1-minute Mesoscale Domain Sector GOES-18 (GOES-West) “Red” Visible (0.64 µm) images (above) include plots of Derived Motion Winds (DMW) — and showed the rapid offshore transport of cold arctic air across the southern Cook Inlet, Shelikof Strait and northwestern Gulf of Alaska on 12 February 2023. The fastest low-level (Surface – 900 hPa) DMW speed was 64 knots in the far southern portion of Cook Inlet at 2305 UTC (below).

A Heavy Freezing Spray Warning had been issued for that entire offshore region (light blue), including the Shelikof Strait (above) — and Buoy 46077 in the Shelikof Strait was recording Ice Accretion rates in excess of 1.0 inches per hour (below). Buoy air temperatures had fallen into the 10-12F range during that time, with wind gusts of 40-50 knots — providing ideal conditions for rapid ice accretion.

RCM/Radarsat-2 SAR winds at 1630 UTC on 12 February (source) are shown below — a NW-to-SE oriented swath of strong offshore winds (40-50 knots, darker shades of red) was seen extending from the far southern end of Cook Inlet (where the aforementioned 64-knot GOES-18 DMW speed was located) into the northwestern Gulf of Alaska. Another pocket of similarly-strong wind speeds was evident farther to the north, in the vicinity of Homer, Homer Spit and the mouth of Kachemak Bay.

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Sentinel-1A overflew the Samoan Islands shortly after sunset on 11 February and captured a packet of very strong winds just to the west of Tutuila Island of American Samoa. The toggle above compares Band 13 imagery with two different enhancements of the SAR Winds: one (here) highlighting the different Beaufort Scales, and one (here) showing just gridded fields (from 0-65 knots). Is there a good relationship between the wind structures and the Band 13 brightness temperatures?

The first thing to do is to examine the strong winds to the west of Tutuila for any possible artifacts that might arise from ice within the clouds. The toggle below (between this wind image and this Normalized Radar Cross Section — NRCS — image from this website) shows that features north of Tutuila, between 13.4 and 13.6^oS and around 170.5^oW, for example, have bright white, diffuse NRCS features that suggest ice; such features are absent in the clouds around western Tutuila, however, suggesting they contain little ice to affect the SAR return (with the exception of features at the very southern edge of the domain). The SAR data suggest modest northerly winds (around 10 knots) over most of the domain. That is in agreement with ASCAT data from MetopB (0800 and 2100 UTC on 12 February) and MetopC (2030 UTC on 12 February), taken from this website.

The animation below shows persistent cooler cloud top features moving northward over western Tutuila. The feature over western Tutuila at 0550 UTC spawns the very strong winds that show up in the SAR data. Immediately in the lee of the island (if one assumes southwesterly outflow!), relatively calm winds are diagnosed.

GOES-18 Band 3 Near-infrared (“Veggie Band”, 0.86 µm) imagery, below, also shows the northward motion of cloud features over western Tutuila. The absence of widespread strong winds in other features within this line of enhanced cloudiness suggests that interaction with the topography of Tutuila was a possible contributing factor to the outflow generation.

What was the weather like at Pago Pago International Airport around sunset? The screen capture below (from this site) shows a shower near 7 PM on the 11th (Samoa Standard Time), which is 0600 UTC on 12th.

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JMA Himawari-9 Infrared Window (10.4 µm) images (above) showed Cyclone Freddy in the South Indian Ocean as it was rapidly intensifying from a Category 1 to a Category 3 storm by 12 UTC on 11 February 2023 (JTWC discussion), eventually presenting a ragged eye for several hours — then briefly reaching Category 4 intensity at 00 UTC on 12 February (SATCON | JTWC discussion). Overshooting tops within the eyewall of Freddy occasionally exhibited cloud-top infrared brightness temperatures of -90ºC and colder (yellow pixels embedded within darker purple regions).

A NOAA-20 VIIRS Day/Night Band (0.7 µm) image valid at 1825 UTC on 11 February (below) showed the eye of Freddy.

Himawari-9 Infrared Window (11.2 µm) images from the CIMSS Tropical Cyclones site (below) include contours and streamlines of deep-layer (850-200 hPa) wind shear. The intensification of Freddy occurred in spite of this unfavorable environment of moderate to strong eaterly shear — however, the storm was moving over relatively warm water (with Sea Surface Temperature values around 29ºC).

RCM/Radarsat-2 SAR winds at 1136 UTC on 11 February (source) are shown below — a maximum wind speed of 106 knots was indicated within the northeast quadrant.

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A modest snow storm affected southern/central Wisconsin on 9 February (WFO ARX story), depositing 6-12″ of snow in a band from northeast Iowa to central Lake Michigan, as shown above: the snowfall caused an obvious change in the Suomi NPP imagery between 08 February and 10 February 2023, shown above using imagery taken from the VIIRS Today website. (A similar before/after pair of images using GOES-16 data is below).

A variety of NOAA/STAR snowfall products gave information about this snowstorm during and after its passage through the midwest. The image below shows the Snowfall rate (data source) over Wisconsin and Michigan at 1816 UTC on 9 February (derived from ATMS data on NOAA-20), and a NEXRAD radar image (from the ‘radar’ tab here) over Wisconsin at about the same time. There is very good agreement between the two representations of falling precipitation over Wisconsin!

The NOAA/STAR cryosphere team developed AMSR-2 snow products using data from JAXA’s GCOM-W polar orbiter. The snowdepth product, below, shows the change in snowdepth over Wisconsin between 9 February (left) and 11 February (right). The deepest diagnosed snow on 11 February in the new snow band is about 35 cm (around 13″).

The GOES-16 Fractional Snow Cover product, below, from 1900 UTC on 11 February 2023, shows the snow band as well.

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Thanks to Yinghui Liu, NOAA/STAR at CIMSS, and Huan Meng, NOAA/STAR in College Park, for imagery for this blog post. Thanks also to Peter Romanov, CUNY/CREST, for contributions.

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