When a warm front approaches a mountain range, the upper section of the frontal surface is above the effects of the mountain range and does not come under its influence (view A of fig. 4-7-2). As the lower portion of the frontal surface ap-proaches the range, the underlying cold wedge is cut off, forming a more or less stationary front on the windward side of the range. The inclina-tion of the frontal surface above the range decreases and becomes more horizontal near the mountain surfaces, but the frontal surface main-tains its original slope at higher altitudes (view B of fig. 4-7-2). While the stationary front on the windward side of the range maybe accompanied by prolonged precipitation, the absence of ascend-ing air on the leeward side of the range causes little or no precipitation. The warm air descending the leeward side of the range causes the cloud system to dissipate and the warm front to travel as an upper front.

Figure 4-7-2.Orographic effects on a warm front.

Frontogenesis (the formation of a new front or the regeneration of an old front) may occur in the pressure-trough area that accompanies the front. The frontal surface then gradually forms downward as the frontal system moves away from the mountain and extends to the earths surface again (views C and D of fig. 4-7-2). 

The effect of the mountain range on a warm front is to widen and prolong the precipitation on the windward side of the range, while on the leeward side the precipitation band is narrowed and weakened, or is nonexistent.

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