{"id":64142,"date":"2025-04-15T06:45:30","date_gmt":"2025-04-15T06:45:30","guid":{"rendered":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/?p=64142"},"modified":"2025-04-16T00:13:49","modified_gmt":"2025-04-16T00:13:49","slug":"forecasting-for-flag-day-activities-on-american-samoa","status":"publish","type":"post","link":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/archives\/64142","title":{"rendered":"Forecasting for Flag Day Activities on American Samoa"},"content":{"rendered":"\n

Thursday 17 April is the 125th anniversary of the beginning of American Sovereignty over American Samoa. Part of the celebration includes the Fautasi<\/a>, a regatta that depends on weather. What tools are available to help forecast in the long-term and in the short-term? The animation above (from the CSPP Geosphere site<\/a>) shows the Night Microphysics RGB during the early morning of 14 April, and periodic convection is occurring. The low clouds — white and pink against the blue background — are occasionally spawning deeper convection that is a deep red in the RGB. These convective towers are short-lived. This kind of information might be useful in the short-term for diagnosing where precipitation is occurring the night before the regatta.<\/p>\n\n\n\n

GOES-18 Night Microphysics RGB, 1100-1500 UTC on 14 April 2025<\/figcaption><\/figure>\n\n\n\n
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MIMIC Total Precipitable Water fields, 1800 UTC 13 April – 1700 UTC 14 April 2025 (Click to enlarge)<\/figcaption><\/figure>\n\n\n\n

MIMIC Total Precipitable Water fields<\/a>, above, show the Samoan Islands along the southern edge of deeper moisture that is the South Pacific Convergence Zone between the islands and the Equator. Animations such as these can help a forecaster anticipate the approach (or retreat) of deep moisture that could support heavy rains.<\/p>\n\n\n\n

The GOES-18 Clear-Air estimates of Total Precipitable Water, below, similarly show a gradient just south of the Samoan Islands, with values dropping from over 2″ over American Samoa (the purple enhancement) to about 1.5″ to the south (the yellow enhancement).<\/p>\n\n\n\n

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GOES-18 Total Precipitable Water fields, 1500 UTC on 14 April 2025 (Click to enlarge)<\/figcaption><\/figure>\n\n\n\n

The green points above show where Legacy Profiles exist, and the animation below highlights the profile very close to Pago Pago in the center of Tutuila. The low-level stability in the sounding and mid-level dry air are consistent with the occasional convective development (and quick dissipation) observed in the Night Microphysics imagery above.<\/p>\n\n\n\n

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Legacy Profiles over Tutuila, 1000, 1100, 1200 and 1500 UTC on 14 April 2025 (Click to enlarge)<\/figcaption><\/figure>\n\n\n\n

LightningCast probabilities (available at this site<\/a>) can be used in the short term for Decision Support for outdoor activities. The animation below shows probability fields for the overnight hours, 1200-1500 UTC on 14 April 2025, matching the animations above. Probabilities with these showers to not become large, and GLM lightning observations are not present.<\/p>\n\n\n\n

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LightningCast probabilities over the Samoan Island chain, 1200-1500 UTC on 14 April 2025 (Click to enlarge)<\/figcaption><\/figure>\n\n\n\n

GREMLIN fields (1000-1500 UTC) that estimate radar returns are shown below (from here<\/a>). Hit-or-miss showers are diagnosed.<\/p>\n\n\n\n

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GREMLIN radar estimates, 1000-1500 UTC on 15 April 2025 (Click to enlarge)<\/figcaption><\/figure>\n\n\n\n
\n\n\n\n

What can be used for winds and waves? Two PacIOOS<\/a> buoys exist around American Samoa, and both are producing data. The plots below show that higher waves 7-7.5 feet, are at the Auunu buoy to the east. Waves are closer to 6 feet at King Poloa. At both locations, waves are approaching from the southeast, and the King Poloa site is somewhat in the lee of Tutuila. Altimetric wave heights are also available (link<\/a>) — but at present the observations at that website are not up to date.<\/p>\n\n\n\n

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Wave observations at the King Poloa site (left) and Aunuu (right) on 14 April 2025 (Click to enlarge)<\/figcaption><\/figure>\n\n\n\n

Scatterometry data gives sea-surface winds; strong winds near the Samoan Islands might build higher waves. Those data are available here<\/a>, but as with the wave heights at that site, they are not up to date today. Scatterometry data are available at this osisaf website<\/a>, however. ASCAT winds from MetopC (over the Samoan Islands) and from MetopB (to the east, i.e., upstream of the Samoan Islands) show values between 15 and 20 knots.<\/p>\n\n\n\n

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ASCAT winds from MetopC (left, 0930 UTC on 14 April 2025) and from MetopB (right, 0830 UTC on 14 April 2025) (Click to enlarge)<\/figcaption><\/figure>\n\n\n\n
\n\n\n\n

Update, 15 April 2025: Today I noticed a feature (at the CSPP Geosphere<\/a> site) in the Night Microphysics RGB that was moving over American Samoa, i.e., that hazy lighter blue, highlighted here<\/a>. What does this feature in the RGB represent?<\/p>\n\n\n\n

Night Microphysics RGB, 1230-1420 UTC on 15 April 2925<\/figcaption><\/figure>\n\n\n\n

To answer that, I first loaded up the Band 7 (shortwave IR, 3.9 \u00b5m) infrared imagery, and one noteworthy feature is the lack of cloud development in\/around that feature. This suggests the RGB image is detecting a region of dry air; it might show up in the RGB because the Split Window Difference<\/a> — that highlights moisture differences in the atmosphere — is part of the Night Microphysics RGB<\/a>. <\/p>\n\n\n\n

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GOES-18 Shortwave Infrared (Band 7, 3.9 \u00b5m) imagery, 1410 UTC on 15 April 2025 (Click to enlarge)<\/figcaption><\/figure>\n\n\n\n

The inset Split Window Difference field (source<\/a>) has a darker smudge signifying dryer air in the same region as the lighter region in the RGB.<\/p>\n\n\n\n

\"\"<\/a>
GOES-18 Night Microphysics RGB, 1410 UTC on 15 April 2025 with a Split Window Difference field inset in the upper right (Click to enlarge)<\/figcaption><\/figure>\n\n\n\n

Of course, there are GOES-18 level 2 products that can help reveal the answer. The Total Precipitable Water field overlain on top of the Band 7 imagery, below, shows a distinct dry patch (TPW of less than 1.25″!) right where the feature in the RGB appears.<\/p>\n\n\n\n

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GOES-18 Shortwave Infrared (Band 7, 3.9 \u00b5m) imagery toggled with GOES-18 Total Precipitable Water, 1410 UTC on 15 April 2025 (Click to enlarge)<\/figcaption><\/figure>\n\n\n\n

MIMIC Total Precipitable Water fields at about the same time (1400 UTC\/15 April) show the dry patch as well.<\/p>\n\n\n\n

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MIMIC Total Precipitable Water, 1400 UTC on 15 April 2025 (click to enlarge)<\/figcaption><\/figure>\n\n\n\n

For more information on the weather around Pago Pago, refer to the National Weather Service website<\/a> and\/or the Facebook page of the Pago Pago office.<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

Thursday 17 April is the 125th anniversary of the beginning of American Sovereignty over American Samoa. Part of the celebration includes the Fautasi, a regatta that depends on weather. What tools are available to help forecast in the long-term and in the short-term? The animation above (from the CSPP Geosphere site) shows the Night Microphysics […]<\/p>\n","protected":false},"author":19,"featured_media":64154,"comment_status":"closed","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[114,58,25],"tags":[],"class_list":["post-64142","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-goes-18","category-metop","category-satellite-winds"],"acf":[],"_links":{"self":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts\/64142","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/users\/19"}],"replies":[{"embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/comments?post=64142"}],"version-history":[{"count":8,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts\/64142\/revisions"}],"predecessor-version":[{"id":64190,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts\/64142\/revisions\/64190"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/media\/64154"}],"wp:attachment":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/media?parent=64142"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/categories?post=64142"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/tags?post=64142"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}