diff --git a/Dockerfile b/Dockerfile
index 742231e7..52de4e5f 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -73,20 +73,22 @@ RUN wget -q --content-disposition "https://github.com/NOAA-EMC/NCEPLIBS-g2c/arch
 
 WORKDIR /tmp/wgrib2
 
-# wgrib2 flags:
-#   * USE_PNG / USE_OPENJPEG / USE_AEC each turn on the matching
-#     encoder path inside wgrib2 and add the compression name to the
-#     -config output. With g2c v2.1.0 underneath (which exports the
-#     g2c_enc_* / g2c_dec_* new-API symbols) all three can be enabled
-#     together — the earlier build failure was not about the flag
-#     combination, it was g2c v1.9.0 shipping without BUILD_G2C.
+# wgrib2 3.8.0 cmake flags:
+#   * USE_AEC turns on AEC compression decode for HRRR files.
+#   * USE_G2CLIB_LOW links wgrib2 to the external g2c lib for png +
+#     jpeg2000 decode. The previous USE_PNG / USE_OPENJPEG flags don't
+#     exist in wgrib2 3.8.0 — they were silently ignored, leaving
+#     wgrib2 unable to read GRIB2 packing type 40 (HRDPS uses it).
+#     Verified 2026-04-30 by ldd showing only libc/libm linked.
+#   * The g2c v2.1.0 build above carries the actual OpenJPEG support;
+#     wgrib2 dispatches to g2c via USE_G2CLIB_LOW.
 RUN wget -q --content-disposition "https://github.com/NOAA-EMC/wgrib2/archive/refs/tags/v${WGRIB2_VERSION}.tar.gz" \
       -O wgrib2.tar.gz \
   && tar xzf wgrib2.tar.gz \
   && cd wgrib2-${WGRIB2_VERSION} \
   && mkdir build && cd build \
   && cmake .. -DCMAKE_INSTALL_PREFIX=/usr/local -DCMAKE_BUILD_TYPE=Release \
-       -DUSE_PNG=ON -DUSE_OPENJPEG=ON -DUSE_AEC=ON \
+       -DUSE_AEC=ON -DUSE_G2CLIB_LOW=ON \
   && make -j$(nproc) \
   && make install \
   && strip /usr/local/bin/wgrib2
diff --git a/rust/prop_grid_rs/Dockerfile b/rust/prop_grid_rs/Dockerfile
index a87ae4de..83a83014 100644
--- a/rust/prop_grid_rs/Dockerfile
+++ b/rust/prop_grid_rs/Dockerfile
@@ -51,13 +51,22 @@ RUN wget -q --content-disposition "https://github.com/NOAA-EMC/NCEPLIBS-g2c/arch
  && make install
 
 WORKDIR /tmp/wgrib2
+# JPEG2000 decode (GRIB2 packing type 40 — used by HRDPS) comes from the
+# external g2c lib via USE_G2CLIB_LOW. Without that flag wgrib2's stock
+# build links neither g2c nor any JPEG2000 backend (verified 2026-04-30
+# by `ldd /usr/local/bin/wgrib2` showing only libc/libm and a runtime
+# `*** FATAL ERROR: packing type 40 not supported ***` from HRDPS files).
+#
+# wgrib2 3.8.0 does NOT have USE_OPENJPEG or USE_JASPER cmake options —
+# the only knob is USE_G2CLIB_LOW for png/jpeg2000 decode. The g2c lib
+# we built above carries the actual OpenJPEG support.
 RUN wget -q --content-disposition "https://github.com/NOAA-EMC/wgrib2/archive/refs/tags/v${WGRIB2_VERSION}.tar.gz" \
       -O wgrib2.tar.gz \
  && tar xzf wgrib2.tar.gz \
  && cd wgrib2-${WGRIB2_VERSION} \
  && mkdir build && cd build \
  && cmake .. -DCMAKE_INSTALL_PREFIX=/usr/local -DCMAKE_BUILD_TYPE=Release \
-      -DUSE_PNG=ON -DUSE_OPENJPEG=ON -DUSE_AEC=ON \
+      -DUSE_AEC=ON -DUSE_G2CLIB_LOW=ON \
  && make -j$(nproc) \
  && make install \
  && strip /usr/local/bin/wgrib2