IMERG Adds Up Ida's Rainfall
All eyes were on Hurricane Ida as it made landfall in Louisiana on Aug. 29, 2021, but many people were taken by surprise by the power of Hurricane Ida's remnants when they reached Virginia during the day on Sept.1 and New York City late at night on Sept. 1 into early morning on Sept. 2. The below animation shows the precipitation that fell during the entire lifecycle of Ida from before landfall in Louisiana through the impacts on New York City.
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This animation uses data from the near real-time version of NASA's IMERG algorithm, a data product that includes input from multiple satellites, and climatological calibration from rain gauges and other datasets where available. One of the strengths of IMERG is that it can estimate rainfall over the world's oceans as well as over land, giving a fairly complete picture of global rainfall.
During the 24-hour period ending at 8:00 a.m. on Sept. 2, Ida brought a total of 6 to 10 inches of rain to a narrow strip of land stretching from eastern Pennsylvania to Manhattan, according to preliminary analysis of ground-radar observations by the National Weather Service (NWS). During the prior 24 hours, Ida brought 3 to 4 inches of rain to parts of northern Virginia according to the NWS. The 7-day total accumulation from Hurricane Ida in some locations in and near New Orleans was 15 to 20 inches, according to the same preliminary analysis of ground-radar data. This rainfall far exceeded the forecasted rainfall just a day prior to landfall. These ground-based observed accumulations are broadly consistent with the satellite-based IMERG estimates.
Major flooding associated with systems after they lose hurricane strength is not unprecedented. In some respects, the track and impact of Hurricane Ida this year are similar to those of Hurricane Camille in 1969.
For example, major flooding occurred in Virginia when Hurricane Camille's remnants made there way there after first making landfall in Mississippi. According to the National Hurricane Center (NHC), Camille's impacts were relatively modest after leaving Mississippi and before arriving in Virginia. In a small portion of Virginia's Blue Ridge Mountains, 27 inches of rainfall fell due to the passage of Camille in August 1969.
NOAA, 2 September 2021: Current 1-day observed precipitation valid on 2 September 2021 at 1200 UTC; and Last 7-day observed precipitation valid on 1 September 2021 at 1200 UTC
National Hurricane Center, 1969: Preliminary Report on Hurricane Camille, August 14-22, 1969
Animation by Jason West (NASA GSFC / PPS)
Story by Owen Kelley (NASA GSFC / PPS)