April Climate Data

Looking back at April's climate anomalies, the mid-atmosphere circulation around Alaska was dominated by the contrast between a big east-west ridge stretching across the mid-latitude North Pacific and a strong trough to the north, over the eastern Arctic Ocean.

The ridge was closely linked to ongoing very warm SSTs from the Sea of Japan to the waters south of the Aleutians.  That oceanic warm anomaly has been entrenched for well over 3 years now, reflecting a negative PDO phase (even though the waters close to the North American coastline aren't particularly cool).

The strong north-south pressure gradient across Alaska in April produced unusually strong westerly flow, and that manifested itself in - yet again - very wet conditions in western Alaska.  It was the 4th wet month in a row from the southern Seward Peninsula southward across the Y-K Delta and down to around Cold Bay, according to ERA5 data.  In contrast, southern parts of Southeast Alaska have been unusually dry every month this year so far.

The statewide April temperature wasn't too unusual in the end, although there was a lot of up and down during the month.  Eastern Bristol Bay, the upper Alaska Peninsula, and Kodiak Island were colder than normal, but most of the east and north was warmer than normal, representing the western edge of a vast area of warmth extending across most of the continent.  The spatial pattern across both Alaska and the continent was very similar to the September-December average: compare the maps below.

April's unsettled weather in western Alaska also involved above-normal wind and cloudiness - also similar to recent months.

The persistently wet weather in western Alaska is more than just a quirk of 2024.  Rick Thoman shows in his latest blog post that most years since 2018 have seen unusually high precipitation from western Alaska to far eastern Russia in March and April, and the anomaly is becoming very statistically significant:

https://alaskaclimate.substack.com/p/is-early-spring-precipitation-higher

I'll take a stab at possible causes for the recurring wet pattern in a future post, but for now here's the average 500mb height anomaly in March-April of the last 7 years.  Looks like the oft-recurring North Pacific Ridge is a big factor...

Cold and Snow

Here's a bit of follow-up on the unusually late cold and snow.  According to Rick Thoman (and relayed via Brian Brettschneider on Twitter), Monday's half-inch of snow in Fairbanks was the first time since 1978 that measurable snow occurred after green-up.

The late snow in 1978 was much later - May 27 - but it was also more marginal, with mixed rain and snow reported, and officially-measured accumulation of only 0.1".  A more legitimate late snowfall occurred in 1966 in Fairbanks, with 1.6" measured on May 15-16; that was 8 days after breakup at Nenana, and almost certainly after green-up too (although the NWS green-up history only goes back to 1974).

Not to be left out, Anchorage also saw accumulating snow on Wednesday night.  This is a top-10 latest date for snow, and it's actually the latest on record with measurable snow on the ground at the daily observation (data back to 1953).  The previous record was May 6, 1955.  Interestingly last year there was snow on the ground on May 4.

The cold hasn't stopped breakup from proceeding, however.  The ice went out on the Yukon River at Tanana on Wednesday, and the Kuskokwim tripod at Bethel stopped the clock on Wednesday morning, despite a temperature of 25°F with snow falling.  Unfortunately significant flooding is now occurring in Bethel and elsewhere on the lower river, as the ice is jammed up downstream.

https://www.kyuk.org/public-safety/2024-05-09/as-lower-kuskokwim-river-breaks-up-bethel-sees-highest-river-gauge-level-in-almost-20-years

On the other side of the Arctic, very unusual cold is also affecting western Russia; it was snowing in Moscow yesterday for their big parade.  Severe freezes have also occurred in the Baltic states in recent days - a rare event for this late in the season.  As I commented in my last post, I think the "perturbed" flow pattern can be traced back to the lingering circumpolar disruption caused by the March stratospheric warming event.

Late Snowfall

Green-up was declared in Fairbanks on Saturday, but that distinctive first shade of green on the hillsides was obscured by white today, as light but steady snow moved into the area mid-morning.  With temperatures dropping slightly below freezing for a while, there was some minor accumulation on grassy and elevated surfaces; the NWS posted this photo:

Accumulating snow in early May in Fairbanks isn't too unusual.  There have been three May snows of more than half an inch since 2013; but of course it quickly becomes a more uncommon event as the month advances.  May 1992 was the exceptional outlier, with 9.4" on the 12th and another several inches in the subsequent days.  1992 also produced an absurdly cold and snowy September in Fairbanks, and it seems rather likely that this "coincidence" was somehow linked to the Pinatubo eruption (1991) and temporary global cooling.

But back to more recent events: read about the 2022 and 2013 May snows here:

https://ak-wx.blogspot.com/2022/05/spring-snow.html

https://ak-wx.blogspot.com/2013/05/big-snowfall-and-precip-differences.html

In the northern interior, Bettles saw a daily high temperature below freezing yesterday, marking the coldest day so late in the season since the (much colder) conditions of 2013.  Bettles also saw a bit of snow, but that's less uncommon: accumulating snow occurs in May in more than half of years up there.

And in Nome it's a very chilly day for the time of year: the temperature hasn't yet risen above 20°F today.  Again the last time that happened in May was in 2013.

The immediate cause of the cold weather is a trough that dropped down from the Canadian Arctic Ocean over the weekend and "joined forces" with another trough moving across the Aleutians into the Gulf of Alaska.  Here's the 500mb analysis from 4am this morning, courtesy of Environment Canada:

Arguably the chain of events was set in motion by a strong ridge stretching across the central Arctic Ocean late last week; and that in turn reflects the lingering influence of a "sudden stratospheric warming" event way back in early March.  The map below shows last Thursday's 500mb height anomaly, with generally above-normal heights from the Arctic Ocean to the northern North Atlantic, i.e. a negative Arctic Oscillation.  This is a typical outcome in the wake of a wintertime stratospheric disruption, although the timing and persistence of the anomaly varies widely from case to case.  Meteorologists watch for these events closely, because they tend to presage mid-latitude cold outbreaks and generally volatile weather owing to the Arctic "blocking" patterns.

April Snow and Temperature

This year in Fairbanks, the date of snowpack meltout - the first day with less than 50% snow cover, or less than an inch depth reported - was April 23.  This is just a day or two ahead of the long-term normal, despite April being distinctly warmer than normal: the month was nearly 3°F warmer than the 1991-2020 average, and fell just inside the warmest 33% of the historical distribution.

However, the snow depth at the beginning of April wasn't greater than normal - in fact it was slightly less (15" versus a 1991-2020 median of 20") - so it's not immediately clear why meltout didn't occur sooner.  But of course there's a certain amount of randomness in the meltout dates at a single location, and the measuring site hasn't remained the same over the years.

One other note on the meltout date: two of the earliest meltout dates in Fairbanks history were in 2016 and 1998 (April 9 and 10 respectively), and those two years were coming out of intense El Niño episodes.  Given that we also had a very strong El Niño this winter - almost on a par with those earlier events - it's again interesting that meltout wasn't sooner.

Let's consider briefly the relationship between April snow cover and April temperatures.  Of course this is a chicken-and-egg problem: temperature affects snow cover, but snow cover also influences temperature.  The basic relationship is obvious, with a tendency for less snow on the ground in warmer Aprils.

Notice that there's a better correlation for daily minimum temperatures than for daily highs.  This makes sense, as nighttime temperatures are prone to dropping more sharply over snow-covered ground than over bare ground.

The major outlier with record April snow depth was 1991 (42" remaining on the 15th!), but the monthly mean temperature was only slightly cooler than this year.  The incredible snow pack that month lingered in spite of the monthly temperature, not because of it.

If we examine the 850mb temperature in relation to April snow depth, the correlation is weaker: see below.  This chart more cleanly illustrates the influence of air mass temperature on snow depth, rather than the other way around, because we wouldn't expect the snow cover to greatly affect the air temperature several thousand feet above the ground in the "free" atmosphere.

In an attempt to extract the reverse causation - the influence of snow depth on surface temperature - the following chart looks at the temperature difference from the surface to 850mb.

It's a weak relationship overall, but better for daily low temperatures than for high temperatures.  Note that 1991 is less of an outlier here.  In April 1991, particularly at night, the snow pack seems to have held back the surface temperatures compared to what the 850mb air mass temperatures would otherwise suggest.

I also started looking at daily temperatures in an effort to see the seasonality (calendar timing) of snow's influence on temperature in Fairbanks, but I'll discuss those results in another post.