*******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.**********
...THURSDAY MAY 25 2023 11:55 PM EDT...
Broad surface low pressure area beginning to materialize offshore of the southeastern United States... continuing to monitor this feature for possible acqusition of tropical characteristics through the weekend as it later pivots toward the Carolinas. See area of interest #4 section below for more information on this system.
As done on this site starting last year... I will be sequentially numbering up areas of interest for possible Atlantic tropical development throughout the year... resetting back to #1 at the start of each year. This scheme is so that each area of interest retains a numeric identity from update to update... which reduces confusion when simultaneous areas of interest begin and end when tropical activity increases during the Atlantic Hurricane Season. The current area of interest is tagged #4 as the first three were assigned in earlier birdseye view posts on this site.
AREA OF INTEREST #4... A broad surface low pressure area is beginning to materialize offshore of northeastern Florida with the support of increasing divergence on the east side of the amplifying base (southern part) of the longwave eastern North America upper trough. Meanwhile a strong surface ridge and amplifying upper ridge over the Great Lakes region of North America are beginning to become stacked into a classic warm core deep-layer ridge... the existence of this materializing adjacent deep-layer ridge has been responsible for the amplification of the longwave upper trough's base so far. In the next 24 hours the deep-layer ridge will ultimately cut-off the base of the upper trough from the rest of the trough... effectively resulting in a cut-off upper vortex centered over the southeastern US. The position of the upper vortex is expected to be near enough to the coast such that its eastern divergence zone will continue to support the offshore surface low pressure area. Given the warm 26 deg C Gulf stream waters and low shear environment underneath the upper vortex's east side... will continue to watch for the development of core thunderstorm activity and resulting possible tropical characteristics of the surface low (i.e. the latent heat release of the thunderstorms generating warm core upper outflow that further aids the surface low pressure's strength... in addition to the initial non-tropical support provided by the upper vortex's eastern divergence zone). There is also a chance the surface low pressure area does not acquire tropical characteristics if it ingests too much dry air to be generated by the western convergence zone of the upper vortex. To start with... the currently-forming surface low pressure area does not have core thunderstorm activity while the nearest activity is displaced well to the east (to the northeast of the Bahamas). In addition the reliable ECMWF as well as the CMC models have recently trended away from showing a circular and potentially tropical core when the surface low pressure reaches its peak strength... instead trending toward a more elongated non-tropical frontal low or frontal cyclone. The GFS meanwhile has remained consistent in showing a circular potentially tropical core... and the NAVGEM has recently joined the GFS albeit the NAVGEM is usually not the most reliable model. I have dropped my peak odds of subtropical cyclone formation to 20% in this update given the current lack of core thunderstorm activity and the less tropical apperance to this system in the more reliable model camps.
Regardless of acquisition of tropical characteristics or not... the track and impacts of the strengthening surface low pressure area will largely remain the same. This system is expected to pinwheel north and then westward in the flow around the east and north sides of the forecast southeastern US upper vortex... and around the south side of the anticyclonic Great Lakes deep-layer ridge. This well-defined steering pattern should allow the surface low to arc northwestard into the Carolinas... followed by arrival into the core of the inland upper vortex as typically seen with a post-mature subtropical or non-tropical surface low pressure system. Impacts for the southeastern US coast will begin as early as tomorrow (Friday) as the surface low pressure area gains an initially weak/broad structure in its ongoing formative phase... however their will be a tight pressure gradient between the northwest side of the broad surface low and south side of the Great Lakes deep-layer ridge. This gradient will drive a fetch of northeasterly onshore surface gusty winds and resultant coastal surf from the coastal Carolinas all the way to northeastern Florida. As this system gains a more consolidated frontal cyclone or subtropical cyclone structure... an area of increasing winds and coastal surf will likely develop for the coastal Carolinas by the latter part of this upcoming weekend. Periods of heavy rainfall are likely for coastal and inland areas of the Carolinas as well.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z May 27)... 5% chance of subtropical cyclone formation (offshore of the southeastern US near 31N-79W)
IOH 48 Hr Outlook (0000Z May 28)... 20% chance of subtropical cyclone formation (vicinity of Georgetown South Carolina near 32.5N-79.5W)
IOH 72 Hr Outlook (0000Z May 29)... 0% chance of subtropical cyclone formation (northwestern Georgia near 33N-84W)
******National Hurricane Center (hurricanes.gov) official outlook as of 8 PM EDT*****************************
Formation chance through 48 hours...10%
Formation chance through 7 days (168 hours)...10%
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z (May 25) CMC Model Run...
**For area of interest #4... broad frontal low forms offshore of northeastern Florida near 29.8N-77.5W at 30 hours... becomes elongated frontal cyclone offshore of the Georgia/South Carolina border and near 31.5N-79.5W through 66 hours... frontal cyclone weakens to a remnant frontal low that drifts north-northeast to Cape Hatteras North Carolina through 96 hours... reverses east-southeastward into offshore waters through 120 hours.
1200Z (May 25) ECMWF Model Run...
**For area of interest #4... broad frontal low forms offshore of northeastern Florida near 29.5N-77.5W at 42 hours... while strengthening into a frontal cyclone makes landfall on the central South Carolina coast at 66 hours... lifts northward across the Carolinas through 96 hours while de-generating into an elongated east-west remnant low.
1800Z (May 25) GFS Model Run...
**For area of interest #4... broad frontal low forms offshore of northeastern Florida near 30N-77.5W at 24 hours... strengthens into possible subtropical cyclone just offshore of the Georgia/South Carolina border and near 31.2N-79.5W at 45 hours... makes landfall on central South Carolina coast at 54 hours... after landfall curves northwestward and inland remnant low dissipates over northwestern South Carolina at 69 hours.
1200Z (May 25) NAVGEM Model Run...
**For area of interest #4... broad frontal low forms offshore of northeastern Florida near 29N-78.5W at 18 hours... strengthens into possible subtropical cyclone just offshore of the central South Carolina coast through 54 hours after which time it makes landfall... inland remnant low drifts north-northeast into eastern North Carolina through 78 hours... shifts east-northeast into offshore waters (offshore of southeastern Maryland) while transitioning into a more elongated remnant frontal low through 102 hours... through 114 hours the remnant frontal low loses its identity while the lowest pressure along the front becomes established further west over the North Carolina Outer Banks.