National Hurricane Center has issued a Special Tropical Weather Outlook this afternoon for the area of disturbed weather over the eastern Gulf of Mexico.
An area of showers and thunderstorms has become slightly better organized today. Global forecast models continue to suggest that this area will gradually develop into a tropical or sub-tropical cyclone over the eastern/northeastern Gulf of Mexico sometime between Monday and Wednesday.
A broad surface low or elongated trough is slowly forming on the eastern flank of an upper level trough located over the central Gulf of Mexico. Forecast models suggest the broad surface low will gradually intensify and drift N/NNE along the eastern flank of the upper level trough over the next 72 hours.
There remains some uncertainty on whether the system is more sub-tropical in nature (with winds and rain well away from the center) or more tropical in nature (with strongest winds and rains closer to the center). The latest European Centre for Medium-Range Weather Forecasts (ECMWF) outlook shows more of a tropical system. ECMWF predicts a surface low of 1003-1005mb making landfall over the Florida panhandle by the middle of this week.
Regardless of development or not, heavy rainfall and squalls will be impacting much of Florida over the next several days and spreading NNW along the NE/N Gulf of Mexico coast, generally east of Mississippi by the middle of the week.
The topical disturbance likely will come inland over the Florida Panhandle later this week. It is not currently a threat to the Texas Coast.
Currently the National Hurricane Center gives this system a 30% chance of development in the next 48 hours and 40% over the next 5 days. Regardless of development or not, heavy rainfall and squalls will be impacting much of Florida over the next several days and spreading NNW along the NE/N Gulf of Mexico coast generally east of Mississippi by the middle of the week.
At this time no significant impacts from this system are expected along the upper Texas coast. Surface winds Tuesday-Thursday may shift back to the ENE/NE, allowing an even drier air mass to advect into the region from the NE. Afternoon high temperatures should rise into the low to mid 90’s. Given the likely small scale nature of any system, do not expect any increase in tides of seas over the NW Gulf of Mexico.
By Jeff Lindner, Director Hydrologic Operations Division/Meteorologist
Harris County Flood Control District
Posted 5/13/18, 4:55 p.m CDT
257 Days after Hurricane Harvey
00adminadmin2018-05-13 16:55:562018-05-13 17:15:37Outlook for Tropical Disturbance: Texas Clear, But Not Florida
A new ADVM river gage is being installed at US59 at the West Fork of the San Jacinto River to more accurately predict floods.
The river gage at US59 and the West Fork of the San Jacinto River is being upgraded. A new Acoustic Doppler Velocity Meter (ADVM) should be operational by Monday, May 14, 2018, according to Jeff Lindner of Harris County Flood Control.
Concerns About Previous Gage
Some flood forecasting professionals suspected that the previous meter was not accurately predicting flood height because of several factors at this location. They felt sedimentation, scouring at the base of the bridge, backwater from Lake Houston, and unsteady stream flow all affected the “rating curve.”
How Rating Curves Work
Rating curves show the correlation between the river discharge (flow volume as measured in cubic feet per second) and the river stage (height). Velocity multiplied by the area of the cross section of the river for any given height equals the discharge rate. Such rates are expressed on a mathematical curve that correlates height and discharge.
These curves can change frequently, especially on a river where both sedimentation and scour can result in a change in the amount of flow at a location, as it does at US59.
Need for More Accurate Gage
One meteorologist suspected that – because the rating curve had shifted – we were seeing higher river levels with less water than pre-Harvey. That impacted the forecast accuracy for the Humble gage. A river forecaster noted that in the last two flood events, initial forecasts were “underdone” another indicator of a shifted rating curve.
ADVMs deliver real-time flow data that would help us better understand what is going on with the rating curve and have more accurate forecasts for the Humble/Kingwood area.
An ADVM measures water velocity by using the Doppler principle applied to sound transmitted under water. Acoustic Doppler systems rely on SONAR, which uses sound waves to determine the distance to targets. They bounce acoustic signals of a known frequency off sediment in the water and measure the shift in frequency when the signals return. By measuring the time between the original pulse and the return signal, forecasters can compute the velocity of flow.
Velocity is crucial in computing the volume of water flowing past a measurement station. River forecasters use the formula:
The faster the velocity for any given cross section, the more water that is flowing past that point.
Lindner cautions that although the new gage will begin collecting data immediately, it will take the river rising and falling several times to get enough data to accurately predict flow rates at different levels.
“A river is always in flux,” says Lindner. “The USGS has already published a new post-Harvey rating which takes into account sedimentation from Harvey near this location.”
Dredging along the West Fork would likely change that rating curve again. “The rating will have frequent changes over the next several months, as the shape of the river evolves both from natural and man-made causes,” said Lindner.
USGS has already sent the new post-Harvey rating for US59 at the West Fork to the West Gulf River Forecast Center to incorporate into their modeling efforts.
Harris County Flood Control District just issued an alert about unsettled weather in the Gulf. Chances of it developing into a subtropical storm system are less than 30% at this time. But nature just issued a wake-up call to get those hurricane kits ready. Here is the exact text of the alert.
A weak low pressure center…likely sub-tropical in nature…may develop over the eastern Gulf of Mexico this week.
Overnight an area of showers and thunderstorms have developed over the southeastern Gulf of Mexico on the eastern side of a developing upper level trough over the central Gulf. Upper level winds are currently unfavorable for the continued development of any sort of surface circulation given strong wind shear aloft. However as this system drifts NNW over the next few days conditions may become slightly more favorable for the formation of either a surface or mid level center near/under the upper level trough. If this were to happen the system would likely develop some sub-tropical characteristics with most of the wind and rainfall over the eastern flank of the feature. Some of the forecast models have been showing off and on some sort of sub-tropical feature moving inland over the northern US Gulf coast by the middle of next week.
At this time development chances of a sub-tropical storm system over the NE Gulf of Mexico appear less than 30% and the formation of any “true” tropical system is unlikely. Given the current forecast model solutions a dry air mass will likely remain in place over SE TX and surface winds may become more ENE/NE by mid week should some sort of surface feature develop likely resulting in some very warm conditions by the middle of next week.
At this time no impacts are expected across SE TX or the coastal waters from anything sort of sub-tropical develop over the eastern Gulf of Mexico.
Jeff Lindner, Director Hydrologic Operations Division/Meteorologist
Harris County Flood Control District
Posted May 12 at 11:15 a.m. CDT
256 Days since Hurricane Harvey
00adminadmin2018-05-12 11:22:132018-05-12 11:22:13Unsettled Weather In the Gulf
Outlook for Tropical Disturbance: Texas Clear, But Not Florida
National Hurricane Center has issued a Special Tropical Weather Outlook this afternoon for the area of disturbed weather over the eastern Gulf of Mexico.
An area of showers and thunderstorms has become slightly better organized today. Global forecast models continue to suggest that this area will gradually develop into a tropical or sub-tropical cyclone over the eastern/northeastern Gulf of Mexico sometime between Monday and Wednesday.
A broad surface low or elongated trough is slowly forming on the eastern flank of an upper level trough located over the central Gulf of Mexico. Forecast models suggest the broad surface low will gradually intensify and drift N/NNE along the eastern flank of the upper level trough over the next 72 hours.
There remains some uncertainty on whether the system is more sub-tropical in nature (with winds and rain well away from the center) or more tropical in nature (with strongest winds and rains closer to the center). The latest European Centre for Medium-Range Weather Forecasts (ECMWF) outlook shows more of a tropical system. ECMWF predicts a surface low of 1003-1005mb making landfall over the Florida panhandle by the middle of this week.
Regardless of development or not, heavy rainfall and squalls will be impacting much of Florida over the next several days and spreading NNW along the NE/N Gulf of Mexico coast, generally east of Mississippi by the middle of the week.
The topical disturbance likely will come inland over the Florida Panhandle later this week. It is not currently a threat to the Texas Coast.
Currently the National Hurricane Center gives this system a 30% chance of development in the next 48 hours and 40% over the next 5 days. Regardless of development or not, heavy rainfall and squalls will be impacting much of Florida over the next several days and spreading NNW along the NE/N Gulf of Mexico coast generally east of Mississippi by the middle of the week.
At this time no significant impacts from this system are expected along the upper Texas coast. Surface winds Tuesday-Thursday may shift back to the ENE/NE, allowing an even drier air mass to advect into the region from the NE. Afternoon high temperatures should rise into the low to mid 90’s. Given the likely small scale nature of any system, do not expect any increase in tides of seas over the NW Gulf of Mexico.
By Jeff Lindner, Director Hydrologic Operations Division/Meteorologist
Harris County Flood Control District
Posted 5/13/18, 4:55 p.m CDT
257 Days after Hurricane Harvey
New River Gage at I-69 Will Measure Flow Accurately Despite Sediment
A new ADVM river gage is being installed at US59 at the West Fork of the San Jacinto River to more accurately predict floods.
The river gage at US59 and the West Fork of the San Jacinto River is being upgraded. A new Acoustic Doppler Velocity Meter (ADVM) should be operational by Monday, May 14, 2018, according to Jeff Lindner of Harris County Flood Control.
Concerns About Previous Gage
Some flood forecasting professionals suspected that the previous meter was not accurately predicting flood height because of several factors at this location. They felt sedimentation, scouring at the base of the bridge, backwater from Lake Houston, and unsteady stream flow all affected the “rating curve.”
How Rating Curves Work
Rating curves show the correlation between the river discharge (flow volume as measured in cubic feet per second) and the river stage (height). Velocity multiplied by the area of the cross section of the river for any given height equals the discharge rate. Such rates are expressed on a mathematical curve that correlates height and discharge.
These curves can change frequently, especially on a river where both sedimentation and scour can result in a change in the amount of flow at a location, as it does at US59.
Need for More Accurate Gage
One meteorologist suspected that – because the rating curve had shifted – we were seeing higher river levels with less water than pre-Harvey. That impacted the forecast accuracy for the Humble gage. A river forecaster noted that in the last two flood events, initial forecasts were “underdone” another indicator of a shifted rating curve.
According to the U.S. Geological Service (USGS), ADVMs are indispensable for backwater-influenced gages. The USGS in recent years has built hundreds of index-velocity gages with an ADVM for the measurement of streamflow. They are especially valuable in reaches where unsteady (varied, nonuniform) streamflow is prevalent that prevents the development of a conventional stage-discharge rating.
ADVMs deliver real-time flow data that would help us better understand what is going on with the rating curve and have more accurate forecasts for the Humble/Kingwood area.
An ADVM measures water velocity by using the Doppler principle applied to sound transmitted under water. Acoustic Doppler systems rely on SONAR, which uses sound waves to determine the distance to targets. They bounce acoustic signals of a known frequency off sediment in the water and measure the shift in frequency when the signals return. By measuring the time between the original pulse and the return signal, forecasters can compute the velocity of flow.
Velocity is crucial in computing the volume of water flowing past a measurement station. River forecasters use the formula:
Q = VA where
The faster the velocity for any given cross section, the more water that is flowing past that point.
Lindner cautions that although the new gage will begin collecting data immediately, it will take the river rising and falling several times to get enough data to accurately predict flow rates at different levels.
“A river is always in flux,” says Lindner. “The USGS has already published a new post-Harvey rating which takes into account sedimentation from Harvey near this location.”
Dredging along the West Fork would likely change that rating curve again. “The rating will have frequent changes over the next several months, as the shape of the river evolves both from natural and man-made causes,” said Lindner.
USGS has already sent the new post-Harvey rating for US59 at the West Fork to the West Gulf River Forecast Center to incorporate into their modeling efforts.
For those interested in learning more about gages and flood forecasting, the USGS has an excellent high-level, non-technical intro to measuring stream discharge. Scientists and engineers may be interested in a more technical discussion of the advantage of ADVM’s in developing more reliable real-time discharge estimates.
Posted by Bob Rehak, May 13, 2018
257 Days Since Hurricane Harvey
Unsettled Weather In the Gulf
Harris County Flood Control District just issued an alert about unsettled weather in the Gulf. Chances of it developing into a subtropical storm system are less than 30% at this time. But nature just issued a wake-up call to get those hurricane kits ready. Here is the exact text of the alert.
A weak low pressure center…likely sub-tropical in nature…may develop over the eastern Gulf of Mexico this week.
Overnight an area of showers and thunderstorms have developed over the southeastern Gulf of Mexico on the eastern side of a developing upper level trough over the central Gulf. Upper level winds are currently unfavorable for the continued development of any sort of surface circulation given strong wind shear aloft. However as this system drifts NNW over the next few days conditions may become slightly more favorable for the formation of either a surface or mid level center near/under the upper level trough. If this were to happen the system would likely develop some sub-tropical characteristics with most of the wind and rainfall over the eastern flank of the feature. Some of the forecast models have been showing off and on some sort of sub-tropical feature moving inland over the northern US Gulf coast by the middle of next week.
At this time development chances of a sub-tropical storm system over the NE Gulf of Mexico appear less than 30% and the formation of any “true” tropical system is unlikely. Given the current forecast model solutions a dry air mass will likely remain in place over SE TX and surface winds may become more ENE/NE by mid week should some sort of surface feature develop likely resulting in some very warm conditions by the middle of next week.
At this time no impacts are expected across SE TX or the coastal waters from anything sort of sub-tropical develop over the eastern Gulf of Mexico.
Jeff Lindner, Director Hydrologic Operations Division/Meteorologist
Harris County Flood Control District
Posted May 12 at 11:15 a.m. CDT
256 Days since Hurricane Harvey