10-minute JMA Himawari-9 AHI Infrared Window (10.4 µm) images (above) showed an explosive eruption of Mount Ruang in Indonesia on 17 April 2024. Note that a thunderstorm was reported at Menado (station identifier WAMM) from 1400-1430 UTC, due to abundant lightning activity within the dense volcanic umbrella cloud.

In the Himawari-9 Infrared Window (10.4 µm) image at 1240 UTC (above), the coldest pixel exhibited a brightness temperature of -88.8ºC (dark purple enhancement) — with an adjacent warm pixel of -46.4ºC (green enhancement), possibly a result of compensating subsidence immediately downwind of the robust updraft (which penetrated the local tropopause and extended into the lower stratosphere).

Maximum radiometrically retrieved Ash Height values (source) reached 16-18 km. In a plot of rawinsonde data from Menado at 1200 UTC (below), this height corresponded to the 80-110 hPa pressure level.

In Himawari-9 Ash RGB images created using Geo2Grid (below), shades of pink around the the umbrella cloud represented higher concentrations of volcanic ash, while shades of green to yellow represented increasing concentrations of volcanic SO2.

27 hour long SO2 RGB loop from Himawari-9 over the #Ruang volcano eruption. Yellow and red are indicative of upper level SO2 which has been dispersed since the large plume was emitted beginning around 11:00 UTC yesterday (04/17/2024). Made with McIDAS-V. pic.twitter.com/4bNtINlqMA

— Robert Carp (@rmcarp) April 18, 2024

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1-minute Mesoscale Domain Sector GOES-18 (GOES-West) “Clean” Infrared Window (10.3 µm) images with an overlay of the Total Precipitable Water derived product (in non-cloudy areas) (above) showed that an axis of moisture was initially in place across all the Samoan Islands — peak TPW values were in excess of 2.0 in (brighter shades of magenta). The axis of moisture began to diminish somewhat over American Samoa after 0600 UTC — but as another lobe of higher moisture approached from the east around 1300 UTC, scattered showers began developing in the vicinity of the Manu’a Islands and Tutuila.

Since there is no radar coverage across the islands, 1-minute GOES-18 imagery was requested to support the 124th annual Flag Day celebration in American Samoa (which included numerous VIP visitors from the US Department of Defense and Legislature, Hawai’i State Department and local government).

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A sequence of Suomi-NPP VIIRS Visible (0.64) images (above) displayed the cyclonically-curved cloud structure associated with an Arctic low pressure system (surface analyses) located between Alaska and the North Pole on 14 April 2024. A blend of Atmospheric Motion Vectors derived from NOAA-20 and NOAA-21 (below) showed the flow pattern at various altitudes around this Arctic low. Real-time polar winds derived from a variety of satellites are available here.

The pressure gradient along the southern edge of this low was responsible for strong west-southwest winds across parts of Alaska’s North Slope — for example, a plot of surface report data from Utqiagvik (Barrow, station identifier PABR) showed wind gusts of 20-24 knots, with intermittent reductions in visibility due to blowing snow (below).

A closer view of Suomi-NPP VIIRS Visible images centered over the Beaufort Sea (below) showed the effect of these strong westerly winds on the deformation of numerous ice leads off the north coast of Alaska. In addition, narrow cloud streets highlighted areas where blowing snow was likely occurring over the sea ice.

A toggle between Suomi-NPP VIIRS Sea Ice Thickness and Sea Ice Temperature derived products at 0000 UTC on 15 April — displayed using RealEarth — is shown above. Probing these two products indicated values of 3.3 m and 257.3 K, respectively (below).

Daily composite images of the Sea Ice Temperature, Sea Ice Thickness and Sea Ice Age derived products from NOAA-20, NOAA-21 and Suomi-NPP (source) on 14 April are shown below.

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he NWS Forecast Office in Honolulu late on Thursday (8:33 PM local time 11 April, or 0633 UTC on 12 April) issued a Severe Thunderstorm warning for the island of Niihau to the southwest of Kauai. Subsequently, flood warnings were issued for Kauai. The animation above (from the southwest corner of the GOES-18 PACUS domain) shows the development of the convection to the southwest of the Hawai’ian Islands that intensified as it approached Niihau and Kauai. Note especially the increase in FED that occurred after 0616 UTC just east of Niiahu, as shown below in an animation that steps more slowly from 0621 to 0636 UTC, overlapping the time of warning issuance. Peak FED occurs at 0636 UTC (87), having ramped up from 19 at 0606 UTC!

The thunderstorms accompanied a significant moistening of the atmosphere as depicted below by MIMIC Total Precipitable water fields. Deep moisture moved northward through the western Hawai’ian islands; the moisture was drawn northward by a strong storm northwest of the main Hawai’ian Islands readily apparent as the cyclonic swirl in the TPW fields.

The moistening is also apparent in the soundings from Lihue on the island of Kauai, shown below. Total Precipitable Water jumps from 1.3 to 1.7″ in the 12 hours shown; note also how the low-level winds become more favorable for severe weather with the increase of low-level southeasterlies.

Microwave estimates of Rain Rate computed from Direct Broadcast data in Honolulu (at this site), shows the significant increase in rain rates after 0700 UTC, with heavy rain diagnosed over Kauai!

MetopC overflew the Hawai’ian islands at around 0750 UTC (Rain Rate from that overpass is in the animation above); NUCAPS profiles from MetopC are now in AWIPS, complementing the fields from NOAA-20 (and soon (sometime after 1 May), NOAA-21) that occur closer to 00/12 UTC. The Total Totals index from those profiles, below, shows considerably instability to the southwest of the Hawai’ian Islands. Although this pass was too late for the Severe Weather warning issuance, information in the profiles can be used to ascertain how the convection might evolve in the near future.

The rain over the island of Kauai was very heavy, as might be expected given the MIMIC TPW fields and the snapshot views of Rain Rate shown above. Rainfall totals are discussed here.

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An interesting question was posed at the online Honolulu forecast office weather briefing on Friday: Does the 0000 UTC Sounding at Lihue (shown above in a toggle, and also here) have an Elevated Mixed Layer (EML)? The very steep lapse rate between about 720 and 600 mb and the nearly constant moisture content are both consistent with the presence of an EML. The GOES-18 Band 10 infrared imagery (7.34 µm, the ‘low-level’ water vapor band), below, is also suggestive of an EML; in the color enhancement used, deep reds are often associated with EMLs. Convection over Niihau and Kauai develops along the leading edge of the warm region in the water vapor imagery. The Band 10 weighting function, at bottom (from this website), shows a peak contribution in Band 10 (drawn in dark magenta) from about 620 mb, in the middle of the steep lapse rate shown at Lihue at the same time.

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