As of mid-day Sunday, all of East Texas now faces some level of excessive rainfall and flash flood risk from Tropical Storm Beta.
The projected path of the storm has shifted inland, so rainfall will reach father north. The latest track now takes the storm on a path right up US59 from Sugar Land to Humble and Kingwood. However, Beta likely be a tropical depression by the time it reaches the north side.
Harris, Montgomery and Liberty Counties (and the whole Houston region) face a high to moderate threat of flooding rains. The first rainbands from Beta have already started to move onshore today, but the heaviest rain will not occur until Monday into Tuesday.
North Houston Under Moderate Flooding Rain Threat
Flash Flood Watch in Effect
The entire region is under a flash flood watch. Coastal areas also face storm surge and tropical storm warnings.
Slow Moving Storm Will Produce Prolonged Rains
Beta is meandering slowly at about 3 mph northwest toward the coastline west of Houston and will then curl back over Houston. It should move inland sometime Monday or Monday night and turn from a storm into a depression by the time it reaches Houston.
However, Beta could dump rain on us from later today all the way through Wednesday.
40-50% Chance of Tropical Storm Force Winds Reaching North Houston
The North Houston Area has a 40-50% chance of seeing tropical storm force winds from Beta.
If the high winds reach this far inland, they would likely arrive Monday evening. Winds extend outward from the center for 195 miles as of Sunday morning.
Feeder Bands Extend Outward 190 Miles
In fact, at 11 a.m. Sunday morning, the first feeder band was approaching the eastern side of Houston, as you can see in the radar image below.
This image shows how the wind and rain from Beta could remain with us for days as the storm’s center wanders around the Gulf.
These feeder bands reach out 190 miles. That is roughly the distance from Houston to San Antonio. So Beta will produce long-duration rainfall from the middle Texas coast to southern Louisiana.
Flash, Urban, and River Flooding Likely
The National Weather Service warns that coastal flooding will remain a threat through midweek with the worst of the storm surge anticipated on Monday and Tuesday.
Elsewhere impacts from the excessive rain include:
Rainfall flooding may prompt evacuations and rescues.
Rivers and tributaries may rapidly overflow their banks.
Small streams, creeks, canals, and ditches may become dangerous rivers.
Flood control systems and barriers may become stressed.
Flood waters may cover escape routes.
Streets and parking lots may become rivers of moving water with underpasses submerged.
Driving conditions may become dangerous.
Many roads and bridges may close with some weakened or washed out.
Lake-Level Situation
According to the SJRA, Beta has the potential to dump up to 8 inches of rain in Montgomery County.
SJRA is operating under standard protocols for a severe weather event and will be onsite at Lake Conroe throughout the duration of Tropical Storm Beta.
Lake Conroe remains 18 inches low, and based on the current forecast, no reservoir releases are expected.
You can monitor stream and river levels in near-real time at Harris County Flood Warning System. Make sure you check out the inundation mapping feature.
Posted by Bob Rehak on 9/20/2020 based on information from the National Weather Service and National Hurricane Center
1118 Days after Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/09/HGXFloodingRainThreat.png?fit=492%2C745&ssl=1745492adminadmin2020-09-20 12:53:402020-09-20 13:32:59Excessive Rainfall Threat from Beta Spreads Inland
As Tropical Storm Beta bears down on the Houston Area, many people in the Elm Grove/North Kingwood Forest area worry that they might flood again. How likely is that, given the current predictions of 6-10 inches? After all, on May 7 last year, Elm Grove flooded on what was officially a six inch rain according to the nearest gage at West Lake Houston Parkway and the West Fork.
…But Detention Based on Pre-Atlas 14 Rainfall Rates
Even though that’s far more than Woodridge Village had during the May or September floods, the detention calculations by LJA Engineering were based on pre-Atlas 14 rainfall rates. A 100-year rainfall then was about 40% less than the official 100-year rainfall now.
So, the questions is, “How much rain would Beta have to dump on Woodridge Village before it overwhelmed the detention ponds that exist today?”
Figures Used by LJA
The chart below shows the rainfalls that the ponds were designed to hold without flooding. The bench mark its the 24-hour, hundred year rain.
These figures come from the hydrology report submitted by LJA to Montgomery County. A table buried on page 32 of the PDF shows that they based their analysis on a pre-Atlas 14, 100-year storm that dropped 12.17 inches of rain in 24 hours.
From Page 2.1 of LJA Hydrology Report Addendum, 8/28/2018 (page 32 of pdf.)
The ponds should also hold any of the shorter-duration rainfalls in the last column above.
Assumptions Underlying the Answer
To answer the question – How much would it take to flood Elm Grove again? – we need to make several assumptions:
LJA correctly calculated the volume needed.
Contractors actually excavated the volume promised.
With those caveats in mind, it would take 12+ inches of rain in 24 hours to exceed the capacity of the detention ponds currently on Woodridge Village. After that, water would start to overflow.
Short, High-Intensity Downpours Can Cause Different Type of Flooding
However, consider the other durations in the chart above. Seven inches in three hours or nine inches in six hours could also exceed the capacity.
Actually, as you get into these short-duration, high-intensity rainfalls, you introduce the risk of flooding from a second source: overwhelming the capacity of storm drains.
Storm Drains Designed for 2″ Per Hour
The storm drains in Kingwood are designed to convey about two inches of rain per hour. When you exceed that, water begins to back up in the streets. Exceed it enough, and water could actually enter homes – without sheet flow from Woodridge Village.
NHC Rainfall Prediction Spans 5 Days
The six-to-ten inch prediction issued by the National Hurricane Center for Beta spans five days. That’s good news. If ten inches were evenly spread out over five days, the streets, drains and ditches could easily handle two inches per day.
But those short, high intensity rainfalls – when you get two inches in five or ten minutes – represent a real danger. There’s just nowhere for the water to when it comes down that quickly.
Perhaps the Biggest Danger
Even if we got the predicted 6-10 inches all in one day, that’s still, at most, about 80% of the old 100-year rain which the detention was designed for.
I suspect the biggest danger from Beta may be those short, high-intensity cloud bursts or training feeder bands that dump a couple inches in five or ten minutes.
So keep your eye on the rain gage. Sign up for alerts at the Harris County Flood Warning System. Also, keep your eye on the forecasts; uncertainty still exists with Beta, its track and rainfall potential.
Posted by Bob Rehak on 9/19/2020
1117 Days after Hurricane Harvey and 1 year after Imelda
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/09/Rainfall-Table-Page-32-LJA-report.png?fit=730%2C350&ssl=1350730adminadmin2020-09-19 20:11:192020-09-19 20:21:44How Much Rain Would It Take To Flood Elm Grove Again?
Beta has turned toward the Texas coast. Adverse conditions will likely begin Sunday along coast and spread inland. The main forecast change since this morning? An increase in rainfall amounts, which could be significant. See rainfall section below.
A Flash Flood Watch will likely be issued on Sunday for the area.
Jeff Lindner, Harris County Meteorologist
Discussion
There has been little change in the overall organization of Beta today. Forecasters expect slow intensification as Beta approaches the TX coast.
Track:
There have been no significant changes in the track reasoning today and the official forecast remains similar from earlier this morning. With that said, there is some uncertainty on when Beta will turn toward the N and NE near the mid Tx coast early next week, but generally impacts will be the same for much of SE TX.
Intensity:
Given the lack of organization of the system and the lurking dry air to the west, it is now unlikely that Beta will attain hurricane intensity. The National Hurricane Center now keeps the system as a strong tropical storm as it approaches the TX coast. It should slowly as it moves NE or ENE up the coast.
Impacts
Storm Surge:
Water levels will rise along the upper TX coast beginning Saturday and worsen Sunday into Monday due to the expansion of the tropical storm force wind field. Expect swells reaching 15-25 feet over the NW Gulf. Four- to eight-foot waves could break on beaches. These large swells will lead to wave run up on top of the elevated tides.
Results:
Significant beach erosion
Damage to fragile dune systems.
Damage to coastal infrastructure
Inundation of low lying roads near the coast.
Once the water begins to rise on Saturday evening, some low lying areas may remain flooded into the middle of next week.
Overwash is likely on Hwy 87.
Ferry operations at both the Galveston-Bolivar and the Lynchburg sites may be impacted.
Rainfall:
While there is still some uncertainty, the storm could produce very heavy rainfall rates and amounts.
Jeff Lindner, Harris County Meteorologist
The rainfall forecasts have been adjusted upward, even though some models suggest only meager amounts of rainfall with Beta. Jeff Lindner, Harris County Meteorologist, currently predicts:
Coast: 12-18 inches
Coastal counties: 10-15 inches
South of I-10: 10-15 inches
South of Hwy 105: 6-10 inches
North of Hwy 105: 4-6 inches
Note that the rainfall amounts above are for a five-day total.
Given the forecast rainfall amounts…flooding of creeks and bayous across Harris County may flood depending on how the rain falls and how much falls at one time.
By Tuesday, when the ground is saturated, NOAA gives the Lake Houston Area a slight chance of flash flooding.
Six inches could fill smaller creeks; 7-8 inches could fill larger streams and bayous such as Buffalo.
Addicks and Barker Reservoirs are currently empty.
Winds:
Tropical storm force winds will be possible starting Sunday evening around Matagorda Bay with sustained 40mph winds likely spreading northward along the upper TX coast on Monday including Galveston Bay. Sustained winds of 55-65mph will spread up the TX coast Tuesday into Wednesday. Across Harris County wind will increase into the 25-35mph range on Monday and possibly 40-45mph range on Tuesday into Wednesday with higher gusts, especially near Galveston Bay.
Note: Tropical storm force winds will begin to reach the coast on Monday…well ahead of any landfall of the center…due to the expanding wind field. Tropical storm force winds now extend 175 miles outward from the center of the storm.
Prepare now for a long period of coastal flooding and periods of heavy rainfall.
Voluntary evacuations have been recommended for:
Western end of Galveston Island, west of the seawall
Bolivar Peninsula
Low-lying areas of Chamber, Brazoria and Matagorda Counties (outside levee protection)
Low-lying areas of Seabrook.
AlertHouston Recommends Taking These Steps Now
Ensure your family has 5-7 days of food, water, and necessary supplies.
This includes enough prescription medication for at least this duration, in case pharmacies and doctor’s offices are closed.
Consider the unique needs of small children, seniors, family members with access and functional needs, and pets.
Decide what you and your family will do if the storm impacts your area. Most City of Houston residents are not vulnerable to storm surge and do not need to evacuate before a hurricane or tropical storm. Vulnerable residents who require electricity may also consider evacuation in advance of a major storm. For a map of hurricane evacuation zones and mandatory evacuation areas, visit: houstonoem.org/hurricanes
Develop a family communication plan, so that you know who to check in with after a storm. Visit readyhoustontx.gov for more information on developing a plan.
Know how to turn off your utilities. This includes electricity, water, and gas. Only turn off gas if instructed by local officials or by CenterPoint Energy.
Posted by Bob Rehak on 9/19/2020 with data from the NHC, HCFCD, and City of Houston
1117 Days since Hurricane Harvey and 1 Year since Imelda
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2020/09/210245WPCERO_sm.gif?fit=1090%2C692&ssl=16921090adminadmin2020-09-19 18:06:232020-09-19 18:07:07Beta Rainfall Estimates Increasing; 6-10 Inches Now Predicted for Lake Houston Area
Excessive Rainfall Threat from Beta Spreads Inland
As of mid-day Sunday, all of East Texas now faces some level of excessive rainfall and flash flood risk from Tropical Storm Beta.
The projected path of the storm has shifted inland, so rainfall will reach father north. The latest track now takes the storm on a path right up US59 from Sugar Land to Humble and Kingwood. However, Beta likely be a tropical depression by the time it reaches the north side.
Six to 15 Inches of Rain Now Predicted
According to the local National Weather Service office and the National Hurricane Center, most of the Houston region can expect 6 to 15 inches with the highest totals closer to the coast.
Harris, Montgomery and Liberty Counties (and the whole Houston region) face a high to moderate threat of flooding rains. The first rainbands from Beta have already started to move onshore today, but the heaviest rain will not occur until Monday into Tuesday.
North Houston Under Moderate Flooding Rain Threat
Flash Flood Watch in Effect
The entire region is under a flash flood watch. Coastal areas also face storm surge and tropical storm warnings.
Slow Moving Storm Will Produce Prolonged Rains
Beta is meandering slowly at about 3 mph northwest toward the coastline west of Houston and will then curl back over Houston. It should move inland sometime Monday or Monday night and turn from a storm into a depression by the time it reaches Houston.
However, Beta could dump rain on us from later today all the way through Wednesday.
40-50% Chance of Tropical Storm Force Winds Reaching North Houston
The North Houston Area has a 40-50% chance of seeing tropical storm force winds from Beta.
If the high winds reach this far inland, they would likely arrive Monday evening. Winds extend outward from the center for 195 miles as of Sunday morning.
Feeder Bands Extend Outward 190 Miles
In fact, at 11 a.m. Sunday morning, the first feeder band was approaching the eastern side of Houston, as you can see in the radar image below.
This image shows how the wind and rain from Beta could remain with us for days as the storm’s center wanders around the Gulf.
These feeder bands reach out 190 miles. That is roughly the distance from Houston to San Antonio. So Beta will produce long-duration rainfall from the middle Texas coast to southern Louisiana.
Flash, Urban, and River Flooding Likely
The National Weather Service warns that coastal flooding will remain a threat through midweek with the worst of the storm surge anticipated on Monday and Tuesday.
Elsewhere impacts from the excessive rain include:
Lake-Level Situation
According to the SJRA, Beta has the potential to dump up to 8 inches of rain in Montgomery County.
SJRA is operating under standard protocols for a severe weather event and will be onsite at Lake Conroe throughout the duration of Tropical Storm Beta.
Real-time information on lake level can be found on the SJRA homepage at the “Lake Operations and Rainfall Dashboard” link.
At of this writing, Lake Houston is down a foot compared to its normal pool level (41.38 vs 42.4). You can monitor Lake Houston levels via the Coastal Water Authority website.
Monitor Downstream River Levels
You can monitor stream and river levels in near-real time at Harris County Flood Warning System. Make sure you check out the inundation mapping feature.
Posted by Bob Rehak on 9/20/2020 based on information from the National Weather Service and National Hurricane Center
1118 Days after Hurricane Harvey
How Much Rain Would It Take To Flood Elm Grove Again?
As Tropical Storm Beta bears down on the Houston Area, many people in the Elm Grove/North Kingwood Forest area worry that they might flood again. How likely is that, given the current predictions of 6-10 inches? After all, on May 7 last year, Elm Grove flooded on what was officially a six inch rain according to the nearest gage at West Lake Houston Parkway and the West Fork.
Additional Detention in Woodridge Village Now…
First of all, understand that upstream conditions have changed. On May 7th, only about 11% of the planned detention pond capacity had been constructed. And only 23% was constructed by Imelda. Today, 100% is in place.
…But Detention Based on Pre-Atlas 14 Rainfall Rates
Even though that’s far more than Woodridge Village had during the May or September floods, the detention calculations by LJA Engineering were based on pre-Atlas 14 rainfall rates. A 100-year rainfall then was about 40% less than the official 100-year rainfall now.
So, the questions is, “How much rain would Beta have to dump on Woodridge Village before it overwhelmed the detention ponds that exist today?”
Figures Used by LJA
The chart below shows the rainfalls that the ponds were designed to hold without flooding. The bench mark its the 24-hour, hundred year rain.
These figures come from the hydrology report submitted by LJA to Montgomery County. A table buried on page 32 of the PDF shows that they based their analysis on a pre-Atlas 14, 100-year storm that dropped 12.17 inches of rain in 24 hours.
The ponds should also hold any of the shorter-duration rainfalls in the last column above.
Assumptions Underlying the Answer
To answer the question – How much would it take to flood Elm Grove again? – we need to make several assumptions:
With those caveats in mind, it would take 12+ inches of rain in 24 hours to exceed the capacity of the detention ponds currently on Woodridge Village. After that, water would start to overflow.
Short, High-Intensity Downpours Can Cause Different Type of Flooding
However, consider the other durations in the chart above. Seven inches in three hours or nine inches in six hours could also exceed the capacity.
Actually, as you get into these short-duration, high-intensity rainfalls, you introduce the risk of flooding from a second source: overwhelming the capacity of storm drains.
Storm Drains Designed for 2″ Per Hour
The storm drains in Kingwood are designed to convey about two inches of rain per hour. When you exceed that, water begins to back up in the streets. Exceed it enough, and water could actually enter homes – without sheet flow from Woodridge Village.
NHC Rainfall Prediction Spans 5 Days
The six-to-ten inch prediction issued by the National Hurricane Center for Beta spans five days. That’s good news. If ten inches were evenly spread out over five days, the streets, drains and ditches could easily handle two inches per day.
But those short, high intensity rainfalls – when you get two inches in five or ten minutes – represent a real danger. There’s just nowhere for the water to when it comes down that quickly.
Perhaps the Biggest Danger
Even if we got the predicted 6-10 inches all in one day, that’s still, at most, about 80% of the old 100-year rain which the detention was designed for.
I suspect the biggest danger from Beta may be those short, high-intensity cloud bursts or training feeder bands that dump a couple inches in five or ten minutes.
So keep your eye on the rain gage. Sign up for alerts at the Harris County Flood Warning System. Also, keep your eye on the forecasts; uncertainty still exists with Beta, its track and rainfall potential.
Posted by Bob Rehak on 9/19/2020
1117 Days after Hurricane Harvey and 1 year after Imelda
Beta Rainfall Estimates Increasing; 6-10 Inches Now Predicted for Lake Houston Area
Beta has turned toward the Texas coast. Adverse conditions will likely begin Sunday along coast and spread inland. The main forecast change since this morning? An increase in rainfall amounts, which could be significant. See rainfall section below.
Discussion
There has been little change in the overall organization of Beta today. Forecasters expect slow intensification as Beta approaches the TX coast.
Track:
There have been no significant changes in the track reasoning today and the official forecast remains similar from earlier this morning. With that said, there is some uncertainty on when Beta will turn toward the N and NE near the mid Tx coast early next week, but generally impacts will be the same for much of SE TX.
Intensity:
Given the lack of organization of the system and the lurking dry air to the west, it is now unlikely that Beta will attain hurricane intensity. The National Hurricane Center now keeps the system as a strong tropical storm as it approaches the TX coast. It should slowly as it moves NE or ENE up the coast.
Impacts
Storm Surge:
Water levels will rise along the upper TX coast beginning Saturday and worsen Sunday into Monday due to the expansion of the tropical storm force wind field. Expect swells reaching 15-25 feet over the NW Gulf. Four- to eight-foot waves could break on beaches. These large swells will lead to wave run up on top of the elevated tides.
Results:
Rainfall:
The rainfall forecasts have been adjusted upward, even though some models suggest only meager amounts of rainfall with Beta. Jeff Lindner, Harris County Meteorologist, currently predicts:
Given the forecast rainfall amounts…flooding of creeks and bayous across Harris County may flood depending on how the rain falls and how much falls at one time.
Six inches could fill smaller creeks; 7-8 inches could fill larger streams and bayous such as Buffalo.
Addicks and Barker Reservoirs are currently empty.
Winds:
Tropical storm force winds will be possible starting Sunday evening around Matagorda Bay with sustained 40mph winds likely spreading northward along the upper TX coast on Monday including Galveston Bay. Sustained winds of 55-65mph will spread up the TX coast Tuesday into Wednesday. Across Harris County wind will increase into the 25-35mph range on Monday and possibly 40-45mph range on Tuesday into Wednesday with higher gusts, especially near Galveston Bay.
Note: Tropical storm force winds will begin to reach the coast on Monday…well ahead of any landfall of the center…due to the expanding wind field. Tropical storm force winds now extend 175 miles outward from the center of the storm.
Summary of Watches/Warnings In Effect
As of 4PM Saturday, 9/19/20:
A STORM SURGE WARNING IS IN EFFECT FOR
A STORM SURGE WATCH IS IN EFFECT FOR:
A HURRICANE WATCH IS IN EFFECT FOR:
* PORT ARANSAS TEXAS TO HIGH ISLAND TEXAS
A TROPICAL STORM WARNING IS IN EFFECT FOR:
A TROPICAL STORM WATCH IS IN EFFECT FOR:
Protective Actions
AlertHouston Recommends Taking These Steps Now
Posted by Bob Rehak on 9/19/2020 with data from the NHC, HCFCD, and City of Houston
1117 Days since Hurricane Harvey and 1 Year since Imelda