GOES-18 infrared imagery (Band 13, 10.3 µm) and derived Total Precipitable Water, above, shows tropical Invest 91P several hundred miles south of American Samoa within a corridor of moisture associated with the South Pacific Convergence Zone (SPCZ). (Note that the default color scale for Total Precipitable Water (TPW) has been rescaled so the “dryest” value is 1.0 inches). The Samoan islands are just at the northern edge of the deepest moisture — observed TPW at the Pago Pago sounding has decreased from 55 mm at 1200 UTC on 12 March to 44 mm at 1200 UTC on 14 March, the start of the animation above. MIMIC Total Precipitable water for the 24 hours ending 1300 UTC on 14 March, below, (source) shows dryer air very slowly moving over the Samoan islands from the north and northeast. (SAR and Scatterometry surface winds are all northerly around Samoa, as shown in this blog post).

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A Suomi-NPP VIIRS Day/Night Band (0.7 µm) image valid at 1252 UTC is shown below — ample illumination from the Moon (in the Waning Gibbous phase, at 56% of Full) provided a useful “visible image at night”. No bright lightning streaks were seen within the convective cluster associated with Invest 91P.

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What does the future hold for Invest 91P? The toggle below shows imagery from the SSEC/CIMSS tropical weather site (link). Shear values are low and Sea Surface Temperatures are warm. However, winds in the atmosphere (shown below, from here) are moving the invest area to the south, towards a less favorable environment.

For more information on Invest 91P (or 90P to its west!), refer to the SSEC/CIMSS Tropical Weather website (link) or the webpages of the Joint Typhoon Warning Center (link).

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The special SAR observation period over American Samoa, allowing extra observations from RADARSAT and RCM satellites (see CIMSS Blog posts here, here, here, here, here, here, here and here!) has ended, but Sentinel-1A will still routinely send back SAR observations over the Samoan Island chain. The pass above show GOES-18 Clean window imagery (10.3 µm) with SAR winds overlain.

A zoomed-in view of the SAR winds to the west of Samoa shows how parallax must be considered. GOES-18 navigation assumes clear skies, and if a tall (GOES-18 cloud-top heights — not shown — diagnose heights exceeding 44000 feet) cumulonimbus cloud is present, it will be navigated such that it is displayed farther from the sub-satellite point (0^oN, 137.2^oW) than its true position. Ice within that cumulonimbus cloud is likely altering the SAR return so that stronger winds are diagnosed. As noted in previous SAR blog posts, the presence of ice can be inferred by feathery features within the Normalized Radar Cross Section (NRCS).

The image below toggles between the NRCS and Wind Speed (from this page) and highlights the feathery structures (suggestive of thick ice within the cumulonimbus cloud) in the regions of strongest diagnosed winds. Note that the wind direction is northerly, and a wind shadow is obvious to the south of Savai’i (Salafai).

Part of the earlier of the SAR observations on this ascending Sentinel-1A pass is shown below. The coldest cloud top, in the northeastern part of the domain (orange/red enhancement) also shows a parallax shift away from the GOES-18 sub-satellite point (at about 137.2^oW) such that the cold cloud top is shifted west of the strong SAR winds (40-50 knots in the white enhancement) that are likely an artifact of the ice within that cloud. The large area of strong winds (20-25 knots, yellow/green in the color enhancement) over the center of the domain do not arise from cloud ice.

A toggle between NRCS and wind speed at 06:00:18 is shown below.

MetopC ascended over the Samoan islands after 0900 UTC on 13 March 2023, and Advanced Scatterometer (ASCAT) winds from that pass are shown below. ASCAT also diagnoses relatively weak winds just south of Western Samoa, 20-25 knot winds south of the lighter winds.

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Dora Meredith from the NWS Forecast Office in Pago Pago asked the excellent question: Would the Parallax shift be different from Himawari-9? GOES-18 is over the equator at 137^oW, about 35^o east of the Samoan Islands, and Himawari-9 is over the equator at 140.7^oE, about 45^o west of the Samoan Islands. The toggles below shows two examples of SAR winds south of Samoa in three panels; the right panel also showing GOES-18 Band 13 infrared imagery — and a parallax shift in the imagery to the west, away from GOES-18’s sub-satellite point is apparent; the left panel shows Himawari-9 Band 13 infrared imagery — and a parallax shift in the imagery to the east, away from Himawari-9’s sub-satellite point is likewise apparent.

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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) included an overlay of GLM Flash Extent Density — which showed thunderstorms that produced flash flooding, hail and an EF-1 tornado (SPC Storm Reports) in the Central Valley of California on 11 March 2023. At 2236 UTC (1 minute prior to the Tornado Warning being issued), an overshooting top along the Calaveras/Tuolumne County line exhibited an infrared brightness temperature of -53.3ºC — which roughly corresponded to the altitude of a Most Unstable Maximum Parcel Level (MU MPL) as analyzed from the 0000 UTC Oakland rawinsonde (source).  Note that for the relatively low-topped convection over California on this day, the coldest value of the default infrared enhancement was modified to -70ºC, to aid in the identification of colder overshooting tops (shades of white embedded within dark black regions).

The corresponding GOES-18 Cloud Top Height derived product at 2236 UTC — just before the issuance of the Tornado Warning — was around 32,482 feet (below). For this storm, the maximum value of the default Cloud Top Height enhancement was set to 35,000 feet.

During the period leading up to convective initiation of the hail/tornado-producing thunderstorm, 1-minute GOES-18 Visible and Infrared images (below) include contours of LightningCast Probability — a >50% LightningCast Probability (green contour) over western San Joaquin County at 2009 UTC provided a 17-minute lead time to the start of a prolonged period of GLM-indicated lightning activity for that storm, which commenced at 2026 UTC (2000 – 0100 UTC animation).

These thunderstorms developed in an environment of modest moisture and instability, as shown by 1-minute GOES-18 Visible and Infrared images combined with Total Preciptitable Water (TPW) and Convective Available Potential Energy (CAPE) derived products in cloud-free areas (below). Satellite-derived TPW values up to 0.90 inch and CAPE values as high as 400 J/kg were observed in the general vicinity of the strongest convection. Note that the default enhancements for TPW and CAPE were also modified for use with these particular storms — the maximum TPW value was set at 1.1 inches, while the maximum CAPE value was set at 500 J/kg.

We have determined a brief EF-1 tornado touched down 6SW of Tuttletown in Tuolumne County during yesterday's severe thunderstorms that also produced 1" hail and significant flash flooding. See map and press release for more info. #CAwx pic.twitter.com/bEHYHzolI4

— NWS Sacramento (@NWSSacramento) March 12, 2023

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The MIMIC Total Precipitable Water product during the period 0400 UTC on 06 March to 1200 UTC on 08 March 2023 (above) displayed the poleward transport of an atmospheric river (AR) that moved across Alaska and then the North Pole — eventually passing over Svalbard (located between Norway and Greenland). As this AR was emerging from the north coast of Alaska and approaching the North Pole, Total Precipitable Water (TPW) value anomalies were as high as 5-6 standard deviations above normal (source).

As the AR moisture plume was moving over Utqiagvik (formerly “Barrow”; station identifier PABR) Alaska at 0000 UTC on 07 March, the rawinsonde-derived TPW value was 0.62 inch (which might have been a monthly record high TPW for PABR, according to this tweet). At that particular time there was good agreement with the MIMIC TPW product, whose value in the vicinity of PABR was 0.65 inch (below).

The NESDIS Snowfall Rate product from the CICS site (below) showed the AR plume of clouds that was likely producing snowfall at the surface as it moved northward from the coast of Alaska to the Arctic Ocean.

Here’s a special image! This shows a snowfall rate product, giving liquid equivalent of snow in inches/hr. What’s special is that the remote north pole typically doesn't have observations of snowfall, but now we have a satellite product that can provide this information! #akwx pic.twitter.com/wPn4wVLpvX

— GINA (@uafgina) March 7, 2023

As the AR was passing across the North Pole, the maximum MIMIC TPW value at that location was 0.25 inch at 0900 UTC on 07 March (below).

As the leading edge of the AR eventually began moving over Svalbard (station identifier ENSB) — a distance of approximately 2000 miles from the northern coast of Alaska — light snow was observed at 2000 UTC (below) and 2100 UTC on 07 March, and then again at 0900-1000 UTC on 08 March.

Note the gradual increase in temperature and dew point as the AR continued to traverse Svalbard (below).

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