Sunday, October 12, 2014

Year-to-Date Precipitation Update (Oct. 11)

How has aggregate precipitation looked so far in 2014?

Daily Average:

The chart below (Figure 1) shows the long-term daily average precipitation in the Contiguous U.S. along with the 2014 values (through October 5th) in red. In addition, the long-term 10th and 90th percentile lines are shown. In theory, each month should have three days exceed the 90th percentile and three days below the 10th percentile. If you add up the 2014 precipitation, it comes out to 25.25". The long-term average should be 25.52". Therefore, 2014 is just barely below average year-to-date. Nearly two months passed (mid-January to mid-March) without a single day exceeding the 90th percentile.

Perhaps more interesting are the normal and percentile curves. June is the wettest month of the year in the Contiguous U.S. but the winter months have the greatest variability. This is likely a result of summer precipitation occurring as thunderstorms on fewer days (with less spatial coverage) and winter precipitation being more stratiform (less intensity but greater spatial coverage).

The chart in Figure 2 shows the year-to-date daily precipitation averages for Alaska. Similar to Figure 1, the 90th and 10th long-term percentile lines are shown in Figure 2. A large number of days between June 1st and August 15th had precipitation values greater than the 90th percentile. Conversely, the period from mid-March through the end of May saw many days with low precipitation values.The summed daily precipitation values for 2014 are 24% higher than the long-term average. Therefore, we can confidently state that 2014 has been significantly wetter than average in Alaska through October 5th. A note of caution about the Alaska chart (Figure 2). It is not spatially weighted. Areas with a higher density of stations (e.g., Anchorage area, Fairbanks area, and all of Southeast) are disproportionately represented. 

Figure 1. Long-term average daily precipitation for the Contiguous U.S. with 2014 data (though October 5th) shown in red. 90th and 10th long-term percentiles are also shown.

Figure 2. Long-term average daily precipitation for the Alaska with 2014 data (though October 5th) shown in red. 90th and 10th long-term percentiles are also shown.


The summary snapshot of the year-to-date precipitation is shown in Figure 3. Areas from Texas and Oklahoma westward to California and northward are running far below their annual normal precipitation through October 11th.Throughout the rest of the country, departures from normal are generally nondescript with the exception of portions of Alaska and Hawaii which are significantly wetter than normal. The nationwide leader in year-to-date precipitation departure among WBAN ("first order") stations is Molokai, Hawaii (203% of normal). In second place is Fairbanks, Alaska (179% of normal). The third place station is Twin Falls, Idaho (170% of normal). In last place is Imperial Beach, California (16% of normal). As noted in the earlier section, the nationwide average is only slightly below normal. The average grid cell value in Figure 2 is 98%.

Figure 3. Percentage or normal precipitation though October 11th.

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