Hurricane Helene spent time percolating in the northwestern Caribbean over the past week. The animation above show true-color imagery from the CSPP Geosphere site at 1-day intervals from 13 to 24 September. Convection over the southern Gulf of Mexico on 18-20 September moved northward and expanded greatly before being designated a tropical storm at 1500 UTC on 24 September, shortly before the end of the stepped animation above.

Helene’s genesis was captured well by data from the direct broadcast antenna at AOML in Miami FL (link). The toggles below show GCOM-W1 AMSR-2 imagery at 36.5 and 89.0 GHz; the images are centered on the cyclone. The 89.0 GHz imagery, below, shows a strong signal of convective clouds shown in red. These cold features show up because 89.0 GHz energy is strongly scattered by ice. At all three times, curvature is apparent, but a definite center is not until the 1832 UTC imagery from 24 September.

The 36.5 GHz imagery, below, is more affected by cloud and rain water features below the freezing level. The background ocean shown to be very cold because ocean water has low emissivity at 36.5 GHz; energy is added to the upwelling signal from the cloud and rain droplets. For Helene, as it developed, this was mostly over the eastern half of the storm. By 1832 UTC on 24 September, there are pronounced curved bands near the center of newly-named Tropical Storm Helene.

Those in the southeast US should closely monitor the approach of Helene. For the latest on Hurricane Helene, and the multiple dangers it poses, refer to the National Hurricane Center.

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The Fall Equinox in 2024 happened on September 22. According to the astronomical definition of the seasons, the autumnal (or fall) equinox marks the end of summer and the beginning of autumn, which lasts until the winter solstice (around December 21 or 22 in the Northern Hemisphere). This is the astronomical definition of the changing of seasons as the tilt of the earth on its axis will lead to summer in the southern hemisphere as we head into winter up here in the northern hemisphere.

The GOES satellites can provide a unique view of the earth as we pass through the various seasons. The angle of the shadow at sunrise or sunset (satellite time, or at the satellite nadir point) shows how earth progresses through the seasons from summer solstice to fall equinox. Watch the reflection of the sun (aka “sunglint”, the shiny spot which starts west of Mexico over the eastern Pacific in the GOES-East animation) migrate toward the equator over the course of summer; it ends up predictably right over the equator on the fall equinox.

Webapps about the Seasons

These images were made using NOAA data with geo2grid software, from UW-Madison, SSEC. T. Whittaker is thanked for the webapps.

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Day Night band visible imagery (0.7 µm) from NOAA-21 shows the characteristic signal of Aurora Borealis over southern Canada early in the morning (0722 UTC, below and 0903 UTC, above) on 12 September 2024 (Imagery is taken from the CIMSS VIIRS Imagery Viewer). Lunar illumination on 12 September was not present because the waxing gibbous moon was below the horizon. The light emitted from the Aurora, and from cities, and from various wildfire (for example, the Short Draw fire along the Wyoming/Montana border) is easily detected by the VIIRS imager in the view above. VIIRS has a bit more difficulty viewing the smoke plumes over the west, and the clouds with Francine over the mid-South, because of the relative lack of reflected light. The clouds from Francine are absorbing/scattering emitted light from cities in Louisiana and Mississippi making the city lights there less obvious.

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LightningCast probabilities (from this website) over the Central Dense Overcast of Tropical Cyclone Bebinca in the western Pacific to the northwest of Guam, above, show increasing values (the island of Guam is center bottom of the imagery above): magenta are values of 75%. Lightning activity within the central core is often a precursor to strengthening in a tropical cyclone, and the JTWC Forecast (below) for the system suggests just that.

Bebinca is over very warm SSTs and in a region of low shear. The projected path of the storm continues over warm water.

Himawari-9 Clean Window imagery, below (courtesy of the NWS Forecast Office in Pago Pago) overlain with lightning shows the cold cloud tops associated with the convection. Lightning becomes more common at the end of the animation, especially after 1800 UTC. This is in agreement with the LightningCast predictions shown above.

MIMIC Total Precipitable Water fields (source), below, show the spinup of the Tropical Storm. The circulation of this system continues to draw very moist air over the Marianas. The National Weather Service in Guam has issued Flood Watches, High Surf advisories and a wind advisory related to this event on 11 September.

Timely overpasses from Metop-B and Metop-C at around 1200 UTC on 11 September, shown below (from this site), helped benchmark the strength of the storm at that time.

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