Cardiff Blues Under-18s produced a battling display in horrendous conditions before a late try from Scarlets Under-18s saw them claim a 20-16 victory in a tense Regional Age Grade Championship encounter at BT Sport Cardiff Arms Park.
The Under-18s had started the season in fine form with victories over Ospreys and the Dragons to move to the top of the table.
But on a night where the two unbeaten sides went head to head it was Scarlets who struck late on with a try from Ellis Thomas securing the win.
A tight and evenly matched opening saw both teams trade kicks for possession in the opening quarter before running rugby slowly took over.
Scarlets wing Ryan Conbeer was first to make an impression with a long range break that threatened to open the game up while all-action home skipper Callym Bradbury made an early run up field as he bounced off tacklers.
Centre Cameron Lewis unloaded a big tackle on the visiting full back to win a penalty for the hosts, for not releasing, and fly half Ben Jones slotted the first points of the night.
Slick handling, after an initial break from playmaker Ioan Hughes, saw the visitors strike back soon afterwards with centre Corey Baldwin sliding over for a well-worked converted try.
The home side, though, were not to be denied and hit back in style as, despite the conditions gradually worsening, prop Rhys Carre burst past men to break the Scarlets defensive line.
The home midfield duo then did the damage as Declan Williams weaved past several men and centre partner Cameron Lewis spotted a great line to race into space and score.
Jones added a touchline conversion for a 10-7 lead after half an hour before strong work at the breakdown saw Blues win a series of penalties to kick for territory.
And it was Ben Jones who almost created a superb second with a 40-metre break down the middle as the elusive No.10 glided past tacklers only to be hauled up short.
The start of the second half saw fly halves Jones and Hughes take it in turns to trade kicks as the teams drew level at 13-13, despite Scarlets being a man down after Lloyd Pike was yellow carded.
The rain was coming down harder than ever as the game entered the final 20 minutes on a wet knife-edge. Replacement Ben Warren led a counter from deep for the home side, but not surprisingly both teams were now looking to kick for territory as a torrential downpour made handling difficult.
Hughes dragged a drop goal wide and then hit the upright with a penalty as Scarlets pushed for a late advantage.
And nine minutes from time it was the home side who went ahead with a kick from Jones.
However, there was to be one late twist and a final flurry of attacks finally paid dividends for the visitors as Thomas just got the ball down under pressure and Hughes adding the extras to wrap up the win.
Cardiff Blues Under-18s: 15 Garyn Payne, 14 Aaron Clark, 13 Cameron Lewis, 12 Declan Williams, 11 Connor Jones, 10 Ben Jones, 9 Dane Blacker; 1 Tom Mably, 2 Corrie Tarrent, 3 Rhys Carre, 4 Caleb Docking, 5 Callum Bradbury (c), 6 Sion Jones, 7 Sion Edwards, 8 Aled Ward
Replacements: 16 Eddie Drake, 17 Iestyn Harris, 18 Ben Warren, 19 Ben Sier, 20 Jim Botham, 21 Dane Blacker, 22 Ben Thomas, 23 Dafydd Smith