Flood Event Unfolding along the Upper Texas Coast

Very serious situation unfolding along the Texas Coast this morning...

Training of intense thunderstorms will bring flash flooding to areas such as Freeport, Galveston and Beaumont/ Port Arthur...

Training of heavy thunderstorm bands along the Upper Texas Coast on Wed Sep 18

DETAILS

1) Areas from Freeport to Galveston should already have saturated surface soil. Galveston has picked up around 6.5 inches of rain so far as of 0600AM this morning. Training of heavy rain over these areas can produce flash flooding...

Parts of Sargeant...near Freeport were observing roads under water earlier...

2) League City/ Friendswood area experienced numerous hours of heavy rain yesterday afternoon and evening, but the training of thunderstorms should be south of you this morning. Nonetheless, some periods of heavy rain could hit you off and on this morning and the ground is already saturated...expect street flooding in heavy rain...

Flooding in Galveston on Wed Sep 18 near 45th Street and Avenue O....source Sara Asocar Facebook

3) Bolivar/ Chambers County- I have received reports of 5-7" of rain already....the training of heaviest thunderstorms may mostly stay south of you this morning but will lift back up to you this afternoon....ground should be saturated...and periods of heavy rain this morning could quickly produce localized flooding...

Parts of Hwy 87 were already underwater yesterday from heavy rain and high salt water levels.

4) Beaumont/ Port Arthur/ Orange Golden Triangle...you've been later to get into the game. As of 0600AM this morning much of the metro area saw only around 2 inches of rain so far. That will change in a hurry this morning, as training of intense thunderstorms could drop 2-3" of rain per hour...

Even though surface soils are relatively dry, that much rain so quickly can cause flash flooding...

As I forecast yesterday, the Golden Triangle area could be the bullseye for heaviest rain totals with Imelda....expect very heavy rains and flooding this morning..

Flooding in Galveston on Wed Sep 18 near 45th Street and Avenue O....source Sara Asocar Facebook

5) Be very careful about many of the graphics/ maps and news reports spreading. Many maps plot out the center of circulation- and this implies that the areas near circulation center are most dangerous. This is not true!

The center of circulation is near metro Houston now. I marked it with an "L" (low pressure) on the map. However, the training of intense thunderstorms is offset to the south and east of this circulation center.

I drew arrows where the training of intense thunderstorms are setting up this morning....as you can see it's offset to south and east of center of circulation.

Updated rainfall forecast for Tropical Storm Imelda

Be careful not to be deceived by reports that say Imelda has weakened or is "only" a tropical depression. These classifications are based on wind speeds, but our biggest hazard here is heavy rain....it doesn't matter how "strong" this storm is...if you see training of intense thunderstorms, flash flooding can happen quickly...it doesn't matter if the storm is a hurricane, tropical storm or tropical depression.

I think the areas with training of intense thunderstorms could start to lift north through the day today, so League City, Friendswood, Kemah, Pasadena and metro Houston may all see training of thunderstorms later today...for now these areas should expect periods of steady rain with some downpours.

Stay safe everybody! Please comment on this post to share any local observations of what you are seeing.

I will try to send another update later.

-HH

Disturbance to Bring Substantial Rain to Upper Texas Coast and Metro Houston

BIG PICTURE

- A disturbance will bring heavy rain to the Upper Texas Coast and Metro Houston area from Tuesday- Thursday, bringing drought relief but causing localized street flooding;

- We should not expect flood levels to approach anything similar to hurricanes Harvey (2017) or Ike (2008);

- Storm surge levels should remain from 1.5 - 2.5 feet...producing minor coastal flooding and impeding the drainage of heavy rainfall;

- Rainfall totals could exceed 8" in much of the Houston-Galveston metro area and exceed 6" in the Beaumont- Port Arthur - Orange metro area. Thunderstorms could produce as much as 2-3" of rain per hour...be careful if driving near flooded areas especially at night.

IN DETAIL

A disturbance approaching the western Gulf coast will bring substantial rain to the Upper Texas Coast and Metro Houston from Tuesday through Thursday. While this system will bring much-needed drought relief, areas of prolonged heavy rainfall could lead to street flooding and creeks/ bayous running over their banks.

Rainfall map from the National Weather Service in Lake Charles, Louisiana

The rainfall is associated with an upper level low pressure area. The National Hurricane Center forecasts a 30% chance that this system will develop a surface low and become a named tropical storm before it moves ashore. Whether it officially becomes a tropical storm or not, the main impact will be prolonged heavy rain.

The rainfall map above, provided by the National Weather Service Office in Lake Charles, Louisiana, forecasts more than 8" of rain possible for the Houston-Galveston corridor and Bolivar Peninsula, with 4-6" generally expected in the Beaumont-Port Arthur-Orange metro area.

Tuesday morning weather radar shows widespread rain has already moved into coastal areas.

Some of the early banding of this system seems to indicate rainfall estimates may be higher than forecast for Beaumont-Port Arthur-Orange metro area, and I would not be surprised if some locations in that region observe rainfall amounts exceeding 6 inches.

Prolonged 15-25 mph winds from the southeast will generate minor coastal flooding along the Upper Texas Coast and Southwest Louisiana. Water levels at the Galveston North Jetty tide gauge have already exceeded 2 feet above normal astronomical tides. In general, we should expect storm surge levels to remain between 1.5 - 2.5 feet above normal for the next several days.

The graphic below, from NOAA Tides and Currents, shows the difference between observed (red) water levels and predicted (blue) water levels at Galveston North Jetty. We can see the prolonged nature of this "minor" coastal flood event.

Water level graph for Galveston North Jetty, from NOAA Tides and Currents

While this amount of flooding will bring saltwater over some beaches and a few low spots on coastal roads, the biggest impact of a long-duration storm surge event like this comes from "compound flooding", as elevated salt water levels impede the drainage of heavy rainfall.

Elsewhere in the tropics, Hurricane Humberto is tracking to the east-northeast and may impact Bermuda this week. The only impact to the U.S. coast should be higher than normal surf and rip currents along the eastern seaboard.

The five-day graphical tropical weather outlook from the National Hurricane Center

The National Hurricane Center forecasts a 90% chance that a disturbance between the Caribbean and Africa will develop into a tropical storm within the next 48 hours. It would be named Tropical Storm Imelda, but it will most likely track east of the United States.

TS Humberto Likely to Form- Could Bring Flooding Rains to SE United States

Potential Tropical Cyclone 9 is likely to form into Tropical Storm Humberto near the Bahamas and track towards the Florida coast. As of 0800AM EDT on Fri Sep 13, the National Hurricane Center forecasts an 80% chance that this low pressure system, which is now centered near the Bahamas, will develop into a tropical storm within 48 hours and a 90% chance that it will develop within the next five days. The disturbed area is now named "Potential Tropical Cyclone 9", but will be named Humberto if it becomes a tropical storm.

Five-Day Graphical Tropical Weather Outlook from the National Hurricane Center. Potential Tropical Cyclone 9 shown with a red X.

The best track forecast from the National Hurricane Center depicts a tropical storm approaching Florida's east coast, and then tracking from south to north very close to the coastline....a track quite similar to Hurricane Dorian's last week. However, until this system forms a center of circulation, we should expect storm track and intensity forecasts to change considerably.

The biggest concern with this system is the forecast stall near the Florida/ Georgia/ South Carolina coastline from Monday through Tuesday morning. If this forecast verifies, it could mean prolonged flooding rain for portions of the Southeast U.S. next week.

National Hurricane Center best track forecast for Potential Tropical Cyclone 9, as of Fri Sep 13 at 0800AM EDT

Most of the GFS (American) model tracks have shifted east over the past 24 hours. Yesterday, most ensemble members forecast Humberto to form and track into the Gulf of Mexico, but today most tracks forecast Humberto to form and track into the Florida Peninsula.

After tracking back out to sea before reaching Georgia, the GFS best track stalls the system in the Atlantic and forecasts the system to stall and then retrograde back towards the U.S. coastline close to North Carolina. Several models over the past two days have agreed with this idea...supporting a big picture of building high pressure over the Atlantic pushing potential Humberto back towards land after tracking into the Atlantic.

GFS ensemble models for Potential Tropical Cyclone 9- image from Tropical Tidbits

We should not focus on the details or exact track of such forecasts at this point, but keep in mind the big picture....it is possible that a tropical storm could form, potentially strengthen into a hurricane and impact the southeast U.S. The idea that this storm could possibly track out to sea and then be pushed closer to the coast again means that people north of the Carolinas should pay attention to this storm, as there is a possibility it could impact areas from the Chesapeake Bay to New England.

Global and hurricane model forecast runs for Potential Tropical Cyclone 9

Image: Tropical Tidbits

Most of the global and hurricane models depict this disturbed area to track to the northwest but remain offshore, then curve to the northeast and track off the Atlantic seaboard. Many of these models predict future conditions out to 120 hours, so they do not yet forecast what could happen past next Wednesday.

Marine Weather and Climate, Flood Information Systems and the U-Surge Project have initiated the first Hazard Area Likeliness (HAL) map for potential wind and flood hazards associated with this storm.

This map depicts that the biggest threat right now appears to be flooding from heavy rainfall. The HAL index forecasts the possibility of moderate damage from rainfall exceeding 8 inches from Mayport (near Jacksonville Beach, Florida) through Wrightsville Beach, NC. 

The possibility of moderate wind damage for the Mayport/ Jacksonville Beach area has also been added to this map.

Expect these forecasts to change considerably in the next 24-48 hours until a center of circulation forms and we have a named storm. All interests along the eastern seaboard of the U.S. should monitor the progress of this storm.

Tropical Storm Humberto Could Form and Approach Florida This Weekend

A broad area of showers and thunderstorms centered over the Southern Bahamas has an increasing chance of developing into a tropical storm over the next 48 hours. This area, now classified as INVEST-95L,  will likely track towards Florida.

Infrared satellite image of Invest-95L on the morning of Thursday, September 12. 

Image: Tropicaltidbits.com

The National Hurricane Center forecasts that this area has a HIGH chance of developing into Tropical Storm Humberto, with the chance of development at 70% in the next 48 hours and 80% in the next five days.

National Hurricane Center Five-Day Graphical Tropical Weather Outlook

Atmospheric models differ considerably on the potential track for Humberto, with the GFS/ American model forecasting a track that takes the storm farther west, towards the Gulf of Mexico, and the European model and others forecasting a track along Florida's east coast, and perhaps the eastern seaboard.

The GFS model tracks are shown on the map below. All of these tracks are for the same model...different tracks are forecast by slightly adjusting the initial conditions, like storm position and pressure, to initialize, or begin the model forecast.

https://www.tropicaltidbits.com/storminfo/95L_gefs_latest.png

GFS model ensemble tracks for Tropical Storm Humberto

Image: www.tropicaltidbits.com

The tracks for global and hurricane models are shown below. In general, most of these model runs forecast Humberto to track towards Florida's east coast if it develops. 

Global and Hurricane model tracks for Tropical Storm Humberto

Image: www.tropicaltidbits.com

Regardless of the exact track, windy, rainy weather should be expected in Florida over the weekend and possibly into early next week. NOAA's Quantitative Precipitation Forecast (QPF) rainfall maps support this general idea, as 5-day precipitation totals are forecast to exceed 3 inches in parts of Florida and Georgia.

We should not focus on the exact locations of this precipitation forecast, but rather focus on the big picture. At this point it appears that some locations in Florida, and possibly along the Gulf Coast and Southeast U.S., are likely to observe heavy rain in the next five days.

NOAA's QPF Rainfall Forecast Map depicts rainfall potential for the next 5 days

Stay tuned for more updates and keep in mind that track forecasts are likely to change considerably until a low-level circulation center develops. Yesterday evening's EURO model predicted that Humberto would develop into a hurricane and track up the Eastern Seaboard. This possibility is not out of the question...nor is potential hurricane development in the Gulf.

Potential for Heavy Rainfall Flooding Increasing for Northern Gulf Coast/ Southern Mississippi Valley

The potential for heavy rainfall flooding is increasing for the Northern Gulf Coast and Southern Mississippi Valley later this weekend and early next week.

An area of disturbed weather north of Hispaniola is forecast to track to the west-northwest and approach Florida. The National Hurricane Center gives a 60% chance of tropical storm development in the next five days, particularly over the eastern Gulf of Mexico.

The major hazard associated with this system will be prolonged heavy rainfall. The Euro model predicts this system will slow down considerably as the storm center tracks over the Florida Panhandle and Alabama, while the GFS predicts an extended stall as the system is centered near coastal Louisiana.

Both of these scenarios would spread torrential rain over multiple states for a prolonged period. Locations from Louisiana through Georgia and the Florida Panhandle, including Mississippi, Alabama and Tennessee should closely monitor this system.

Keep in mind that a storm becomes "named" based on one metric alone- that maximum sustained winds reach 39 miles per hour. (The system must also have closed circulation and be tropical in nature). But the presence of a name tells us nothing about flood potential. Even if this storm remains unnamed, it has the potential to inflict widespread flooding, especially along the northern Gulf Coast.

Just three years ago an "unnamed" storm stalled in this region of the country and dumped more than 30 inches of rain in southeast Louisiana. We will continue to monitor this system for future development.

Peak of Hurricane Season Arrives as We Watch Three Systems for Development

Here we are at September 10th, the climatological peak of the Atlantic Hurricane season. Climate records indicate that nearly one named storm exists per year at this time on the calendar. Another way to look at this statistic is that in 100 years of climate records, we would expect nearly 100 named storms to have existed around this date.

This graph shows the frequency of hurricanes and tropical storms over 100 years. Source: NOAA

The graphic above shows the number of named storms per 100 years and depicts a strong seasonal influence on hurricane activity, with the busiest period of Atlantic hurricane and tropical storm activity occurring from mid-August to mid-October. A strong peak is visible around September 10.

So we shouldn't be surprised that we are watching three areas of disturbed weather for potential tropical development over the next five days. 

National Hurricane Center map of potential tropical development over the next five days.

Text added by Hal Needham.

The area of greatest concern is centered north of Hispaniola. This system is forecast to move to the west-northwest and approach the Bahamas/ Florida and then move into the Gulf of Mexico. Although development of this system into a named storm is unlikely over the next several days, the National Hurricane Center forecasts a 30% chance of development in the Gulf of Mexico over the weekend.

Regardless of development into a named storm, this system has ample moisture and should bring heavy rain to the Gulf Coast over the weekend. NOAA's Quantitative Precipitation Forecast (QPF) provides rainfall estimates that could exceed three inches from the Bahamas through the Northern Gulf Coast, including areas near the Florida Peninsula, over the next seven days.

NOAA's QPF map forecasts 3+ inches of rain could fall near the Northern Gulf Coast over the next seven days. Link: https://www.wpc.ncep.noaa.gov/qpf/p168i.gif?1568119485

Stay tuned for more updates as we track this system.

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