Favorable conditions for light to moderate flights begin and ends the period for the West as Black-bellied Plover, Lesser Yellowlegs, Willow Flycatcher, Swainson’s Thrush, Townsend’s Warbler, Virginia Warbler, and Blue Grosbeak are on the move, while the moderate to heavy flights that will include White-rumped Sandpiper, Short-billed Dowitcher, Least Flycatcher, Philadelphia Vireo, Blackpoll Warbler, Chestnut-sided Warbler, Wilson’s Warbler, Rose-breasted Grosbeak, Savannah Sparrow and White-crowned Sparrow navigate around some serious weather in the first half of the period and quiet markedly with that weather’s departure to end the period.
Arrows show wind speed and direction (arrow points in the direction to which wind is blowing) 100 m above ground level. Areas with southerly winds are colored red; northerly winds colored blue. Accumulated precipitation (in 6 hour intervals) is green, outlined by white. Broadly speaking, areas of the map in red will experience conditions that are favorable for migration, and areas where red and green (and red and blue) intersect and overlap may experience migrant concentrations and fallouts as migrants interact with precipitation.
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A swath of favorable conditions spans the region from the Mississippi River valley east and north through the Appalachians into New England at time during the first half of the period, though peppered and doused, at times, with precipitation. Where and when rain is not falling, moderate to heavy flights will occur as migration peaks in the region, including some very heavy flights. But these movements will not be widespread given the forecast for storms. By Tuesday night and Wednesday the atmospheric disturbances generating the storms begin to depart, as does the influence of the spinning subtropical storm in the Atlantic, ushering in a cooler blast of unfavorable conditions that will put the brakes on these larger movements of birds for the latter portion of the forecast period. Note, the effects of subtropical storm Ana will most certainly shut down movements in the coastal mid Atlantic during the early to middle parts of the period, but birders should watch carefully, particularly in coastal locations for storm-driven waifs (although most of this action, if there is any, will be farther south) that include tropical pelagic species and concentrations of inbound Neotropical migrants.
Moderate to heavy flights will be widespread in the western portions of the region for the first half of the week, while the effects of subtropical storm Ana will shut down movements in more eastern areas until it departs. Birders in southeastern coastal plain should watch for potential effects of this storm in the arrivals both of pelagic species inshore and downed inbound Neotropical migrants. Additionally, birders should watch the distribution of precipitation carefully, as localized concentrations of inbound migrants may occur after their flights over the Gulf of Mexico and Caribbean Sea may encounter scattered precipitation. The latter half of the period brings a substantial change to the region, with a frontal boundary once again approaching and entering the Gulf of Mexico, creating late season fallout potential, and with the departure of low pressure and the subtropical storm ushering in much less favorable migration conditions of scattered light to moderate movements.
A slowly building and slow moving low pressure system advancing through the region brings a mix of cooler and unfavorable conditions to the northern Plains and favorable warm conditions to the southern Plains to start the weekend as the disturbance passes. The associated moderate to heavy flights in the southern Plains will gradually lose ground to the light to moderate flights of the northern Plains as the unfavorable conditions spread across the region by Sunday and Monday. The effects of this system’s passage will continue through most of the week, so birders should expect light to locally moderate flights during this period. Note that some areas where winds are calmer will experience higher intensity movements, given the time of year and the need for birds to arrive on breeding territories. Thursday night will see a return of moderate to very heavy flights that are widespread across the region.
A quiet start is on tap for the region, as scattered light movements will occur in largely marginal and unfavorable conditions for the weekend. By Monday night, moderate movements return with more favorable conditions, particularly in the Desert Southwest and intermontane regions. As these conditions become increasingly widespread for the remainder of the period, so to will moderate movements become more widespread. Some of these movements in the northern Rockies and grasslands of Montana may be heavy.