diff --git a/README.md b/README.md
index ae641bc..1dea28c 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,45 @@
-# copilot-workshop
+# NWFSC Survey Grid – SST Shiny App
 
-create a shiny app
+An interactive R Shiny application that visualizes **Mean Sea Surface Temperature (SST)** on the NWFSC survey grid points along the US West Coast.
+
+## Features
+
+* Leaflet map centered on the WA/OR coast.
+* Date picker populated with the valid date range from the NOAA ERDDAP server.
+* Grid points coloured by SST using the **viridis** colour palette with a legend.
+* Click any point to see its **Grid Cell ID**, coordinates, and SST value in a popup and sidebar panel.
+* Data are fetched from ERDDAP only when the selected date changes (reactive).
+
+## Data Sources
+
+| Source | Details |
+|--------|---------|
+| **SST** | NOAA ERDDAP – dataset `ncdcOisst21Agg_LonPM180` via the `rerddap` package |
+| **Grid** | `surveyjoin::nwfsc_grid` (filtered to `survey == "NWFSC.Combo"`) |
+
+## Prerequisites
+
+* R ≥ 4.1
+* System libraries (Ubuntu/Debian): `libcurl4-openssl-dev`, `libssl-dev`, `libxml2-dev`, `libgdal-dev`, `libgeos-dev`, `libproj-dev`, `libudunits2-dev`
+
+Install R packages:
+
+```r
+install.packages("pak", repos = "https://r-lib.github.io/p/pak/dev/")
+pak::pkg_install(c("shiny", "leaflet", "rerddap", "sf", "dplyr", "viridis"))
+pak::pkg_install("DFO-NOAA-Pacific/surveyjoin")
+```
+
+## Running the App
+
+```r
+shiny::runApp("app.R")
+```
+
+Or from the terminal:
+
+```bash
+Rscript -e "shiny::runApp('app.R')"
+```
+
+The app will open in your browser. Use the **date picker** in the sidebar to choose a date; the map will update automatically with SST values for that day.
diff --git a/app.R b/app.R
new file mode 100644
index 0000000..7b84f99
--- /dev/null
+++ b/app.R
@@ -0,0 +1,213 @@
+library(shiny)
+library(leaflet)
+library(rerddap)
+library(surveyjoin)
+library(viridis)
+library(dplyr)
+
+# ── Static data loaded once at startup ──────────────────────────────────────
+
+# Load NWFSC.Combo grid points
+grid_pts <- surveyjoin::nwfsc_grid |>
+  dplyr::filter(survey == "NWFSC.Combo") |>
+  dplyr::mutate(cell_id = dplyr::row_number()) |>
+  dplyr::select(cell_id, lon, lat)
+
+# Fetch available date range from ERDDAP metadata
+erddap_url  <- "https://coastwatch.pfeg.noaa.gov/erddap/"
+dataset_id  <- "ncdcOisst21Agg_LonPM180"
+
+get_date_range <- function() {
+  tryCatch({
+    meta <- rerddap::info(dataset_id, url = erddap_url)
+    time_row <- meta$alldata$time[
+      meta$alldata$time$attribute_name == "actual_range", ]
+    if (nrow(time_row) == 0) stop("No time range found")
+    # actual_range is two doubles (seconds since 1970-01-01)
+    vals <- as.numeric(strsplit(time_row$value, ",\\s*")[[1]])
+    list(
+      min = as.Date(as.POSIXct(vals[1], origin = "1970-01-01", tz = "UTC")),
+      max = as.Date(as.POSIXct(vals[2], origin = "1970-01-01", tz = "UTC"))
+    )
+  }, error = function(e) {
+    list(min = as.Date("1981-09-01"), max = Sys.Date() - 2)
+  })
+}
+
+date_range <- get_date_range()
+
+# ── UI ───────────────────────────────────────────────────────────────────────
+
+ui <- fluidPage(
+  titlePanel("NWFSC Survey Grid - Sea Surface Temperature"),
+
+  sidebarLayout(
+    sidebarPanel(
+      width = 3,
+      dateInput(
+        inputId = "selected_date",
+        label   = "Select Date",
+        value   = date_range$max - 1,
+        min     = date_range$min,
+        max     = date_range$max,
+        format  = "yyyy-mm-dd"
+      ),
+      hr(),
+      helpText("Click a grid point on the map to see its SST details."),
+      hr(),
+      verbatimTextOutput("click_info")
+    ),
+
+    mainPanel(
+      width = 9,
+      leafletOutput("map", height = "600px")
+    )
+  )
+)
+
+# ── Server ───────────────────────────────────────────────────────────────────
+
+server <- function(input, output, session) {
+
+  # Fetch SST from ERDDAP – only re-runs when the date changes
+  sst_data <- eventReactive(input$selected_date, {
+    date_str <- format(input$selected_date, "%Y-%m-%d")
+
+    # Bounding box around the nwfsc_grid extent
+    lon_min <- floor(min(grid_pts$lon))   - 0.5
+    lon_max <- ceiling(max(grid_pts$lon)) + 0.5
+    lat_min <- floor(min(grid_pts$lat))   - 0.5
+    lat_max <- ceiling(max(grid_pts$lat)) + 0.5
+
+    withProgress(message = "Fetching SST data…", value = 0.5, {
+      raw <- tryCatch(
+        rerddap::griddap(
+          datasetx  = dataset_id,
+          url       = erddap_url,
+          time      = c(date_str, date_str),
+          longitude = c(lon_min, lon_max),
+          latitude  = c(lat_min, lat_max),
+          fields    = "sst",
+          fmt       = "csv"
+        ),
+        error = function(e) NULL
+      )
+    })
+
+    if (is.null(raw)) return(NULL)
+
+    sst_df <- raw$data |>
+      dplyr::rename(lon = longitude, lat = latitude) |>
+      dplyr::filter(!is.na(sst)) |>
+      dplyr::select(lon, lat, sst)
+
+    sst_df
+  })
+
+  # Spatially match each grid point to the nearest SST observation
+  joined_data <- reactive({
+    sst_df <- sst_data()
+    if (is.null(sst_df) || nrow(sst_df) == 0) return(NULL)
+
+    # Round to OISST grid resolution (0.25°) for fast join
+    res <- 0.25
+    sst_df <- sst_df |>
+      dplyr::mutate(
+        lon_r = round(lon / res) * res,
+        lat_r = round(lat / res) * res
+      )
+
+    grid_pts |>
+      dplyr::mutate(
+        lon_r = round(lon / res) * res,
+        lat_r = round(lat / res) * res
+      ) |>
+      dplyr::left_join(
+        sst_df |> dplyr::select(lon_r, lat_r, sst),
+        by = c("lon_r", "lat_r")
+      ) |>
+      dplyr::select(cell_id, lon, lat, sst) |>
+      dplyr::filter(!is.na(sst))
+  })
+
+  # ── Base map (rendered once) ──────────────────────────────────────────────
+  output$map <- renderLeaflet({
+    leaflet() |>
+      addProviderTiles(providers$CartoDB.Positron) |>
+      setView(lng = -124.5, lat = 46.0, zoom = 5)
+  })
+
+  # ── Update coloured points whenever joined_data changes ──────────────────
+  observe({
+    df <- joined_data()
+
+    proxy <- leafletProxy("map")
+    proxy |> clearGroup("sst_points") |> removeControl("legend")
+
+    if (is.null(df) || nrow(df) == 0) return()
+
+    pal <- colorNumeric(
+      palette = viridis::viridis(256),
+      domain  = df$sst,
+      na.color = "transparent"
+    )
+
+    proxy |>
+      addCircleMarkers(
+        data        = df,
+        lng         = ~lon,
+        lat         = ~lat,
+        radius      = 4,
+        color       = ~pal(sst),
+        fillColor   = ~pal(sst),
+        fillOpacity = 0.85,
+        stroke      = FALSE,
+        group       = "sst_points",
+        layerId     = ~cell_id,
+        label       = ~paste0("Cell ", cell_id, " | SST: ", round(sst, 2), " °C")
+      ) |>
+      addLegend(
+        position = "bottomright",
+        pal      = pal,
+        values   = df$sst,
+        title    = "SST (°C)",
+        layerId  = "legend"
+      )
+  })
+
+  # ── Click popup ──────────────────────────────────────────────────────────
+  observeEvent(input$map_marker_click, {
+    click <- input$map_marker_click
+    df    <- joined_data()
+    if (is.null(df) || is.null(click$id)) return()
+
+    row <- df[df$cell_id == as.integer(click$id), ]
+    if (nrow(row) == 0) return()
+
+    mean_sst <- round(row$sst, 2)
+
+    output$click_info <- renderText({
+      paste0(
+        "Grid Cell ID : ", row$cell_id, "\n",
+        "Longitude    : ", round(row$lon, 4), "\n",
+        "Latitude     : ", round(row$lat, 4), "\n",
+        "SST          : ", mean_sst, " °C\n",
+        "Date         : ", format(input$selected_date, "%Y-%m-%d")
+      )
+    })
+
+    leafletProxy("map") |>
+      addPopups(
+        lng     = click$lng,
+        lat     = click$lat,
+        popup   = paste0(
+          "<b>Cell ID:</b> ", row$cell_id, "<br>",
+          "<b>SST:</b> ", mean_sst, " °C<br>",
+          "<b>Date:</b> ", format(input$selected_date, "%Y-%m-%d")
+        )
+      )
+  })
+}
+
+# ── Launch ───────────────────────────────────────────────────────────────────
+shinyApp(ui = ui, server = server)