TAMPA — Not a ton of people have heard of Ralph Vaughan Williams' A London Symphony, the title piece in this weekend's Florida Orchestra concert series. That might help explain why every seat at the David A. Straz Jr. Center for the Performing Arts on Friday was not taken. Nor is Sir Edward Elgar's In the South a well known crowd pleaser in the mold of Beethoven's Symphony No. 5. Both are considered tone poems — more mood than story, a single movement, free of traditional exposition-development-recapitulation symphonic structure.Yet both pieces helped bring England out from under the shadow of the countries that had a firmly established national brand, namely Germany, France and Russia. So music fans who skipped this concert missed out on a pair of sensual treats, each with a relatively uncommon bonus in extended viola solos.Don't get me wrong. Ferguson Hall at the Straz, smaller than the Morsani, was nearly full. But the second piece of the evening, Felix Mendelssohn's Violin Concerto, might be one audiences will look back on later with an "I-knew-her-when" nostalgia. Esther Yoo, just 24 and setting an impressive pace in performances around the world, played magnificently (and almost constantly) through the 26-minute piece famed for its warmth and celebratory spirit."She really seeks to communicate what the composer's intentions are, which seems a very obvious thing to say," music director Michael Francis said in an interview Thursday. "But actually it's very important because otherwise, you can have people come in and put their ego on it and say, 'Well, I want my version of Sibelius or my version of Tchaikovsky.' What she's trying to do is actually look at what's being written and then make musical sense of it."Elgar's inspiration came in a 1903 family vacation to a coastal Italian town (hence In the South is also commonly known as Alassio). This is a romantic explosion of short but emphatic musical sentences, a continual stream of ideas and moods. Woodwinds and strings expand and contract, one grieving, another consoling. A booming brass dominates, replaced by viola solo (by principal Derek Mosloff). The Mendelssohn concerto begins in agitation and blossoms quickly into exquisite harmonies between soloist and orchestra. Yoo attacked its furious passages and emphasized its subtler dynamics just as much, every shift in tone. An unaccompanied cadenza showed off her technical skill while lovingly dissecting the composer's themes, as if splitting atoms.After three curtain calls before intermission, she performed an encore, an arrangement of a Korean folk song.A London Symphony treats listeners to the sounds and moods of the city — awakening in the fog, bustling with intensity, the chimes of Big Ben or the occasional jingle of a horse-drawn taxicab. A sudden shift in horns and strings marks an underbelly of open secrets, the start of a complex message that ends with a wistful lamentation and hints of doom. The viola, clarinet, oboe and swelling brass and judicious percussion all contribute to a chill as a senescent city marks the end of an empire. Contact Andrew Meacham at [email protected] or (727) 892-2248. Follow @torch437 .IF YOU GOVAUGHAN WILLIAMS 'A LONDON SYMPHONY'Concerts start at 8 p.m. Saturday and 2 p.m. Sunday at the Mahaffey Theater, 400 First St. S, St. Petersburg. A preconcert talk starts one hour before performances. $18-$48. (727) 892-3337. floridaorchestra.org.