Florence Update

As expected, the path of Florence has changed, however the threats mentioned before will be no less impactful. I will get right into it:

Where


The latest path forecast based largely on model guidance has Florence making landfall in Wilmington as a Category 2 hurricane. Shortly after landfall and after many hours of actually interacting with the land (as this storm is HUGE), wind speeds will slow to category 2 strength (max sustained up to 110 mph.

The likely path of the storm will be to take a turn to the southwest shortly after making landfall as it gets caught in the northeasterly (from the North East) flow from the synoptic ridge building into the northeastern United States. This, in addition to friction, will also cause the storm to slow down in it's forward propagation, which will allow for large precipitation totals along the coast of NC and into central NC and central SC.

The storm, at this point, looks like it will continue its southwestward trend through SC, and once the ridge (mentioned above) clears out to the northeast, the storm will get caught in the lifting trough and rapidly move northward. Remember, this storm is very, very big from an areal extent.... over 400 miles wide. Rain will be falling over a huge portion of the United States for a number of days.

When

Right now the National Hurricane Center has Florence making landfall early morning Friday. This time of landfall (or nearest proximity) to Wilmington has been slowly moving later in the week as the storm approaches and we get a better idea of how fast and to where it is moving. Part of the reason so much has been closed and will be closed for a few days ahead of forecasted landfall is to give residents a chance to get out, deal with traffic, and get to their desired safe spot with enough time prior to the storms initial impacts. The storm will continue to wash out the Carolinas, GA, and southern VA through the weekend and into early next week.

At that point, the first phase of Florence will have passed, but the potential worst for many lower-lying communities along the major rivers traversing the NC/SC border (images to the left). As 10-20" of rain falls throughout the duration of Florence, that water eventually has to flow back downhill. This means the flooding experienced by North and South Carolinians during and after Matthew in October 2016 will be felt again, and this time with more of a negative impact because of the amount of water. It will take a couple of weeks for all the water in western NC to make its way to the SC coast, flowing through major cities like Charlotte, NC, Columbia, SC, Florence, SC, and eventually out to the coast.

Florence from GOES-East GEOCOLOR
13 Sep 2018 07:07 UTC

On the left I have provided the latest flash flooding potential map from the National Oceanic and Atmospheric Administration. Keep in mind, flash flooding refers to floods relating to localized heavy rainfall, not the traversing downhill which will result days and weeks after that localized heavy rainfall occurs. Flood warnings will be issued as necessary by the National Weather Service, as our forecasters have been and will continue to spend major overtime hours pouring over model guidance and the latest trends to keep us as informed as they can. Overlapping 12-hr shifts take a toll after a while and they have been doing amazing work keeping us informed.

What/How Much

Precipitation:
The heaviest precipitation will fall in Wilmington, NC and surrounding communities. In these areas, I'm expecting upwards of 30"+ of rain. SE NC and NE SC will receive between 10-20" of rain. Central SC-Northwestern NC, around to Central NC and out to the coast and that band will likely receive 3-5" of precipitation. Basically the rest of NC, South Central SC, NE GA, and SE/SCentral VA, and Eastern TN will likely receive 1-5" of rain.


Wind:
One thing about this storm that has me awestruck is the sheer size of the storm... While it is losing some strength as dry air is getting entrained and thus hindering the storms ability to restrengthen (VERY good in many ways for the coast). The current max sustained winds are 110 mph and those speeds extend about 50-75 miles out from the storm center. Tropical Storm force wind speeds (>45 mph) extend over 200 miles from the storm center to the West, North, and East. Winds will have an impact along the coast and will produce notable storm surge along the coast of primarily NC and southeastern VA.

Storm Surge Storm surge from this system will range from 13-18 feet closest to the center of the storm and in the NE quadrant of the storm. Surge will be highest in SE NC, the southern Outer Banks, and the Pamlico Sound/Neuse River. Early storm surge estimates have surge equivalent to 9' AGL (above ground level, as opposed to above sea level) reaching as far as Greenville, NC along the Neuse River, which is normally very shallow anyway. Wave heights will aid storm surge potential and fortunately a full moon will not. Another thing to watch out for is the storm making landfall at high tide. That adds 3-4' of water as a foundation for the waves to build off of and move inland. The longer the storm stalls before moving inland to SC, the worse the flooding from storm surge will become. Rain, as discussed earlier, will continue through the event as the storm progresses east/southeastwards.


Hazards:

The hazards for the storm remain largely the same from the last post:

1. Flash Flooding - The flash portion of this will result mostly at the coast of North Carolina and Virginia
2. Wind - Winds are strong enough and the water tables saturated enough that it won't be an outlandish event to see a tree or tall structure topple over.
3. Power Outages - Once this storm comes ashore, winds will batter trees and structures, and trees could topple over.
4. Potential Isolated Tornadoes. As the rain band moves ashore, the friction from the land causes a change in the force balance diagram and will become mini areas of maximum vorticity, or atmospheric rotation.
5. Flooding down the road: If this storm will be analogous or worse than Matthew, It will take weeks to see how much water is actually going to flow down hill and over road surface. 

Recap

For now just stay hunkered down. A few matches and candles can't hurt anymore. Stay tuned to your local weather station for the latest information. Florence is a Category 2 hurricane and will likely stay that way through its landfall and path over land. There will be lots of rain, some wind, and very high storm surge.

Be safe, and be smart out there during this storm!