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!

Hurricane Florence: What is it about "F"-named hurricanes and North Carolina?

Hurricanes affect North Carolina fairly regularly (52 storms since 2000), however not many of those hurricanes have been major hurricanes. The strength of hurricanes are weighted against the Saffir-Simpson scale which determines a hurricanes strength based on it's maximum sustained wind speed.

*For the ins and outs of hurricanes and other information in this blog, continue reading. However if you just want to read the forecast, jump down to "Florence: Where, When, How Strong?". I apologize for my long-winded writing as it has been a while since I have written a post and I am out of practice with succinct writing.*

Terminology, Dynamical Explanation

Why Wind Speed?
Great question! Above is our equation for the geostrophic wind, or the wind that is balanced with respect to Pressure Gradient Force (caused from a difference in atmospheric pressure at two locations) and the Coriolis Force (the force enacted on an object as a result of our Earth spinning on it's axis), While each hurricane can have it's own major impacts, may it be flooding, wind damage, long-term power outages, or coastal erosion, the Saffir-Simpson Hurricane Wind Scale is a fantastic way to determine the strength and potential hazards of a hurricane. Diving shallowly into our dynamical equations for the atmosphere, the horizontal wind is related to the horizontal pressure gradient, or the change in atmospheric pressure over a given distance. A hurricane is essentially a localized, very strong low pressure system, with extremely low pressure at the surface. The lowest pressure recorded in a hurricane was the Labor Day Hurricane of 1935 that struck the Florida Keys with 892 millibars of central pressure. Generally pressure at the surface ranges from 980 millibars to 1040 millibars in North Carolina depending on the season and the location in a weather pattern. A bar is the metric unit of pressure and is equivalent to the weight of the atmosphere above a location at sea level, hence the surface pressure being ~1000 millibars. In essence, the stronger the pressure gradient force, the faster the geostrophic winds will be, and the more opportunity the storm has to have more devastating impacts.

History of 'F'-Named Hurricanes in North Carolina

While in recent memory, Hurricanes beginning with the letter 'F' have had an incredible impact on North Carolina (Hurricane Fran in 1996, Hurricane Floyd in 1999, Hurricane Florence in 2000, Hurricane Fabian in 2003, and Hurricane Frances in 2004), the only one of that list to actually be a major hurricane upon landfall was Hurricane Fran. If you have lived in North Carolina for a while and remember these storms, you may recall that Fran was the only storm of that list in which major destruction was caused by wind. Floyd and Frances both caused catastrophic flooding from record-shattering rainfall totals, and Florence and Fabian both affected the coast with rip currents and beach erosion (Fun Story: Hurricane Fran is how I gained my interest in meteorology. The dream and fascination with the weather began back in 1996). With the exception of Fran, all the major hurricanes to make landfall in North Carolina had names that did not begin with 'F' (Emily 1993, Diana '84, Donna '60, Helene '58, Ione '55, Connie '55, Hazel '54, Great Atlantic Hurricane '44...). The complete list can be found here if you're interested.

Florence Again?

You may have noticed from the above paragraph that a hurricane named 'Florence' made landfall in 2000. Why is there another Hurricane named 'Florence'? The World Meteorological Organization implemented a list in 1953 of six years worth of names starting with 'A' to 'W' and with the exception of 'Q', 'U', 'X', 'Y', and 'Z' because names beginning with those letters are generally difficult to pronounce. The WMO implemented these lists because prior storms were referred to based on their coordinates in longitude and latitude, making it difficult to differentiate storms both during the event and in historical archives. The National Hurricane Center also mentions that a name can be removed from the lists in the event that the "storm is so deadly or costly that the future use of its name on a different storm would be inappropriate for reasons of sensitivity." Sandy, Katrina, Irma, Andrew, Hugo, Fran, Floyd, Hazel, and others have been removed from this list.

Florence: When, Where, How Strong

Hurricane Florence is one of three named tropical Cyclones in the Atlantic basin: Florence, Isaac, and Helene (Gordon formed in the Gulf and made it's way up through the belly of the United States as a Tropical Storm last week). As of 11:00 p.m. AST (Atlantic Standard Time, analogous to Eastern Daylight Time), Hurricane Florence was located at 24.6°N 57.7°W moving west at 7 mph with a minimum pressure of 974 mb and max sustained winds of 90 mph. Over the course of the next 4-5 days, it will move over warmer waters and into an area of lower wind shear, both conditions favorable for strengthening. Florence is expected to increase in strength to a major hurricane prior to making landfall. 

Levi Cowan, meteorologist and owner of "Tropical Tidbits" posted this latest GFS Ensemble track for Florence. As is apparent, most of the ensemble members show a trend for Florence to become a major hurricane and make landfall somewhere in North Carolina. While it is still too soon to determine exactly where landfall will occur, it is safe to say that the coasts of North Carolina, Virginia, and likely South Carolina will be affected by strong winds, heavy rains, and storm surge.

At this point, North Carolina Governor Roy Cooper has wisely declared a state of emergency in order to allow for numerous days of preparation ahead of a potentially devastating storm. The National Hurricane Center believes that the most likely path for Florence to take will be into the southern beaches of NC, which could mean direct landfall in Wilmington, however it is still too soon to pinpoint an exact beach as to where the biggest impacts will occur. At this point the best thing to do is prepare for the worst. Once Florence makes landfall, interaction with the land will cause the storm to weaken substantially, however even with weak winds this storm will still have an impact

Expected Hazards

1. Wind: As Florence will likely be a Category 3 or stronger storm at landfall, winds in excess of 110 mph can be expected within a couple dozen miles of the storm center. These winds could easily cause downed trees, power outages, damage to roofs and other structures, and damage from flying debris. The best thing to do would be stay away from windows (or even board up windows at the coast) and remain in interior rooms of the home throughout the initial stages of the event. 

2. Rain/Flooding: This storm is moving relatively slowly right now, but will pick up speed as it rounds the south end of the High pressure system establishing itself over the New England States. However, once the storm makes landfall, it will slow down substantially and the moisture nearby will be converted to rain within the storm. Some early precipitation forecasts show over 12" of rain to fall through the event in some areas (but still too early to tell exactly how much and where). Flooding could also lead to pollution of water used by the city, so be sure you have bottled water stocked up. 

3. Power Outages: Hurricane Florence might not produce as widespread or long-lasting power outages that Fran did back in 1996, however I suspect that due to the nature of this storm there will be extensive, widespread power outages along the coast and inland along the path Florence inevitably takes. Be sure you have your batteries and external batteries charged, flashlights at the ready, and candles where you can easily access them. 

4. Beach Impacts: Ahead of Florence, due to the expected wind direction and increased wave action along the beach, it would be wise to stay out of the water due to possible rip currents. In addition, through Florence's trip, I would expect some beach erosion along the the Carolinas and Virginia. 

*I will have a subsequent post early this week as Florence continues it's west-westnorthwest path toward the east coast.*