Author: okelley

Hurricane John Gives Southern Mexico a Surprise One-Two Punch

Hurricane John (2024) rainfall accumulation

On Monday evening, Sept. 23, 2024, an unusual hurricane struck southern Mexico, dumping 20 inches of rainfall along the coast according to estimates from NASA’s IMERG precipitation product. The following image shows these rainfall estimates.

Observations from an international constellation of satellites were the starting point for calculating these estimates, and the data confirm that forecasters in prior days were right to warn of possible heavy rainfall. Forecasters did have trouble predicting the storm’s track and intensity, though. In the rural area of southwestern Mexico where Hurricane John struck, there are few rain gauges that provide public data in real-time, so having calibrated multi-satellite near real-time precipitation estimates like IMERG available is particularly valuable for understanding this storm as it unfolded.

The contours in the image cover the areas where the IMERG algorithm estimated that 5 inches (blue) to 36 inches (bright red) of rainfall fell during a four-and-a-half-day period ending on Friday, Sept. 27, at 3 a.m. CST (0900 UTC). The dashed yellow line in the image shows the U.S. National Hurricane Center’s estimate of the path taken by the storm’s low-pressure center.

According to NOAA hurricane track data, approximately once a year during the past 50 years, one major hurricane over the eastern Pacific has passed within 800 miles of this part of southern Mexico. Almost all of these hurricanes moved roughly west over the ocean, parallel to the coast. Before Hurricane John this year, only two other times during the past 50 years has a major hurricane made landfall within 150 miles of where Hurricane John hit Mexico on Monday. On Sept. 23, John made landfall near the small, coastal village of Punta Maldonado, which is located about 100 miles east of the town of Acapulco. The two other hurricanes that had made landfall in the same region in prior years were Hurricane Pauline in 1997 and Hurricane Otis in 2023.

John was just a 40-knot tropical storm at 3 a.m. CST on Monday, and forecasters had only expected it to become a slightly stronger tropical storm during the following 24 hours before making landfall and dissipating. Instead, John experienced extremely rapid intensification to become a 105-knot Category 3 hurricane on the Saffir-Simpson scale. This intensification occurred during a period that was merely 18 hours long, immediately before landfall. Following landfall, societal impacts were moderated by the fact that the storm hit a low-population region of Mexico.

According to the National Hurricane Center, John made landfall around 9:15 p.m CST in Mexico on Monday evening (Sept. 23, 2024), which was 0315 UTC on Tuesday morning, Sept. 24, in universal time.

On Tuesday, Sept. 23, Hurricane John was forecast to dissipate over land, and for part of Tuesday, the National Hurricane Center stopped issuing advisories for it. However, the storm’s remnants moved over the Pacific Ocean again where the system regained hurricane strength (65-knot winds) by Thursday, Sept. 26. The next day, John weakened to tropical-storm strength before striking Mexico a second time at a location west of its first landfall earlier in the week.

For more information about the IMERG algorithm and the NASA Global Precipitation Measurement (GPM) mission that IMERG is part of, please visit the GPM website: https://gpm.nasa.gov. Credits: Image and caption by Owen Kelley.

References:

NOAA Historical Hurricane Tracks webpage, https://coast.noaa.gov/hurricanes.

NHC, Discussion #3, https://www.nhc.noaa.gov/archive/2024/ep10/ep102024.discus.003.shtml. Intensity was 40 kt at 3AM CST on Monday, Sept. 23, 2024. Intensity was forecast to increase 20 kt in the following 24 hours.

NHC, Discussion #7, https://www.nhc.noaa.gov/archive/2024/ep10/ep102024.discus.007.shtml. Intensity increased to 105 kt, making John category 3 as of 9PM CST Monday, Sept. 23. The intensity estimate was influence by passive-microwave satellite data (SSMIS) and by T-values from infrared geosynchronous satellite data.

NHC, Discussion #8, https://www.nhc.noaa.gov/archive/2024/ep10/ep102024.discus.008.shtml. Landfall time was 9:15PM CST Monday, Sept. 23, with 105-kt category-3 winds. This time is the same as 0315 UTC, Tuesday, Sept. 24, 2024.

NHC, rain potential images, posted on https://www.nhc.noaa.gov. These images were issued between Sept. 23, 2024, 1200 UTC and Sept. 25, 2024, 1530 UTC. These images showed a narrow area with 20-30 inches of 3-day rainfall accumulation along the coast of the Mexican states of Oaxaca or Guerrero.

Hurricane Debbie (2024) Stalls off the Coast of South Carolina

Hurricane Debby precipitation accumulation 12UTC 3 Aug to 0UTC 9 Aug 2024

Since 2014, NASA has run the IMERG multi-satellite algorithm to estimate how much precipitation falls over the globe every 30 minutes on a grid with 0.1-degree resolution in latitude and longitude. Each grid box is approximately a square seven miles to a side at the Equator.

Every few years, the team of scientists that develop the IMERG algorithm deliver a new version of the software. They give the algorithm code to the NASA Goddard Precipitation Processing System (PPS) to run. Analysts and computer programmers with George Mason University’s CEOSR assist with running the algorithm and detecting anomalies in its output. When a new version of IMERG is delivered, NASA goes back over 23 years of satellite observations to give updated precipitation estimates that cover every 30 minutes of that entire period.

In 2024, NASA released Version 7 of the IMERG algorithm. An example accumulation from this version of the algorithm is shown above, covering five and a half days of observations of Hurricane Debby. Debby impacted the United States East Coast in August 2024. For part of that time, Debby was stalled off the coast of South Carolina, which means that the hurricane brought on shore a great deal of moisture which fell as large rainfall accumulations.

CEOSR team members used ancillary datasets to help evaluate the quality of the IMERG precipitation accumulations of Hurricane Debbie. The red triangles are locations of preliminary tornado reports associated with Hurricane Debbie according to the NOAA Storm Prediction Center. The violet-colored bow ties are locations where the USGS measured major
flooding at a steam gauge, as reported by the NOAA Water Prediction System. The orange and red squares are locations where rain gauges measured accumulation in excess of 14 or 17 inches, respectively. These gauge data were published in a discussion by the NOAA Weather Prediction Center.

The image’s filled contours in the image show precipitation estimates from the near real-time run of Version 7 IMERG. These near real-time estimates are known as the Late IMERG run. Weather forecasting agencies, disaster management organizations, and researchers can download these estimates approximately 16 hours after the satellite observations were collected. The path taken by the low-pressure center of Hurricane Debby is show with the white dashed line as reported in real-time by the National Hurricane Center.

As can be seen in the image, the maximum accumulation reported by the rain gauges occurred mostly over South Carolina within 50 miles of the coast. Many of the tornadoes and flooding stream gauges were also located this close to the sea. In contrast, the near real-time accumulations from IMERG were largest further inland or off the coast.

Because IMERG relies heavily on satellite estimates, it does not always capture the fine-scale structure of precipitation from severe storms with sharp precipitation gradients. Over much of the world’s oceans and even parts of continents, ground-based precipitation estimates are sparse, so there is interest in improving the IMERG algorithm based on comparisons with ground data where plentiful, such as over much of the United States.

CEOSR researcher Owen Kelley examined Version 7 IMERG accumulations from other hurricane impacting the United States coast in prior years to see if the new Version 7 algorithm always has errors matching the spatial pattern seen here with Hurricane Debby (2024). He determined that the new algorithm does not habitually make this mistake. NASA and CEOSR researchers will continue to evaluate the new version of IMERG with the aim of helping NASA to release an improved version in approximately 2 years.

Hurricane Ian Crosses Florida and Heads Toward South Carolina

Hurricane Ian rainfall accumulation through 9UTC Sept. 30, 2022

Hurricane Ian’s rainfall accumulation up to 5AM EDT on September 30, 2022

As Hurricane Ian impacts Florida and South Carolina, NASA IMERG algorithm provides estimates of Ian’s rainfall over both land and sea. CEOSR team members assist NASA with visualizing this data and with monitoring the performance of the IMERG algorithm. Such evaluations assist with improving the algorithm, a new version of which is due to be released later this year or in early 2023.

The image above shows the rainfall accumulation from Hurricane Ian from when it formed in the Caribbean on September 25, 2022, through when it approached the coast of South Carolina on September 30. On September 28 and 29, Ian cross Florida going west to east from Ft. Myers to Cape Canaveral.

The image also show the locations of tornadoes and river flooding in Florida that were associated with the passage of Hurricane Ian. There were preliminary reports of tornadoes over southern Florida the night before Ian’s Florida landfall, which are marked by red triangles in the image. The tornadoes were associated with a large, persistent band of heavy rain that maintained a position
to the east of Hurricane Ian’s inner core during the days prior to the Florida landfall.

On September 29, the day Ian left Florida, numerous USGS stream gauges reach major-flood stage, meaning significant economic damage was expected. These stream gauges are marked with purple
bows in the image. Florida’s inland flooding was caused by Ian’s heavy rainfall. Along the coast, storm surge contributed to the height of the flood water.

Lightning before Landfall

Prior to making landfall in Florida on September 28, Hurricane Ian intensified over the Gulf of Mexico. During this time, its inner core dropped a considerable amount of rain and also exhibited many lightning flashes. This period is show in the image below.

Hurricane Ian’s rainfall accumulation during its approach to landfall in Florida

In the Gulf of Mexico, Ian’s eyewall was seen to contain particularly vigorous storm cells. Lightning was observed in Ian’s eyewall, whereas lightning had been near absent from Ian’s eyewall prior Ian passing over Cuba. Lightning strokes from the World Wide Lightning Location Network (WWLLN) are displayed in the image as small yellow dots. Over the Gulf of Mexico, the period of active eyewall lightning can be identified using the white dots in the image that indicate six-hourly locations of Hurricane Ian’s low-pressure center, as estimated by the National Hurricane Center.

Once in the Gulf of Mexico, Hurricane Ian intensified and underwent an eyewall replacement cycle early on September 28, according to the National Hurricane Center. During this period, NASA’s IMERG algorithm estimates that the rainfall accumulation under Ian’s inner core reached 12 to 20 inches. This accumulation is much higher than the 5-to-12-inch accumulation that Ian’s inner core had produced the day before, when the storm was located south of Cuba.

On September 27, stretching for several hundred miles to the east of Hurricane Ian’s inner core was a band of heavy rain storms and thunderstorms. These storms are seen in the IMERG rainfall accumulation, covering the southern tip of Florida during this period. During the night of September 27 and morning of September 28, these storms spawned a number of tornadoes according to preliminary reports from NOAA. These tornadoes are displayed as red dots in the image.

Credits: WWLLN data from the University of Washington with support from Jeremy Thomas and Natalia Solorzano at DigiPen Insitute of Technology and NorthWest Research Associates. Visualization Owen Kelley (CEOSR/NASA).

Hurricane Fiona from Caribbean to Canada

Hurricane Fiona and Its Landfall in Canada

Rainfall In Nova Scotia, Canada

CEOSR team members tracked Hurricane Fiona as it approached Canada in September, 2022. One of the tools they used was NASA’s IMERG algorithm, which provides estimates of global rainfall rates every 30 minutes with 0.1 degree latitude-longitude resolution. IMERG estimates of Hurricane Fiona’s rain accumulations in Nova Scotia were similar but somewhat lower than rain-gauge accumulations for the event. IMERG’s rainfall estimates are only approximate because they rely on morphing satellite microwave observations and on more frequent but less direct estimates of rainfall based on satellite infrared observations. None the less, over the open ocean, IMERG’s precipitation estimates are among the best available because the ocean is none instrumented with rain gauges or ground radars.

At approximately 4 AM local time (Atlantic Daylight Time) on Saturday, Sept. 24, Hurricane Fiona made landfall in Canada with a surface pressure of only 931 hPa at its center according to the National Hurricane Center (NHC). By 6 AM that morning, the NASA IMERG algorithm estimated that Hurricane Fiona had dropped in excess of 6 inches of rainfall off the coast of Nova Scotia. Rain was still accumulating over Atlantic Canada at that time. The image below shows these IMERG rainfall estimates and the NHC estimate of the distance that tropical-storm strength winds (39 mph, 34 knots) extend from the hurricane’s center.

Fiona’s 3-day rainfall accumulations in Canada were generally in the 2-to-8-inch range, according to rain-gauge data provided by the Canadian government: historical data. Selected gauges are shown in the image using circles that are color-coded with the same color table as the IMERG data. The IMERG-estimated 3-day accumulation over Nova Scotia is, for the most part, in the same 2-to-8-inch range as the rain-gauge data.

On September 23, which was one day before the Canadian landfall, IMERG estimated higher rainfall totals of over 12 inches when Hurricane Fiona passed near Bermuda. The storm was moving forward more slowly at that time, which increased rainfall totals. Rainfall totals were even higher when Hurricane Fiona passed through the Caribbean, as described in the section below.

September 23, a north-south-oriented cold front moved east off the shore of New England and the US Mid-Atlantic region. Rain and thunderstorms associated with that cold front are seen in the IMERG rainfall image, specifically along the coast of Maine and Massachusetts. The blue contour indicates 2-to-4-inch accumulation from the cold front along the Maine coast.

Rainfall in the Caribbean

Hurricane Fiona (2022) in the Caribbean Sea

Between Sept. 16 and 19, 2022, Hurricane Fiona struck Guadeloupe, Puerto Rico, and the Dominican Republic. NASA estimated precipitation in the region in near real-time using data from an international constellation of satellites processed by the IMERG algorithm.

IMERG estimated that Fiona dropped 12 to 18 inches of rainfall over Guadeloupe and over the ocean just south of Puerto Rico. Media sources and the National Weather Service reported that heavy rainfall accumulation extended somewhat further north, exceeding 20 inches over part of southeastern and central Puerto Rico. Some locations with particularly high accumulation from the storm are indicated with white dots in the image.

IMERG’s strength is its global reach, including regions of land and ocean where ground-based weather radar data is unavailable. Where ground radar or rain gauges are present, they may do better at capturing small, intense rain cells and the local effects of mountainous terrain. These strengths and limitations are illustrated by IMERG’s ability to depict Fiona’s full swath of rain across the Atlantic and its underestimation in Puerto Rico.

The National Hurricane Center reported that tropical-storm strength winds extended out from the center of the storm at the distances indicated in the three wind figures shown in the image. At the first two times shown (September 15 and 17), Fiona was a tropical storm with maximum sustained surface winds 39-73 mph. By September 19, Fiona had strengthened to a category-2 hurricane (96-110 mph winds).

Credits: Visualization by Owen Kelley (CEOSR/NASA).

Hurricane Henri Fights Wind Shear on Approach to New England (August 2021)

In the North Atlantic, the tropical system known as Henri reached hurricane status on Saturday, August 21, 2021. At the time, it was approaching a landfall in New England. Between Friday and Sunday, Henri was observed three times by the Dual-frequency Precipitation Radar (DPR) on the core satellite of NASA’s Global Precipitation Measurement (GPM) mission.

The GPM satellite’s first two flyovers of Henri occurred just 10 hours apart and revealed a remarkably unchanging structure that was being impacted by wind shear. A third overflight occurred a day later, when wind shear had abated and Henri had become more organized. The GPM mission’s observations, such as these, are processed in near real-time for use by weather agencies and researchers.

Above, the 3D image from the first overflight shows the volume of light precipitation associated with Henri at 1646 UTC (12:46 PM local time) on Friday (August 20), and the structure tells this chapter of Henri’s story. At this time, Henri’s center was approximately 750 miles south of the Connecticut coast.

Light precipitation is detected by a 20 dBZ radar reflectivity, with color-coded stripes indicating which portion of the precipitation reached an altitude of 8.5 km (green), 11 km (yellow), or 14.5 km (red) above the ocean surface. A letter “L” indicates the location of the system’s low-level center of rotation.

For several days, Henri had been battling strong wind shear. Wind shear is when the horizontal wind near the Earth’s surface is blowing in a different direction or speed than the horizontal wind higher in the troposphere, the layer of the atmosphere that most storms are confined to. Henri’s low-level center of circulation was at its northern edge, with most of the precipitation occurring in a 100-kilometer-wide area to the south of the low-level center of circulation. Wind shear can encourage the formation of strong convective cells in the inner core of a tropical system, but shear can also hamper those convective cell’s ability to strengthen that system.

Hot Towers and Heat Engines

In the first overflight of Henri on Friday morning, a “hot tower” convective cell was observed adjacent to the low-level circulation center, and this hot tower had strong enough updrafts to lift ice precipitation more than 16 kilometers (10 miles) above the ocean’s surface. Hot towers are convective cells with updrafts strong enough to lift precipitation up through the top of the troposphere.

A series of hot towers in the inner core of a tropical storm or hurricane indicates that a considerable amount of heat is being released inside the storm. This heat could result in wind intensification if wind shear or other factors do not short circuit the process.

The second overflight of Henri (not shown) occurred late on Friday night, local time, which was early on Saturday morning in universal time (0232 UTC). At that time, shear was still high and Henri still had a hot tower near its low-level center of rotation. By 11 AM Saturday morning (1500 UTC), shear had weakened and Henri had strengthened 5 knots to becoming a category-1 hurricane according to the National Hurricane Center.

Hurricanes are often said to contain “heat engines” because they convert some of the energy in the sun-warmed surface of the ocean into kinetic energy of the hurricane’s strong winds that rotate around its low-level center of rotation. In this analogy, strong convective cells in the inner core of a hurricane are the pistons of its heat engine.

When a factor such as wind shear inhibits this energy conversion, one might say that all the pistons of the heat engine were firing but the engine wasn’t in gear. In other words, vigorous convective cells were injecting heat into the atmosphere, but the hurricane could not convert that heat into a well defined eye circled by rapid winds. This diagnosis of Henri is consistent with the two GPM satellite overflights during the 10-hour period on Friday, August 20.

Wind Shear Weaken,
Henri Strengthens into a Hurricane

Hurricane Henri (21 Aug 2021) — Larger Image: click here

As pictured above, the Dual-frequency Precipitation Radar flew over Henri for a third time late on Saturday night, local time (9:37 PM EDT), which was early Sunday morning in Universal Time (0137 UTC). At this time, Henri’s center was approximately 230 miles south of the Connecticut coast. The precipitation of Henri’s inner core at this time was consistent with that of a weak hurricane. In this overflight, the DPR captured most but not all of the eye. The precipitation along the southwest edge of the eye was outside of the field of view of the DPR.

On Sunday, Henri had an eye, an area at its center that was free from precipitation, and an eyewall, an arc of heavy precipitation around that eye. Henri’s eyewall was relatively weak, with updrafts able to lift precipitation to less than 8 km altitude in most places.

In the Sunday morning overflight, there was, again, a hot tower near Henri’s center. Given time and favorable circumstances, a series of hot towers in a hurricane’s eyewall can lead to intensification, according to NASA research. For Hurricane Henri, further intensification was not forecast, nor did it occur, following this overflight. Further intensification likely did not occur in part because Henri entered cold water off the coast of New England. Credits: Visualization and caption by O. Kelley.

Two Weeks of Heavy Rain and a Year of Drought Over Parts of China (July 2021)

During July 17 to 28, 2021, several storm systems brought heavy rain to parts of China and surrounding countries, while a nine-month-long drought persists in an adjacent part of China. NASA’s multi-satellite precipitation algorithm has been monitoring this rainfall in near real-time, and the estimates are distributed to weather-forecasting agencies and disaster-monitoring organizations. This algorithm is called IMERG, the Integrated Multi-satellitE Retrievals for GPM. GPM is the NASA / JAXA Global Precipitation Measurement mission, which launched its Core Observatory satellite in 2014.

Two Typhoons

IMERG precipitation estimates from July 17 to 28, 2021, are shown in the image above. Among the weather systems impacting the region were two landfalling typhoons – Cempaka and In-fa.

One can see that over a foot of rain has fallen during this 12 day period along most of the coast from Hong Kong west to Hanoi, Vietnam. This rain came mostly from Typhoon Cempaka during July 17 through 19.

Over a foot of rain has also fallen during this time near Shanghai, which is China’s most populous city. That rain came mostly from Typhoon In-fa during July 25 to 28. According to Buckingham (2021), parts of Zhoushan City, near Shanghai, received at least 30 inches of rain from In-fa between July 23 and 27. Over the Pacific Ocean near China, two rain gauges in the Global Historical Climatology Network (GHCN) reported 12-inch rain accumulations during the July 17 to 28 period at locations where NASA’s IMERG algorithm also reported similar accumulations. These gauges are located on two small islands, Japan’s Nago and Kumejima islands. Point estimates of rainfall are represented in the image as circles that is colored using the same color scale used to display the IMERG data that covers the globe. Nearby over the ocean, where no rain gauges exist, the IMERG algorithm reported significantly more rain accumulation along the path of Typhoon In-fa, over 30 inches of accumulation.

Two Additional Storm Systems

During the same time, a synoptic system brought over 30 inches of rain accumulation to the Chinese city of Zhengzhou and surrounding areas of Henan Province during July 19 to 22. According to Childs (2021), over 31 inches fell during a three-day period.

Also separate from the two typhoons, a storm system brought accumulations in excess of 20 inches to the western coast of the Philippines, near Manila, during July 21 to 24. These accumulations were detected by the IMERG algorithm and are consistent with rain gauge data in Manila.

While such accumulations are impressive, they are not unprecedented. For example, Hurricane Harvey (2017) stalled over Texas and brought over 60 inches of rainfall to a small area over a period of four days (Blake and Zelinsky, 2018). IMERG was also used to analyze the rainfall from Harvey.

A Persistent Drought

Meanwhile, a drought in Fujian Province in coastal China has persisted since October 2020 with rainfall accumulation less than 25% of the typical accumulation for part of Fujian Province. Deviation from normal rainfall can be estimated using the long-term record of IMERG estimates that stretches back to the year 2000.

Wang (2021) and Ye (2021) report that China’s coast south of the Yangtze River, a region that includes Fujian Province, only received 20% to 50% of the normal amount of rainfall between October 2020 and February 2021. IMERG data shows that summer rains have relieved the drought in the provinces adjacent to Fujian, but not in Fujian itself.

The regional rainfall patterns seen during the past two weeks over China are significantly different than the Meiyu-Baiu pattern of rainfall that IMERG detected at this time last year. Credit: Visualization and caption by O. Kelley.

Rainfall that Matters: A Convective System over Arizona within the North American Monsoon (July 2021)

North American Monsoon region — **Figure 1.** Geographic extent of the North American Monsoon. *Larger Image: click here*

There is a monsoon that occurs in the southwestern U.S. each summer, and it brought heavy rain to the deserts of Arizona this week. This monsoon is less well known than India’s Summer Monsoon, but both monsoons have similar causes [1, 2, 3].

In western Mexico and the southern edge of the southwest U.S., most of the year’s rain typically falls in just three months: June, July, and August. The region is shown in light blue in the below climate map, which shows where summer rainfall predominates (Figure 1). This seasonal pattern is known as the North American Monsoon. The map was generated using the long-term record of NASA’s IMERG multi-satellite precipitation analysis. Climatologists and forecasters have used various dates to define the exact start and stop dates of the North American Monsoon [4].

This year’s monsoon season has officially begun, and the week of July 12-16 has been an active monsoon week for Southern Arizona [5].

On the morning of July 15, 2021, the National Weather Service issued a warning for severe thunderstorms over Arizona’s southern desert with rain rates of 1 inch per hour over ground that was already saturated by recent storms [6]. Under these circumstances, flash flooding is a concern. The location of this warning is indicated by the red circle on the climate map (31.72N latitude and 112.13W longitude in Figure 1).

The NASA / JAXA GPM Core Observatory satellite captured a 3D view of a convective storm system at this location at 8:37 a.m. local time on July 15. The data was collected by the satellite’s Dual-frequency Precipitation Radar (DPR). The 3D view is shown in Figure 2. This overflight of the storm system showed several convective cells embedded within it with precipitation rates at the Earth’s surface in excess of 1.4 inches per hour. The strongest of these cells contained updrafts that were sufficiently strong to lift ice precipitation overshooting through the top of the troposphere. The troposphere is the lowest layer of the atmosphere that normally contains the weather.

Shown in red in Figure 2, the Dual-frequency Precipitation Radar detected a 20-dBZ radar-reflectivity signal at an altitude 14 kilometers above the Earth’s surface and over 14.8 km above sea level. In Figure 2, the yellow and green areas identify where 20-dBZ radar reflectivity was detected at 11 and 8.5 km altitude, respectively. Commercial airlines cruise at 9 to 13 kilometers. At lower altitudes, this convective cell contained radar reflectivity in excess of 50 dBZ, suggesting hail was present.

This GPM satellite overflight of the storm system occurred at 8:37 a.m., approximately half an hour before the National Weather Service issued its thunderstorm discussion for the area [6].

This portion of Arizona receives only 10 to 15 inches of precipitation in an entire year, on average, according to the long-term record from NASA’s IMERG multi-satellite precipitation algorithm. Local accumulation exceeded 2 inches in places due to the convective storm system that passed through this area on July 15, 2021. This storm total was estimated both by the near real-time IMERG algorithm and also by the National Weather Service’s preliminary analysis of ground-radar data [https://water.weather.gov].

It is typical of this area’s climate that heavy rainfall from convective systems such as this one contribute much of the area’s annual rainfall accumulation. According to the IMERG record, this portion of Arizona receives approximately 42% to 50% of its annual accumulation in June through August. Visualization and caption by Owen Kelley (NASA/GMU).

References:

(1) NASA, 2017: NASA Looks at the North American Monsoon. 1:26 minute video featuring NASA Scientist Dr. George Huffman, https://gpm.nasa.gov/resources/videos/nasa-looks-north-american-monsoon.

(2) Branon, M., 2021 June 17: How the Weather Service cleared the air about Southwest monsoon season. Capital Weather Gang, Washington Post, https://www.washingtonpost.com/weather/2021/06/17/southwest-monsoon-sea….

(3) Adams, D. K., and A. C. Comrie, 1997: The North American Monsoon. Bulletin of the American Meteorological Society, 78, 2197-2213.

(4) June 15 is currently the National Weather Service’s official start date for the North American Monsoon. Adams and Comrie (1997) propose June-August or July-September as possible date ranges for the monsoon.

(5) Arizona Daily Star, 2021 July 14: Tucson passes 2020 monsoon rainfall total; flash flood warning in effect, https://tucson.com/news/local/tucson-passes-2020-monsoon-rainfall-total….

(6) NWS, 2021 July 15: Mesoscale Precipitation Discussion #0546 issued at 9:10AM EDT on Thursday, July 15, 2021. Weather Prediction Center (WPC), College Park, MD, https://www.wpc.ncep.noaa.gov/metwatch/metwatch_mpd_multi.php?md=0546&y….

Australia’s Heavy Rainfall: Desert and Coast (March 2021)

larger image: click here

During the week ending on March 23, 2021, two locations in Australia experienced unusually high rainfall totals. In the news, a persistent system brought flooding rains to Australia’s east coast from Brisbane to Sydney and points further south.

The preliminary estimate from NASA’s multi-satellite global precipitation analysis is that more than 24 inches fell just off the coast of Australia in 7 days with accumulations in coastal areas exceeding 16 inches. Near the Strzelecki Desert in central Australia, a storm system brought 8 inches of precipitation during the same 7-day period. Most of the rain fell during a 3-day period (0000 UTC on 20 March to 2359 UTC on 22 March).

These estimates come from the near-realtime version of NASA’s IMERG algorithm which combines microwave and infrared satellite observations with climatological datasets including rain-gauge
observations. In addition to near global coverage, another advantage of the IMERG algorithm is that it provides estimates covering approximately a 20-year period. This long-term record allows for comparisons of current precipitation events with long-term averages.

According to IMERG records, the past week’s rainfall accumulation along Australia’s eastern coast is more than 25% of the average annual accumulation there. The past week’s rainfall accumulation near the Strzelecki Desert is greater than the average annual accumulation there.

Data from the Australian Bureau of Meteorology confirms at least the main conclusions from these IMERG observations. Central Australia typically receives only 4 to 8 inches (100 to 200 millimeters) of precipitation a year, while a thin band of land along Australia’s eastern coast typically receives about ten times as much precipitation: 40 to 60 inches (1000 to 1500 millimeters) in a year.

In terms of 7-day storm totals, the Australian Bureau of Meteorology reports that there is a 1% chance each year of an event exceeding 498 or 177 millimeters (20 or 7 inches) of precipitation accumulation in Sydney or in the desert where New South Wales, Queensland, and South Australia meet. The desert rainstorm during the week ending on March 23, 2021, exceeded this threshold, while the flooding over Australia’s east coast did not.

Credits: IMERG data from NASA at https://gpm.nasa.gov/. Average annual accumulation from the Australia Bureau of Meteorology at http://www.bom.gov.au/jsp/ncc/climate_averages/rainfall/index.jsp. One percent annual chance of 7-day rainfall accumulation from the Australian Bureau of Meteorology’s intensity-depth-frequency (IDF) data in the Design Rainfall Data System (2016) at http://www.bom.gov.au/water/designRainfalls/revised-ifd/. Visualization and caption by O. Kelley.

Hurricane Sally in Context (Sept. 2020)

Hurricane Sally (2020) and Hurricane Harvey (2017)

One way to put a storm into context is to compare its precipitation to the average annual accumulation at that location. The IMERG multi-satellite precipitation algorithm estimates that Hurricane Sally brought over 20 inches of precipitation to the coast of Florida’s Panhandle during the 7 days ending September 18, 2020, at 0000 UTC. That accumulation was equivalent to 25% to 33% of that area’s average annual precipitation. This area is indicated in purple on the left half of the image.

The coast of Florida’s Panhandle is the wettest area along the entire US East Coast and Gulf Coast. It receives approximately 60 to 68 inches of precipitation in an average year, based on 19 years of estimates made by the IMERG algorithm.

When it comes to hurricane flooding along the Gulf Coast, Hurricane Sally does not top Hurricane Harvey which made landfall in late August, 2017. Like Sally, Harvey also dumped over 20 inches of rainfall, but over a larger area that included a major metropolitan area. Specifically, this area of southeastern Texas includes Houston, Texas, a city that typically averages approximately 55 to 60 inches of precipitation a year. In Houston, the 7-day accumulation from Hurricane Harvey was 50% to 75% of the city’s average annual accumulation. This area is indicated in red on the right half of the image. Houston area flooding contributed to Hurricane Harvey being one of the most costly hurricanes in US history.

Our ability to interpret near real-time estimates of precipitation over both land and ocean is assisted by the availability of a reference data set of 19 years of global precipitation estimates made by the same IMERG algorithm. Visualization and caption by O. Kelley.

A Year’s Worth of Rain in a Week for a City in Pakistan (Aug. 2020)

In the last week of August 2020, Pakistan’s largest city, Karachi, received over 8 inches of rainfall according to NASA’s IMERG dataset, causing destructive flooding in the region. The amount of rain that fell that week is roughly equivalent to the amount that Karachi typically receives in an entire year, based on IMERG’s 19-year global climatology. In a typical year, most of Karachi’s rain will fall in July and August, but the rainfall during the week of August 23rd was unusually heavy.

The top panel of the three panels in this image shows the depth of the 7-day rainfall accumulation in inches (August 23 to 30, 2020). The light green color indicates at least 8 inches of rainfall in Karachi and locations further inland and to the east.

The middle panel divides the rain that fell during this seven day period by the average annual accumulation at each 0.1 x 0.1 degree grid box of the IMERG product. Values in excess of 1 (red) indicate locations where a full year’s worth of rain fell in this one-week period. When such an extreme event occurs, it can be difficult for the built environment to function normally.

The bottom panel gives a sense of what is normal for the last week of August. It shows what fraction of the annual total precipitation falls during the last week of August of a typical year. Regions where at least 1/24 of a “year’s worth” of rain typically falls during the last week of August normally experience part of their rainy season during the last week of August. This is the case for the eastern portion of Pakistan and India. Regions where less than 1/100 of a “year’s worth” of rain typically falls in the last week of August normally experience part of their dry season during the last week of August. This is the case for western Pakistan and Afghanistan.

IMERG rainfall estimates are automatically generated in near real-time as part of NASA’s effort to monitor the Earth. The estimates are based on observations from an international constellation of satellite including the GPM satellite. Several months after an event occurs, the rainfall estimates are improved when NASA re-runs the IMERG algorithm using additional data sets that are not available in near real-time.