Sep. 21, 2020 – NASA Looks at Rainfall Potential in Tropical Storm Beta
Tropical Storm Beta is forecast to be a soaker because of its rainfall potential and its forecast slow movement along the Texas coast. NASA calculated rainfall rates within the storm to give forecasters an idea of its soaking potential.
Using a NASA satellite rainfall product that incorporates data from satellites and observations, NASA found Beta was producing heavy rainfall off the coast of Texas.
Watches and Warnings on Sept. 21
NOAA’s National Hurricane Center posted a Storm Surge Warning from Port Aransas, Texas to Sabine Pass, Texas including Copano Bay, Aransas Bay, San Antonio Bay, Matagorda Bay, and Galveston Bay.
A Tropical Storm Warning is in effect for Port Aransas, Texas to Morgan City, Louisiana. A Tropical Storm Watch is in effect for Baffin Bay to Port Aransas, Texas.
Beta’s Status on Sept. 21
At 11 a.m. EDT (1500 UTC), the center of Tropical Storm Beta was located by an Air Force Reserve reconnaissance aircraft and NOAA Doppler weather radars near latitude 27.9 degrees north and longitude 95.7 degrees west. That is 55 miles (90 km) southeast of Port O’Connor, Texas, and about 75 miles (120 km) south-southeast of Freeport, Texas.
Beta was moving toward the west-northwest near 7 mph (11 kph), and this general motion is forecast to continue today. Data from the aircraft and Doppler radars indicate that maximum sustained winds are near 50 mph (85 kph) with higher gusts.
NOAA’s National Hurricane Center (NHC) noted that tropical-storm-force winds extend outward up to 175 miles (280 km) from the center. The estimated minimum central pressure based on data from the reconnaissance aircraft is 996 millibars.
Estimating Beta’s Rainfall Rates from Space
NASA’s Integrated Multi-satellitE Retrievals for GPM or IMERG, which is a NASA satellite rainfall product, estimated on Sept. 21 at 8 a.m. EDT (1200UTC), Beta was generating as much as 30 mm (1.18 inches) of rain per hour around the center of circulation. That area was off the coast of Texas and over Gulf of Mexico.
Rainfall throughout most of the storm was estimated as falling at a rate between 5 and 15 mm (0.2 to 0.6 inches) per hour. At the U.S. Naval Laboratory in Washington, D.C., the IMERG rainfall data was overlaid on infrared imagery from NOAA’s GOES-16 satellite to provide a full extent of the storm.
What Does IMERG Do?
This near-real time rainfall estimate comes from the NASA’s IMERG, which combines observations from a fleet of satellites, in near-real time, to provide near-global estimates of precipitation every 30 minutes. By combining NASA precipitation estimates with other data sources, we can gain a greater understanding of major storms that affect our planet.
What the IMERG does is “morph” high-quality satellite observations along the direction of the steering winds to deliver information about rain at times and places where such satellite overflights did not occur. Information morphing is particularly important over the majority of the world’s surface that lacks ground-radar coverage. Basically, IMERG fills in the blanks between weather observation stations
NHC Key Messages
The National Hurricane Center issued several key messages for Beta:
STORM SURGE: The combination of a dangerous storm surge and the tide will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline. The water could reach the following heights above ground somewhere in the indicated areas if the peak surge occurs at the time of high tide:
- Port Aransas, TX to Sabine Pass, TX including Copano Bay, Aransas Bay, San Antonio Bay, Matagorda Bay, and Galveston Bay…2-4 ft
- Sabine Pass, TX to Ocean Springs, MS including Sabine Lake, Lake Calcasieu, Vermilion Bay, Lake Borgne, Lake Pontchartrain, and Lake..1-3 ft
- Baffin Bay, TX to Port Aransas, TX including Corpus Christi Bay and Baffin Bay… 1-3 ft
- Mouth of the Rio Grande to Baffin Bay, TX…1-2 ft
The deepest water will occur along the immediate coast in areas of onshore winds where the surge will be accompanied by large and dangerous waves. Surge-related flooding depends on the relative timing of the surge and the tidal cycle, and can vary greatly over short distances.
WIND: Tropical storm conditions are expected to begin later this morning in portions of the tropical storm warning area. Tropical storm conditions are possible within the tropical storm watch area later today.
RAINFALL: Through Friday, Beta is expected to produce rainfall accumulations of 5 to 10 inches with isolated totals of 15 inches from the middle Texas coast to southeast Louisiana. Rainfall totals of 3 to 5 inches are expected northward into the ArkLaTex region and east into the Lower Mississippi Valley through the end of the week. Flash and urban flooding is likely, as well as isolated minor river flooding.
TORNADOES: A tornado or two could occur today and tonight, near the middle to upper Texas coast or the southwestern Louisiana coast.
SURF: Swells generated by a combination of Beta and a cold front over the northern Gulf of Mexico will continue along the coasts of Louisiana and Texas during the next couple of days. These swells are likely to cause life-threatening surf and rip current conditions. Please consult products from your local weather office.
Beta’s Forecast
A decrease in forward speed and a sharp turn to the north and northeast are expected on Tuesday. On the forecast track, the center of Beta will continue to move toward the central coast of Texas today and will likely move inland by tonight. Little change in strength is forecast before Beta reaches the Texas coast. Weakening is anticipated once Beta moves inland. Beta is forecast to remain close to the coast of southeastern Texas on Tuesday and Wednesday.
NASA Researches Tropical Cyclones
Hurricanes/tropical cyclones are the most powerful weather events on Earth. NASA’s expertise in space and scientific exploration contributes to essential services provided to the American people by other federal agencies, such as hurricane weather forecasting.
For more than five decades, NASA has used the vantage point of space to understand and explore our home planet, improve lives and safeguard our future. NASA brings together technology, science, and unique global Earth observations to provide societal benefits and strengthen our nation. Advancing knowledge of our home planet contributes directly to America’s leadership in space and scientific exploration.
For more information about NASA’s IMERG, visit: https://pmm.nasa.gov/gpm/imerg-global-image
For forecast updates on hurricanes, visit: www.hurricanes.gov
By Rob Gutro
NASA’s Goddard Space Flight Center