This will be a long and technical post specifically designed for the weather geek. We look forward to answering any questions that you have regarding the more scientific phrases that will be mentioned in our article. For the non-weather geeks, this is interesting stuff and you may learn something.
After a week of well below temperatures across much of the Great Lakes and eastern US, Mother Nature has decided to flip a switch and much of the eastern third of the US will enjoy a period of well above normal weather through Wednesday-Thursday.
The reasons for this warm up are rather apparent on the image below, depicting the pattern at about 18,000 feet above sea level.
- The NAO is currently positive, as illustrated by the large polar vortex near Iceland.
- The PNA is currently negative, as shown by the trough diving into the western US.
- The AO has also briefly gone positive as ridging over the North Pole temporarily weakens.
This all supports a deep trough over the western US and a large ridge over the eastern US. Expect temperatures to rise to 15-25 degrees above average across much of the eastern third of the US at some point between Monday and Wednesday. However, all good things must come to an end, and there are forces at work over the Tropical Pacific that will usher in that change by the end of the week.
Convection continues to fester near and just west of the dateline. At this time, the location of the convection corresponds to a “phase 7 MJO” which correlates to the –PNA and eastern US ridging we will see over the coming days. Note the MJO composite surface temperature correlations for late January below.
Although the MJO isn’t the end all be all of North American winter weather, it is one of the main factors we look at when determining what may happen when we try to forecast the pattern more than a week out in time, and the current pattern developing across the country matches a late January phase 7 MJO to a “t”.
This is important because there is strong agreement on the MJO propagating east later this week, ushering a much colder regime into the eastern US.
As one can see above, a phase 8 and phase 1 MJO (which we will in all likelihood see by later this week) both correlate to a +PNA (ridging/warmth over the western US) and colder weather over the eastern US. This MJO pulse is supported by the global wind pattern or “GWO” for short, moving into more positive El Nino like ranges recently.
For the past year, the GWO has consistently been “negative” or “La Nina” like, which makes sense given we are coming off of a multi-year La Nina. When the GWO is negative, it supports a stronger west-east flow across the globe and also generally corresponds to weaker MJO convection in the central and eastern Tropical Pacific.
When the GWO goes positive, it means that the flow across the globe is very meridional, or more north-south oriented than east-west, which supports more extreme warmth/cold and storminess in general. When the GWO is positive it often corresponds to stronger MJO convection and also stronger high-latitude blocking thanks to the flow not being too flat.
When the GWO is negative, it tends to destructively interfere with any MJO convection that tries to establish itself over the central-eastern Tropical Pacific, as generally there are stronger mid-latitude patterns at work that the MJO often cannot overpower. However, when the GWO is “positive” the jet stream becomes much more snake like, and the MJO can influence the positioning of the bends of the jet stream.
To help us understand how the MJO will impact the pattern over the coming days and weeks, let’s first understand how the MJO even influences the pattern.
Essentially, convection is a large heat engine, and it gives off a lot of heat in the upper levels of the atmosphere. So, this heat will effectively push the jet stream farther north and compress it to the north of where the convection is occurring…to the east of the convection, the jet stream “breaks,” and where the jet stream breaks there is usually great upper level divergence/lift which allows for storms to occur underneath where the jet breaks. Since storms rotate counter-clockwise in the Northern Hemisphere, storms will draw up warm air on their east side.
So, on the eastern edge of the convection the jet breaks and storms pull warm air up east of the convection, allowing for a ridge of high pressure to occur there. As you can see above in the analysis of the jet-stream at around 30,000 feet as of Sunday morning, the jet is farther north than normal over the western Pacific, which is collocated with the longitude of the convection occurring. To the east of this, the jet stream clearly “breaks,” and a ridge develops thanks to warm air being pulled to the north.
As we fast forward to Wednesday morning, as the MJO moves east (which is shown on the wheeler plots above as moving into phase 8), the whole process shifts east. Instead of seeing a ridge of high pressure off the west coast and troughing over the western US/Canada, the ridging is essentially on the west coast and the troughing is forced eastward to first the central and then by Friday the eastern US, supporting a quick end to the warm up we are going to be seeing over the coming days.
The questions that remain now are:
1. How long will we remain cold over the eastern US after the initial shot late this week?
2. Will there be any snowstorms to track with this colder weather moving back in?
When looking at the forecast upper air pattern off of the GFS ensembles for Wednesday morning, we can see that while the trough is moving east quite nicely, the trough is very broad. There will be a lot of energy rotating through the trough and when this occurs it is typically difficult for any one piece of energy to dominate and harness the energy available and generate a significant storm system.
The NAM/DGEX models have been consistently hinting at a potential major snowstorm for portions of the TN/Ohio Valleys into the lower Great Lakes and potentially parts of western NY/PA Wednesday into Thursday. However, the rest of our guidance is going against this idea which makes sense to me given the broad nature of the trough. Thus, right now I’m expecting a few weaker waves of low pressure to ride up the cold front as it progresses east Wednesday-Friday with no major snowstorms…however I can’t rule out a couple of these lows dropping a swath of generally light snow behind the front from the Great Lakes east to the Mid-Atlantic/New England coastlines.
As the trough settles in over the eastern US through next weekend (February 2-3) several weak pieces of energy will continue to drop into the upper Midwest and then translate eastward across the lower Great Lakes/Upper Ohio Valley and towards the Mid-Atlantic states. Although the trough will be broad, making things difficult for a large storm, each wave may produce swaths of light snow across these areas. Lake effect snow will also be ongoing downwind of the Great Lakes through at least February 3rd.
As we head farther into February however, there are a few conflicting signals as to what will happen across the US.
The first uncertainty is the state of the MJO as we head past the first week of February.
The Euro Wheeler plot above shows the MJO beginning to approach phase 3 (warmth in the eastern US) in about 2 weeks while the GFS is generally slower with the propagation of the pulse. The CFS, shown immediately above, shows the MJO remaining favorable for a +PNA and eastern US trough through around February 10th. There are a couple of points I’d like to make before guessing what the MJO will do past the first week of February.
The GWO is quickly making an orbit through the El Nino-like phases, and is approaching the point where it may swing back towards La Nina like or at the very lease neutral phases for a time. It is very difficult to predict the future GWO, however, given that momentum is continuing to increase in the tropics and sub-tropics per the total AAM graphic shown well above, I believe the GWO will not simply die and go back into La Nina like octants…I’d expect it to remain in more of an El Nino state of mind through at least mid-February.
The Wheeler plots often have trouble diagnosing the MJO once it gets into the eastern hemisphere as it often times will decouple a little bit, but still propagate east. Given this I believe the ECM is more correct in continuing to propagate the MJO eastward into the second week of February, but also believe that we may see another pulse begin to develop over the Indian Ocean and attempt to propagate east in late February.
What this all adds up to is continued cold through the first 10 days of February before a potential relaxation for the third week of February before perhaps another reload near the end of February. This is supported by the warm stratosphere, which suggests that the –AO/-NAO may try to make returns and help lock colder air over the eastern US.
The models are in good agreement in bringing in a very active sub-tropical jet to the west coast of the US by the 4th/5th of February as the MJO enhanced jet continues to move eastward. The European model ensembles, shown above for Monday February 5th show this subtropical jet arriving while the eastern US remains cold. This means more moisture and potential storms may be in the mix by February 6th-7th over the eastern US. So, while I believe there won’t be any major snowstorm threats across the eastern US through February 5th, there may be a window between about February 6th and February 14th across the eastern US for a potentially significant snow storm as the sub-tropical jet and cold air potentially work together for a time.
That’s all for today, enjoy the warmth and get ready for more cold later this week!