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...