Sentinel-1A overflew the Hawai’ian islands twice, once on 26 February and again on 27 February. The animation above shows the AWIPS presentation of SAR winds at 1616 UTC on 26 February. Images from the NOAA/STAR website for those two footprints are shown below as toggles between the derived wind speed and the Normalized Radar Cross Section at 16:16:24 (top) and 16:16:55 (bottom), i.e., for about 1 second of imagery from Sentinel-1A! The orginal images can be found at the NOAA/STAR Sentinel-1A website link above for 26 February.

One feature stands out in the toggle from 16:16:55 directly above: the band of strong winds seemingly emanating from a point near 18.8^oN, 156.67^oW. That hourglass appearance in the derived winds indicates a small-scale error in the winds used at the start of the retrieval (as noted in Quick Guide for this product); note the feature is absent in the NRSC fields. The derived winds there are most likely not correct.

A zoomed-in view over the Alenuihaha channel between Hawai’i to the south and Maui to the north, below, shows the very strong winds characteristic of that body of water. Winds of 25-30 knots are widespread, with a small area of winds exceeding 40 knots just off the south coast of Maui! Winds are much weaker in the lee of the islands.

The slow animation below shows a region in the SAR winds where (highlighted) isolated pockets of strong winds are diagnosed. Those strong winds have the look of regions where ice contamination in the retrieval is possible. Note, however, that the GOES-13 Brightness Temperature is only just below 0^o C (in the -2^o to -4^oC range); Cloud-top phase shows only liquid cloud in that region. However, the Level 2 Cloud Top Temperatures shows values colder than -50^oC!! (Here’s an AWIPS-sampled example near the coldest cloud top with a large difference between the Band 13 Brightness Temperature and the Derived Cloud-Top temperature!). Consider that It’s possible that the ice features are not quite at the size range of a pixel.

Part of the zoomed-in feature of the downwind-from-Hawai’i NRCS image (available online here) has an anticyclonic swirl that is very reminiscent of a von Kármán vortex feature (such as here), but at much smaller scale!

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Shortly before 0500 UTC on 27 February, Sentinel-1A again overflew the Hawai’ian island chain, but this time farther west. The toggle at bottom shows the wind distribution downwind of Kauai along with the GOES-18 Band 2 (Visible) imagery (that has been brightened considerably). A noticeable tail of relatively calm winds extends downwind of the island. At the time, based 0000 and 1200 UTC soundings from Lihue, below, from this site, a strong inversion was present at around 700 mb.

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1-minute GOES-16 (GOES-East) Mesoscale Domain Sector “Red” Visible (0.64 µm) images (above) included time-matched (+/- 3 minutes) plots of SPC Storm Reports — which showed severe thunderstorms that moved east-northeastward across parts of Texas, Oklahoma and Kansas on 26 February 2023. The hazy signature of blowing dust was apparent along the New Mexico / Texas border region, along and ahead of an approaching cold front.

1-minute GOES-16 “Clean” Infrared Window (10.3 µm) images with plots of time-matched SPC Storm Reports (below) indicated that the coldest cloud tops associated with many of the storms exhibited infrared brightness temperatures around -60ºC range (darker red enhancement). These thunderstorms produced numerous tornadoes (the 12 in Oklahoma was a record for the month of February), hail up to 1.75 inches in diameter in Texas and Oklahoma, and damaging wind gusts as strong as 114 mph in Texas.

The GOES-16 Lifted Index (LI) derived product (below) displayed a corridor of instability with LI values as low as -5ºC (brighter red enhancement) extending northward from Texas into Oklahoma.

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1-minute Mesoscale Domain Sector GOES-18 (GOES-West) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.3 µm) images (above), with and without an overlay of GLM Flash Extent Density, showed convection moving onshore in Southern California during the late morning hours on 24 February 2023 — which prompted NWS Los Angeles to issue a Tornado Warning at 1807 UTC. There was a notable lightning jump that began at 1803 UTC, 4 minutes prior to the issuance of the Tornado Warning.

1-minute GOES-18 Visible images (above) include contours of LightningCast Probability. Cursor-sampled maximum values of LightningCast Probability within each contour (below) indicated that while the offshore probability values were fairly low (ranging from 12% to 22%), they did begin to appear as early as 1748 UTC (19 minutes prior to the Tornado Warning) — and probability values increased to a more modest 33% at 1815 UTC as the tornado-warned storm moved farther inland (and the aforementioned lightning jump was still underway).

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A second pair of SAR observations occurred on 23 February, one at 0544 UTC (above) and one at 1647 UTC (below). The character of the two scenes is quite different. The 0544 UTC wind imagery and NRCS imagery (from this site) shows a mostly quiescent scene with widespread northeasterly winds. One arc of winds just west of 12.25^oS, 168.5^oW is present with peak winds in excess of 40 knots; the NRCS does not show any of the structures that might suggest that high wind observations is influenced by ice within the cloud.

In contrast to previous SAR wind scenes over American Samoa (here, here, here, here, here), the scene above shows a long downstream ‘tail’ to the island influence on the wind from both Tutuila and the Man’ua Islands (Ofu/Olosega and Ta’u). Perhaps this is due to the lack of convection at 0544 UTC. However, the 0000 UTC and 1200 UTC soundings from Pago Pago (from this site), shown below, show a developing low-layer stable layer — between 700-800 mb — that might trap the effect of the wind at lower layers, allowing it to persist.

The 1647 UTC SAR winds, below, (both the winds and the NRCS data, both from this site), show a much more chaotic scene suggestive of widespread shower activity. In addition, the NRCS data at this time does include features that suggest ice within clouds (widespread, in fact, near 13.25^oS, 170.6^oW that will affect derived wind speeds.

Given the SAR winds above, what do you think the GOES-18 Clean Window infrared (Band 13, 10.3 µm) imagery looks like? The animation below, from 0400 to 1800 UTC on 23 February, including toggles that show the SAR winds (click here for an animation without the SAR wind overlays) spans the two times above. A couple of features stand out, especially an low-level boundary that becomes apparent near 1200 UTC and propagates southwestward to near Ta’u at 1800 UTC. Intermittent convection forms along its leading boundary. Intermittent convection is producing orphan anvils to the south of the Samoan islands during much of the animation.

Added: one might make an argument that the arced feature at 0544 UTC propagates eastward and is assoicated with the north-south band that intersects Tutuila around 1240 UTC and Upolu (the eastern of the two main islands of Western Samoa) around 1530 UTC. In both cases, the line results in convection that is a bit more vigorous.

The orphan anvil features are easily seen in the Night Microphysics RGB, below (using imagery from the CSPP Geosphere site; direct link to the animation), as bright red evanescent features that move northward (in contrast to the southwestward motion of the lower clouds).

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The imagery below compares the GOES-18 clean window infrared imagery to SAR winds. AT 0540 UTC, it is very difficult to relate surface SAR features to infrared imagery.

Careful inspection of the Band 13 (10.3 µm) animation between 0500 and 0600 UTC, below, does reveal a cooler cloud top aligned with the northern segment of the arc of stronger SAR winds. It would be difficult indeed to highlight a region of enhanced surface winds with just the GOES-18 data however!

There is a better-defined relationship between the GOES and SAR data at 1647 UTC? Wind maxima are linked to features in the GOES-18 imagery. However, not all cloud-top features in the ABI data are linked to SAR wind maxima. The challenge for a forecaster is to learn when the linkage exists.

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