The Met Office has warned that heavy, thundery downpours are expected to develop during Monday afternoon.
They are warning the public that as over an inch of rain could fall in as little as three hours,with the potential for minor disruption due to surface water flooding.
A spokesman said: "Conditions are unstable at present, and this will allow some heavy showers to develop across parts of Devon and Cornwall on Monday afternoon.
"Southern parts of the peninsula will be favoured as sea breezes meet the prevailing northerly wind, producing some slow-moving, thundery downpours here.
"These may produce 10 to 15 mm in an hour, and, perhaps very locally, 30 mm in three hours. This may be enough for some surface water flooding. However, many places will miss the worst of the showers."
The showers will slowly die out through Monday evening.