[M T]

Regarding the nexstrain link you posted, dig deeper at this link.
Note the earlier California yellow circles at the bottom and about 1/3 the way up, that predate the NYC blue-green circles, and are split off from the orange WA branches

Since you started a thread on this topic, here's something I posted on April 4 in another thread.  Northern California SARS-CoV-2 is much wider than those 2 sources.   I dn't know of a similar workup of Southern California.

A Genomic Survey of SARS-CoV-2 Reveals Multiple Introductions into Northern California without a Predominant Lineage

Basically, it examines cases in northern California and decides that there have been a number of different introductions of the disease into the area.

Phylogenetic analyses revealed at least 8 different SARS-CoV-2 lineages, suggesting multiple independent introductions of the virus into the state. ...
In Santa Clara County, cases with onset dates within two weeks of each other were associated with a large facility with multiple employers, large areas of shared space, and heavy pedestrian traffic. ...
Consistent with this epidemiological data, the genomes (UC13-15) from the three workers in the large facility are distinct from each other.

[/quote]

and more detail...

These viruses included 2 viruses likely related to lineages from Wuhan or other regions of China, 4 viruses related to those circulating in Europe, 11 viruses positioned in small independent clusters and not part of a larger clade, and 13 viruses associated with the Washington State (WA1) clade

UC12 was a resident of San Mateo County with a travel history to Switzerland. The genome from UC12 fell within a large lineage containing many sequences from European residents or travelers from Europe  and therefore it is likely the infection was acquired there. The sequence from individual UC29 (MT027062 is from the same patient) was a returning traveler from China and has an identical COVID-19 sequence to a household contact (MT027063). Their viral genomes were more closely related to ancestral lineages from China. Interestingly, three further genomes (UC24, UC26 and UC27) also grouped within a viral lineage with widespread circulation in Europe. Individual UC27 was diagnosed after returning from a New York trip, raising the possibility of interstate transmission. Individual UC26 also reported domestic travel, while local resident UC24 had no known travel history, illustrating the existence of undetected virus transmission in the community.

Two patients from San Mateo County (UC18 and UC25) were close household contacts (a husband and wife), and their viruses comprised a separate cluster with a virus from an infected person from China.

[M T]

Regarding the nexstrain link you posted, dig deeper at this link.
Note the earlier California yellow circles at the bottom and about 1/3 the way up, that predate the NYC blue-green circles, and are split off from the orange WA branches

Since you started a thread on this topic, here's something I posted on April 4 in another thread.  Northern California SARS-CoV-2 is much wider than those 2 sources.   I dn't know of a similar workup of Southern California.

A Genomic Survey of SARS-CoV-2 Reveals Multiple Introductions into Northern California without a Predominant Lineage

Basically, it examines cases in northern California and decides that there have been a number of different introductions of the disease into the area.

Phylogenetic analyses revealed at least 8 different SARS-CoV-2 lineages, suggesting multiple independent introductions of the virus into the state. ...
In Santa Clara County, cases with onset dates within two weeks of each other were associated with a large facility with multiple employers, large areas of shared space, and heavy pedestrian traffic. ...
Consistent with this epidemiological data, the genomes (UC13-15) from the three workers in the large facility are distinct from each other.

[/quote]

and more detail...

These viruses included 2 viruses likely related to lineages from Wuhan or other regions of China, 4 viruses related to those circulating in Europe, 11 viruses positioned in small independent clusters and not part of a larger clade, and 13 viruses associated with the Washington State (WA1) clade

UC12 was a resident of San Mateo County with a travel history to Switzerland. The genome from UC12 fell within a large lineage containing many sequences from European residents or travelers from Europe  and therefore it is likely the infection was acquired there. The sequence from individual UC29 (MT027062 is from the same patient) was a returning traveler from China and has an identical COVID-19 sequence to a household contact (MT027063). Their viral genomes were more closely related to ancestral lineages from China. Interestingly, three further genomes (UC24, UC26 and UC27) also grouped within a viral lineage with widespread circulation in Europe. Individual UC27 was diagnosed after returning from a New York trip, raising the possibility of interstate transmission. Individual UC26 also reported domestic travel, while local resident UC24 had no known travel history, illustrating the existence of undetected virus transmission in the community.

Two patients from San Mateo County (UC18 and UC25) were close household contacts (a husband and wife), and their viruses comprised a separate cluster with a virus from an infected person from China.