Simplest answer is a very large warm water lake in the centre (Around the size of the great lakes if you can, but even a lake half as big will work, even as little as 25km in diameter is good enough to have some effect). The lake will change the pressure in the area, creating some small amounts of rain. The water table can be close to the surface, allowing trees to grow, and there can be frequent flooding of this lake (makes sense, the water has no where to go when it rains).
And that gives you a realistic fertile land that rains sometimes.
Here is a link explaining some of the effects the great lakes have on the weather.
The Great Lakes influence local and regional weather and climate.
The Great Lakes affect weather and climate by impacting the basin’s energy and water cycles. Changes in the Great Lakes’ water
circulation, water temperatures and ice cover can produce changes in
The Great Lakes warm by absorbing solar radiation. Lake temperatures are also affected locally by the temperature of inflowing
river waters. The Great Lakes lose heat by evaporation and by warming
the overlying air when the atmosphere is cool. After water vapor is
released into the atmosphere, it condenses and forms precipitation,
some of which falls within the Great Lakes Basin.
The Great Lakes modify the local weather and climate. Because water temperatures change more slowly than land temperatures, lake
waters gain heat in summer and release heat during cooler months. This
results in cooler springs, warmer falls, delayed frosts and lake
The Great Lakes have a significant influence on regional climate by absorbing, storing and moving heat and water. Lake effect
precipitation can occur downwind when major weather systems move over
The Great Lakes are influenced by larger climate change patterns affecting North America and the world. Climate patterns in the Great
Lakes are changing, with warmer and drier conditions predicted.
Depending on the height of the mountains, you have an interesting secondary concern, shorter days. This will help keep the area cool during the day, allowing for people to work harder and allowing plants to grow without being dried out.