Convective suppression before and during the United States Northern Great Plains flash drought of 2017

Tobias Gerken,Skylar Williams,Benjamin L Ruddell,Gabriel T Bromley,Paul C Stoy

doi:10.5194/hess-22-4155-2018

Tobias Gerken, Skylar Williams + Show 3 more

Open Access

https://doi.org/10.5194/hess-22-4155-2018

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Abstract

Abstract. Flash droughts tend to be disproportionately destructive because they intensify rapidly and are difficult to prepare for. We demonstrate that the 2017 US Northern Great Plains (NGP) flash drought was preceded by a breakdown of land–atmosphere coupling. Severe drought conditions in the NGP were first identified by drought monitors in late May 2017 and rapidly progressed to exceptional drought in July. The likelihood of convective precipitation in May 2017 in northeastern Montana, however, resembled that of a typical August when rain is unlikely. Based on the lower tropospheric humidity index (HIlow), convective rain was suppressed by the atmosphere on nearly 50 % of days during March in NE Montana and central North Dakota, compared to 30 % during a normal year. Micrometeorological variables, including potential evapotranspiration (ETp), were neither anomalously high nor low before the onset of drought. Incorporating convective likelihood to drought forecasts would have noted that convective precipitation in the NGP was anomalously unlikely during the early growing season of 2017. It may therefore be useful to do so in regions that rely on convective precipitation.

Highlights

Rapid onset “flash” droughts (Otkin et al, 2017) due to extreme heat or precipitation deficit (Mo and Lettenmaier, 2016) are difficult to predict and prepare for and tend to be disproportionally destructive (Ford and Labosier, 2017)
Even though abnormally dry conditions were reported by agricultural producers in northeastern Montana as early as late April (Montana DNRC, 2017), the 2017 Northern Great Plains (NGP) flash drought was not foreseen in the seasonal forecast
The US National Weather Service Climate Prediction Center’s (CPC) 3-month (JJA) seasonal forecast issued on 18 May 2017 reported above-average precipitation probabilities across the NGP

Summary

Introduction

Rapid onset “flash” droughts (Otkin et al, 2017) due to extreme heat or precipitation deficit (Mo and Lettenmaier, 2016) are difficult to predict and prepare for and tend to be disproportionally destructive (Ford and Labosier, 2017). Even though abnormally dry conditions were reported by agricultural producers in northeastern Montana as early as late April (Montana DNRC, 2017), the 2017 NGP flash drought was not foreseen in the seasonal forecast. The US National Weather Service Climate Prediction Center’s (CPC) 3-month (JJA) seasonal forecast issued on 18 May 2017 reported above-average precipitation probabilities across the NGP. Severe drought conditions (D2 as classified by the US Drought Monitor, USDM, Svoboda et al, 2002) began in late May to early June 2017 in the central Dakotas and extended westward toward Montana (Fig. 1; Table 1). At the peak of the drought in early September, nearly three-fourths of Montana was under extreme (D3) or exceptional (D4) drought. Two-thirds of North Dakota was under severe to exceptional drought in mid-August, and more than half of South Dakota was under severe to exceptional drought in mid-July, causing acute and ongoing economic and ecological impacts across the region

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Discussion

Conclusion