*******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 1 2023 11:00 AM EDT...
The birdseye view chart below has been updated to now include the surface analysis from 0600Z September 30 and upper level analysis from 1200Z September 30... which represents the setup of the atmosphere at the time I completed the forecasts in the full update below.
...SATURDAY SEPTEMBER 30 2023 8:20 PM EDT...
Note the usual surface analysis and upper air charts for the above birdseye view chart are still being assembled and will be released within the next several hours. This update is released now without those parts of the chart to ensure a timely release of this update.
The primary focus for the Atlantic tropics at this time are a pair of central Atlantic tropical cyclones interacting with each other... see Tropical Storm Philippe and Rina sections below for more details on the two cyclones. See area of interest #42 section below for more information on possible subtropical development offshore of the eastern United States and northwest of Bermuda over the next couple of days.
TROPICAL STORM PHILIPPE... Since my previous update on Philippe on Thursday... the tropical storm has been drifting southwestward on Friday and today while remaining located to the east of the northern Lesser Antilles. The tropical storm also briefly dipped to a strength of 45 mph maximum sustained winds but has since recovered and held 50 mph maximum sustained winds. The southwest drift is likely due to the fujiwhara interaction with Tropical Storm Rina now passing to the northeast... and also as Philippe may be trying to regenerate toward its thunderstorm mass which has been lopsided to the southeast of its center where the outflow of the central tropical Atlantic upper ridge cell has been optimal.
For the next 24 hours... it appears Philippe is setup for a stall as I predicted in the previous update while tugged by both Rina and the surface ridge weakness associated with Area of Interest (AOI) #42... its just that the stall is going to occur further east as Philippe is east of the previous forecast. In the upper-levels... warm southerly flow ahead of AOI #42 will continue to support a mid-latitude warm core upper ridge that moves from the western to central Atlantic. So far the western part of the nearby upper vorticity string has retrograded westward and away into the central Caribbean and waters northeast of the eastern Bahamas under the influence of the mid-latitude upper ridge... while the northeastern part of the upper vorticity string has been pushed eastward to a position northeast of Philippe by the inflating mid-latitude upper ridge and southwest side of the current north Atalntic upper trough. Both parts of the upper vorticity have been far enough away from Philippe to not detrimentally affect the storm... however in the next 24 hours models indicate the mid-latitude upper ridge will shove the portion of upper vorticity northeast of the eastern Bahamas southeastward toward Philippe. Thus I forecast some weakening of Philippe as the incoming upper vorticity squashes thunderstorm development in Philippe's north half.
After 24 hours... the models have now trended with Philippe having a longer life by having Philippe's thunderstorm latent heat release weaken the disrupting upper vorticity. During this period Philippe accelerates northward in the surface ridge weakness created by Rina and AOI #42. The global models agree on Philippe becoming a hurricane during this time as Philippe becomes exposed to lower shear beneath the mid-latitude upper ridge described in the prior paragraph. However I agree with the NHC official forecast from 11 AM EDT today which showed the hurricane not exceeding an intensity above 85 mph maximum sustained winds as Philippe will tend to be toward the west side of the upper ridge instead of being tucked underneath it... exposing Philippe to some westerly shear from the weakening upper vorticity... followed by some westerly shear from the upper trough associated with AOI #42. Between 96 and 120 hours I show Philippe weakening while the storm is turned eastward by the upper trough... as the storm is exposed to stronger westerly shear while moving closer to the trough. During this time the upper trough will be splitting into southwestern and northeastern halves... with Philippe potentially caught in linear upper westerlies on the south side of the northeastern upper trough that have a lack of divergence... and the lack of divergence would also contribute to the weakening. However note that after the 120-hour period... Philippe's northwestern quadrant would have potential to pull down cold air associated with the upper trough to make a local upper trough with a divergence zone that would help Philippe continue on as a non-tropical frontal cyclone continuing eastward toward the direction of the Azores.
With the updated forecast below... impacts to the northeastern Caribbean islands will be limited to the northern Lesser Antilles over the next 48 hours and in the form of possible coastal surf.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Sep 30)... 50 mph maximum sustained wind tropical storm centered east of the northern Lesser Antilles at 16.9N-56.3W
IOH 24 Hr Forecast (1200Z Oct 1)... 45 mph maximum sustained wind tropical storm centered east of the northern Lesser Antilles at 17N-56W
IOH 48 Hr Forecast (1200Z Oct 2)... 60 mph maximum sustaind wind tropical storm centered east-northeast of the northern Lesser Antilles at 19N-56W
IOH 72 Hr Forecast (1200Z Oct 3)... 75 mph maximum sustained wind hurricane centered in the central Atlantic at 22.5N-56W
IOH 96 Hr Forecast (1200Z Oct 4)... 85 mph maximum sustained wind hurricane centered in the central Atlantic at 27.5N-56W
IOH 120 Hr Forecast (1200Z Oct 5)... 70 mph maximum sustained wind tropical storm centered in the central Atlantic at 32.5N-50W
******National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT***************************
5-Day Position (1200Z Oct 5)... 85 mph maximum sustained wind hurricane centered in the central Atlantic at 28.5N-57W
TROPICAL STORM RINA... Infrared satellite image shortly after Rina formed on September 28... as of 1630Z that day. The center of Philippe is marked with the western (left) yellow +... and the center of Rina is marked with the eastern (right) yellow +:
The tropical wave of low pressure that has been chasing Philippe became Trpoical Storm Rina on 11 AM EDT September 28. Since then... Rina has accelerated northwestward while in a fujiwhara interaction with Philippe. Both Rina and Philippe are sharing the same central tropical Atlantic upper ridge cell for reliance of upper outflow... but it is clear that Philippe has the upper hand on Rina as Philippe's outflow has been shearing Rina's thunderstorms southeast of Rina's center. Despite this Rina over-achieved and reached 50 mph maximum sustained winds at 11 PM EDT September 29 as seen by a satellite scan of winds. The shear has more recently overwhelmed Rina as evidenced by Rina degenerating into a shallow cloud swirl that lacks any thunderstorms nearby... and the intensity has been downgraded to 45 mph maximum sustained winds in recent hours. The northwesterly shear will be exacerbated by upper vorticity northeast of the eastern Bahamas getting pushed toward Rina and Philippe in the next 24 hours... and I forecast Rina to dissipate into a remnant low in the short term. After 24 hours... the circulation of Rina is expected to curve northward and then northeastward in the flow ahead of Area of Interest (AOI) #42 and its upper trough... and during this time I forecast Rina to make a comeback. At around 30 hours the comeback would be helped by passage beneath the mid-latitude upper ridge axis noted in the above Philippe forecast discussion where wind shear is lower... and by 48 hours Rina could be boosted by divergence on the southeast side of the upper trough associated with AOI #42. I forecast only slow strengthening through 72 hours due to shear imparted by the upper trough. By 96 hours the upper trough will be splitting into southwestern and northeastern halves... with Rina caught in linear upper westerlies on the south side of the northeastern upper trough that have a lack of divergence... thus I forecast Rina to collapse around that time under the combo of shear and lack of upper divergence.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Sep 30)... 45 mph maximum sustained wind tropical storm centered in the central Atlantic at 21.2N-50.4W
IOH 24 Hr Forecast (1200Z Oct 1)... Remnant low centered in the central Atlantic at 24N-55W
IOH 48 Hr Forecast (1200Z Oct 2)... 35 mph maximum sustained wind tropical depression centered in the central Atlantic at 27.5N-56W
IOH 72 Hr Forecast (1200Z Oct 3)... 45 mph maximum sustained wind tropical storm centered in the central Atlantic at 33N-53.5W
IOH 96 Hr Forecast (1200Z Oct 4)... Dissipating remnant low in the central Atlantic centered at 35N-46W
******National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT***************************
Loss of Tropical Cyclone Status (0000Z Oct 2)... 35 mph maximum sustained remnant low centered in the central Atlantic at 25N-55.3W
AREA OF INTEREST #42... An upper trough has been progressing from the Great Lakes region of North America... across the northeastern US... and now heading into the northwestern Atlantic. Divergence on the east side of this upper trough has triggered an offshore broad frontal low pressure. Over the next 24 hours the upper trough is likely to become a cut-off upper vortex that stacks over the surface frotnal low pressure as the northwest quadrant of the frontal low pulls cold air associated with the trough southward which will contribute to amplification of the upper trough... and also due to ongoing amplified upper ridging over Canada. The cut-off upper vortex will provide a low shear environment over the surface frontal low... and combined with sufficiently warm waters will watch to see if the surface frontal low acquires core thunderstorms and tropical characteristics in the next 24 hours. However I assign a low 10% chance of this happening as so far no thunderstorm activity has developed near the frontal low's center. Beyond 24 hours... the strength of the ongoing surface ridge protruding from eastern Canada will tend to keep the surface low from accelerating eastward with the upper vortex... thus the surface low is forecast to end up beneath the west side of the upper vortex where it will be pulled southeastward by the vortex. And with suppressing upper convergence beneath the west side of the upper vortex... subtropical development will no longer be possible and thus I end the outlook at 48 hours by dropping development odds to 0%. Beyond 48 hours... noting the upper vortex will merge with upper vorticity diving southeastward from its current northern Canada position... creating a southwest-northeast tilted upper trough. The elongated and tilted upper trough's divergence zone will result in a southwest-to-northeast elongation of whatever remains of the surface low pressure circulation. In the long range... the tilted upper trough splits into southwestern and northeastern halves... with the divergence zone of the northeastern upper trough transforming a part of the elongated surface circulation into a frontal cyclone that moves east-northeast into the north Atlantic.
Noting the pressure gradient between the current surface frontal low offshore of the eastern US and surface ridge protruding from eastern Canada is still driving winds blowing toward shore... resulting in coastal surf for parts of the eastern US. Coastal flood advisores are in effect for southwestern Connecticut... southeastern New York including Long Island... New Jersey... Delaware... southeastern Maryland... and southeastern Virgnia as of this writing. As the surface low moves southeast through 48 hours as discussed above... surf generated by this system may later reach the shores of Bermuda.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Oct 1)... 10% chance of subtropical cyclone formation (offshore of the eastern United States near 37.5N-69W)
IOH 48 Hr Outlook (1200Z Oct 2)... 0% chance of subtropical cyclone formation (north of Bermuda near 36N-65W)
******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/).
0000Z (Sep 30) CMC Model Run...
**For Tropical Storm Philippe... moves slowly west-northwest to a position just northeast of the northern Lesser Antilles and near 19N-60.5W through 60 hours... subsequently accelerates north-northeast while becoming a hurricane and reaches 28N-58.8W at 120 hours
**For Tropical Storm Rina... moves northwest to 25N-54W through 36 hours while weakening to a remnant low... remnant low subsequently accelerates northeast in flow ahead of area of interest #42 and transitions into a remnant frontal cyclone that passes just north of the Azores by 120 hours.
**For Area of Interest #42... broad frontal low currently offshore of the eastern US moves east-northeast to 37.5N-67.5W through 30 hours... subsequently moves southeast to Bermuda through 84 hours where it then dissipates.
0000Z (Sep 30) ECMWF Model Run...
**For Tropical Storm Philippe... moves west-southwest and then west-northwest to a position just northeast of the northern Lesser Antilles and near 18N-60.5W through 60 hours while holding on to tropical storm status... subsequently moves northwest while weakening to a tropical depression and then regaining tropical storm status and reaches 23.5N-64.5W at 126 hours
**For Tropical Storm Rina... moves northwest to 23.5N-53.5W through 30 hours while weakening to a remnant low... remnant low then moves north and becomes absorbed by area of interest #42 at 72 hours while located near 31N-53.5W
**For Area of Interest #42... broad frontal low currently offshore of the eastern US splits into a southwestern and northeastern half at 54 hours while located near 37.5N-61W... northeastern half dominates while becoming a frontal cyclone that moves east-northeast into the north Atlantic while southwestern half becomes a cold front attached to the frontal cyclone.
1200Z (Sep 30) GFS Model Run...
**For Tropical Storm Philippe... drifts east then north through 30 hours in an initial fujiwhara interaction with Rina and reaches 17.5N-55W as a strengthening tropical storm... while becoming a hurricane accelerates north-northeastward and reaches 32.5N-50W at 120 hours.
**For Tropical Storm Rina... turns northwest and then northward and weakens to a remnant low near 29.5N-56.5W at 54 hours... remnant low becomes absorbed by area of interest #42 while located near 34N-51W at 72 hours
**For Area of Interest #42... broad frontal low currently offshore of the eastern US splits into a southwestern and northeastern half at 48 hours while located near 36N-60W... southwestern half quickly dissipates while northeastern half continues east-northeast into the north Atlantic as a frontal low.
0600Z (Sep 30) NAVGEM Model Run...
**For Tropical Storm Philippe... moves northwest to 19.5N-57.5W at 54 hours while strengthening... while becoming a hurricane accelerates north-northeast and then northeastward and reaches 35N-44.5W at 120 hours
**For Tropical Storm Rina... moves northwest to 24N-53.5W at 24 hours while weakening to a remnant trough... remnant trough accelerates northeast in the flow ahead of area of interest #42 such that it passes just north of the Azores by 120 hours
**For Area of Interest #42... broad frontal low currently offshore of the eastern US splits into a southwestern and northeastern half at 66 hours while located near 39N-55W... through 102 hours the northeastern half dominates while becoming a frontal cyclone that moves east-northeast into the north Atlantic while southwestern half becomes a cold front attached to the frontal cyclone.