*******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.**********
...SATURDAY MAY 27 2023 2:06 AM EDT...
Broad surface low pressure area offshore of the southeastern United States becoming better defined and organized on satellite pictures... continuing to monitor this feature for possible acquisition of tropical characteristics over the next 24 hours and before it makes landfall on the South Carolina coast. 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... The south end of the upper trough that was previously over eastern North America (now heading for the eastern Canada coast) has become cut-off from the mid-latitude westerlies by a deep-layer ridge that has solidified over the Great Lakes region... and now prevails over the southeastern United States as a cut-off upper vortex. The eastern divergence zone of the upper vortex continues to support a frontal surface low pressure area parked just offshore of the southeastern US which was initially large/broad Friday morning while having little activity near its core and most of the thunderstorm activity being displaced to the east side of the system. As we progressed through Friday... activity near the core area of lowest surface pressures increased in a north-south region while the displaced activity to the east has gradually subsided. In the most recent nighttime satellite pictures... there are indications the surface low pressure area is becoming less broad and more focused as the core area of activity has organized into a comma shape... with increasing thunderstorm activity at the head of the comma and near a better-defined swirl center. Models remain split on the potential for this system to become a subtropical cyclone before it makes landfall tonight on the South Carolina coast... with the ECMWF continuing to show a more elongated and less tropical system while the GFS... as well as intermittent runs of the CMC and NAVGEM... show this system quickly ramp up with the development of a circular and potentially tropical core. Since 8 PM EDT the NHC tropical weather outlook product has leaned toward the less tropical idea while dropping odds of subtropical cyclone formation to 0%. In this update I am maintaining peak subtropical cyclone formation odds of 20% due to the above observations of a better-defined swirl center to the surface low pressure with increased nearby thunderstorm activity.
Regardless of acquisition of tropical characteristics or not... the track and impacts of the strengthening surface low pressure area will largely remain the same. As far as track... the core of this system is expected to arc northwestward into South Carolina while arcing toward the core of the parent upper vortex as tradtionally seen with a maturing subtropical or non-tropical system. Once the surface low and upper vortex become stacked inland... weakening and dissipation of the surface low by Sunday and onwards is expected due to a lack of upper divergence at the upper vortex's core. In general a slow eastward drift of the decaying system is expected as a fast-moving upper trough from northwestern Canada and current upper trough energy over the western US attempt to erode the Great Lakes deep-layer ridge while also trying to grab the upper vortex while injecting some upper westerly flow into the region. For what is now the 24 to 48 hour period immediately following the landfall... models have been trending more east with forecast track projections while showing the western US upper trough energy getting an earlier start in pushing the upper vortex more eastward... and with the fast-moving upper trough/surface front from western Canada in the next 48 hours being amplified enough to make a stronger deep-layer ridge weakness that may try to grab this system from the norhteast. With these model signals... 24 to 48-hour forecast track in the outlook below has been adjusted eastward relative to my previous outlook accordingly.
Impacts already began Friday and are continuing as of this writing as a tight pressure gradient has already developed between the northwest side of the surface low pressure system and south side of the Great Lakes deep-layer ridge. This gradient has been driving a fetch of northeasterly onshore surface gusty winds and resultant coastal surf from the coastal Carolinas all the way to northeastern Florida. For today and tonight... 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... with gusty winds spreading as far inland as southeastern North Carolina and eastern South Carolina. An area of increasing rainfall has already overspread eastern South Carolina and southeastern North Carolina from Friday through now... expect periods of heavy rainfall to continue to spread inland across much of the Carolinas in the next 24 hours.
Here are some National Weather Service station reports of wind (in mph) from the last several hours... available at weather.gov:
**Jacksonville FL... sustained 18... gust 35 (Friday 8:53 PM EDT)
**Brunswick GA... sustained 20... gust 36 (Friday 4:45 PM EDT)
**Savannah GA... sustained 17... gust 28 (Friday 11:55 PM EDT)
**Charleston SC... sustained 16... gust 28 (Friday 11:55 PM EDT)
**Wilmington NC... sustained 23... gust 32 (Friday 11:53 PM EDT)
**Hatteras NC... sustained 16... gust 29 (Friday 11:51 PM EDT)
**Nags Head NC... sustained 20... gust 30 (Saturday 12:30 AM EDT)
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (0000Z May 28)... 20% chance of subtropical cyclone formation (vicinity of Georgetown South Carolina near 32.5N-79.5W)
IOH 48 Hr Outlook (0000Z May 29)... 0% chance of subtropical cyclone formation (western North Carolina/South Carolina border near 35N-81W)
******National Hurricane Center (hurricanes.gov) official outlook as of 2 AM EDT*****************************
Formation chance through 48 hours...0%
Formation chance through 7 days (168 hours)...0%
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z (May 26) CMC Model Run...
**For area of interest #4... frontal low currently offshore of northeastern Florida strengthens into possible subtropical cyclone offshore of the Georgia/South Carolina border and near 31.5N-79.5W at 36 hours... continues north and makes landfall over northeastern South Carolina at between 48 and 54 hours... inland remnant low dissipates over southeastern North Carolina through 90 hours.
1200Z (May 26) ECMWF Model Run...
**For area of interest #4... frontal low currently offshore of northeastern Florida strengthens into broad frontal cyclone that makes landfall on the central South Carolina coast at 48 hours... while weakening to inland remnant frontal low moves north-northeast into eastern North Carolina through 66 hours where it then dissipates through 90 hours.
1800Z (May 26) GFS Model Run...
**For area of interest #4... frontal low currently offshore of northeastern Florida strengthens into possible subtropical cyclone just offshore of Georgetown South Carolina at 30 hours... through 36 hours makes landfall over northeastern South Carolina... weakening remnant low subsequently curves northeastward across eastern North Carolina and dissipates in vicinity of the northern Outer Banks of North Carolina at 66 hours.
1800Z (May 26) NAVGEM Model Run...
**For area of interest #4... frontal low currently offshore of northeastern Florida strengthens into possible subtropical cyclone offshore of the Georgia/South Carolina border and near 31.5N-79.5W at 24 hours... makes landfall on the central South Carolina coast at 42 hours... weakening inland remnant low reaches south-central North Carolina through 72 hours... remnant low drifts east into southeastern North Carolina where it then dissipates by 90 hours.