diff --git a/backend/src/Command/ComputeForecastsCommand.php b/backend/src/Command/ComputeForecastsCommand.php
index 17d53ee..7c37f6a 100644
--- a/backend/src/Command/ComputeForecastsCommand.php
+++ b/backend/src/Command/ComputeForecastsCommand.php
@@ -29,32 +29,60 @@ class ComputeForecastsCommand extends Command
 
     protected function configure(): void
     {
-        $this->addOption('limit', 'l', InputOption::VALUE_OPTIONAL,
-            'Nombre max d\'observations à traiter', 5);
+        $this
+            ->addOption('limit', 'l', InputOption::VALUE_OPTIONAL,
+                'Nombre max d\'observations à traiter', 5)
+            ->addOption('force', null, InputOption::VALUE_NONE,
+                'Supprime les forecasts existants et les recalcule (utile après changement de modèle)');
     }
 
     protected function execute(InputInterface $input, OutputInterface $output): int
     {
         $io    = new SymfonyStyle($input, $output);
         $limit = (int) $input->getOption('limit');
+        $force = (bool) $input->getOption('force');
 
         $io->title('Calcul forecasts + ImpactScores');
 
-        // Observations récentes sans forecast
-        $observations = $this->em->getRepository(SargassumObservation::class)
+        // Charge les N observations cibles (avec ou sans forecasts existants)
+        $qb = $this->em->getRepository(SargassumObservation::class)
             ->createQueryBuilder('o')
-            ->leftJoin('o.forecasts', 'f')
-            ->where('f.id IS NULL')
             ->orderBy('o.detectedAt', 'DESC')
-            ->setMaxResults($limit)
-            ->getQuery()
-            ->getResult();
+            ->setMaxResults($limit);
+
+        if (!$force) {
+            $qb->leftJoin('o.forecasts', 'f')->where('f.id IS NULL');
+        }
+
+        $observations = $qb->getQuery()->getResult();
 
         if (empty($observations)) {
             $io->info('Aucune observation sans forecast trouvée.');
             return Command::SUCCESS;
         }
 
+        if ($force) {
+            $ids = array_map(fn($o) => (string) $o->getId(), $observations);
+            $io->note(sprintf('--force : recalcul pour %d observation(s)...', count($ids)));
+
+            $this->em->getConnection()->executeStatement(
+                'DELETE FROM sargassum_forecast WHERE source_observation_id IN (:ids)',
+                ['ids' => $ids],
+                ['ids' => \Doctrine\DBAL\ArrayParameterType::STRING]
+            );
+            // ImpactScore n'a pas de FK vers l'observation — on purge tout et on recalcule
+            $this->em->getConnection()->executeStatement('DELETE FROM impact_score');
+            $this->em->clear();
+
+            // Recharge les observations détachées
+            $observations = $this->em->getRepository(SargassumObservation::class)
+                ->createQueryBuilder('o')
+                ->where('o.id IN (:ids)')
+                ->setParameter('ids', $ids)
+                ->getQuery()
+                ->getResult();
+        }
+
         $io->progressStart(count($observations));
         $errors = 0;
 
diff --git a/backend/src/Service/Forecast/DriftSimulationService.php b/backend/src/Service/Forecast/DriftSimulationService.php
index e23ffd3..aca8883 100644
--- a/backend/src/Service/Forecast/DriftSimulationService.php
+++ b/backend/src/Service/Forecast/DriftSimulationService.php
@@ -22,10 +22,11 @@ use Psr\Log\LoggerInterface;
 class DriftSimulationService
 {
     private const HORIZONS          = [6, 12, 24, 48];
-    private const SAMPLE_POINTS     = 30;
+    private const SAMPLE_POINTS     = 40;
     private const WIND_FACTOR       = 0.03;   // 3% du vent = dérive Stokes
-    private const MODEL_VERSION     = '1.0.0';
-    private const CONCAVITY         = 0.8;    // paramètre ST_ConcaveHull (0=convex, 1=très concave)
+    private const MODEL_VERSION     = '1.1.0';
+    private const CONCAVITY         = 0.7;    // paramètre ST_ConcaveHull (0=convex, 1=très concave)
+    private const DIFFUSION_M       = 600;    // m/heure — diffusion turbulente horizontale (marche aléatoire)
 
     public function __construct(
         private OpenMeteoClient      $weather,
@@ -155,9 +156,16 @@ class DriftSimulationService
             $dy = $speed * cos($dirRad) * 3600; // Nord-Sud  (m)
 
             foreach ($points as &$pt) {
-                // Conversion m → degrés
+                // Advection : conversion m → degrés
                 $pt['lat'] += $dy / 111320;
                 $pt['lng'] += $dx / (111320 * cos(deg2rad($pt['lat'])));
+
+                // Diffusion turbulente : marche aléatoire indépendante par point
+                // Simule la dispersion naturelle du patch sous l'effet des courants de méso-échelle
+                $perturbLat = (mt_rand(-1000, 1000) / 1000.0) * self::DIFFUSION_M / 111320;
+                $perturbLng = (mt_rand(-1000, 1000) / 1000.0) * self::DIFFUSION_M / (111320 * cos(deg2rad($pt['lat'])));
+                $pt['lat'] += $perturbLat;
+                $pt['lng'] += $perturbLng;
             }
             unset($pt);
         }