Tropical Invest 96P and Tropical Cyclone Vicky near American Samoa

February 18th, 2020 |

GOES-17

GOES-17 “Clean” Infrared Window (10.35 µm) images, 16-18 February [click to play animation | MP4]

GOES-17 (GOES-West) “Clean” Infrared Window (10.35 µm) images (above) showed the movement of numerous thunderstorms across American Samoa during the 16-18 February 2020 period. This deep convection was being forced by an active South Pacific Convergence Zone or “Monsoon Trough” (surface analysis) and the presence of Tropical Invest 96P (named TD07F by the Fiji Met Service / Nadi Tropical Cyclone Centre) northwest of Samoa. Due to outflow from a nearby thunderstorm, winds gusted to 60 knots at Pago Pago, American Samoa (NSTU) at 11 UTC on 17 February.

With an increasing probability of Invest 96P becoming better organized (aided by low values of deep-layer wind shear along with modest upper-level divergence), a GOES-17 Mesoscale Domain Sector was positioned over the Samoan Islands on 18 February — providing “Red” Visible (0.64 µm) and “Clean” Infrared Window images at 1-minute intervals (below). During this period, the coldest convective overshooting tops exhibited infrared brightness temperatures in the -80 to -85ºC range (which corresponded to the tropopause temperatures seen in Pago Pago rawinsonde data).

GOES-17 "Red" Visible (0.64 µm) and "Clean" Infrared Window (10.35 µm) images, 18 February [click to play animation | MP4]

GOES-17 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images with surface plots for Pago Pago, American Samoa on 18 February [click to play animation | MP4]

===== 20 February Update =====

 GOES-17 "Red" Visible (0.64 µm) and "Clean" Infrared Window (10.35 µm) images with surface plots for Pago Pago, American Samoa on 18 February [click to play animation | MP4]

GOES-17 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images with surface plots for Pago Pago, American Samoa on 20 February [click to play animation | MP4]

Another tropical depression (Invest 97P/TD09F) developed along the active Monsoon Trough on 20 February (surface analyses), intensifying just south of American Samoa to become Tropical Cyclone Vicky (TC 17P) as of 18 UTC (JTWC advisory). Once again a GOES-17 Mesoscale Sector was positioned over the region — “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed the gradual organization of Vicky; the coldest cloud-top infrared brightness temperature of convective overshooting tops was -90ºC. Surface observations revealed a wind gust to 65 knots at Pago Pago, American Samoa just before 20 UTC.

GOES-17 Infrared Window (11.2 µm) images from the CIMSS Tropical Cyclones site (below) showed that Vicky was moving through an environment characterized by of low values of Deep Layer Wind Shear, a favorable factor for further intensification.

GOES-17 Infrared Window (11.2 µm) images with contours of Deep Layer Wind Shear (click to enlarge]

GOES-17 Infrared Window (11.2 µm) images with contours of Deep Layer Wind Shear [click to enlarge]

Hourly MIMIC Total Precipitable Water images during the 16-20 February period (below) displayed the northwest-to-southeast oriented band of elevated moisture along the South Pacific Convergence Zone (or Monsoon Trough). The Samoan Islands are centered near 14.3° S latitude, 170.1° W longitude.

Hourly MIMIC Total Precipitable Water product during the 16-20 February period [click to play animation | MP4]

Hourly MIMIC Total Precipitable Water product during the 16-20 February period [click to play animation | MP4]