Follow Me from Inside Florence

Follow me from inside Hurricane Florence. I’m in a parking garage in North Myrtle Beach, SC. 

I’m updating everything on my Facebook feed:

https://www.facebook.com/profile.php?id=100010445988869

I have to download an app to Post a pic here and I can’t do that now- so just text on today’s post...

Hal

Florence Threatens to Inflict Unusual and Multi-Dimensional Flood

Main Points about Flood Potential

1) Hurricane Florence is forecast to slow down and drift near coastline. This would inflict a prolonged compound flood event from storm surge and heavy rain.

2) Possible southward drift along coastline would inflict an unusual, rapid storm surge event, as winds immediately reverse from offshore to onshore when Florence's eye passes.

3) Heavy rain impacts are likely for extended distances inland and to the north of Florence's center. The decrease in forward speed will enable Florence to spread torrential rain over a widespread area for many days.

4) Storm surge flood threat most pronounced in the Carolinas. Inland flood impacts extend as far west as the Appalachians and as far north as Pennsylvania; ground from Virginia through Pennsylvania is already saturated.

Action Items

1) If you live along East Coast, from South Carolina through Pennsylvania, stay tuned to the National Hurricane Center, National Weather Service and your local media outlets for updated information, warnings and mandatory evacuations. Florence threatens to inflict disastrous wind and flood impacts across a large region.

2) Two Facebook groups are connecting people people threatened by Hurricane Florence with volunteers. Flood Survivors (Harvey and Others) Helping Hurricane Florence Friends connects flood victims with flood survivors and provides practical discussions on everything from evacuation to mold remediation. Person2Person4Relief directly connects flood victims with volunteers who help with flood relief. 

Meteorological Analysis in Detail

Hurricane Florence is a category-4 hurricane on the Saffir-Simpson Hurricane Wind Scale. Florence will move into a favorable environment with low wind shear and sea surface temperatures exceeding 84 degrees F (29C) over the next day, enabling some strengthening Wednesday. Florence may begin to slightly weaken on Thursday, as the storm encounters increased wind shear, cooler sea surface temperatures from upwelling and drier air drawn in from the continent. Nonetheless, Florence should approach the coastline as a major hurricane with widespread impacts.

Florence to Drift near the Coastline

A major change from yesterday's forecast is that it is becoming more likely that Florence will slow down and drift before making landfall. This is an ominous development, as warm ocean waters fuel hurricanes. A hurricane centered offshore will weaken at a slower rate than one that stalls inland.

Another development in today's forecast is that Florence is expected to drift west, or even southwest, possibly enabling it to track along the North Carolina and South Carolina coastline. A slow-moving hurricane tracking near a coastline is bad news indeed, as it enables the storm to inflict destructive storm surge along an extended area.

The latest run of the European model shows the unbelievable- Florence nearly making landfall near Wilmington, North Carolina on Thursday evening....then slowly tracking along the entire South Carolina coast through the entire weekend and making a landfall near the Georgia/ South Carolina border on Sunday evening. This scenario would devastate the Carolinas.

European model depicting Florence's position on Thursday evening. Source: Tropical Tidbits/ Levi Cowan.

European model depicting Florence's position on Sunday evening. Source: Tropical Tidbits/ Levi Cowan.

It is important not to focus on the exact track, as Florence's extended forecast is still quite uncertain. It becomes difficult to forecast a hurricane's path when there is a drastic decrease in the upper-level steering currents, rivers of air in the upper levels of the atmosphere that drive these storms.

This makes sense to us because we have all experienced a change in steering when riding a bicycle. At a high speed it is easy to choose a straight path, but when we stop pedaling our path becomes less certain as we approach our stopping point. Hurricanes that lose their upper-level steering sometimes wobble or drift in an erratic pattern for extended periods.

Unusual, Rapid Storm Surge Possible

Most hurricanes that track along the East Coast move from south to north. Because circulation around hurricanes in the Northern Hemisphere is counterclockwise, onshore winds steadily increase as the storm approaches. This enables storm surge to gradually build over time.

It is quite unusual for a hurricane to track from north to south along this coastline, and if this happens, as the European model suggests, the wind and storm surge patterns will be unusual, and inflict rapid storm surge flooding in southern North Carolina and along the South Carolina coast.

This coastline would experience increasing offshore winds, blowing from land to water, as Florence's center approaches. This would create chaotic seas, as Florence is still displacing tremendous amounts of water towards land, but a powerful offshore wind would serve to mitigate water levels.

Super Typhoon Haiyan generated a rapid storm surge in the Philippines as winds in the typhoon's eyewall shifted from offshore to onshore. Haiyan's surge hit more like a tsunami than a typical surge. Footage of this event is available in the first minute of this video from the PBS/NOVA documentary Killer Typhoon.

However, as soon as Florence's center passed a location, powerful winds in the hurricane's eyewall, the most intense part of the storm, would immediately shift from offshore to onshore, producing a destructive storm surge in the matter of minutes.

This phenomenon was observed in Super Typhoon Haiyan in the Philippines in 2013. Haiyan's storm surge rushed in more like a typical tsunami than a hurricane storm surge. See the video above (first minute of footage).

It should be noted that Haiyan was among the most intense tropical cyclones to ever make landfall, and the sharp boundaries of the bay near Tacloban exacerbated this surge event. I do not expect a Florence's surge would move in as rapidly.

Nonetheless, a southward moving hurricane tracking along the coastline, would produce a sudden reversal of winds from offshore to onshore, rapidly pushing a storm surge into the coastline. This is unfortunate, as rapidly-moving flood events kill more people than gradual floods.

Compound Flood Threat Increasing 

While Florence's storm surge and rainfall forecasts are constantly updated, it is important to realize that Florence threatens to inflict widespread compound flooding, from the combination of storm surge and heavy rain, particularly near the coastline.

During compound flood events, prolonged onshore winds elevate the ocean and fill bays to capacity with salt water, reducing the efficiency for heavy rainfall to drain. This phenomenon enables less rain to produce serious flooding, as the drainage is slowed considerably.

In 2012, Hurricane Isaac generated a widespread compound flood in southeast Louisiana, flooding some areas that were not flooded by Katrina. This surprised many people, as Isaac was a category-1 hurricane. However, Isaac stalled along the coast, pumping saltwater into Lake Pontchartrain for days, which slowed the drainage of 10-15 inches of rain in the region.  

Hurricane Isaac (2012) inflicted widespread compound flooding in southeast Louisiana. The prolonged nature of the event enabled storm surge and heavy rain to flood some areas not flooded by Katrina. Image: https://www.youtube.com/watch?v=ZMSe75GIo98.

Thomas Wahl, Assistant Professor in the Department of Civil, Environmental and Construction Engineering at University of Central Florida has studied this phenomenon extensively. He is lead author on a recent paper published in EOS Earth and Space Science News that investigates such complex environmental relationships and takes a broad viewpoint on how multiple factors can amplify the effect of severe storms and other extreme events.

Wahl was also lead author on a paper in Nature Climate Change that found the threat of compound flooding from storm surge and heavy rainfall was increasing for major U.S. cities.

Wahl expects impacts from compound flooding in Florence and stated that his biggest concerns come from, "A possibly stalling system that could bring unprecedented rainfall and storm surge to the area, not just in terms of intensity but also duration." 

Rainfall Forecast

National Hurricane Center rainfall forecast from this morning's advisory.

Widespread and long-duration rainfall will likely generate unprecedented flooding through the Carolinas. The graphic above, from this morning's National Hurricane Center advisory, depicts a broad area with 6-10 inches of rain, with rainfall exceeding 20 inches near the North Carolina coast. The advisory states that 20-30 inches could fall in this area, and mentions the possibility of 40-inch totals.

Keep in mind that the rainfall patterns will be highly dependent on Florence's track, which is difficult to pinpoint for a hurricane that stalls near the coastline. Localized maximum rainfall can considerably exceed such forecasts, as training of squalls over the same areas can produced extraordinary rainfall totals for slow-moving tropical systems.

I expect rainfall forecasts may increase considerably across central and western North Carolina, if Florence does indeed drift west, with the center of circulation passing south of this area. Prolonged east winds, which would gain lift as they upslope up the eastern side of the Appalachians, could generate phenomenal rainfall totals.

Heavy Rainfall Displacement in Stalled-out Hurricanes

We all make assumptions when we jump to a conclusion that seems obvious. If your friend tells you she lost 10 pounds, the assumption is that she has become healthier. But maybe she has been stressed and not eating, or began smoking heavily. A dramatic increase in salary with a job in a new city may imply increased wealth, until we realize the cost of living in the new city is double.

A major assumption people make with hurricanes is that the flood impacts will be worse the closer we are located to the eye of the storm. This seems intuitive. However, did you realize someone 100 miles from the storm's center may experience more severe flooding than someone 20 miles from the center?

Hurricane Harvey (2017) produced heavy rainfall that extended far from the center of circulation on the onshore side of the storm, but dry conditions near the eye on the offshore side. Adapted from Youtube Channel: Washington 1067. Markups by Hal Needham.

The counter-clockwise flow around Northern Hemisphere hurricanes that stall out near the coastline, produce onshore winds to one side and offshore winds on the other side.

Heavy rainfall commonly extends for more than 100 miles on the onshore side, but can rapidly diminish on the offshore side.

I have personally experienced this phenomenon during both Hurricanes Isaac (2012) and Harvey (2017).

The image above shows the rain pattern in Texas as Harvey was centered inland, between Corpus Christi and San Antonio. Harvey whipped bands of torrential rain more than 100 miles east of the circulation center for several days, while areas near the circulation center, on the "left" side of the storm stayed relatively dry.

People in Corpus Christi were shocked by the relative lack of flooding, as hurricane maps predicted a category-4 hurricane to strike not far east of them, while people displaced more than 100 miles to the east, near Houston/ Galveston and Beaumont/ Port Arthur, were taken off guard that a storm that "missed them" could inflict unprecedented flooding. 

NOAA radar loop from Hurricane Isaac (2012). Image courtesy Charles Kuster Youtube Channel.

The image above is taken from a Hurricane Isaac (2012) radar loop. Note the extent of squalls wrapped around the "right" (onshore) side of circulation, while areas just west of the eye were observing light to moderate showers.

Isaac drifted slowly northwest and the center of circulation passed not far from my home in Baton Rouge. However, Isaac's powerful squalls were displaced considerably to right side of the storm, inflicting more flooding in coastal Mississippi, more than 100 miles to the east, than at my location near the center of the storm.

Rainfall map from Hurricane Isaac (2012) shows heavy rainfall displaced to the right of the storm track

A map of Isaac's observed rainfall shows a substantial displacement of heavy rainfall to the "right" of the storm track. In fact, it appears that Isaac's track provides a western boundary on the area of heavy rain.

We cannot generalize that all hurricanes produce this rain pattern. However, when a hurricane stalls out near the coast for prolonged periods, the side with onshore winds is pulling warm, moist air off the ocean, while the side with offshore winds is pulling drier air off the continent.

The application to Florence is that areas well north of the storm track could observe surprising and unprecedented rainfall totals. At the same time, people in places like South Carolina and even Georgia should remain vigilant...although the southern boundary of the rain shield will probably have a sharp gradient, we cannot know where those rain totals will drop off because of uncertainty in the storm track.

Saturated Soils will Enhance Any Rainfall across Virginia and Mid-Atlantic States

Finally, a look at recent precipitation and soil moisture content is important for us to get a picture of the ability for the soil to retain heavy rainfall. Fortunately, September has been relatively dry so far for much of the Carolinas and Georgia, as depicted in this PRISM rainfall map below. Northern Virginia and the Mid-Atlantic States, however, have had a wet month so far, with flooding recently reported in northern Virginia, Maryland and Pennsylvania.

Cumulative rainfall map for September, from PRISM.

A wet September has unfortunately led to saturated soils from northern Virginia through Pennsylvania and New Jersey. The map below from the Climate Prediction Center/ NCEP/ NOAA shows that soil moisture in this region is unusually wet. Therefore, it will not take much rainfall in these areas to induce rapid flooding. 

Map of Soil Moisture Anomalies. Climate Prediction Center/ NCEP/ NOAA

The Latest on Florence's Storm Surge and Overall Flood Potential

Main Points

1) Hurricane Florence has intensified to a category-4 hurricane and will likely maintain category-4 strength as it approaches the Carolinas;

2) Florence will generate a large storm surge near and to the east of the landfall location;

3) Wind, rain and storm surge impacts will increase through the day on Thursday...landfall is likely Thursday night or early Friday;

4) Global weather models agree that Florence will slow down and drift. There is still considerable uncertainty in the timing and location of Florence stalling. If the drift occurs near the coast or offshore, the geographic extent and magnitude of storm surge flooding could increase dramatically;

5) The truly extraordinary and most catastrophic impacts of Florence will likely come from days of torrential rain near and well north of Florence's center from Thursday through early next week. Regardless of the exact track and intensity, extraordinary rainfall totals could impact locations from South Carolina through Pennsylvania/ New Jersey causing a widespread catastrophe.

Action Points

1) If you live along East Coast, from South Carolina through Pennsylvania, stay tuned to the National Hurricane Center, National Weather Service and your local media outlets for updated information. Florence threatens to inflict a widespread catastrophe.

2) I created a Facebook group yesterday to enable people threatened by Hurricane Florence to connect with survivors from previous floods. People are sharing lessons learned here- covering everything from evacuation to mold remediation. Click here to go to this group.

Meteorological Analysis in Detail

Florence has intensified to a category-4 hurricane and is forecast to maintain that intensity through landfall. The storm will increase in forward speed and approach the Carolinas over the next two days. The latest National Hurricane Center (NHC) advisory is shown below.

Latest advisory from the National Hurricane Center

Florence will be tracking over a large region with favorable conditions to support a category-4 hurricane or even strengthen to category 5. Wind shear remains relatively light in this region and Florence is forecast to track over a large area where sea surface temperatures reach 86 degrees F (30 degrees C). The image below, adapted from a tropical cyclones website at the CIMSS/ University of Wisconsin, depicts this large area of warm waters over which Florence will traverse.

CIMSS/ University of Wisconsin tropical cyclone website depicts that Florence will track over a large area of warm water exceeding 86 degrees F (30 degrees C). Storm position added by Hal Needham.

Florence's Storm Surge Potential

Florence threatens to generate a large and destructive storm surge near and to the east of the landfall location. Given the morning NHC advisory, this region will most likely occur along the North Carolina coast.

Major (category 3 and 4) hurricanes in the Carolinas have historically generated storm surges in the 15-20 foot range. Hurricane Hugo (1989) made landfall as a category-4 hurricane and generated a 20-foot storm tide (surge + tide) above Mean Sea Level (MSL) in South Carolina. Hurricane Hazel (1954) made landfall as a category-4 hurricane near the border of North Carolina and South Carolina and generated a storm tide of 18 feet above Mean Low Water, but when this is converted to height above MSL is reduces to 15-16 feet. Hurricane Fran (1996) made landfall as a category-3 hurricane in this same region and also generated a 15-16 foot storm tide.

Storm tide maps of these three events are shown below:

Hurricane Hugo (1989) made landfall as a category-4 hurricane in South Carolina and generated a 20-foot storm tide.

 Hurricane Fran (1996) made landfall as a category-3 hurricane and generated a storm tide ranging from 15-16 feet in North Carolina.

Hurricane Hazel (1954) generated a storm tide reaching 18 feet above Mean Low Water. When converted to Mean Sea Level, this storm tide reached 15-16 feet. Source: Harris (1963). The two lines near left of the map mark the boundaries of Hazel's eye, as it moved from south to north.

Although category-4 hurricanes in this region generate large, destructive storm surges, Florence has two characteristics that may serve to somewhat limit storm surge flooding compared to other storms with comparable intensities.

Characteristic #1: Fast Forward Motion

Florence is moving forward at 15 miles per hour and this rate will likely increase over the next two days. Fast-moving hurricanes have less potential to generate storm surge than slow moving hurricanes, because it takes time for strong winds to displace massive amounts of water.

This characteristic has one major caveat, however. The models are uncertain about the timing and location of Florence's stall, and last night's European model run suggests the stall could begin offshore. If the stall occurs offshore, this will serve to dramatically increase the storm surge magnitude and geographic extent of coastal flooding.

Characteristic #2: Relatively Small Wind Field Compared to Historic Hurricanes in Region

A second factor that may limit the magnitude and geographic extent of Florence's storm surge is the relatively small wind field, at least compared to historic hurricanes that have impacted the Carolinas. I was lead author on a paper that found the radius of pre-landfall 50-knot (58 mile-per-hour) wind fields correlated better with storm surge heights than other wind fields.

As Florence approaches the coastline, the NHC forecasts the radius of 50-knot winds to extend 80 nautical miles (nm) to the northeast. This extent of 50-knot winds is nearly half the extent observed in both Hugo (1989) and Fran (1996), which both generated 50-knot winds to extend 150 nm from the center of circulation.

Hurricane Hugo (1989) generated 50-knot (58-mph) winds 150 nautical miles from the center of circulation as it approached the coastline.

Hurricane Fran (1996) generated 50-knot (58-mph) winds 150 nautical miles from the center of circulation as it approached the coastline.

The NHC forecasts Hurricane Florence's radius of 50-knot (58-mph) winds to reach 80 nautical miles as it approaches the coastline (11pm advisory Mon Sep 10). However, if Florence stalls offshore, the wind field may drastically increase.

Smaller wind fields can limit both the total magnitude and geographic extent of storm surge inundation, because strong winds reach the coast later than hurricanes with large wind fields. Also, strong winds in larger storms are displacing water over a greater area than smaller storms.

If the NHC's size forecast holds and Florence can make it through landfall without stalling, I expect Florence could generate a localized storm surge level of approximately 14-16 feet east of the landfall location, but surge levels could taper off quickly outside this area.

However, global models suggest that Florence's size could increase as it slows down, so if the stall and drift take place offshore, these size estimates could change dramatically. Florence is a powerful and dangerous hurricane and interests from South Carolina through the mid-Atlantic states should be on guard for coastal impacts.

Florence's Stall and Drift

Global weather models agree that Florence will likely stall and drift later in the week. The timing and location of this stall is still quite uncertain.

If this stall occurs while Florence is centered near the coast or just offshore, it will increase the storm surge extent and impacts dramatically. Monday evening's European model run forecasts Florence's stall to begin offshore. The images below show the unthinkable- Florence making landfall on Thursday evening (top image).....and Florence still making landfall on Friday evening (bottom image).

The European model run from Monday evening depicts Florence's stall starting offshore....with the storm making landfall on Thursday evening (above) and still making landfall Friday evening (below). Images courtesy: Tropical Tidbits/ Levi Cowan

Regardless of where the stall takes place, the truly extraordinary impact from Florence will come from days of torrential rain near and far north of the storm's center. These impacts could extend as far north as Pennsylvania and New Jersey.

Unfortunately, this summer has been unusually wet along much of the East Coast, and soils are already saturated in many areas- particularly in Virginia, Maryland and Pennsylvania.

Prolonged onshore winds will fill Chesapeake and Delaware Bays to capacity, slowing the drainage of heavy rainfall across a broad area.

Florence threatens to take us into unknown territory. NOAA's Quantitative Precipitation Forecast (QPF) shows the potential of 10 or more inches of rain across a broad area from eastern North Carolina through much of Virginia over the next seven days, but these values could increase dramatically as this event unfolds.

It is common for QPF maps to start with a broad forecast of rain potential on a conservative basis. Much of the flooding from hurricanes can be localized from training of squalls over the same areas, and these localized maximums are unknown at this point and cannot be accurately forecast.

Last year during Hurricane Harvey, early precipitation forecasts depicted a large area of rain exceeding 15 inches.....in the end, training of rain bands over the same area generated rainfall totals exceeding 50 inches in the most extreme areas.

It's certainly possible that Florence's rainfall totals could exceed 25 inches to the north of where the system stalls out. This scenario threatens to inflict an unprecedented catastrophe across a widespread area.

REFERENCES

Harris, D.L., 1963. Characteristics of the Hurricane Storm Surge. United States Weather Bureau, Technical Paper No. 48. 

Gordon's Storm Surge Ranged from 2-3 Feet

Tropical Storm Gordon made landfall near the Mississippi/ Alabama border last night. Highest storm surge levels ranged from 2-3 feet along the stretch of coast from Shell Beach, Louisiana through Apalachicola, Florida.

Gordon's track, lopsided nature and relatively fast forward speed kept storm surge levels modest through the region.

Map of Gordon's peak storm surge along the Gulf Coast and Southeast Atlantic

Gordon generally maintained a forward speed of 14-15 mph as it tracked across the Gulf of Mexico. This is slightly higher-than-normal for the region and did not allow enough time for Gordon to generate substantial storm surge.

As Gordon approached landfall, the heavier convection was clearly located on the northern and eastern areas around the eye, with much less activity to the west and south. This kept storm surge levels relatively low to the west of the storm's track.

This radar image from Weather Underground shows the lopsided nature of Gordon's convection as it approached landfall. Most of the activity was located to the north and west of the eye, with less activity found on the south and west sides.

Had Gordon tracked farther west, making landfall in southwest Mississippi or southeast Louisiana, the stronger side of the storm would have pushed higher storm surge levels into Mississippi that would have likely reached the 3-5 foot range. However, Gordon's strongest winds impacted Alabama and the western Florida panhandle, which are less efficient than Mississippi and southeast Louisiana for generating storm surge.

Here are selected storm surge observations, measured in maximum difference between observed and predicted tide levels:

Apalachicola, Florida:               2.19 ft
Panama City Beach, Florida:     2.0 ft
Pensacola, Florida:                    2.5 ft
Dauphin Island, Alabama:         2.88 ft
Mobile State Docks, Alabama:  2.53 ft
Pascagoula, Mississippi:            1.87 ft
Bay Waveland Yacht Club
(Bay St Louis, Mississippi:)       2.0 ft
Shell Beach, Louisiana:              2.88 ft

The next concern is inland flood potential, as Gordon's track is forecast to slow down. Even after Gordon is downgraded to a tropical depression, flood potential remains high from the Lower Mississippi Valley through much of Arkansas.

From a storm surge perspective, all eyes now turn to Florence, well offshore in the Atlantic. Both American and European models suggest it could approach the Eastern Seaboard as a powerful hurricane next week.

Gordon to Generate Storm Surge Flooding along Northern Gulf Coast

Tropical Storm Gordon continues to track towards the Northern Gulf Coast....as of 0700 Eastern Time this morning, maximum sustained winds were 65 mph and movement was west-northwest at 15 mph. This means overnight Gordon strengthened and the forward speed slightly decreased. Gordon's forward speed is still faster than average for this region, but is forecast to continue decreasing through landfall.

GOES EAST Infrared Satellite Imagery shows that Gordon's convection has increased overnight. Darker colors indicate colder temperatures and higher cloud tops.

Gordon is a relatively small tropical cyclone, with tropical storm force winds extending 80 miles from the center of circulation. The National Hurricane Center's surface wind field map, depicted below, shows the extent of Gordon's tropical storm force winds (shaded orange) - showing us that these winds cover a relatively small area. Small storm size and relatively fast forward speed will reduce the extent of storm surge flooding. 

The National Hurricane Center forecasts storm surge inundation potential of 3-5 feet above normally dry ground from Shell Beach, Louisiana, through Dauphin Island, Alabama. This includes the entire Mississippi Coast. To the east, from Dauphin Island through Navarre, Florida, including Pensacola, storm surge potential is 2-4 feet. The same level is forecast from Shell Beach through the mouth of the Mississippi River.

Given Gordon's small size and forecast track, I would be surprised to see storm surge levels exceeding 3 feet east of the Florida/ Alabama line, and I expect any coastal flooding in the western Florida Panhandle, including Pensacola, to be minor.

Storm surge inundation could reach 3-5 feet from Shell Beach, Louisiana through Dauphin Island, Alabama, including the entire Mississippi Coast. 

While Gordon should not generate an extensive storm surge, in terms of geographic extent, it is forecast to become a hurricane, with the center of circulation approaching the Mississippi Coast, one of the most efficient areas in the world for storm surge generation. Coastal residents should take caution- storm surge can build very rapidly in this region- particularly in Southeast Louisiana, like near Shell Beach, and the entire Mississippi Coast. Where hurricane-force winds are blowing on shore, storm surge levels can increase rapidly.

A word of caution about Southeast Louisiana, from near the Rigolets to mouth of the Mississippi River....when hurricanes strike the Northern Gulf Coast, we generally think of onshore wind direction as blowing from south to north, however, in extreme southeast Louisiana, the coastline shape throws a wrench in this generalization. Where the Mississippi River Delta juts out into the Gulf of Mexico, strong northeast or even north winds are actually blowing "onshore" and displacing water from Mississippi Sound to wetland areas and small towns along the Mississippi River levee.

As of 0700AM Eastern Time on Tue Sep 04, storm surge levels have not exceeded 2 feet at any coastal location along the U.S. Gulf Coast. Visit the U-Surge Project for updates.

As Tropical Storm Ida made its closest approach to Mississippi Sound in November 2009, water levels at Shell Beach, Louisiana, actually peaked as winds turned from the north. The coastal geography in this area exposes Shell Beach to a north wind, pushing water into town.

So friends in Southeast Louisiana- don't let your guard down if Gordon's center of circulation passes east of you....you can still generate quick storm surges on the "left" side of the storm.

Last night I looked into storm surge history for Lake Pontchartrain and I'm not concerned about substantial storm surge entering the Lake. We would need a slower moving storm centered south of New Orleans to push substantial water into the Lake, and Gordon's forecast track, small size and rapid forward speed will all serve to keep storm surge levels relatively low in this area.

As of 0700AM Eastern Time, no coastal locations were observing a storm surge exceeding 2 feet along the U.S. Gulf Coast. I will keep this map updated on the U-Surge website as Gordon approaches the coast.

Tropical Storm Gordon Eyes the Northern Gulf Coast and May Threaten Inland Areas Later in the Week

Tropical Storm Gordon forms over South Florida and is forecast to approach the Northern Gulf Coast...

Key Messages:

1) Gordon may produce some localized flooding in South Florida on Labor Day;

2) Gordon will track northwest and impact Southeast Louisiana, Southern Mississippi and Southern Alabama late Tue/ early Wed;

3) Gordon's movement is faster than average and its fast pace is forecast to continue. Faster moving storms do not have time to generate substantial storm surges, so expect coastal flooding to be minor from Southeast Louisiana through Mississippi and Alabama;

4) Gordon is forecast to slowly strengthen over the Gulf and will likely bring tropical storm force winds near and east of the landfall location;

5) After Gordon is well inland it is forecast to become a tropical depression but slow down considerably. This may pose a threat for life-threatening flooding to inland locations later this week...

Gordon's rain bands were pushing into South Florida this morning...my location is shown as the blue dot on the map- we experienced some decent squalls this morning

In Detail:

Tropical Storm Gordon formed this morning near where the southern tip of the Florida Peninsula meets the Florida Keys. I live in Southwest Miami, about 45 miles northeast of the center of circulation. We awoke to squally weather this morning, with periods of strong winds and heavy rain.

As expected, some banding of the squalls is occurring in South Florida, from Key Largo to Palm Beach, and persistent squalls training over the same locations could quickly bring several inches of rain to locations in South Florida today. Expect street flooding in low-lying areas.

Gordon is moving towards the west-northwest at 17 mph, and is forecast to make a turn towards the northwest, enabling the storm to approach the Northern Gulf Coast by Tuesday night. This forward speed is faster than average, which is beneficial for several reasons. Fast-moving hurricanes and tropical storms do not have as much time to build storm surge, so expect coastal flooding to be minor near and to the east of landfall.

Gordon's persistent squalls have generated some street flooding in South Florida this morning. I took this picture in the Kendall area of Miami. Photo: Hal Needham

Also, in recent years we have experienced several hurricanes/ tropical storms stalling out near the coastline, enabling these storms to dump massive amounts of rain just inland and produce compound flooding from rain and storm surge. Hurricane Harvey was a classic example of this.

Gordon is forecast to move quickly across the Gulf of Mexico and maintain a fast pace through the time of landfall. This is great news for minimizing flood impacts from both storm surge and heavy rain.

Unfortunately, Gordon is forecast to slow down considerably later this week. Although Gordon will likely be downgraded to a tropical depression and be centered well inland by the time this happens, life-threatening flooding is a potential threat across a swath from eastern Oklahoma through Arkansas and Northern Louisiana later in the week.

The National Hurricane Center 11AM Eastern advisory on Mon Sep 3 forecasts Gordon to track near the Northern Gulf Coast at a rapid pace and then slow down considerably after being downgraded to a tropical depression later this week

If Gordon does indeed stall once it is inland, the greatest threat to life and property would likely be from inland flooding hundreds of miles from the Gulf Coast.

We should note that Gordon comes on the heels of substantial rainfall in portions of Southeast Texas and Southwest Louisiana. Some locations in Southeast Texas have picked up 3-5 inches of rain in the past 24 hours, from Galveston Bay through the Louisiana border.

Even before Gordon's approach to the Northern Gulf Coast, portions of SE Texas and SW Louisiana have observed big-time rains, with some areas observing 3-5" over the past 24 hours. It appears that Gordon's heaviest rain will pass east and north of this area. Source: National Weather Service Radar.

It appears that Gordon will stay far enough east that torrential rains will hopefully not be repeated for SW Louisiana and SE Texas. However, later in the week, as Gordon slows down, things could get interesting for Northeast Texas, Northern Louisiana, Arkansas and Oklahoma.

NOAA's Quantitative Precipitation Forecast (QPF) rainfall map (above) shows rain potential for the next 7 days (Mon Sep 3- Mon Sep 10). Notice that the heaviest rain bypasses Arkansas. However, if Gordon slows down and tracks through SW Arkansas as a tropical depression, we have to wonder if the updated forecast will dramatically increase the rainfall in that state. We will see how things play out through the week.

Gordon will likely bring tropical storm force winds to locations near and east of the landfall location on Tuesday night. While this is better news than hurricane-force winds, tropical storm force winds can still cause some problems. Expect downed trees, loss of power and even minor structural damage on items like sheds, car ports and other structural features that are not secured.

Gordon threatens to produce flood and wind impacts from Florida through the Northern Gulf Coast and even the Mississippi River Valley/ Southern Plains later in the week

Last year during Hurricane Harvey I stayed on Galveston Island. Galveston's peak sustained (10-min) winds were 39 mph, which is the minimum threshold for tropical storm force winds. The highest wind gust (2-min) was 49 mph. While these winds don't sound extraordinary, they caused a loud whistling sound through the power lines and brought down a car port on some cars in a nearby apartment complex.

Tropical storm force winds tore down a carport on Galveston Island during Hurricane Harvey on August 27, 2018. While not as destructive as hurricane-force winds, tropical storm force winds can still cause some structural damage, bring down trees and cause power outages. Photos: Hal Needham

I have been receiving messages from dear friends in Louisiana and Texas over the past day. Given the forecast track, I expect minimal storm impacts for Baton Rouge and points west along the Gulf Coast, but inland flooding could spread as far west as Oklahoma later this week.

I will post some maps soon on Gordon's storm surge potential, and, like other storms, I will create a map that enables us to get a birds-eye-view of Gordon's storm surge as it approaches the coast.

Feel free to comment on this blog or email me at hal@marineweatherandclimate.com if you would like to reach out.

Stay safe everyone!
Hurricane Hal