US Space Force EWS-G1 (formerly GOES-13) Infrared Window (10.7 µm) images (above) showed Category 3 Cyclone Freddy making landfall along the east coast of Madagascar on 21 February 2023.

According to products from the CIMSS Tropical Cyclones site, during the 12-15 hours leading up to landfall Freddy was traversing warm water with Sea Surface Temperature values of 28C, and moving through an environment characterized by low values of deep-layer wind shear (below).

View only this post Read Less

Heavy Rains overspread the Samoan Islands late on 20 February/early on 21 February 2023. ASCAT scatterometry from 0845 UTC on 21 February, above, (from this website), shows strong northerly winds Equatorward of the Samoan Islands, and weaker winds to the south: surface convergence is indicated. The animation below shows GOES-18 Visible imagery overlain on top of GOES-18 K-Index, one of the derived stability indices routinely produced. At this time, a large area of disturbed weather is located to the east of American Samoa.

The disturbed weather was associated with a trackable feature, a lowered tropopause, in gridded NUCAPS fields (available at this site). The lowered tropopause is highlighted in green values below and surrounded by higher tropopause values that are blue/cyan. This feature was attached to the lower tropopauses poleward of 30^oS (see this image from 2126 UTC on 20 February)

The animation below shows GOES-18 Band 13 Clean Window infrared (10.3 µm) imagery underlain with Clear-Sky-only estimates of K Index. Vigorous and persistent convection develops near Tutuila (America Samoa’s largest island) around 0700 UTC and continues through the end of the animation as it also builds northwestward over Western Samoa.

Flash Extent Density observations from the Geostationary Lightning Mapper (GLM) on GOES-18 show abundant lightning with the convection around the Samoan Islands.

What were the impacts from this convection? Flash Flooding occurred on American Samoa, as almost five inches of rain fell at the airport between 9 PM and 4 AM Samoa Standard Time (that is, 0800 UTC to 1500 UTC). In addition, Hawaiian Airlines Flight 465 (from Honolulu to Pago Pago) landed on time — but lightning prevented passengers from disembarking for 2 hours. Consequently, the return flight (Hawaiian Airlines Flight 466) was delayed 16 hours because of crew time constraint]

———————————————————————————————————————————–

Another heavy rain event across the islands of American Samoa was documented in February 2021.

View only this post Read Less

SAR data collection over Cyclone Freddy in the Indian Ocean has led to a remarkable collection of imagery. To date, more than twenty eye overpasses have occurred from RADARSAT-2 (3 overpasses), Sentinel 1A (1 overpass) and the three satellites of the RADARSAT Constellation Mission (17(!) overpasses). The animation above shows images tracking the system as it moves across the Indian Ocean towards a projected landfall in Madagascar. Eye-centered imagery, below, shows the organization and strengthening of the system.

Because 5 satellites are involved, occasionally imagery that is very close in time is acquired. For example, RCM-3 and RADARSAT-2 both sampled the between 2236 and 2240 UTC on 11 February 2023; RCM-1 and RADARSAT-2 both sampled the eye near 0000 UTC on 16 February (Freddy was near its peak intensity at that time, as noted in this blog post); RCM-1 and Sentinel-1A both sampled the eye near 0020 UTC on 17 February 2023. Three toggles showing these comparisons are below. Note that winds at the edge of a scan are likely weaker than they actually are.

SAR Winds and analyses for Cyclone are Freddy are also available here. For more information on Freddy, refer to the JTWC webpages, or the RSMC in La Reunion (map of all RSMCs is here), or the SSEC/CIMSS Tropical Weather Website.

View only this post Read Less

GOES-18 (GOES-West) Air Mass RGB images during the 15-19 February 2023 period (above) displayed the migration of a Kona Low from north of the main Hawaiian Island chain on 15 February toward Johnston Island on 19 February. The MIMIC Total Precipitable Water product (below) showed that the circulation of this Kona Low helped to draw a broad plume of moisture northwestward from the ITCZ  and across Hawai’i — providing the fuel for a prolonged period of heavy rainfall.   

Plots of rawinsonde data from Hilo (below) revealed the deep tropical moisture that was present over the eastern portion of the Big Island from 18-19 February.

GOES-18 “Clean” Infrared Window (10.3 µm) images (above) provided a closer look at the Big Island, where the heaviest rainfall occurred across the Kau, Puna and South Hilo districts (NWS Honolulu PNS). On 18 February, Hilo experienced 2 consecutive hours with rainfall in excess of 3 inches (below).

Hilo, Hawai'i, with over 3" rain in consecutive hours. The 2-hour total of 6.16" has a recurrence interval of > 50 years. Looking at the hourly records (since 1962), only one other instance of consecutive hours each having 3"+ rain (Nov 2, 2000). pic.twitter.com/41fnxiUojA

— Brian Brettschneider (@Climatologist49) February 18, 2023

View only this post Read Less