The following is an excerpt from a forecast briefing prepared for a client in advance of Hurricane Florence in 2018.
Hurricane Florence 1 PM Briefing 9-9-18
Overview: Hurricane Florence has strengthened into a major category four hurricane as of 12 PM EDT 9-9-18 with maximum sustained winds of 130 mph, and additional strengthening is forecast prior to landfall which will occur Thursday afternoon or evening.
|Wind||Thursday AM – Friday AM||Medium||High||S/Coastal NC|
|Storm Surge||Thursday PM – Saturday PM||Medium/High||High||Coastal NC/S Coastal VA|
|Flooding From Heavy Rains||Thursday – Tuesday||High||Very High||All of Southern VA and all of NC|
Two graphics have been attached to highlight geographical locations more precisely.
Wind Discussion: After the storm reaches its peak intensity over water Wednesday night or Thursday morning, it is expected to weaken slightly on final approach to the North Carolina coastline Thursday. Expect the intensity at landfall to be a category three, with maximum sustained winds between 115 and 130 mph. Note that these maximum winds will be confined to a very small area of the immediate coast in the storm’s core, which may be only 20-30 miles across. However, a much broader area will see hurricane force (75 mph) winds, which while not catastrophic, will cause significant disruptions. Most of eastern North Carolina will see tropical storm force (40 mph) winds, which will cause power outage and tree damage concerns. Due to the large area impacted, extended outages should be expected. Wind threats will subside quickly on Saturday and Sunday as the storm weakens over inland NC.
Surge Discussion: Onshore winds will push storm surge waters into low lying parts of the North Carolina coast as Florence makes landfall on Thursday. The high tide cycle with the greatest impacts will be centered around 1:38 AM Friday morning (time given for Wilmington). The Outer Banks should expect significant storm surge as well as large battering waves that will create erosion concerns. If the storm’s track shifts farther northeast, surge impacts would be much higher for the Outer Banks. Inundation estimates will depend on the exact track, and will be nailed down with more precision later tonight/early tomorrow morning. However, low lying parts of the North Carolina coast, including inlets, should be preparing for dangerous storm surge flooding. Surge threats will subside Saturday and Sunday as the storm weakens over inland NC, but low lying areas could continue to see issues with spashover/minor inundation through Monday as onshore winds continue.
Heavy Rain Discussion: Florence will slow down after it moves onshore Friday, and will eventually stall over central or eastern North Carolina by Sunday. This lack of forward motion will mean a prolonged period of heavy rain for all of North Carolina, as well as much of Virginia (with the exception of far northern parts of the state). Much of this area will see rainfall totals in the 10-15” range, with localized amounts in excess of 24” expected. This excessive rain will cause major flooding concerns both on small streams and on large rivers. Even though Florence will weaken into a tropical storm or tropical depression by Saturday, it’s important to note that it will be no less dangerous of a storm! It’s also important to note that locations typically considered safe for coastal evacuees may be compromised by flooding for prolonged periods of time. Safe evacuation locations include areas north or northeast of the Chesapeake Bay, and areas west of the Appalachian Mountains. Georgia and western SC will also be far enough southwest to avoid significant impacts from Florence. The heavy rain threat will continue into the early part of next week, with the exact end time remaining uncertain. Regardless, the storm will have either dissipated or moved away from the region by the end of next week at the absolute latest.
Forecast Confidence Discussion: Overall confidence in this forecast is medium-high, but the exact location of most severe impacts remains uncertain due to questions about the exact track.
What elements of the forecast have high confidence?
– Florence landfall somewhere in S Coastal NC
– Landfall as large and extremely dangerous hurricane
– Widespread major flooding due to heavy rain extending well inland from the point of landfall.
– Widespread power outage issues due to tropical storm force winds extending well inland from the point of landfall. Outage duration will be extended (>1 week) in some/many areas especially near the coast.
What elements of the forecast have medium confidence?
– Florence landfall somewhere between Bald Head Island and Hatteras – Swath of serious wind damage extending somewhat inland (75-100 miles) due to hurricane force winds.
– Serious storm surge impacts along coastal NC/Southern VA (Norfolk area) What elements of the forecast have low confidence?
– Exact point of landfall (current idea is somewhere NE of Wilmington, near Jacksonville) – Exact point of landfall determines locations that see devastating wind damage from >110 mph winds and storm surge potentially exceeding 10 feet.
– Exact point of landfall and rain bands determines locations that see devastating inland flooding from >20” of rain.
– End time for rain/flooding impacts as storm stalls and meanders early next week.