Heavy Rain and Potential for Dangerous Flooding for northeast SC and southeast NC

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1 US National Weather Service Wilmington ***Life Threatening Flooding Possible*** Heavy Rain and Potential for Dangerous Flooding for northeast SC and southeast NC Briefing #9 Saturday October 3, PM EDT National Weather Service Wilmington Office, NC State Steven Pfaff, WCM Name of Briefer

2 Updates A Flash Flood Watch has replaced the Flood Watch and it has been extended through Monday Evening. Given a higher potential for flowing water the change to a Flash Flood Watch has been made. The area of heavy rainfall plaguing portions of central and southern SC will begin to shift northward during tonight through Monday. As a result, round two of this event is projected to begin by this evening across our southern and southwest areas and work its way northward into southeast NC by Sunday night. Remember, scattered showers and thunderstorms are expected ahead of the main area of heavy rainfall. Several areas across northeast SC and southeast NC were hit hard from the rainfall yesterday and last night. These areas will be especially vulnerable as the second round of the event occurs. Rain gauge reports of over 15 inches were reported near Longs and the Little River area. Doppler estimates also show a significant amount of rain has fallen across western Brunswick, southern Columbus, and Darlington counties. To remain unified in our messaging we need to focus on several aspects including flood driving dangers - especially at night and the Turn Around Don t Drown campaign. Also need to stress that the potential for earthen dam failures will exist, water will quickly come out of the banks of streams and creeks, any property left in flood prone locations (ie. vehicles) will be at risk, and earthen bridge supports may fail. 2

3 Expected Threats/Impacts Matrix Hazards Specific to southeast NC and northeast SC Extreme High Moderate Elevated Little to None Flooding remains the biggest concern with highest potential across SC initially through tomorrow. The threat will shift across northeast SC and southeast NC by Sunday night and persist into Monday. Coastal issues are also likely during each high tide through Monday. Large/powerful rip currents are also expected. Large waves are occurring across the Coastal Waters, and very dangerous maritime conditions are expected farther offshore. An isolated tornado will also be possible through tonight. 3

4 Potential Rainfall Amounts ***Life threatening flooding possible*** possible Although scattered showers and thunderstorms are expected through tonight, the brunt of the additional heavy rainfall is expected later Sunday into Monday The highest amounts shown will impact several areas that were already hit hard from heavy rainfall yesterday and last night. These values are basin averages, so expect a few areas to receive higher amounts. Most of the rain will come to an end late Monday or early Tuesday. WPC 3 day rainfall prediction valid 8 PM tonight through 8 PM Tuesday 4

5 Excessive Rainfall Outlook from WPC There is a slight to moderate risk that rainfall will exceed flash flood guidance across northeast SC through tonight. The focus of the heaviest rainfall will shift back across southeast NC from SC tonight through Sun and Mon. Weather Prediction Center Day 1 Excessive Rainfall Outlook valid through 8 AM Sun. In the hardest hit areas flowing run-off may create road scours or complete loss of the roadway. Some buildings, especially those near vulnerable low-lying locations may receive flooding as well. Earthen dams are also at risk given the amount of rainfall expected. If any dams fail, those just downstream of the dams could experience life threatening flooding in a very short period of time. Day 2 Day 3 Weather Prediction Center Day 2 (left) and Day 3 (right) Excessive Rainfall Outlook valid 8 AM Sun through 8 AM Mon and from 8 AM Mon through 8 AM Tue Vehicles and other property left near flood prone areas such as drainage ponds, creeks, and streams are at risk as water levels rise. 5

6 Day 1 Storm Prediction Center Convective Outlook Storm Prediction Center Day 1 Categorical Convective Outlook valid through 8 AM Sun. The Storm Prediction Center continues to highlight a marginal risk for tornadoes through tonight across portions of the Carolinas. Although the risk is low the greatest risk exists from near Charleston, SC to the Brunswick County coast. 6

7 Max Wave Swath from CERA utilizing 11 AM Advisory Note this CERA swath image of Significant Wave Height is from the 11 AM EDT NHC Advisory today. 7

8 Key Points The risk for excessive rainfall ranges from slight to high across SC through today, but the risk will shift back into southeast NC Sunday into Monday. As a result, the potential for dangerous flooding will persist with a Flash Flood Watch in effect through Monday. The combination of saturated ground conditions and wind gusts could lead to some downed trees through Sunday. Basin-wide heavy rainfall, locally and upstream, will allow all rivers to experience significant rises. Additional rivers could begin to exceed flood stage early next week while others are a little slower to respond to the rainfall. Some of the rivers could reach moderate, and possibly major stages. Refer to the River Forecast briefing for details. Flooding will make driving difficult as run-off collects across intersections and lowspots. In addition, some small creeks may quickly get out of their banks and spill across the roadway. Any location where water is flowing across the road may result in a scour, complete wash-out, or sink hole. The dangers of flooding are even higher at night as the hazards become harder to recognize. 8

9 Key Points Large waves will continue across the open ocean and large breakers will continue in the surf zone along with strong rip currents. Waves will become very steep near inlet entrances, especially with the falling tide. Wave run-up and higher than normal tide levels are expected into Monday. During each high tide expect water rises into the dunes resulting in minor to moderate coastal flooding and additional beach erosion. MVFR/IFR conditions expected at KFLO, KLBT, KMYR, KILM, and KCRE, especially in/near areas of heavy rainfall. Other terminals across the Carolinas will likely be impacted by the areas of heavy rainfall through Monday. Winds at the coast: Through tonight E G30 MPH. Sunday E-NE G25 mph Winds farther inland: Through tonight NE-E G25 MPH. Sunday NE G20 mph The next conference call is planned for 4 PM EDT today. The next PDF briefing will be issued by 730 AM later Sunday. 9