Batten down the hatches.

Confidence continues to grow that Major Hurricane Florence will bring cataclysmic conditions to vast stretches of South Carolina, North Carolina, and Virginia. If the forecast continues on track, Florence will submerge much of the Southeast with extremely torrential rainfall and life-threatening storm surge in addition to generating destructive sustained winds. All residents or those with family, friends or other interest here should make preparations for life-threatening conditions—that some coastal areas could become inhospitable for an indefinite period of time following the departure of Florence is becoming a serious possibility.

The situation is beginning to look very dire. Major Hurricane Florence was upgraded to Category 4 status early Monday afternoon after aircraft reconnaissance measured sustained wind speeds near 130 mph and minimum central pressure of 946 mb. The storm’s rapid intensification occurred at a rate much faster than each run of the entire suite of numerical models before Monday afternoon. In other words, Florence is stronger than any global or hurricane deterministic weather model had predicted, and she is still intensifying. That’s some scary stuff.





As of 5pm EDT September 10, Florence was a 140 mph Category 4 Hurricane with minimum central pressure of 939 mb. Infrared satellite imagery depicts cohesive organization and a large eyewall. The intensifying bright colors indicate higher cloud tops, reflecting increasing convective organization.

Florence is no regular Major Hurricane, if such a concept were to exist. The storm will soon become the farthest northern Category 4 or Category 5 Hurricane ever recorded in the Atlantic Basin, provided the storm continues to closely follow the forecast. The records extend back to 1851 and can be referenced here. The reason Florence has intensified so rapidly is because the storm recently entered a moist, steam bath-esq environment with minimal wind shear. Steam bath is not much of an exaggeration. Sea Surface Temperatures (SSTs) are near 30 °C (86 °F) off of the Southeast Coast. As the anomaly plot depicts below, these SSTs are over 4°C above normal.

Numerical guidance has tightened its spectrum of solutions since the weekend. Deterministic models have remained steadfast on Florence making landfall as a strong Category 4 or possibly weak Category 5 Hurricane between northern South Carolina and northern North Carolina. From there, guidance diverges with respect to what happens after Florence makes landfall, with solutions ranging from slowly dissipating over the Appalachian mountains to stalling just on or off shore before recurving into the Northeast US. The GFS is an exception, since it stalls Florence just east of the outerbanks as a strong Category 5 hurricane. The former scenario would be catastrophic to Appalachia. It does not take much rain in flat land areas for devastating floods and mudslides to submerge and bulldoze valleys and mountainsides. The GFS solution would be catastrophic to the coast, as continuously building storm surge inundates the shore and travels through bays, rivers, and estuaries.




The complete suite of possible tracks from the 18 UTC September 10 global and hurricane models, courtesy of Tropical Tidbits, is presented below and illustrate the discussion above. Note that the tracks labeled “AVNO” and “AEMN” refer to the GFS and the GFS Ensemble (GEFS) mean. The suite of forecast tracks from these models has been steadfast for the past several days, greatly boosting confidence.


Although one now may say with reasonable certainty that Hurricane Florence will bring significant impacts to the Southeast coast, ideally the details ought to address the specific timing and magnitude of those impacts.  But with so many possible tracks, localized details are still difficult to pinpoint. However, that Florence is already a 140 mph Category 4 hurricane presents a unique opportunity to clear up confusion before even acknowledging the future state of the atmosphere.

Looking back at the 12z September 10 global model runs, the depiction of Florence just 3 hours after they were initialized already yields significant differences from reality. The GFS and ECMWF both depicted a hurricane with minimum central pressure of 961 mb and 967 mb, respectively. Such egregious errors of more than 20 mb must indubitably result in cascading errors later in the forecast, so these models can be discarded. As of this writing, the 18z GFS had not finished processing so at least for Monday’s forecast, these discrepancies reduce the range of possibilities to those of only the hurricane models.




Even exclusive in exclusive consideration of the suite of American hurricane models from NOAA and the Navy,  a surprisingly large amount of spread still exists. Though North Carolina will unanimously face the worst of the impacts, the Navy COMAPS-TC model (CTCX) is much farther north and east than NOAA’s HWRF and HMON. The AVNO is the GFS and is discounted from Monday’s analysis. Each point along the track represents a six hour time step. With each successive time step practically on top of the previous, the COMAPS, like the GFS, slows Florence to a near halt before making landfall near the Outerbanks. The HWRF and HMON slow their approach, but they continue northwestward into Appalachia. All models weaken Florence as she approaches the coastline, most likely due to upwelling of cold water and eye-wall intensification. The plots below are from NOAA Environental Modeling Center and illustrate the above discussion.

The diverse solutions even among just three models still causes confusion. Which track is more likely? The COMAPS likely stalls Florence like the GFS if it overamplifies feedback to a ridge of high pressure to the north of Florence. It is also further north and east of the forecast mean. Though further north and east tracks typically reflect a stronger storm, and Florence certainly is stronger than originally forecast, the position of the ridge is more important. Since the HWRF and the HMON depicted Florence to have a similar strength as was found Monday morning and afternoon and since they are both closer to the forecat mean track, a forecast similar to their projection is favored at this juncture.

Major Hurricane Florence is expected to become a Category 5 Hurricane by Tuesday afternoon and continue northwest toward the North Carolina coast. Florence will likely slow down, if not temporarily stall, shortly after making landfall late Thursday night due to friction and high pressure preventing the storm from continuing northward. Thereafter, she will gradually weaken into a tropical depression as she treks into Appalachia or possibly the Northeast US.




Regardless of where Florence makes landfall, a deadly storm surge will inundate the coast and travel up major rivers and estuaries. In conjunction with vicious hurricane-force winds weakening trees and man-made structures, the strongest storm surge will destroy almost everything in its path. Death will be a likely outcome for those along coastal waters near the path of Florence who do not evacuate. The strongest storm surge could exceed 10-15 feet, and is most likely to occur somewhere along the southern half of North Carolina, but devastating storm surge is nonetheless possible from South Carolina to Virginia. Otherwise, moderate storm surge could produce coastal flooding from central Florida to Southern New Jersey as Florence’s swells spread up and down the coast.

Extreme rainfall will accumulate 18-24″ or more near the center of the storm. Many areas of the South have recorded their wettest summers on record, so it will not take much rain to produce widespread flooding. This excessive rain will create vast inland seas in low-lying areas and near bodies of water. The force of the water will also be considerable as quiet creeks will become raging rivers. Further west in the mountains, valleys will quickly fill with water. Mudslides will destroy structures along mountainsides. This effect will be maxmimized if Florence were to travel westward after making landfall.

Hurricane-force winds will penetrate far away from the coast, given the storm’s large radius. Millions are expected to lose power. Wind gusts exceeding 100 mph are likely to spread across eastern North Carolina and flat-line many structures in open areas without trees or dense structures. Tropical Storm Force wind could spread as far west as the Appalachian Mountains, which would block evacuation routes by downing trees on roadways escaping flooded areas.

A disaster is likely in the works. Specific timing and magnitude of impacts is still uncertain, but confidence has grown to reasonably expect devastation somewhere in the Carolinas and Virginia. Prepare for an extended period without electricity or fresh water if residing in the storm’s path. Else, be sure friends, family and other interests have a plan ready.



Author

Josh is a lifelong nature and weather enthusiast as well as the Head Meteorologist at WeatherOptics. He began regularly forecasting for New Jersey, Long Island and New York City in 2014 on social media, contributing to community pages such as SBU Weather. He holds degrees in Physics and in Atmospheric and Oceanic Sciences from Stony Brook University, from which he graduated in 2018. In the Fall of 2018 Josh will start graduate school for his M.S. in Marine and Atmospheric Sciences at Stony Brook, continuing his research on approaches to non-convective wind gust forecasting.

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