MIMIC TPW fields for the first half of 22 October (at which point the data feed from NOAA/NESDIS was interrupted) shows the Samoan Islands within an isolated region of relatively dry air with moisture moving in from the east. GFS fields showing the Galvez Davison Index (GDI) that is sometimes used to anticipate rain in the tropics, below (from this site), shows a predicted increase in index values (suggesting an increase in rain probabilities; GDI has been discussed on this Blog previously here and here).

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Given this background, what might you examine to determine if rain is imminent? Visible imagery from GOES-18 might be one example, as shown in the animation below from the CSPP-Geosphere site. Convective clouds approach the Samoan Islands from the east, coming very close to American Samoa but not overspreading the islands. Convection does overspread Upolu by the end of the animation.

GREMLIN is a product that predicts what radar would show given a distribution of GOES data (from bands 7, 9, and 13). The animation below shows predicted radar echoes getting very close to Tutuila (the island including Pago Pago), and overspreading Upolu, the island of Samoa that includes its Capital city Apia.

A meteorogram for Pago Pago, American Samoa (station NSTU) from 0300 UTC 22 October through 1500 UTC 23 October, below, shows that light rain was observed at Pago Pago airport at 22 UTC on 22 October. Heavier rain overspread the region on 23 October. A similar meteorogram for Apia, Samoa (station NSFA), follows, showing a similar tale, but does it show the rain that might have been expected (at Apia especially) given the imagery above?

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Use large-scale products such as MIMIC or model fields of GDI to get a general feel for how precipitation might be approaching and/or evolving, then examine satellite imagery, either individual bands or derived products, as guidance for your precipitation forecast in the short-term.

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(Added, 24 October)

You will note above that the meteorogram for Pago Pago shows rain starting at 1200 UTC on the 23rd. What did the GREMLIN fields look like at that time? The animation below spans 1100-1850 UTC on the 23rd. GREMLIN gradually increases its estimations of radar echoes during this time, including the times in the meteorogram when rain was observed at NSTU.

The National Weather Service in Pago Pago used GREMLIN imagery (from 1850 UTC) in the Facebook post, below, to alert their followers to the extent of the rain.

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GREMLIN fields are now available at the CIRA SLIDER.

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VIIRS Day Night Band visible imagery (at 0.7 µm) on 23 October, above, shows a dark patch over the Mid-Atlantic right to the east of New Jersey and the Delmarva peninsula. Chesapeake Bay waters are also much darker than most of the adjacent Atlantic Ocean. Dark patches such as these are associated with light (or no) winds (as noted here and here on this Blog). A lack of winds means the unperturbed sea surface will reflect less moon light back to the satellite. Winds are indeed light around Chesapeake Bay, as shown below. Sparse wind observations over the ocean also suggest light winds in/around the dark patch; this surface analysis shows that the region of the oceanic dark patch is within a ridge of High Pressure where light winds would be expected.

RAP13 model winds (at 10m Fixed Height Above Ground) were also very light in the vicinity of the dark patches of water — with speeds around 3-4 knots over the Atlantic, and 3-4 knots (or less) over the Chesapeake Bay and just east of the Delmarva Peninsula (below).

 

Metop-C ASCAT data from early on 23 October, below (from this site), also show very light winds near the dark spots.

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The VIIRS ACSPO SST analysis toggled below with the Day Night Band shows that the dark oceanic patch is in a region of relatively cooler water east of the North Wall of the Gulf Stream. That cooler surface could suppress any kind of atmospheric convection that might transport higher velocity down to the ocean surface.

Day Night Band imagery in this post is actually from NOAA-20, not Suomi-NPP. Thanks to Kathy for alerting me to this great case!

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From a Slack Channel (because email is so 2012!) comes a link to RealEarth (imagery shown below) that shows Advanced Clear-Sky Processor for Oceans (ACSPO) estimates of SSTs. The sinuous bendings of the Gulf Stream include a pronounced warm eddy with a ragged thermal edge. (The exercise in determining whether this is inertial instability in this anticyclonic gyre is one that is left to the reader). A zoomed in view of the anticyclone is below. VIIRS has excellent horizontal resolution (375 m) that allows the great imagery shown. A larger view that covers much of the western Atlantic is here.

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This derived-from-VIIRS imagery was processed by CSPP software at the CIMSS Direct Broadcast sites, and ingested into an LDM feed for AWIPS. The image below from AWIPS can be sampled to show temperature values. Gulf Stream SSTs leading into the anticyclonic gyre (red-orange in color) are around 79^oF (they are warmer than 80^oF (red/white in the color) at the western edge of the domain, south of New England); the cooler temperatures (south of the gyre; green in the enhancement) are close to 68^oF.

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Tropical Storm Nadine made landfall in Belize on 19 October (at 1600 UTC), and parts of this system then crossed Central America and entered the Pacific Ocean (Here is the NHC discussion that mentions this — from 1500 UTC on 20 October). Tropical Storm Kristy formed off the coast of Mexico later in the day on the 21st of October. The GOES-16 animation of clean window infrared imagery, below, shows the cloud features associated with both systems. Nadine maintained a detectable cyclonic rotation through about 1200 UTC on 20 October. Subsequent to that time, the clouds associated with Nadine moved into central Mexico as a large convective complex developed over the Pacific Ocean to the south of Mexico — this is the system that became Kristy.

It does happen occasionally that tropical systems in the Atlantic traverse Central America and become tropical systems in the Pacific. Although these two systems, Nadine and Kristy, shared the same airmass, they are not the same system.

Scatterometry data (from the manati site) from OSCAT (below) and from ASCAT (at bottom), show the evolution from strong northerly winds over the Gulf of Tehuantepec that quickly coalesce into a strong tropical circulation.

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