Large-Scale Influences on Summertime Extreme Precipitation in the Northeastern United States

Abstract

Observations indicate that over the last few decades there has been a statistically significant increase in precipitation in the Northeastern United States and that this can be attributed to an increase in precipitation associated with extreme precipitation events. Here we use a state-of-the-art atmospheric reanalysis to examine such events in detail. Daily extreme precipitation events defined at the 75 and 95 percentile from gridded gauge observations are identified for a selected region within the Northeast. Atmospheric variables from the Modern Era Retrospective Analysis for Research and Applications - Version 2 (MERRA-2) are then composited during these events to illustrate the time evolution of associated synoptic structures, with a focus on vertically integrated water vapor fluxes, sea level pressure, and 500 hPa heights. Anomalies of these fields move into the region from the northwest, with stronger anomalies present in the 95 percentile case. Although previous studies show tropical cyclones are responsible for the most intense extreme precipitation events, only 10% of the events in this study are caused by tropical cyclones. On the other hand, extreme events resulting from cut off low pressure systems have increased. The time period of the study was divided in half to determine how the mean composite has changed over time. An arc of lower sea level pressure along the east coast and a change in the vertical profile of equivalent potential temperature suggest a possible increase in the frequency or intensity of synoptic scale baroclinic disturbances.

Mechanisms of Abrupt Extreme Precipitation Change Over the Northeastern United States

“…In contrast with early fall, we find a much smaller contribution (8%) to the annual 1996 EP shift from early summer (June-July) TCs. Collow et al (2016) detected no significant trend in summer EP events associated with TCs from 1985 to 2014. However, we emphasize that fall TCs are essential to explaining the total EP increase over the Northeast in our analysis.…”

Sensitivity of Extreme Rainfall to Atmospheric Moisture Content in the Arid/Semiarid Southwestern United States: Implications for Probable Maximum Precipitation Estimates

“…The synoptic pattern for the 19 August 2014 storm is similar to the 24 July 1990 storm documented by McCollum et al (). Previous studies highlight the role of cut‐off lows in producing extreme rainfall and flooding (e.g., Awan & Formayer, ; Knippertz & Martin, ; Marquardt Collow et al, ; Pinheiro et al, ). At 0600 UTC on 19 August, a strong upper‐level trough extended from −130°E to −110°E longitude at 500 hPa and extended toward 30°N latitude, with the 5,820 m contour cut‐off from the main circulation (Figure a).…”

Contribution of large-scale midlatitude disturbances to hourly precipitation extremes in the United States

“…Note that color bars cover different ranges contribution to 1-h AMP is weaker and falls within the confidence range and is thus proportionate to the jet stream frequency. In the rest of the US, the jet stream contribution to 1-h AMP is nominal as precipitation extremes in these areas are generally set when other extra-tropical frontal systems (Marquardt Collow et al 2016), tropical cyclones (Knight and Davis 2009;Kunkel et al 2010), meso-scale convective systems (Kunkel et al 2012) or purely localized convective processes transit over the region.…”

“…Indeed, meso-scale convective processes responsible for heavy precipitation rates are often connected with large-scale vertical velocity and moisture convergence ) associated with synoptic-scale patterns . Previous studies have shown that daily or multi-day precipitation extremes in the US are often coincident with atmospheric rivers [e.g., (Dettinger et al 2011;Rutz et al 2014)] or extra-tropical cyclones [e.g., Agel et al (2015)] embedded within upperlevel dynamics such as the jet stream (Pfahl and Wernli 2012) and cutoff upper-level low pressure systems (Abatzoglou 2016;Marquardt Collow et al 2016). It is thus of interest to better understand the contribution of midlatitude circulation patterns to precipitation extremes on sub-daily timescales, and how this contribution varies geographically.…”