Sunday, August 9, 2020

Historical Rainfall Extremes

In the wake of last weekend's epic rains in central Alaska, I've been poking around in historical rainfall data from the Alaska RAWS network (data courtesy of Rick Thoman) and a few other sources.  The question is whether any previous events can be identified in interior or northern Alaska that approach or even exceed the 5.35" peak 24-hour total that occurred at the McKinley River site last weekend.

According to Rick's earlier research, the previous record for rainfall in 24 hours is 4.58" at Clear, about 20 miles south of Nenana; this occurred in the same event as the Fairbanks flood of August 1967.  The observation was a standard once-per-day measurement, so it's almost inevitable that a higher peak 24-hour total occurred.

Does the RAWS data reveal anything more extreme than this in recent decades?  The short answer is no - if we're talking about interior Alaska, i.e. north of the Alaska Range.  Epic rains occur along Alaska's southern and southeastern coastline, of course, and I found an event with over 6" of rain in 24 hours at the Ruth Glacier RAWS on the south side of Denali NP (at 3300' elevation).  But it's obviously a very different climate on the south side of the divide.

In terms of a new finding, the highest I uncovered for the interior is 4.20" in 24 hours at the Salmon Trout RAWS, which is at 2200' elevation above the Porcupine River, not far from the Canadian border (and 66.8°N latitude).  This occurred on August 7, 2010, and the peak 12-hour rainfall was 3.05".

Another high-latitude one that surprised me - although it's not quite in the same league - is 3.01" in 24 hours at the Red Dog CRN in far northwest Alaska (68.0°N, elevation 940').  That was in mid-August 2012.

I would certainly be remiss not to mention also the remarkable 4.16" last summer (August again) at Denali NP's Eielson Visitor Center (3760' elevation on the north side of the park).

For context, the record 24-hour rainfall in Fairbanks is 3.44", which was of course in August 1967.  Nothing close to this has occurred at valley-level in Fairbanks since, but up in the hills there have been some 3" rains in 24 hours; for instance, the Fairbanks 11NE CRN reported 3.07" in 24 hours in July 2014, and interestingly the Munson Ridge SNOTEL has reported over 3" in three recent years (3.76" in August 2014, 3.23" in July 2016, 3.09" in August 2019).  Given the frequency with which Munson Ridge sees big rains, it may not be many years before they see 4-5" in a day; we can only imagine what occurred up in the hills in 1967 (e.g. the Gilmore Creek co-op north of town recorded 6.18" in two days).

On a 6-hour time scale, the 3.32" at McKinley River last weekend easily beats anything else I can find for the interior.  The closest is 2.48" in 6 hours on July 6, 2007, at the Preacher Creek RAWS to the west of Circle.  There are just a handful of other events exceeding 2" in 6 hours.

If anyone can suggest other notable historical rain events that might be contenders, I'd be glad to dig into the data a bit more; the RAWS data obviously only goes back a few decades at best.

No comments:

Post a Comment