In past La Nina winters, the mid-level flow has shaped up with a strong negative anomaly in western Canada, in what appears to be a core of cold and snowy weather. Consequentially, I’m seeing the possibility for ridging becoming a rather common feature along the East Coast into eastern Canada, due to that swath of warmer than normal waters near Nova Scotia. For instance, warmer than normal waters will tend to promote ridging in the longer-term, while cooler than normal waters will encourage troughing in the longer-term. As we discussed previously, SST anomalies can promote longer-term ridging or troughing patterns. In general, SST anomalies can be strong predictors of the presence of ridges or troughs over the longer-term. There’s a swath of much warmer than normal SST anomalies centered just south of Nova Scotia, and this has the potential to become an issue for those in the East hoping for a cooler than normal winter.

It is this ridge that shapes the temperature and precipitation anomalies. This is a signal (albeit a very weak one) of the infamous Southeast Ridge that can emerge in La Nina years. The La Nina & negative PDO combination should dictate the broader weather pattern through the winter, with warmer waters in the Bering Sea encouraging meridional flow in the Pacific, and likely an active Pacific jet. As the above image shows, the negative phase of the PDO is identified primarily by a body of warmer than normal waters south of the Gulf of Alaska, surrounded by colder than normal waters along the coast of North America. In a La Nina regime, above-normal precipitation is generally seen in the Pacific Northwest, with drier than normal weather observed from southern California through the southern Plains, Gulf Coast and up into the Mid-Atlantic. Over the last month or so, we’ve seen a persistent blocking ridge in the north-central Pacific into the Bering Sea. This goes to show how, even though the ridge remained relatively constant in the Bering Sea region, the effects downstream are not as certain. Prediction: A ridge present in the Bering Sea would introduce uncertainty to the winter forecast, and likely result in stormier weather along the West U.S., which in turn would most likely result in a warmer forecast for the U.S.

This is opposed by a strong ridge of high pressure, centered south of the Aleutian Islands but slanted so that it stretches from extreme northeast Russia to the waters just offshore of Baja California. If we follow Newton’s Third Law of Motion, which dictates that every action has an equal and opposite reaction, the East US must then have high pressure form, and that is what the negative PNA does. Since the negative phase of the PDO is generally seen with La Nina conditions in the Equatorial Pacific, you might expect the effects of a negative PDO to be similar to that of a La Nina. As the sea surface temperatures suggest, we have seen the PDO as negative since July of this year. It’s also entirely possible this actually promotes colder weather for the East, by encouraging ridging that then pushes north into Greenland to promote the negative phase of the North Atlantic Oscillation (NAO).

The Pacific jet stream rides the line between these two extremes, generally pushing southward from Alaska or the Gulf of Alaska, typically then beginning a northward turn when it hits central California. Once it has been melted, clean the fridge and then turn on the thermostat again. A new kitten will generally take to a litter box “like a duck to water.” But the litter box needs to be clean or they won’t use it. As the sun sets, daytime heating will take a big punch out of storms, but if there remains strong frontogenesis and fair lifting and instability mechanisms, the storms could certainly continue along through the night. I often listen to music while out for riding for fun. This will add some more uncertainty to the forecast for the coming winter, should these warmer than normal waters remain in the north-central Pacific and Bering Sea. So we should just stop here and say this winter will look more or less like a ‘typical’ La Nina winter, we should just stop the article here, right? Not so fast.