As discussed in this blog post, heavy rainfall had continued across the Samoan Islands on 25 December 2023. The South Pacific Convergence Zone (SPCZ) remained in the vicinity of the Samoan Islands on that day (surface analyses) — and 1-minute Mesoscale Domain Sector GOES-18 (GOES-West) “Clean” Infrared Window (10.3 µm) images (above) showed additional pulses of deep convection that produced heavy rainfall over Tutuila and the Manu`a Islands of American Samoa. During the calendar day of 25 December local time (which is 1100 UTC on 25 December to 1100 UTC on 26 December), Pago Pago on Tutuila received 2.45 inches of rainfall.

The GOES-18 Infrared image at 1720 UTC on 25 December (below) included a cursor sample of the 10.3 µm cloud-top brightness temperature and associated Level 2 Rain Rate derived product just southwest of the Pago Pago METAR site on Tutuila — the Rain Rate was 1.92 in/hr, highlighting the heavy rain potential of these convective showers.

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10-minute Full Disk scan GOES-18 Air Mass RGB images (above) showed a cyclone as it intensified to a Hurricane Force Low by 0000 UTC on 24 December 2023 as it moved north across the Gulf of Alaska (suface analyses). Brighter orange-red hues in the RGB images highlighted a Potential Vortcity (PV) Anomaly within the core of the low pressure system — AK-NAM40 model fields indicated that the “dynamic tropopause” (taken to be the pressure of the PV 1.5 surface) had descended to near the 800 hPa pressure level at 0000 UTC on 24 December (below).

A closer view of GOES-18 Air Mass RGB and Mid-level Water Vapor (6.9 µm) images (below) displayed the low as it eventually moved inland across Southcentral Alaska by 1200 UTC on 24 December. Notable peak wind gusts included 70 knots at Buoy 46082 at 0710 UTC, 69 knots at Middleton Island (PAMD) around 0200 UTC and 60 knots at Codova (PACV) around 1100 UTC.

This Hurricane Force Low was rather anomalous in terms of its low Mean Sea Level Pressure (MSLP) and strong 925 hPa wind speeds (source), as shown below.

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Turbulence early on 24 December on a flight (Airbus A-300) from Barbados to Manchester (Scotland) led to a diversion (to Bermuda) and hospitalization (albeit with non-life-threatening injuries) of about a dozen passengers. The graphic below shows the path of the flight with data taken from FlightAware.com. The green parts of the plotted path (that is, the first and last hours) come from observations, the white part of the path, including the region through convection is extrapolated. News articles on this event state (link, link, link) that turbulence occurred between 2 and 2-1/2 hours after take-off.

GOES-16 Clean Window infrared (Band 13, 10.3 µm) imagery, below, shows active convection in the region where the turbulence was observed between 0200 and 0230 UTC.

Upper-level water vapor (GOES-16 Band 8) imagery, below, also shows the convection in the region where turbulence occurred. (Click here to view low-level water vapor — GOES-16 Band 10 — imagery)

The CIMSS Turbulence product is a Machine-Learning tool that relates GOES-R imagery (and GFS estimates of lapse rates) to the probability of Moderate or Greater (MOG) turbulence. It is available online here, and is also available via an LDM feed for AWIPS. What did this product show during this event? That is shown below in an animation from 0140 to 0250 UTC on 24 December 2023. The turbulence was observed between 0200 and 0230 UTC to the south of 30^oN — the latitude of northern Florida — and between 47^oW and 50^oW. Click here to see an annotated image from 0220 UTC.

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The plane was traversing a region where Turbulence Probability was large in places. Without a precise knowledge of where/when turbulence occurred, it’s a challenge to assess precisely how well the product performed for this event.

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Perhaps GLM observations could help identify the convective feature that might have caused the convection. GLM observations between 0200 and 0240 UTC, below, bracket the reported time of the convection. If the turbulence is associated with convection, perhaps the cell near 28^oN, 48^oW, strong enough after all to produce lightning, was responsible.

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GOES-16 (GOES-East) Nighttime Microphysics RGB and Day Snow-Fog RGB images (above) displayed the nocturnal formation — followed by the daytime dissipation — of shallow fog/stratus across snow-covered (shades of red in Day Snow-Fog RGB imagery) parts of southeastern Saskatchewan, southwestern Manitoba and northern North Dakota on 22 December 2023. With surface air temperatures dropping into the teens F, several METAR sites began to report freezing fog (that occasionally restricted surface visibility to near zero). Note that the shallow fog/stratus did not extend to the height of a few of the plateaus in the region (such as Turtle Mountain along the North Dakota/Manitoba border, Moose Mountain Provincial Park in Saskatchewan and Riding Mountain National Park in Manitoba).

A toggle between the GOES-16 Nighttime Microphysics RGB image at 1301 UTC and Topography (below) helped to highlight the 3 aforementioned plateau features (darker shades of tan to brown).

The GOES-16 Cloud Thickness derived product (below) — a component of the Fog and Low Stratus suite — indicated that much of the shallow fog/stratus was generally in the 600-1200 ft thickness range.

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