The month of February has begun on a cold note thanks, in part, to the lingering effects of the recent major stratospheric warming event. It appears that there will be a break in the cold pattern for about 4 or 5 days during the early and middle parts of next week. What about beyond that? There are a few interesting signals that cold weather will return to much of the nation later in February, perhaps most dramatic in the central Plains, and the overall pattern may get stormier as well with more in the way of “southern” storms rather than these fast-moving “clippers”.
To begin, the sea surface temperatures in the tropical Pacific Ocean have been generally slightly below normal over the past several weeks (weak La Nino). However, there is now a growing area of warmer-than-normal sea surface temperatures just off the west coast of South America – perhaps an early indication of of a more neutral signal or even a very weak El Nino. At the same time, an index called the Southern Oscillation Index (SOI), which gives us an idea as to what state the tropical Pacific Ocean is in, has dropped sharply in recent days well into negative territory, and this too suggests some warming is occurring in that region. The impact of warming waters in the tropical Pacific is that it could very well activate the southern branch of the jet stream here in the US which, in turn, could generate a stormier pattern here later in the month. An active southern branch of the jet stream in the southern US has been noticeably absent in recent days.
Another tropical signal that we monitor here that can give us a clue about our weather in the Mid-Atlantic region is called the Madden-Julian Oscillation (MJO). The current and forecasted MJO index values are suggesting that there are some cold, and perhaps stormier, times ahead in the latter part of February following the expected upcoming temporary milder spell. Specifically, the MJO, which is tracked as it goes through different phases from 1 through 8, is forecasted to go through phases that are typically cold and stormy in the central and eastern US (i.e. through phase 8 and into phase 1).
In addition to the signal coming from the tropics, there are signs that a blocking pattern may set up later this month way up in the northern latitudes (near Greenland) and this typically forces cold air into the US that will be quite sustained if the blocking becomes strong and persistent. The latest ensemble runs of the GFS and Euro computer forecast models indeed suggest that a blocking pattern may develop in the next 10-15 days, and this could lead to a return to colder weather after next week’s temporary break. One way to track the potential developing blocking pattern in the northern latitudes is through the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) indices and both are showing signs of turning negative later this month which is indicative of a developing blocking pattern.
Bottom line, Phil has come out with a forecast for an early spring and I can’t say that I agree with him as there are some signals which suggest the month of February could feature a return to cold, and perhaps stormy weather conditions, following a temporary break in the temperature pattern next week.