*******Note that forecasts 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.**********
...THURSDAY MAY 20 2021 11:58 AM EDT...
A subtropical or tropical storm is expected to form in the waters just east of Bermuda over the next day or so. See area of interest #1 section below for additional details.
AREA OF INTEREST #1...As expected...the current broad western Atlantic upper trough is amplifying into a cut-off cold core upper vortex due to adjacent amplification of a warm upper ridge over eastern North America bolstered by warm surface southerly flow between the current eastern US surface ridge and a series of frontal systems pivoting across western and northern North America. The western Atlantic surface trough has intensified into a frontal low while below the eastern divergence maximum of the developing upper vortex...and is therefore expected to absorb the current northwestern Atlantic frontal low and become the dominant feature. Dynamics are in place for the western Atlantic frontal low to acquire tropical characteristics as the southward cold air delivery from the secondary northwest Atlantic frontal low will make upper air temperatures cold enough for instability and thunderstorms...despite the western Atlantic frontal low being a little further north (towards 34N latitude) than I forecasted yesterday and being over 20 deg C waters. Since special update #1A issued yesterday evening...all of the global models (CMC...NAVGEM...GFS...ECMWF) continue to agree that the western Atlantic frontal low will transition into a subtropical cyclone...therefore I have begun a subtropical cyclone formation forecast as outlined below.
In the short-term...the track of this system is expected to hook westward towards the general direction of Bermuda due to the blocking effect of the deep-layered warm ridge to the north and west and as this system whirls into the center of the cut-off upper vortex as we typically see in the early phases of subtropical development. Overall the GFS and CMC propel this system further west than the ECMWF and NAVGEM while suggesting in the short-term that a compact tropical core will develop on the north side of the currently broad western Atlantic frontal low...with this tropical core being ejected westward in the tight pressure gradient between the north side of the broad low pressure field and south side of the deep-layered ridge. My forecast track is not as far west as the GFS and CMC as there are no current signs of a northern compact tropical circulation developing...so I prefer the NAVGEM/ECMWF idea of the current broad surface low pressure field gradually consolidating and more gradually whirling westward into the center of the cut-off upper vortex. However even the ECMWF eventually joins the GFS and CMC in showing a compact and strong tropical core developing at some point. I support this idea by 48 hours when my forecast track brings the circulation toward the slighlty warmer 22 deg C south edge of the Gulf stream...while the cut-off upper vortex is still shown to be a rather cold 1190 dekameters in height at 200 mb. On October 27 2019 Tropical Storm Pablo was able to reach hurricane strength with a similar upper air temperature but with even cooler waters just below 20 deg C...so I do think that by 48 hours there is a high likelihood of this system developing a compact tropical core of thunderstorms with warm core upper outflow driven by the thunderstorm latent heat release being present below the upper vortex that would help intensify this system to strong tropical storm strength...especially given that the GFS...CMC...and ECMWF continue to show this at some point in their latest model runs.
I keep my forecast peak intensity through 72 hours as this system is expected to remain generally stalled above the south edge of the Gulf stream and below the cut-off upper vortex where its upper-level temperature will still remain at a cold de-stablizing level of around 1195 dekameters at 200 mb. Albeit I have some norhtheastward drift in the forecast track at that time as the upper vortex begins to get knocked northeastward by the approach of the next frontal system/upper trough to eject from eastern Canada. The ECMWF...NAVGEM...and CMC are more north in their long-term northeast forecast track while the GFS has a more south solution attributed to the north-south elongation of the upper vortex expected to occurr from the strength of the warm deep-layered ridge to the west...with the GFS having the southern lobe of the elongated upper vortex tugging this system more south. My long-term forecast is a compromise between these two camps...with eventual absorption into the approaching eastern Canadian front by 120 hours. Prior to that...at 96 hours...I show some weakening due to possible southwesterly shear induced by the east side of the front's upper trough...and maintain tropical status as the forecast track keeps this system on the 22 deg C southern edge of the Gulf stream with upper air temperatures still remaining at a cold de-stabilizing 1190 dekameters in height on the east side of the front's upper trough.
On a final note...the forecast track keeps this system just east of Bermuda...therefore gusty damaging winds from this system are not expected over Bermuda at this time. However...expect coastal sea swells to increase through tomorrow and the weekend.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z May 20)....Frontal low centered near 34N-54W
IOH 24 Hr Forecast (1200Z May 21)...50 mph maximum sustained wind subtropical storm centered at 35N-58W
IOH 48 Hr Forecast (1200Z May 22)...65 mph maximum sustained wind tropical storm centered east-northeast of Bermuda at 34N-62W
IOH 72 Hr Forecast (1200Z May 23)...65 mph maximum sustained wind tropical storm centered at 36N-61W
IOH 96 Hr Forecast (1200Z May 24)...50 mph maximum sustained wind tropical storm centered at 39N-59W
IOH 120 Hr Forecast (1200Z May 25)...Absorbed by cold front near 42N-45W
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
0000Z CMC Model Run...For area of interest #1...strong subtropical cyclone formation suggested within the next 24 hours near 35N-56W...closest approach to Bermuda is at 72 hours when system briefly stalls at 34N-62.5W...accelerates northeastward and absorbed by cold front at 114 hours near 40N-51.5W.
0000Z ECMWF Model Run...For area of interest #1...strong subtropical cyclone formation suggested within the next 24 hours near 36N-56W...closest approach to Bermuda is at 48 hours when system briefly stalls at 32.5N-61W...accelerates north-northeastward into open northwest Atlantic and absorbed by cold front between 96 and 120 hours.
0600Z GFS Model Run...For area of interest #1...strong and compact subtropical cyclone formation suggested within the next 12 hours near 36N-55W...closest approach to Bermuda is at 42 hours when system briefly stalls at 32.5N-61.5W...cyclone briefly drifts northwest and stalls a second time just north of Bermuda near 34N-64W at 72 hours...cyclone turns east-northeastward across the open central Atlantic and maintains identity along incoming cold front through 120 hours.
0600Z NAVGEM Model Run...For area of interest #1...subtropical cyclone formation suggested within the next 24 hours near 35N-60W...closest approach to Bermuda is at 30 hours when system briefly stalls at 33.5N-61W...accelerates northeastward across the open northwest Atlantic and absorbed by cold front at 96 hours while located near 40N-53W.