It seems as if winter decided to arrive a little late this year. The groundhog was forecasting an early spring on Feb 2, but that forecast has failed miserably. The only real concrete way to determine how our climate will behave is through something we call teleconnections. There are tons of them, but there are three that play a significant role in U.S. weather during the winter. The North Atlantic Oscillation (NAO), the Arctic Oscillation (AO) and the Pacific-North American Oscillation (PNA). It is a combination of how each of these behaves that determines how the jet stream will be position, consequently determining if we are in a moist, warm, west to southwest flow...or a colder, dry northwest flow.
The NAO is basically an index that is either positive or negative. It is determined either way based on the difference in pressure between a low pressure over Iceland and high pressure over the Azores. Quoting a passage from Wikipedia: "
Although having a less direct influence than for Western Europe, the NAO is also believed to have an impact on the weather over much of eastern North America. During the winter, when the index is high (NAO+), the Icelandic low draws a stronger south-westerly circulation over the eastern half of the North American continent which prevents Arctic air from plunging southward. In combination with the El Niño, this effect can produce significantly warmer winters over the northeastern United States and southeastern Canada. Conversely, when the NAO index is low (NAO-), the eastern seaboard and southeastern United States can incur winter cold outbreaks more than the norm with associated snowstorms and sub-freezing conditions into Florida, in summer, a strong NAO- is thought to contribute to a weakened jet stream that normally pulls zonal systems into the Atlantic Basin, thus contributing to heat waves."
So when the index is negative, it causes the jet stream over the eastern US to buckle and blow from Canada straight to the Gulf Coast. This flow sends cold air from Canada spilling toward the Gulf Coast and Florida. When the index is positive, the jet stream flip flops and becomes more westerly to southwesterly. This locks the cold air up in Canada and prevents it from spilling southward. As a result, during periods of positive NAO, we tend to see unseasonably warm temperatures.
The PNA index is positive or negative as well. Here is an excerpt from Wikipedia on the PNA: "The positive phase of the PNA pattern features above-average barometric pressure heights in the vicinity of Hawaii and over the inter-mountain region of North America, and below-average heights located south of the Aleutian Islands and over the southeastern United States. The PNA pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. The positive phase is associated with an enhanced East Asian jet stream and with an eastward shift in the jet exit region toward the western United States. The negative phase is associated with a westward retraction of that jet stream toward eastern Asia, blocking activity over the high latitudes of the North pacific, and a strong split-flow configuration over the central North Pacific.The positive phase of the PNA pattern is associated with above-average temperatures over western Canada and the extreme western United States, and below-average temperatures across the south-central and southeastern US. The PNA tends to have little impact on surface temperature variability over North America during summer. The associated precipitation anomalies include above-average totals in the Gulf of Alaska extending into the Pacific Northwestern United States, and below-average totals over the upper Midwestern United States.
The negative PNA phase is associated with the opposite."
So in other words, a positive PNA means warmer weather western US and colder weather eastern US and vice versa.
The Arctic Oscillation behaves similarly to the NAO in that it analyzes pressure differences at 20°N latitude. Negative phases tend to have more dips in the jet stream, especially over the eastern US which leads to colder air and positive phases tend to lead to a more flat jet stream or westerly flow keeping the cold air locked in Canada. Here is a graphical example.
Here is the latest forecast from The National Center For Environmental Prediction (NCEP) for the next 6-10 days. It clearly shows a bullseye of below normal temps over the southern and eastern US.
Take a look at these computer model forecasts for the NAO and the PNA. Blue means negative and Red Means positive. Pay close attention to how the NAO is negative at the same time the PNA shows red (positive). This is a strong indicator that we will continue to see a cool, dry, northwest flow pattern over the eastern US through at least the first week of March, possibly even the second week.
This is just looking at one long range computer model, the GFS model, but the ECMWF or European Model indicates a similar pattern.
So...to summarize, it looks like the cooler than average weather will be sticking around for a little while. So keep those jackets handy!
Chief Meteorologist Derek Beasley
