This Winter has been pretty remarkable considering we are in a moderate to strong "la nina" which generally brings much warmer & drier winters to the eastern 2/3 of the country. But just like last winter, starting in December the North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) both went into their negative phases and actually went extremely negative dealing us very harsh winter weather for much of the winter until now. We have finally flipped the pattern and the NAO, which will be the topic of this blog tonight, has gone back to the neutral/positive phase ending our brutal weather and sending a much needed warming trend (more representative of la nina) for most. Later this week I will look at the Arctic Oscillation (AO) and give some thoughts on where we are headed in the next several years. Both the AO and NAO have positive and negative phases and both their phases deal us the same kinda weather. Some consider them to be the "same" but they are different and I will show that as well.
Positive NAO Negative NAO
When the NAO is positive the Low and High Pressure systems over the North and Central Atlantic are much stronger causing the jet stream (storm track) to be flatter across North America. This keeps the cold weather bottled up in the Arctic, across eastern Canada and over much of greenland. Warm dry weather builds across the southeast and mid-atlantic states while the northeast recieve glancing blows from arctic outbreaks. The strong pressure systems also create very strong trade winds which blow from east to west across the tropical atlantic. These strong trade winds stir up cool waters from the ocean depts lowering sea surface temperatures. This plays a big part at reducing hurricane developement from this area of the atlantic (Cape Verde).
When the NAO is negative the Low and High Pressure systems over the Atlantic are much weaker, this cause the jet stream over the eastern United States to buckle, dipping down over the southeast and creating a strong "blocking" High Pressure over eastern canada. This sends all the arctic air spilling out of Canada and into the eastern 2/3 of the country. This explains all of the cold and snowy weather we have seen over the past few months. The NAO has been very negative since December 1st until now. Also, during winters that see a negative phase NAO will see sea surface temperatures rise in the tropical Atlantic, due to decreasing trade winds, increasing the risk of another Major Cape Verde Hurricane season (East Coast we need to watch out this summer).
Current NAO and NAO forecast:
From this image you can see how the NAO has been negative since late november. You can also see the NAO going positive around January 16th. This has a lag time of several weeks. So we began to see very harsh winter weather around December 4-5 which was approxiamtley 2 weeks after the NAO tanked. Also, we are just now seeing warmer temperatures developing which is approxiamtley 2-3 weeks after the NAO when positive. The red lines that are drawn out in advance are the ensemble models prediction of where the NAO will go. In the near term they are clustered tightly in the positive phase but towards the end of the month many of them go towards neutral and even slightly negative (not as negative as December and January) indicating some weak "blocking". We will need to watch this because it may try to send us more winter weather towards the beginning and middle of March.
Current Pattern for the Next 10-15 days:
Forecast:
Mountains- Tomorrow a front comes through during the afternoon and brings our warming trend to a hold for 36 hours. Temperatures will make a run at the 50s before they begin to fall tomorrow afternoon with very windy conditions. Tomorrow night looks very cold with a few flurries possible. Tuesday will be very windy too with near normal temperatures. A strong warming trend will begin Wednesday and run through Saturday. The timing of the next front is still up in the air but late next weekend we could see some rain and cooler temperatures. The mountain areas may make a run at the 60's by the end of the week.
Foothills and Piedmont- Tomorrow will be another nice day with temperatures in the mid to upper 60's. Tomorrow night and into Tuesday look slightly cooler with a front coming through tomorrow evening. It will be a dry front so I am not expecting any precipitation tomorrow evening. Wednesday through Saturday look amazing! Temperatures possibily approaching 70 by the end of the week before a front and some showers move in by late next weekend.
Stay Tuned*** A look at the Arctic Oscillation tomorrow
No comments:
Post a Comment