Originally posted on DanielSchreiber.org.
As winter sets in, a weak La Niña weather pattern has started to take shape across North America. Typically, this would encourage warmer & drier winter weather through the Southern United States, and cooler and wetter winter weather through the north. On November 10th, the National Oceanic & Atmospheric Administration’s (NOAA) Climate Prediction Center (CPC) issued a La Niña Advisory, projecting weak La Niña conditions through the wintertime.
Below was their projection for December, issued on November 17th, 2016.
An unfortunate turn of events occurred later in the month of November and into early December (after the above outlook was published), which caused the CPC to change their mind a bit with the latest December Outlook below, replacing the above graphic for the same valid times.
Woops. Kind of a big change. An “upset”, some may call it. The November 17th Outlook, valid two weeks in the future called for abnormally warm and dry conditions through the west and portions of the southeast, while the November 30th Outlook, valid beginning the next day, nearly flipped the forecast around. This new forecast was issued as much of the Southeast US was inundated by severe weather.
I can’t speak for what happened at the CPC in this forecast, but I can vouch for one thing- the current weather that we’re experiencing in late November and early December through parts of the Southern United States is not very typical.
Let’s look at the satellite from today (December 3rd).
Below is a weather graphic of 500-mb Geopotential Height Anomaly for the same day as the above satellite loop. In English, this graphic shows how “normal” the weather patterns are through North America compared to long-term average, with a value of “0” being “normal”.
In this case, we note an anomalous High Pressure offshore of California pushing 3 standard-deviations higher than “normal”, and an anomalous Low Pressure over Northern Mexico, roughly 5 standard deviations lower than “normal”. And, the fact that it’s sitting this far south, while the typical storm track during La Niña years tends to stick north, is certainly not normal.
So, what are the effects?
Not surprisingly, we see a strong correlation between the 500-millibar (roughly 18,000 feet) geopotential height anomaly and the 850-millibar (roughly 5,000 feet) temperature anomaly – abnormally cool through the southwest and abnormally warm off of California. We also note some abnormal warm air over South Texas- this is due to significant cloud cover and rainfall (courtesy of the strong Low Pressure over Northern Mexico) keeping warm air trapped in the lower troposphere.
The above radar image is more-or-less in-line with the CPC’s latter December Outlook (which encompasses all of the month of December, not just one storm on one day).
December 3rd’s early-morning temperatures…with subfreezing temperatures as far south as Arizona’s Mogollon Rim and much of the State of New Mexico, also match the latest CPC Outlook, but put to shame the miserable November 17th Outlook.
Latest Climate Forecast System model output would indicate continued cooler and wetter conditions more in-line with the most recent CPC outlook. for the remainder of 2016. However, the CPC continues to pull for overall warmer and drier Southern United States conditions through the winter. While the El Niño/La Niña pattern would dictate that to be a reasonable assumption…latest Sea-Surface Temperature observations – a large contributor to global weather patterns – may indicate other plans.
We can observe a negative anomaly (blue) over the Northeast Pacific, exactly where the strong High Pressure in the satellite loop above exists. Since High Pressure and cooler surface temperatures are often related (cool air is more dense that warm air), and ocean water heats and cools slower than dry ground, it’s safe to say that a decently strong High Pressure may hang out for a bit offshore of the West Coast unless the Jet Stream strengthened enough to uproot it. Barring that scenario, the East-Pacific High Pressure becoming semi-permanent over it’s current location would continue to force Low Pressure around it’s periphery…and even into the Southwest United States and Northern Mexico, like we see today, causing cooler and more moist conditions in not-so-typical locations under the given global weather pattern.
Dan Schreiber is an operational meteorologist, with experience