Tuesday, March 26, 2019

Yet More Warmth

Winter is coming to an end with truly exceptional warmth across Alaska this year, and a record early breakup is looking increasingly likely as the pattern persists.  The map below shows temperatures well above freezing and far above normal today across nearly all of the state.  A long list of records could be drawn up, but standing out to me today is the high of 60°F at Yakutat; this is the second time this month that 60°F has been reached there, and it's a record for the month of March.

Fairbanks will probably set a record for highest monthly average temperature in March; Utqiaġvik (Barrow) will certainly do so.

As I noted about 10 days ago, the breakup situation at Nenana is interesting, because the accumulation of thaw heat units (thaw degree days) is so far ahead of normal that there's a chance breakup could occur before the Ice Classic closes to new entries.  The chart below (click to enlarge) shows evidence for this contention; the dashed ascending line shows the accumulation of TDDs through today, and then going forward as predicted by NOAA's probabilistic EKDMOS system (see here).  The expanding gray range shows a plausible range of possibilities based on the 10th and 90th percentiles of the daily probability distribution, and the horizontal lines show percentiles of the TDDs that have been required to produce breakup in the past.

The smallest number of TDDs that have ever been observed prior to breakup is 75, in 2002 and 2006, and the short-range forecast suggests that Fairbanks will probably reach that number by the end of the month; we're already above 50 TDDs as of today, which is easily a record for this early in the season.  Last year's breakup occurred after only 88 TDDs of thaw.  So it's theoretically possible that the tripod could fall this weekend, although I would rate this as highly unlikely because there's a tendency for earlier breakups to require more TDDs to overcome the weaker solar insolation this early in the spring.

A more reasonable estimate for breakup date is based on the median TDDs at breakup, i.e. 141 TDDs, and the latest forecast suggests this may be reached around April 9th; but of course by that lead time there is a lot of uncertainty in the forecast.  Nevertheless, with continued warmth looking likely, and with ice measurements suggesting below-normal ice thickness at Nenana, I think breakup is at least as likely to occur before April 9th as after, and April 5th or before (the contest closing date) is not at all impossible.  And if the general weather pattern holds, then a record early breakup (pre-April 20) will be almost inevitable.

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