Tuesday, September 11, 2018

5pm (Update) Tuesday, Hurricane Florence Information

140 mph winds Hurricane Florence
4:50pm Tuesday
5pm National Hurricane Center: Hurricane Warning Now in Effect from South Santee River, South Carolina north to Pimlico and Albamarle Sounds. 
Dark brown is the hurricane-force winds and amber is the tropical storm-force winds. The wind field will likely grow in diameter between now and landfall. Thus, don't worry too much about the point of landfall. Just about all of the information you need is below.
Basemap by Capital Weather Gang
Closeup of Hurricane Warning Area With Major Cities
First, I hope you have used the checklists in the postings below to prepare yourself and your family for the oncoming storm.

Here is the timing of sustained 40 mph winds (with gusts to 60 mph). You should be gone by these times.


Below, I am going to provide details as to the threat areas for the three types of hazards:
  • Saltwater storm surge
  • Wind damage
  • Freshwater (river and stream) major to catastrophic flooding
Many of these suggestions are already covered by local emergency evacuation information but I want to reiteration and provide some of the weather science behind those evacuation orders.

Storm Surge
Click to enlarge
This map does not include wave action. In addition to a 20' storm surge, there could be 20' waves on top. This is combination is extremely destructive. 

My advice? The Outer Banks should be completely evacuated!
If you live 22' above sea level or lower, GET OUT!! You may want to raise that to 24-25' to be sure. 
Courtesy of Jim Reed, Twitter
This is what storm surge does (this was Hurricane Ike in Galveston in 2008)! You don't want to be there!

Peak Wind Speeds

These are two different but plausible forecasts of peak wind gusts. Find your location and then pick the worst case scenario. If you are in the bright red, dark brown, white or pink areas, you are in an area that could be possibly have power failures or street blockages due to falling trees. Along the South Carolina coast, the highest winds may be delayed until the weekend. 

If you are in the winds of 80 mph or higher, if it were me, I'd get out. Trees could fall on your home or structural damage could occur exposing your family to danger or even extreme danger. 

Below, courtesy of Jim Reed, is what Category 4 wind damage looks like. 
You don't want to be there when winds like this are in progress. 

Flooding
Again, I am going to present two maps. The map immediately below shows a situation where heavy rains falls in the Appalachians. If this occurs, the amounts are likely too low. Near the coast, amounts are more than 30 inches.
click to enlarge
And, another scenario.



At 4:30pm, the NWS issued its rainfall forecast for the next seven days. While it is intentionally smoothed a bit (so it doesn't display the absolute maxima), the pattern is very reasonable.
If this is correct, there will be massive flooding.

Although the locations vary, both maps indicate the potential for more than 30 inches. If the wrong things come together, amounts could be heavier and more widespread. Major to catastrophic flooding is likely, especially given the recent wet conditions. Exact amounts may vary (per the two model patterns depicted). The bottom line: major to catastrophic flooding

While this is from Hurricane Harvey, this is the type of flooding to be expected from Florence. One of the things said right after Harvey was, "there was a failure of imagination" with regard to the amount of flooding. Don't let that be the case here. 

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