Table 1.

Climate variables considered for models of disease incidence by region, along with descriptions and justification of their relevance to disease transmission.

Climate Variable	Description	Biological Relevance
Lagged winter temperature	Average monthly temperatures for Dec - Feb 1.5 years prior. Identified by Subak 2003 as significantly positively correlated with Lyme disease incidence in highly endemic areas.	Colder winter temperatures are associated with reduced host-seeking abilities of the adult tick (Duffy and Campbell 1994, Clark 1995, Carroll and Kramer 2003) and reduced abundance of the white-footed mouse, a highly competent reservoir host (Wolff 1996).
Spring precipitation	Average precipitation in May and June. Identified by McCabe and Bunnell 2004 as significantly positively correlated with Lyme disease incidence in highly endemic areas.	Greater precipitation during the late spring and early summer increases the moisture of the leaf litter, providing conditions which promote the survival and questing activity of the nymphal life stage (Knülle and Rudolph 1982, Berger et al. 2014).
Hot, dry days	The number of days with temperature > 25°C and precipitation = 0 during May – July (or May – June for counties with Ixodes pacificus). Identified by Burtis et al. 2016 as significantly negatively correlated with Lyme disease incidence in highly endemic areas.	Hot, dry conditions are associated with decreased questing activity and questing height of ticks (Randolph and Storey 1999, Schulze et al. 2001), reducing the likelihood of attachment to humans (Arsnoe et al. 2015). The May - July, and May - June, time periods capture the peak nymphal questing periods for I. scapularis and I. pacificus, respectively (Eisen et al. 2016).
Cumulative average temperature	The sum of average daily temperatures (°F) over the entire year	Cumulative temperature appears to control most developmental stages of I. scapularis (Lindsay et al. 1995, Rand et al. 2004). Lower cumulative temperature is associated with longer development periods and/or higher tick mortality (McEnroe 1977, Estrada-Peña 2002, Brownstein et al. 2003, Ogden et al. 2004, Leighton et al. 2012).
Total annual precipitation	The sum of total daily precipitation (mm) over the entire year	Greater precipitation increases the moisture of the leaf litter, providing conditions which favor tick survival and questing activity (Knülle and Rudolph 1982, Jones and Kitron 2000, Berger et al. 2014a).
Daily temperature variability	The variance in average daily temperatures (°F) over the entire year	Frequent temperature variation can decrease tick survival, even beyond that of constant cold exposure, due to energetic costs associated with adapting to changing temperatures (Gigon 1985, Hermann and Gern, 2013); however, effects will vary based on the average temperature of the region.
Daily precipitation variability	The variance in total daily precipitation (mm) over the entire year	Both drought and heavy rainfall are associated with deceased tick questing activity and survival (Randolph 1997, Jones and Kitron 2000, Perret et al. 2004). Variation in precipitation, as opposed to consistent rainfall supplying favorable high relative humidity conditions, may thus be detrimental for tick survival, but will depend on the average precipitation of the region and the magnitude of variation.