Wednesday, April 20, 2016

Seasonal Forecast

Here's a quick update to show the latest CFSv2 model forecast for next month.  The model ensemble is calibrated with the historical re-forecasts and converted into the probabilities that the climate anomalies will be significantly below-normal, near-normal, or significantly above-normal.  The temperature forecast for May shows continued high probabilities of unusual warmth for most of Alaska, exceeding 90% over the warm water in the Gulf of Alaska and Bristol Bay, and 60-70% in most of the interior except the east.

The forecast for the May through July average is similar, although the probability of warmth is not quite as high in the interior.  There is a surprising area of expected cooler than normal conditions in the Chukchi Sea; this is probably related to the CFSv2 sea ice forecast, which is not known to be particularly reliable.

Interestingly the 3-month forecast for sea-level pressure shows a low pressure anomaly across interior Alaska, and the precipitation forecast hints at above-normal rainfall in the southeastern interior.

The probability forecast for Fairbanks shows a modestly enhanced probability of a wet May-July period, and it seems the model expects this anomaly to show up in July; the probability of significantly above-normal rainfall in July is 48%, compared to 36% for the near-normal category and only 16% for below-normal.

The official CPC forecast will come out tomorrow:

Also, I added the March verification at the end of my post from 2 months ago:

Update: here's the CPC forecast; the most interesting aspect may be the expectation for above-normal precipitation across northern Alaska in May-July.  It's also interesting that the CPC doesn't buy the CFSv2 forecast for an increased chance of wet conditions in southeast Alaska.

Wednesday, April 13, 2016

Breakup Update

This is just a quick update to note that Fairbanks thawing degree days exceeded 100 yesterday, which is not far off the record pace of 1940; but a cooler air mass has moved in and the next week appears likely to be cooler; so a record early breakup at Nenana (before April 20) no longer appears quite as likely.  The chart below shows the updated 2016 TDD trajectory (solid line) along with the latest GFS MOS forecast through the 20th (dotted line).

In Fairbanks it appears that the Chena River has gone out or is about to:

Regular readers should note that I'll be traveling in Europe for the rest of this month, so posting will probably be sporadic and brief.

Saturday, April 9, 2016

Snowpack Meltout

Despite a few hours of accumulating snow yesterday morning in Fairbanks, today's climate report shows only a "trace" of snow on the ground, which means the winter snowpack has officially melted out.  In practice it means that less than half of the ground area was snow-covered at midnight last night in the vicinity of the upper-air observation site at the airport.

The chart below shows the date of meltout for Fairbanks since 1930.  As Rick Thoman noted on this blog at meltout 5 years ago, there is no long-term trend.  There have been decadal-scale variations, such as periods of early meltout in the early decades and late meltout around the 1960s, but recent decades have not been unusually early on the whole.  The only possibly noteworthy aspect of the past 20 years is how few years have been very anomalous: only 2 years have seen meltout before April 16, and only 2013 was later than May 2.

The absence of a long-term trend is surprising in view of satellite indications that Northern Hemisphere spring snow extent has decreased in recent decades.  See, for example, the following articles:

It's also surprising that Fairbanks meltout dates have remained steady when we consider that spring temperatures increased substantially in the 20th century (with the main increase occurring at the 1976 PDO phase change).  The chart below illustrates this and also provides at least part of the answer for the meltout mystery: late winter snow depths have also increased over time.  It seems that increasing temperatures have been offset by greater snowpack depth, and so there's been no significant change in the timing of spring snowpack elimination.

Tuesday, April 5, 2016

Nenana Ice Classic

The 100th Nenana Ice Classic closes to entries today, and I'm guessing those entries are heavily front-loaded towards an early breakup, given the warm weather of recent weeks and months in interior Alaska.  The ice is slightly thicker than it was last year at this time, but the visual appearance has been quite poor, judging from the webcam view in recent weeks.  It's actually looking a bit better today thanks to a little fresh snow on Saturday night.

I've mentioned before that April mean temperatures in Fairbanks are highly correlated with the breakup date at Nenana - see the chart below.  The warmest April in Fairbanks history was in 1940, and that year also produced the earliest breakup on record - narrowly ahead of 1998.  However, the early breakup of 1940 was quite an outlier for those early decades; 6 of the earliest 7 breakups have occurred since 1990.  In the past few decades, it's been more common to see breakup in April than in May, in contrast to earlier years.

Another way of looking at the relationship between breakup and temperature is to calculate the thawing degree days up until the date of breakup.  Thawing degree days (TDDs) are the accumulation of temperature excess above freezing; for example, a daily mean temperature of 42°F equates to 10 TDDs.  If we add up the daily TDD values for each year between March 1 and the date of breakup, we get the following chart:

The first thing to note is that there is quite wide variation in the amount of thermal energy needed to produce breakup, as measured by TDDs; since 1930, the Fairbanks TDDs have varied from 75 (in 2002) to 265 (in 1975).  It's slightly counter-intuitive to see that the latest breakups occur after a relatively small accumulation of TDDs; the reason for this (I believe) is that late breakups allow more time for the increasingly intense solar radiation of May to work on the ice, even though air temperatures are low.

In contrast, if breakup is to occur early, then melting has to be accomplished more by the warmth of the air than by direct solar heating, and so warmer temperatures are required.

We can see that recent decades have been characterized more by early breakups in association with large TDDs, in contrast to earlier years that tended to have smaller TDDs and later breakup.  The last two years, 2014 and 2015, were actually a little unusual in having early breakup AND relatively low TDDs - perhaps a consequence of the warm winters and poor ice conditions.

So far this spring Fairbanks is accumulating TDDs more rapidly than in the past 3 years, although it's not a record; in 1965, 45 TDDs were observed already by April 4 (but breakup didn't occur until May 7).  According to today's 7-day NWS forecast, Fairbanks will accumulate another 29 TDDs through April 11, putting the total at 59.  There doesn't seem to be any sign that the warm weather will abate in subsequent weeks, so the melting rate will probably accelerate in the middle of April, especially as snow cover will be gone by then.

Subjectively, then, it seems we are on pace for something pretty close to a record early breakup, but much will depend on just how warm it is after April 15.  I'd say the most likely window for breakup at Nenana is April 21-24; and that's probably close to what everyone else is thinking without any of the temperature analysis.  In other words: your guess is as good as mine.