*******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.**********
...WEDNESDAY MAY 24 2023 8:20 PM EDT...
Global models are in increasing agreement on a subtropical low pressure area with cyclone development potential for the western Atlantic waters offshore of the southeastern United States... to pivot into the Carolinas... by Friday and the weekend. Daily birdseye view posts on the Atlantic tropics will continue on this site over the next few days to provide updates on this situation. See area of interest #4 section below for more information on the forecast subtropical low pressure 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... Global models are in increasing agreement on an upper air pattern conducive for the development of a subtropical low pressure area near the southeastern United States coast within the next few days... consisting of a triggering cold-core upper vortex consolidating over the inland southeastern US and a warm deep-layer ridge consolidating over the Great Lakes region. At present the northwestern convergence zone of a sprawling longwave upper trough over eastern North America is ushering in a southeastward-diving and strong central Canada surface ridge. Through day 4... the flow on the west side of the anticyclonic surface ridge will promote northward warm air transport and amplification of a warm core upper ridge over central Canada...with the surface and upper ridges eventually aligning over the Great Lakes region. The formation of the adjacent deep-layer ridge will in turn cut-off the south part of the current eastern North America longwave upper trough into a cut-off upper vortex over the interior southeastern US. The upper vortex is however expected to be positioned near enough to the Atlantic coast to allow for a regime of upper divergence and low shear over offshore warm Gulf stream waters... allowing the formation of a surface low pressure area with enough thunderstorm activity to be considered subtropical. The latest sea-surface temp analysis from the NHC (https://www.nhc.noaa.gov/sst/) shows the Gulf stream has recently warmed to the tropical development threshold of 26 deg C... therefore subtropical cyclone formation appears increasingly possible by Friday and this weekend. Should the core of the subtropical surface low pressure system develop enough thunderstorm activity to develop latent heat release driven outflow for additional strengthening... it could evolve into a fully-tropical storm. The CMC has recently joined the GFS and ECMWF in showing the development of a compact circular and potentially tropical core at the surface with a tight pressure gradient... the holdout in the major global models at this point is the NAVGEM which still presents a more elongated and weak frontal low pressure area. Therefore in this update I have raised my peak odds of subtropical cyclone formation to 30%.
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 forecast Great Lakes deep-layer ridge. This well-defined steering pattern should allow the subtropical 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. Based on the latest emerging consensus in the models... impacts for the southeastern US coast will begin as early as Friday as the surface low pressure area gains an initially weak/broad structure... 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 potentially gains a more consolidated tropical structure... an area of increasing winds and coastal surf may 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 (1800Z May 25)... 0% chance of subtropical cyclone formation (just offshore of the east Florida coast near 28.5N-80W)
IOH 48 Hr Outlook (1800Z May 26)... 15% chance of subtropical cyclone formation (offshore of the southeastern US near 30.5N-79.5W)
IOH 72 Hr Outlook (1800Z May 27)... 30% chance of subtropical cyclone formation (vicinity of Georgetown South Carolina near 32.5N-79.5W)
IOH 96 Hr Outlook (1800Z May 28)... 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...0%
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 24) CMC Model Run...
**For area of interest #4... broad frontal low forms offshore of northeastern Florida near 30N-77.5W at 66 hours... strengthens into possible subtropical cyclone just offshore of the Carolinas and near 32.5N-78W at 90 hours... drifts north and makes landfall at Cape Fear North Carolina at 102 hours... remnant low weakens over far southeastern North Carolina through 120 hours.
1200Z (May 24) ECMWF Model Run...
**For area of interest #4... broad frontal low forms offshore of northeastern Florida near 29.5N-79W at 66 hours... strengthens into a possible subtropical cyclone just offshore of southern South Carolina and near 32N-79.5W through 90 hours... quickly accelerates northwestward with inland remnant low dissipating over northeastern Georgia at 108 hours.
1800Z (May 24) GFS Model Run...
**For area of interest #4... broad frontal low forms offshore of northeastern Florida near 29.5N-78.5W at 42 hours... strengthens into possible subtropical cyclone just offshore of southern South Carolina and near 32N-79W through 60 hours... while drifting north makes landfall near the North Carolina/South Carolina border at 75 hours... after landfall curves northwest in track and dissipates over northwestern North Carolina at 93 hours.
1200Z (May 24) NAVGEM Model Run...
**For area of interest #4... broad frontal low forms offshore of northeastern Florida near 30N-78W at 72 hours... moves north-northeast and makes landfall just west of Cape Fear North Carolina at 102 hours... frontal low weakens over far eastern North Carolina through 120 hours.