FRIDAY FREE FOR ALL: IT'S NOT JUST EL NINO
Written by: Brian Neudorff
I decided to use the quiet weather pattern and sunshine to open this mornings post up to you the commenters and answer some of the weather question you may have on your mind. So feel free to post your question below, you comment will be emailed to the weather team and we will come back and answer it.
The first comments I want to address occurred yesterday in Scott's Thursday Afternoon post, "SIGNS OF WINTER RETURNING" regarding how much of this current pattern or the pattern we've seen so far has been related to El Nino.
Although this is an El Nino winter, and you can attribute the storminess in California this past week to El Nino, it's not the only player that has or will continue to impact our winter.
The first 2 weeks of January when we were seeing all that cold, it wasn't El Nino but it was the North Atlantic Oscillation (NAO). The NAO is a climate pattern in the North Atlantic Ocean of fluctuations in the difference of sea-level pressure between the Icelandic Low and the Azores High. Through east-west oscillation motions of the Icelandic Low and the Azores High,the NAO controls the strength and direction of westerly winds and storm tracks across the North Atlantic. When there is a large difference in pressure between Iceland and the Azores (positive NAO) westerly winds increase and Europe will get a mild and wet winter. Here in the United State, a positive NAO will cause the Icelandic Low to bring a stronger south-westerly flow of air over the eastern US, preventing Arctic air from plunging south. During this January thaw we've seen over the last week and a half the NAO trended to the positive side. As you can see in the graphic below. (Click to enlarge)
If the difference in pressure between Iceland and the Azores is small (negative NAO), westerly winds decrease and are suppressed, allowing Arctic air to spill southwards into eastern North America more readily. Negative NAO winters tend to bring cold winters to Europe, and the prevailing storm track moves south towards the Mediterranean Sea. This brings increased storm activity and rainfall to southern Europe and North Africa. As you can see in the graphic above, the NAO was very negative as we endured those very cold winter temperatures.
The areas in black are observed value of the NAO and the red lines represent a suite of models' forecast on how the NAO will change in the coming days and weeks. Towards the end of January the start of February, we start to see a good portion of the models begin to trend the NAO negative to reinforce our forecast that very cold air will return into early February.
When talking about the cold weather we had and will see again in the coming week you also can't ignore the Arctic Oscillation (AO). The AO refers to opposing atmospheric pressure patterns in northern middle and high latitudes, it's also a pretty good source on of how much cold air is likely to escape from the North Pole.
Effects of the Positive Phase | Effects of the Negative Phase
of the Arctic Oscillation of the Arctic Oscillation
(Figures courtesy of J. Wallace, University of Washington)
For most of the past century, the Arctic Oscillation alternated between its positive and negative phases. But over the last 3 decades, going back to the 1970s, it appeared the AO was stuck in positive phase, this created strong westerly winds that whipped around the Arctic,and effectively preventing much of the Arctic air from spilling south into the eastern U.S. and much of Europe.
The above graphic, like the chart I showed from the NAO, shows the observed value in black and the forecasted potential in red from several different computer models. During our cold outbreak in January the AO was at it's lowest level in a very long time. As we started to see our January Thaw the AO began to turn positive and now you see that some of the models and a good portion of them start to trend negative as we head into February.
I hope that put some of the factors that impact our winter into perspective. Feel free to ask any question and we will try to get back with you and maybe do a specific blog post on your question or topic.
Nice, understandable discussion. Thanks.
ReplyDeleteHope your models are wrong (I doubt they are).
What is the difference between cyclonic and anticyclonic flow, and what does it have to do with lake effect snow?
ReplyDeleteCyclonic and Anticyclonic describe how air circulates around either a high or a low pressure center.
ReplyDeleteIn the northern hemisphere air circulates and flows around a low pressure center counter-clockwise (clockwise in the southern hemisphere) this is called cyclonic flow.
When the air flows and circulates around a center of high pressure in the northern hemisphere it flows clockwise (counter-clockwise in the southern hemisphere) this is anticyclonic flow.
In basic terms, we can typically get lake effect and lake enhancement taking place in the fall and winter once we are on the back side of a low pressure system or storm system that has moved through.
The winds behind a storm system because they are rotating counter-clockwise as stated above will bring in winds out of the north and then northwest allow for colder air to flow over the lakes.
Although the center of a storm may have passed doesn't mean we are losing the lifting dynamics in the atmosphere needed to develop snow or lake effect snow.
The key thing to know is with cyclonic flow we get air rising or lift from the lower parts of the atmospher to the upper levels. The lifting or rising of air is important so that clouds and precipitation can form.
With an anticyclonic flow that is assoicated with high pressure air tends to sink and this prevents or prohibits any development of clouds or precipitation.
what does the 850mb temperature mean? When it's 0c at 850 does that mean it's cold enough to snow, or does it depend on the time of year?
ReplyDelete850 mb temperature - That's a great question and the 850 mb chart is very important to meteorologist when it comes to forecasting, especially for temperature and of course precipitation type.
ReplyDeleteThe 850mb temperature field shows where warm and cold air are located at about 5,000 ft (1500 m). Unlike the surface temperature that tends to change from morning lows to afternoon highs, which we refer to as diurnal temperature changes, the 850mb can be a little more consistent and this allow us to see if warmer air or colder air are moving into a region. This is also referred to as warm or cold air advection. Many time meteorologist can use the 850mb temperature to forecast the max or min temperature of the day.
We do use the 850 mb level to get an idea on what kind of precipitation can or will fall. Since most precipitation forms at about 5,000 ft. or above, if the temperature at 850 is above freezing (especially if more than 2 C above freezing), precipitation is likely to NOT fall as snow. Temperatures at freezing (0 C) or below can indicate snow, but if temperatures below 850mb are a little warmer or you have a layer of warmer air trapped between the surface and 850 mb then you can get sleet or freezing rain.