As many of you know, Meteorological Spring begins on March 1, which is this Friday. However, in both the short term and long term, the weather will not be cooperating. After this current storm system that has given us our clouds and rain over the past few days moves away, colder weather will be ushered in for Friday and beyond. Temperatures will generally represent average highs and lows for early February, rather than early March.
What’s responsible for this? For the first time this winter, we are going to see sustained NAO blocking. The 18z GFS ensemble mean forecast for 96 hours out, which is valid for the afternoon of March 3, shows a huge closed off positive anomaly of geopotential heights south of Greenland. This is classified as a strong negative NAO event, and these events often lead to very strong storm systems affecting our area, such as Hurricane Sandy. These negative NAO blocks help to force a trough on the east coast, providing sustained, cold air, along with storm systems that can slow down, strengthen, and take tracks just off of our coastline.
So, do we have any storm systems to look out for? Initially, some models were indicating a threat for the March 3 time period, but the problem for that time period is that the trough axis is too far east and too positively tilted. There was a positive PNA ridge on the west coast, but a disturbance moves onto the west coast, which flattens that ridge, represented by that little blue area in Washington State. It also pushes the flattened ridge eastward, pushing our trough eastward. The NAO block will prevent the trough from scooting out to sea, but it cannot keep the trough axis far enough west for a storm system to bring us precipitation. Additionally, and arguably the bigger problem is the upper level low pressure system that is elongated to the southeast of our NAO block. That keeps the flow out in the Atlantic pretty flat, preventing any storm from being able to turn up the coast. Thus, this time period is not favorable for a storm.
However, will this change? Potentially. When?
We are watching the March 6-7 period. As previously mentioned, there is a storm system crashing onto the west coast, which flattens the ridge. This disturbance actually happens to be our next storm threat. That initial upper level low moves out of the way, and the block is very firmly established. The block was still in its transition phase during a lot of the prior “threat’s” development. The 120 hour 00z GEFS mean, which is valid for the night of March 4th shows a potent shortwave in the Plains that is forced southward underneath our block, helping it to potentially take a storm track that would favor snow as opposed to rain, and be a strong system as well. Additionally, the disturbance off the west coast helps to pump up the ridge along the west coast, leading to further amplification. Thus, it is pretty clear that we will have a potent storm system, and that precipitation type will not be an issue. This does not mean, however, that there aren’t potential caveats to this setup
for snow lovers.
For one thing, this is a very strong block. This is almost always a good thing. However, if the block is too strong, it will force the storm too far south, and it will not have the room to run up the coast. Additionally, the ridge to the west is rolling eastward with time, so that combined with a very strong block may yield a flat and eastward trough axis, despite quite the powerful storm.
What becomes key here is the potential interaction with the northern stream. If we can get a phase between our storm system and some energy diving down from the northern stream, that will help to strengthen the system further, and perhaps amplify the pattern enough for our area to see a significant snowstorm. This is where once again, a block can help. A block can help to yield northern stream energy to dive down on the back side of it. And since the block is so strong, it forces the northern stream to follow the progression of its height contours, which are from north to south. This increases the odds of the northern stream phasing with the storm system.
The GFS makes the northern stream energy very weak and undefined, essentially rendering it a non factor. Thus, the GFS is well out to sea. However, the European model for the last several runs has broken off a potent northern stream disturbance and phased it with our storm. This is why this setup is so complicated: we do not even know if that energy will be there to begin with! Fortunately, the European model has been consistent with it, so it is a decent possibility that we will have this source of energy.
Now, let’s compare a run of the European model that showed a major blizzard for our region, versus one that showed a major blizzard for only the Mid Atlantic region. The first image is the 00z European model run that initiated last night, and the 2nd image is the 00z European model run that initiated two nights ago. Both show a beautiful phase of northern and southern streams into extremely potent storm systems. However, notice that while the block is strong on both runs, it is stronger and more oriented to the west on last night’s run. As many of you know, for every ridge, there is a trough, and vice versa. Thus, if you move that blocking ridge further west and make it stronger, you are also forcing any downstream trough and energy to also be further west and stronger, making it closer to our coastline. This often forms an area of confluence, or an area where the prevailing westerlies are “funneled”, and thus increase in strength. If our storm runs into stronger prevailing westerlies, it will have the tendency to scoot off to the east instead of being propelled northward. If that confluence is further east, like it was on the run two nights ago, it does not interfere with the storm, meaning that the storm has more room to climb the coast, and thus give us a major blizzard.
Another way to look at it is this: Both runs look nearly identical with the 500mb lows in strength and orientation, which is a good thing. However, last night’s run was fully phased when the confluence was strong near our area, and the run two nights ago was fully phased when the confluence was well offshore. Thus, last night’s run could not gain any latitude when it was fully phased, like what would be ideal for a major storm. This means that by the time the storm cuts off from the atmospheric pattern, which means it stalls or very slowly drifts eastward, its latitude is not any higher than it was when it initially phased. But two nights ago, there was room for the storm to climb north before it cut off, because the confluence was further east. Thus, last night’s European model run just missed giving us a blizzard, but instead gave one to the Mid Atlantic; whereas the European model run from two nights ago gave our area 1-2 feet of snow.
The ceiling with this event is quite high, because the phasing between our storm and the northern stream disturbance, along with the block being so strong and slowing down the pattern lead to the potential for our storm to “cut off” from the jet stream. This would allow the storm to essentially stall for 12 consecutive hours, and give areas 24 hours of heavy precipitation. Some ensemble data has hinted at this storm’s potential as well, as the GFS ensemble members are stormier than the GFS operational run. However, as noted, this setup is quite complex, and is still a relative long shot considering how far out in time we are talking about. We need a phase between a potent northern stream that we do not even know for sure will be there yet, and we cannot have the block be too strong to not allow it to climb all the way up the coast. That being said, having a strong NAO block, a very potent storm system traversing the country, ridging to the storm’s west, and energy in Canada to potentially phase into our storm is certainly a setup that needs to be carefully monitored. Considering that I favor the European model’s evolution of the northern stream, I favor a monster storm forming somewhere. Assuming the European model’s evolution of the northern stream is correct to begin with; whether the storm crushes the Southeast, Mid Atlantic, or our area, remains to be seen.