Heavy rain and wind over American Samoa
A meteogram of surface observations at Pago Pago International Airport, above, shows a period of heavy rain and strong winds between 1400 and 2300 UTC on 26 May 2023. The strong winds had a big affect on waves at Aunu’u, as shown below (imagery from this website; click here to see the location of that buoy). What kind of satellite products could help a forecaster anticipate this inclement weather?
GOES-18 Night Microphysics RGB imagery from 1000-1950 UTC on 26 May is shown above. It is a challenge to intuit any kind of low- or mid-level feature that might be triggering convection in this imagery because of the preponderance of high clouds (red in this RGB) (Note also that barcode noise artifacts do appear in this RGB and show up as vertical yellow streaks; here are GOES-18 PACUS images that show the noise on 26 May). The window channel imagery (Band 13, 10.3 µm) loaded with the Total Precipitable Water fields is shown below. GOES-18 TPW is a clear-sky only product, and therefore little information is present in the active band of precipitation — but it does show abundant moisture to the north of Samoa, with dryer air to the northeast and southwest. (AWIPS Note: I’ve changed the default TPW values here from 0-2.5″ to 1-2.5″ because values between 0 and 1″ in this part of the Globe are exceptionally rare!). Strong convection develops between Upolu and Tutuila around 1200 UTC and the moves eastward to cover Tutuila. (Click here to see a METAR listing from Pago Pago; heavy rain started at 1330 UTC). Also: there is a parallax shift in the imagery below. GOES-18 sits over the Equator at 137 o W, to the east of the imagery below that has the 170oW longitude line in its center; the true location of the cloud tops will be to the east of their displayed location, so they are developing closer to Tutuila than in the display.
CIMSS creates LightningCast probabilities for the South Pacific surrounding American Samoa, available at this website (choose the ‘American Samoa’ sector). The animation below shows Probabilities of a GLM observation with the next 60 minutes (i.e., LightningCast Probability) increasing to the west of Tutuila starting around 1200 UTC and then persisting at high values around the island. One can use this product to anticipate not only lightning, but also the strong convection that produces lightning.
As noted above, GOES-18 Level 2 TPW fields provided scant information for this event because of abundant clouds. MIMIC TPW, in contrast (shown above), derived from microwave data that is then redistributed with pressure-weighted GFS winds, shows an atmosphere rich in moisture across the Samoan island chain, with largest values of TPW moving over the island chain towards the end of this animation. The upper-air Soundings at Pago Pago, below, from 0000 and 1200 UTC on 26 May (from this site) shows an increase in TPW during those 12 hours as well, from 58 to 62 mm. That is near the maximum observed value for 26 May at this site (source).
What kind of wind observations could be used to anticipate the strong winds?
Microwave estimates of wind from the AMSR-2 instrument on GCOM-W1, above, show the development of strong winds just to the south of the Samoan island chain by the time of the descending pass on 26 May. A similar increase is apparent in the ASCAT winds, below, from MetopB! The peak winds have increased, and the areal extent of the strong winds has definitely expanded!