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8 assessment of climate change in the southwest united statesTable 1.1 Current and predicted climate phenomena trends discussed in this report (Continued)ProjectedChangeParameterDirectionof Change Is it Occurring? Remarks Confidence ChapterAverageannualprecipitationDecreaseNot yet detectable.During 1901–2010there was little regionalchange in annualprecipitation.For all periods and both scenarios,model simulations show bothincreases and decreases in precipitation.For the region as a whole,most of the median values arenegative, but not by much, whereasthe range of changes, amongdifferent models, is high. Annualprecipitation projections generallyshow decreases in the southernpart of the region and increases inthe northern part.Mediumlow6SpringprecipitationDecrease Not yet detectable. By mid-century, all but one modelprojects spring regional precipitationdecreases. By 2070-2099,the median projected decrease is9-29%, depending on the emissionsscenario.Mediumhigh6ExtremedailyprecipitationIncreaseMaybe. Studies indicatethe frequency of extremedaily precipitationevents over the Southwestduring 1901–2010had little regionalchange in extreme dailyprecipitation events.Models project more intenseatmospheric river precipitation;some studies project more frequentintense precipitation during thelast half of the twenty-first century,especially in the northern part ofthe region.Mediumlow5; 7MountainsnowpackDecreaseYes, in parts of theSouthwest.Model projections from this reportand other studies project a reductionof late winter-spring mountainsnowpack in the Southwest overthe twenty-first century, mostlybecause of the effects of warmertemperature.High 6