November is typically characterized by increases in cold air intrusions across much of the United States. This also begins what is considered the dry season for much of the Plains and portions of the inter-mountain west. Rainfall begins to become more prevalent further south along the Gulf and where Gulf flow (southerly winds carrying moisture from the Gulf into the US) can become established before a storm system. This flow typically becomes established ahead of an advancing cold front, but is pushed further east than during the summer months. Multiple cold fronts with very little moisture/flow recovery time between can greatly influence fire activity for the Mississippi Valley and eastward. Wide sweeping cold fronts are common and typically drier as moisture becomes more limited, dominated by Canadian/Arctic air masses.
Vegetation is now dormant across most of the continental US and foliage/grasses can cure easily. The Appalachians have a vigorous deciduous tree canopy and provide new debris and additional uncompacted litter for increased fire occurrence/sustainability. Strong cold fronts tend to bring much drier air and windy conditions that only fuel fire conditions along these slopes and further to the east/southeast. The fuel load depends greatly on the water received from tropical systems during the late summer/early fall. If there is an active tropical season, fuel loads can be greatly enhanced. Therefore, the region is greatly dependent on the tropical/drought status of the area and their subsequent influences on the fire season.
Overnight relative humidity recovery is typically much higher across the US than the summer. This is due in part to the lack of solar radiation. Less sunlight reaches the earth’s surface in the Northern Hemisphere during winter. This in turn prevents the surface from warming, allowing the surface temperature to drop during the overnight, approaching saturation much easier, and providing increased humidity. Therefore, often fires in late fall/early winter may make a big run in the afternoon when the surface is warm, but then easily/quickly lay down once the sun sets. There are some exceptions including strong storm systems and terrain influences, but typically this is the case.
