just offshore of the Belize/Mexico border through 168 hours
0000Z (Oct 25) ECMWF Model Run...
**For Hurricane Tammy... moves north-northeast and reaches 28N-59W at 24 hours... subsequently shifts northwest and then north into the waters just east of Bermuda through 72 hours... reverses to an east and then east-southeast track through 120 hours while weakening and reaches 30N-54.5W
**For area of interest #49... no development shown
0000Z (Oct 25) GFS Model Run...
**For Hurricane Tammy... moves north-northeast and reaches 29N-58.5W at 24 hours... drifts northwest into the waters just east of Bermuda through 81 hours... subsequently reveres to an east-southeast drift and reaches 30.5N-58.8W through 126 hours
**For area of interest #49... develops into a rather broad central Caribbean tropical low near 15N-71W through 111 hours... drifts north across Haiti and the Dominican Republic and the center of the broad system reaches the waters just east of the Bahamas through 168 hours
0000Z (Oct 25) NAVGEM Model Run...
**For Hurricane Tammy... moves north-northeast and reaches 28.5N-59.5W at 24 hours... subsequently turns west-northwest with the center passing just south of Bermuda through 72 hours... reverses to an east-southeast track and reaches 29.5N-58.5W through 120 hours.
**For area of interest #49... no development shown
**Remnants of Tropical Depression Twenty-One redevelop as an eastern Pacific tropical cyclone near 9N-88.8W at 42 hours... through 138 hours tropical cyclone continues northwest and then north into southeastern Mexico and just west of Guatemala... remnant low of the tropical cyclone then reverses to an east-northeast track that takes it into the western Caribbean waters just north of Honduras through 168 hours