Wednesday, June 21, 2017

Cindy Generates Widespread 2-4 Foot Storm Surge with Localized Maximum Exceeding 6 Feet



While Tropical Storm Cindy's torrential rains have grabbed the headlines, coastal flooding in Southeast Louisiana and Mississippi inundated some communities with saltwater.

Cindy generated widespread 2-4 foot storm surges in the region, with a localized maximum exceeding 6 feet at Shell Beach, Louisiana. The highest storm surge levels occurred east of the Mississippi River, even though the center of circulation has been moving to the northwest, with a likely landfall location near the Texas/ Louisiana border.



Peak storm surge levels observed through Wed Jun 21 at 900AM CDT. Data provided from NOAA Tides and Currents

Shell Beach, Louisiana, observed the highest storm surge so far, with a surge level exceeding 6 ft (1.83 m). At 1:06AM local time this morning, the tide gauge observed a water level 6.07 feet higher than predicted astronomical tides. Water levels had been consistently rising through the evening under sustained  northeast winds exceeding 35 mph and often gusting more than 45 mph.

The map above depicts peak storm surge levels through 900AM Central Time today. Bay Waveland Yacht Club at Waveland, Mississippi, and New Canal Station in New Orleans, both observed storm surge levels in the 3.5- 4.0 ft range.

Wind and water observations for Shell Beach provide insight into the localized nature of storm surge. Water levels peaked following a period of sustained winds from the northeast. This wind direction pushes water directly into Shell Beach, which may seem unusual as we may picture onshore winds along the northern Gulf Coast blowing from the south.

Graph depicting observed water levels (top), wind direction and wind speed (middle) and temperature (bottom) for Shell Beach, Louisiana, through the afternoon and evening of Tue Jun 20, 2017. Data provided by NOAA Tides and Currents, markups by Dr. Hal Needham.

However, Shell Beach is actually exposed to the northeast, so Cindy provided an ideal wind direction to pile up storm surge.  As Tropical Storm Ida passed to the east in November, 2009, Shell Beach observed peak water levels as winds blew from the north-northeast, even as this wind direction caused surge levels to fall in nearby coastal Mississippi. Shell Beach broke all kinds of rules during Ida- it even observed the highest storm surge record for the storm, although it was located to the LEFT of the storm track.

Tropical Storm Cindy also produced coastal flooding west of the Mississippi River, on LA-1 heading into Grand Isle. Chris Roberts uploaded the following photograph to a Facebook post yesterday evening, depicting coastal flooding over LA-1 before sunset.

Photo: Chris Roberts https://www.facebook.com/chris.roberts.37669

As Tropical Storm Cindy tracks to the northwest, expect storm surge levels to build in Southwest Louisiana and the Upper Texas Coast. Storm surge levels in this region should reach 2-4 above normal astronomical tides, inundating coastal roads.

I will try to take the ferry to the Bolivar Peninsula this evening to observe any coastal flooding before sunset. This location, northeast of Galveston, has experienced substantial coastal erosion during previous decades, and observed the highest water level for Hurricane Ike.

The table below provides estimated peak storm surges for Tropical Storm Cindy through 900AM on Wed Jun 21. Data provided by NOAA Tides and Currents:

Shell Beach, LA:                             6.0 ft
Bay Waveland Yacht Club, MS:      4.0 ft
New Canal Station (New Orleans): 3.5 ft
Grand Isle, LA:                                2.6 ft
Pascagoula, MS:                              2.6 ft
Galveston, TX:                                2.4 ft
Sabine Pass, TX:                             2.3 ft
Pilot Station (near mouth of Miss): 2.3 ft
Calcasieu Pass, LA:                         2.2 ft



Tuesday, June 20, 2017

Torrential Rains Will Overshadow Minor Coastal Flood Event Along Northern Gulf Coast

Tropical Storm Cindy has developed in the central Gulf of Mexico.  Torrential rains from Louisiana through the Florida Panhandle will grab the spotlight, as destructive winds should not develop and coastal flooding remains minor.


The 7:00AM National Hurricane Center (NHC) advisory places the center of circulation for Potential Tropical Cyclone 3 approximately 300 miles south of Morgan City, Louisiana. The NHC forecasts this system to become better organized into a tropical storm and track towards the northwest.

Note the area of tropical storm force winds depicted on the map above. These winds are offset from the center due to southwesterly wind shear. Typically when winds are offset from the center like this, they do not provide consistent coverage over the entire area, but are more found in the heavier squalls. This means that the system is not producing a wide enough coverage of strong winds to generate substantial coastal flooding.

Southwesterly wind shear was displacing the heaviest convection to the north and east of the center of circulation for Potential Tropical Cyclone 3 this morning. Background image: NOAA, Markups: Hal Needham.

The greatest coastal flood potential exists along the Mississippi Coast and in southeast Louisiana, east of the Mississippi River.  Coastal flood levels near the center of circulation, in south central and southwest Louisiana, should be noticeably lower than locations 100+ miles to the east (east of Mississippi River).

This offset should occur for two reasons: 1) Most of the convection is offset to the north and east; 2) Coastal Mississippi and the area of Louisiana east of the Mississippi River is more efficient at piling up storm surges than areas to the west, near Morgan City or Cameron.

Regardless of coastal flood impacts, the torrential rain of this system will grab the headlines. NOAA's Quantitative Precipitation Forecast (QPF) map depicts a broad area of rain from East Texas through the Florida Panhandle exceeding four inches, the localized amounts exceeding 10 inches through Friday morning.

NOAA QPF- precipitation potential map depicts the possibility of 10 or more inches of rain in localized pockets along the northern Gulf Coast. 

Keep in mind that "weaker" tropical cyclones, such as minimal tropical storms, often generate heavier rainfall totals than better developed tropical storms and hurricanes. This is counterintuitive to most people, as we may assume that all hazards are worse during higher "category" cyclones. However, weaker tropical storms are not as well developed vertically so they do not tap into the upper level steering winds that would generally give them a faster forward motion. So they tend to stick around for awhile.

The biggest concern is in areas that observe "training" of persistent rain bands well to the east of the center of circulation.

Gulf Coast residents should be well aware that we do not need a strong tropical storm or a hurricane to generate substantial flooding. Last August, a hybrid tropical low that was an unnamed storm dumped more than 30" of rain in portions of South Louisiana near Baton Rouge.

Trailer in front of a home near Denham Springs, Louisiana. Photo taken by Hal Needham in May, 2017.

Residents in South Louisiana are still cleaning up from this catastrophic flood, which inundated tens of thousands of buildings. Last month I drove through Denham Springs, Louisiana, one of the worst impacted areas, and was saddened to see many people still cleaning up and living in trailers on their property.


Let's hope the forecasted heavy rains do not repeat flooding in these hardest-hit areas.

Monday, June 19, 2017

Minor Coastal Flood Event Likely Along U.S. Gulf Coast This Week

A developing area of low pressure near the northern Yucatan Peninsula will likely generate a minor coastal flood event along the U.S. Gulf Coast this week. The National Hurricane Center forecasts an 80% chance of tropical cyclone development within the next 48 hours and a 90% chance of tropical cyclone development within the next five days, in 8AM EDT update from Mon Jun 19.

If this system develops it would be the third named storm of the Atlantic Hurricane Season, named Cindy.

Deep convection firing near the northern Yucatan Peninsula will likely develop into a tropical cyclone in the Gulf of Mexico in the next two days, according to the National Hurricane Center.


The two main global forecast models, the European and American (GFS), have consistently shown different solutions for the forecast track. The European model has favored a more westward track, while the GFS has favored a more eastward solution.

The two models are showing more consensus this morning, as they are splitting the difference. The European model favors a forecast track into the west-central Gulf of Mexico by Tuesday evening local time, which would bring onshore winds possibly exceeding tropical storm force into southeast Louisiana.


The European model forecast for Tuesday evening local time. Note strong onshore winds in southeast Louisiana. Image: www.tropicaltidbits.com

Note the strong (30 kt) shore-parallel winds forecast for coastal Texas. Such winds effectively generate coastal flooding due to Texas' coastal profile and Ekman transport, which turns water to the right of the wind direction in the Northern Hemisphere. Therefore, this scenario would cause minor coastal flooding from the Florida Panhandle through coastal Texas.

The GFS model favors a track into the north-central Gulf of Mexico, with strong onshore winds from southeast Louisiana through the Florida Panhandle. Offshore winds are forecast for south Louisiana and the Upper Texas Coast, while strong onshore winds buffet the Florida Panhandle.

The American (GFS) model for Tuesday evening local time. Note strong onshore winds along the Florida Panhandle. Image: www.tropicaltidbits.com

In this scenario, minor coastal flooding would occur from extreme southeast Louisiana through the Florida Panhandle, with offshore winds generating lower-than-normal water levels from south-central Louisiana through much of the Texas coast.

It is important to not follow the precise track at this point, as the forecast will become more refined throughout the day Monday. Regardless of where the system tracks, it is looking more likely that some locations along the U.S. Gulf Coast will observe coastal flooding late Tuesday into early Wednesday.

Tuesday, March 28, 2017

Tropical Cyclone Debbie Pounds Queensland with Powerful Storm Surge

Tropical Cyclone Debbie made landfall along the Queensland coast Tuesday as a category-4 cyclone on the Australian cyclone wind scale. Debbie produced maximum sustained winds of around 175 km/ hr (110 mph), which would place it at the threshold of a category 3 hurricane in the Atlantic.

Debbie generated substantial coastal flooding for areas east and south of Bowen. Photographs coming from the impacted area have depicted storm surge and large waves pounding coastal buildings on Hamilton Island. 

Storm surge and waves pound a building on the coast of Hamilton Island, Queensland, on Tue Mar 28. Photo: http://www.abc.net.au/news/2017-03-28/cyclone-debbie-bom-before-after-hamilton-island-photos/8393164


Near Midge Point, photographs depict compound flooding from storm surge and heavy rain, without the presence of wave action. Witnesses on the ground have reported around 1 m (3.3 ft) of water above ground level, inundating buildings in the town. The region from Midge Point to Conway has a harbor that is open to the southeast, enabling prolonged winds to pile up storm surge in this region.

Compound storm surge and rainfall flooding in Midge Point, Queensland, on Tue Mar 28, 2017. Local reports indicated 1 m (3.3 ft) of water above ground level. Photo submitted by Jodi Lorraway of Proserpine.

Compound storm surge and rainfall flooding in Midge Point, Queensland, on Tue Mar 28, 2017. Local reports indicated 1 m (3.3 ft) of water above ground level. Photo submitted by Jodi Lorraway of Proserpine.

Storm surge levels exceeded 1 m (3.3 ft) above normal astronomical tides from Shute Harbour to Mackay. The highest recorded water level was 2.75 m (9 ft) at Laguna Quays, however, higher water marks may be found after the storm subsides and coastal surveys are conducted.

Storm surge levels exceeded 1 m (3.3 ft) from Shute Harbour to Mackay, with the highest recorded levels coming from Laguna Quays, where the storm surge reached around 2.75 m (9 ft). 

Unfortunately, the peak storm surge for many locations occurred near the time of high tide, enabling total water levels to exceed Highest Astronomical Tide (HAT). HAT represents the highest high tide under "normal" (non-storm) conditions, taking into account astronomical tide levels.

This was due, in part, to Debbie's slow forward speed, which enabled high water to persist for more than one day, ensuring high storm surge would occur near the time of high tide. Locals on the ground reported that one of the major differences between Debbie and Tropical Cyclone Ului, in 2010, was that Debbie's wind and storm surge lasted noticeably longer.

At Laguna Quays, the 2.75 m (9 ft) storm surge occurred near the time of high tide, enabling total water level (storm surge + tide) to exceed HAT  by around 80 cm (2.6 ft). 

Water level graph for Laguna Quays, Queensland. The orange line depicts storm surge (height above normal astronomical tide), while the blue line represents total water level, including tides. Source: https://www.qld.gov.au/environment/coasts-waterways/beach/storm-sites/laguna-quays/

Debbie generated the highest storm surge in Queensland since Tropical Cyclone Yasi produced a 5.33-m (17.5 ft) surge in 2011. According to the U-Surge Project, Debbie generated the 14th highest storm surge in Queensland since 1880, or a storm surge level that should only be expected approximately every 10 years in that state.

Tropical Cyclone Debbie generated a storm surge of 2.75 m (9 ft) at Laguna Quays, Queensland. This is the 14th highest recorded storm surge in Queensland since 1880.

Monday, March 27, 2017

Storm Surge Levels Rising along Queensland Coast

Tropical Cyclone Debbie Update
0230 AEST Tue Mar 28
1630 UTC Mon Mar 27

Storm surge starting to push in from Bowen to Dalrymple Bay. Storm surge at Laguna Quays has reached 1.2 m (3.94 ft) and surge levels at Bowen, Mackay and Dalrymple have all surpassed 0.5 m (1.64 ft).


The highest recorded storm surge level so far is the 1.2 m (3.94 ft) surge at Laguna Quays. This water level is tied for the 37th highest storm surge event in Queensland since 1880.


Although the storm surge is building in these areas, total water levels are falling because high tide has passed. The good news is that these locations made it through high tide without the water level surpassing Highest Astronomical Tide (HAT). This means that Debbie's water level did not exceed the "highest" high tides under normal (non-storm) conditions.


Storm surge levels were rising at Laguna Quays, QLD, after 2AM Tuesday morning, but total water levels were decreasing as the time of high tide had passed. Data source: https://www.qld.gov.au/environment/coasts-waterways/beach/storm-sites/laguna-quays/....annotations provided by Dr. Hal Needham.

The area of greatest concern is the north-facing coast east of Bowen. This area is observing lower-than-normal water levels as Debbie approaches, but as soon as the eye passes, intense winds from the north will quickly push a tsunami-like storm surge into this coastline. Anyone sheltering lower than 3 m (10 ft) above sea level in this area should beware.


Tropical Cyclone Debbie to Generate Large Storm Surge along Coast of Queensland, Australia

Tropical Cyclone Debbie was bearing down on the coast of Queensland, Australia, late Monday evening local time. As of 1300 AEST (0300 UTC) on Mon Mar 27, maximum sustained winds near the center of circulation were sustained at 150 km/ hr (93 mph), making it a category-3 on Australia's tropical cyclone category system. Debbie is forecast to make landfall near Bowen as a category-4 tropical cyclone around 0900 AEST on Tue Mar 28, however, conditions will deteriorate throughout Monday night between Bowen and Mackay.




This map from the Australia Bureau of Meteorology depicts destructive winds from TC Debbie coming ashore near Airlie Beach as of 10PM local time (1200 UTC) on Mon Mar 27.

Storm surge levels were beginning to build on Monday afternoon at several sites along the Queensland coast. Laguna Quays, Mackay and Dalrymple Bay all reported storm surge levels exceeding 0.5 m (1.64 ft).  Water levels should increase through the night as TC Debbie approaches the coastline, with storm surge exceeding 2.5 m (8.2 ft) in localized areas between Bowen and Mackay.




Storm surge levels exceeded 0.5 m (1.64 ft) at Laguna Quays on the Queensland coast late Monday evening, as TC Debbie approached the coastline.

Most locations in this part of the world have a high tidal range, with the difference between high and low tide often exceeding 4 m (13.1 ft). Such large tidal ranges will affect the timing of maximum coastal flooding on the landscape. While storm surge refers to the difference between predicted (astronomical) tides and actual water levels, storm tides combine storm surge with astronomical tides to produce a total water level that is seen on the landscape.

Most areas will experience the greatest coastal flood impact within two hours of high tide, however, storm tide flooding is quite localized, and a difference of several kilometers can make substantial difference in water levels. The table below provides generalized information on tidal ranges and the timing of the greatest flood impact for selected locations.


Waves and storm surge were building along the Queensland coast on Monday morning (local time). This pic shows conditions near Mackay. Image: @meljmaddison on Twitter.


Location                    Tidal Range                Time of Greatest Coastal Flood Impact
Dalrymple Bay        5m+ (16.4ft+)               10PM Mon - 2AM Tue
Mackay                    5m+ (16.4ft+)               10PM Mon - 2AM Tue
Laguna Quays         4m+ (13.1ft+)                10PM Mon - 2AM Tue
Shute Harbour         3m+ (9.84ft+)                10PM Mon - 2AM Tue
Bowen                     2.5m+ (8.2ft+)              10PM Mon - 2AM Tue       
                                                                                 8-10AM Tue

Much of the coast will experience the greatest coastal flood impact between 10PM Mon and 2AM Tue, local time, corresponding with the hours near high tide. However, some areas near or just south of Bowen, could experience a secondary high water event later Tue morning near the time of landfall.

There should be a drastic difference in water levels and timing of high water between north- and south-facing coasts...

South-facing coasts, in areas such as Conway:
Water levels will gradually build as Debbie approaches, reaching maximum levels just before Debbie makes closest approach...

North-facing coats, such as Airlie Beach
Water levels may actually be lower than normal as Debbie approaches due to strong south winds. Just after closest approach, winds will suddenly blow from the north and could rapidly generate storm surge....flooding could come in as we typically think of a tsunami...almost in one sudden wave.

Fortunately, the coast of Queensland has been on high alert and flood/ evacuation maps have circulated through many communities. For example, Townsville City Council circulated a map depicting the most areas most vulnerable to storm surge flooding to help people make evacuation decisions.

Townsville City Council map depicting areas most vulnerable to coastal flooding. The Townsville Bulletin circulated this map to help people make potential evacuation decisions.

The U-Surge Project has identified storm surge levels for 72 tropical cyclones that have struck Queensland since 1880, providing an updated database that builds off the foundational work from Needham et al. (2015). A storm surge level of at least 2.5 m (8.2 ft) would tie Debbie for 15th place since 1880, according to Queensland storm surge records. This would make Debbie's storm surge around a 9-year flood event, or a flood level we should expect on average around every nine years.

As of early Monday afternoon, TC Debbie's high water mark of 0.85 m (2.62 ft) ties it for 50th place all-time since 1880. This blog will be updated frequently through the storm, enabling you to follow Debbie's peak surge level and see how high it ranks in historical context. 



Needham et al. (2015) provided a data-driven frequency analysis of storm surges in Queensland, finding that this region observes an average of 2.5 storm surges per decade exceeding 2 m (6.56 ft) and an average of 1.4 storm surges per decade exceeding 3 m (9.84 ft).

Tropical Cyclone Yasi was the last cyclone to generate a storm surge exceeding 2 m (6.56 ft) in Queensland. This cyclone generated a storm surge of 5 m (16.4 ft) near Cardwell in 2011.