Monday, August 3, 2020

Extreme Rain

Here's a look at observed 48-hour rainfall totals from the weekend's storm; click to enlarge the map below.  As usual the data is sparse, but it suggests that over 3 inches fell across a large area on the northwest side of Denali NP, as well as between Fairbanks and Livengood.

The highest totals on the map are as follows:

5.76"  McKinley River RAWS
4.17"  Wien Lake RAWS
3.88"  Wonder Lake RAWS
3.61"  Wickersham Dome HADS
3.16"  Chatanika RAWS

The high-quality Denali 27N CRN sits in close proximity to the Wonder Lake RAWS and reported 4.10", so that provides strong support for the highest amounts.

Rainfall totals in the middle Tanana Valley were somewhat lower than expected, although Nenana received a hefty 1.71" and Fairbanks airport reported a respectable 1.22".

Part of the reason for the lower totals east of Nenana is that the initial disturbance from the east arrived much later than expected on Saturday evening and did not produce the widespread strong afternoon thunderstorms that were advertised by the NWS.  It wasn't until the front moved off to the west in the early hours of Sunday that it started generating prolific rainfall, as illustrated by the rainfall accumulation chart from the McKinley River RAWS:

In this chart the green dots and the blue line show the same thing, but they pertain to the left and right vertical axes respectively.  Incredibly, the instrument recorded 2.01" of rain in 2 hours and 3.0" in 4 hours.

The NOAA precipitation atlas does not have return interval estimates for this particular site, but for Wonder Lake the 1000-year return interval for a 2-hour total happens to be 2.01".  It's almost the same at Minchumina, and the McKinley River RAWS is half-way between the two sites, so it seems this was indeed about a 1-in-1000 year event for the specific location.

And according to Rick Thoman:

All in all, I'd argue the forecasts were quite good, and the strong wording of the NWS messages was justified even though rainfall underperformed a bit in Fairbanks.  In particular, the forecasters' assessment was highly prescient when it spoke of "very heavy rain... in the Alaska Range especially near Denali National Park where 2 to 4 inches of rain is expected with local amounts pushing nearly 6 inches".  It's not easy to predict extreme and historic weather events, because history provides no guide, so this was quite a triumph of modern technology.

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