March 4-6 Potential Significant Winter Storm

Have upgraded storm title in accordance with latest model trends.

Satellite imagery indicates the storm system in question is currently still mainly offshore of the west coast of North America. The enhanced infrared values over land, however, show that this system is at least partially sampled into the upper air network. This means that weather balloons can collect data from the storm that then enhances model forecast verification chances. This is a point that I will restate in the next several minutes that you read this.

Recent observations of the latest model forecasts show that this system will be putting down some significant snowfall. The two images above show the 84 hour accumulated snowfall forecast for the GFS on top and NAM on bottom. As you can see, both models remain slightly different in placement of snowfall, and there are differences in snowfall amounts. The NAM model has more snowfall than the GFS, which is typical, since it has a wetter-than-normal bias. The GFS model and NAM model both indicate that there will be a heavy snowfall event in North Dakota, possibly into Minnesota. From there, we diverge into two different tracks. One track has heavy snow in Chicago and Milwaukee (essentially the NAM and European models), while the other track has the heaviest snow in Iowa and central Illinois (American model).

Looking at animations of recent model runs, I have seen a clear northward trend in model forecasts, more towards the first track I mentioned in the above paragraph. Because the system is just now getting onshore and into the weather balloon network, changes in the models were bound to occur. However, this last-minute shift north is interesting in more than how the system is now just getting onshore. I could see how the system maintains this north track. It will be coming into the US from the Pacific Jet Stream, which is the jet stream in the North US. As a result, it is more likely than not to be bound to the northern jet stream (essentially the northern track) than completely separating and shifting south, in my opinion. I feel like the recent models are also counting this factor in, but the big question is where the snow will fall. In that aspect, I can't say I'm too sure just yet. I will need to see another 12-24 hours of snowfall forecasts before I'm confident enough to issue an accumulation map. However, I am confident enough to say that I can make a preliminary map, which is shown above.

I may need to extend that 'Heaviest Snow' area further south and east with time, but I'm just not confident enough yet to bring it in that direction.

Andrew

Sudden Stratospheric Warming Imminent

I am seeing increasing signs that a sudden stratospheric warming is about to take place in the upper stratosphere.

Shown above is an animation of 10 millibar temperature anomalies in the stratosphere. As I alluded to in my February 24th post, the Himalayan mountain range is a natural geographic instigator for these sudden stratospheric warmings. The enormous warming event we saw in January originated from the same mountain range we are now seeing this impending mass of warmth. Analyzing the above animation shows us that this warming event has been in the works since the beginning of February. In recent weeks towards mid and late February, we saw strengthening of this warming, and it became clear to me that this was not just a quick warming event. In my opinion, if you see a warming event over the Himalaya Mountains and it is sustained and strengthening, there's a solid chance that a sudden stratospheric warming could emerge from that situation.

The reason I am suddenly on what you may perceive as high alert is because of the last frame of this animation. If you look closely, you can see the pulse of warm temperature anomalies suddenly flare up over the Himalayan mountains, a sign of possibly sudden strengthening of this warming mass. This body of upper stratospheric warmth may very well be in its final stages of formation and the beginning stages of possible propagation to the Arctic, where the sudden stratospheric warming event could then take place. The warm air in the stratosphere would then push cold air down to the surface, and we could see that cold air hit the surface in the next 2-5 weeks, right around the start of severe weather season. If this cold air is able to come close to the US, or even penetrate through the nation, chances are the severe weather risk and winter weather risk would be heightened. I will explain more later on.

Andrew

March 6-8 Potential Nor'easter

Note: This will not be a full-blown Nor'easter.

Both major weather models are pointing towards a late-season partial Nor'easter that would be spit out of the Mid-Atlantic to dump snow in New England before moving out to sea.

Shown above is the European model's (top) forecast for mean sea level pressure about 7 days out. The bottom image shows mean sea level pressure and precipitation, again for 7 days out. Analysis of the two forecasts reveals that they are similar in both strength and placement of this storm. To have both major weather modeling systems in agreement at this point in time is certainly a good sign for those wanting snow in the Northeast and Mid-Atlantic.

With that agreement noted, I have opted to show the 48 hour accumulated snowfall forecast from Hour 156. We can clearly see a major hit of snow for Pennsylvania, New York and maybe a sliver of New Jersey, while coastal states are in the no-snow zone, as a typical Nor'easter will do. We see amounts exceeding one foot of snow, and these totals are centered in southern New York into eastern Pennsylvania. Major amounts of 6-12 inches are still found widely across New York and in the middle of Pennsylvania, as well as western Massachusetts and one or two other coastal states. Lower amounts below half a foot are found widely across the Northeast into the Mid-Atlantic.

Right now, such good model agreement is certainly a good sign for the above forecast to work out but we are still over 5 days away. That's a lot of time for things to change, and the system hasn't even gotten into the nation's upper air network, where the weather balloons can get data from the storm and put it into models to enhance forecasts.

Despite all the caveats, I feel confident in my call for all in the Northeast and Mid-Atlantic to keep one eye open on this system. It has good potential to bring heavy, wet snow to parts of the regions mentioned above, and must be monitored.

Andrew

March 4-6 Potential Winter Storm

Latest model forecasts are indicating the potential for a band of heavy snowfall to set up from the Plains into the Midwest and Ohio Valley during the timeframe of March 4-6.

Shown above is the snowfall forecast from the American model. It shows a corridor of heavy snow extending from Montana through the Plains and into the Midwest as a weakening trend hits. Some more enhanced totals are seen in southern Indiana and Ohio before the system transfers offshore of the East Coast, which is a post for another time. The snow would definitely be heaviest in the Plains as of current thinking, and these totals could very well exceed half a foot, with some spots reporting isolated totals very close to or just over one foot.

Contrary to the current belief, this system is not an Aberta Clipper. It is a regular storm system that happened to come onshore in Canada rather than the Pacific Northwest. Considering we remain a fair time out from this situation, and models are notorious for having issues with heavy snow in narrow corridors (as is to be expected), I think it is best to not make a preliminary map and instead cautiously go with the above snowfall forecast. This is not my forecast, but it does echo where I think the snows will fall. If models can show that they are rock solid with this heavy corridor of snow in the next 24 hours, I will most likely have a map out tomorrow.

Andrew

March 2013 Outlook

This is the March 2013 outlook. This outlook will not include a graphical outlook. Rather, a region-by-region summary will be included in this article.

Dates are shown at the bottom of this image as (day/time), so 4/12 would be March 4th at 12z (6 AM CT)

We're going to start with something called the Arctic Oscillation, or AO. The Arctic Oscillation involves the strength of a massive low pressure system in the Arctic that we call the polar vortex. Think of how you used to swirl your straw around in your drink in restaurants when you were a kid. If you swirled it fast, you could form a tornado-like vortex. If you only swirled it a little, the attraction to the center of the cup and the vortex is weak. This can be applied to the polar vortex. When the polar vortex (swirling your straw around) is weak, cold Arctic air is released down to lower latitudes, and can penetrate into the US. This is known as the negative phase of the AO. In the positive phase, a stronger than normal polar vortex (swirling your straw around fast) is observed. This locks up the cold air in the Arctic and provides for a warmer pattern in the United States. If we look above, we see the long range forecast of the Arctic Oscillation from the American ensemble forecasting system. The black line with intermittent circles is what is called the control run of this ensemble system (an ensemble member that does not have its initial parameters changed, unlike other ensemble members), and the green lines illustrate the mean forecast. The blue lines are the maximum and minimum of all the ensemble members' forecasts for the Arctic Oscillation. Analyzing the forecast above, we see that the AO is negative at this time and has been negative recently. This promotes stronger chances of cold air due to the weak polar vortex losing its grip on cold Arctic air. Looking ahead to the future, we see there is a strong consensus for this negative AO to continue into the middle of March. However, at the end of this forecast, the ensemble mean's green lines spike towards neutral territory, and this would indicate a strengthening polar vortex and thus less chances for cold air. As far as I can see, the first half of March has fair potential to be chilly in parts of the US. I will address this further at the end.

Now shown above is the forecast for the North Atlantic Oscillation, or NAO. The same rules about colors and types of lines in this forecast apply. The NAO has a positive and negative phase (in the following description, the positive and negative phase word colors above will correlate with word colors in the next few sentences). When there is high (low) pressure over Greenland, the NAO is said to be negative (positive). As a result, low (high) pressure forms over the Eastern US, and the subtropical jet stream, which helps Nor'easters form, is strengthened (weakened). Thus, when the NAO is negative, one can anticipate better chances for cold and storms in the East US, while the positive NAO brings about warmer weather. Looking at the forecast above, once again from the American ensembles, we see that we are currently in a weak negative phase that will be intensifying in coming days. Looking towards the longer-term, we see a tendency to go back to that medium-weak negative phase, and this could continue through the first half of March. Like the Arctic Oscillation, we see a spike towards more neutral and positive phase territory at the end of this forecast, which could mean warmer times to come. I want to point out that future forecasts of the NAO could be more negative than what we're seeing above. Based on my extrapolation of forecasts over the North Hemisphere, I feel that the negative NAO could be enhanced more than what is being depicted, but only time will tell if that comes to fruition.

For our last forecasting indice, we have the Pacific North American index, or PNA. The PNA has a positive and negative phase. In the positive phase, high pressure forms in the West US. In accordance with Newton's Third Law of Motion, an action will have an equal and opposite reaction, meaning high pressure in the West US will provoke low pressure in the Central and East US. The positive PNA is the favored stage for storms and cold in these two regions. In a similar fashion, when the PNA is negative, low pressure dominates the West US. Going back to the Third Law of Motion, high pressure then tends to form in the East US. This type of pattern gives the North Plains the big winter storms, a feature we have seen much of the winter (the PNA was negative during much of the winter, so the Plains storm train can be attributed to that). The forecast of the PNA starts out with us in a solid modest positive phase. This looks to continue into the first days of March before we go into neutral territory to round out the first week of March. Beyond this, we drop well into negative territory with the PNA, and you know what that means: storms diverted away from the East US, warmth dominates. And look at those blue maximum-minimum forecast lines- for a couple days at the end of the second week in March, the forecast is off the charts, while the maximum forecast barely breaks neutral territory. That is not a good sign for those in the East and Central US wanting cold weather for March, but an excellent sign for the warm-weather lovers in those areas.

This is a long range forecast from the most recent American ensembles, and this time they are forecasting 500 millibar height anomalies. Reds and oranges tell of high pressure, while blues indicate low pressure. I outlined the three indices we talked about above- the AO, NAO and PNA. We see very strong, blocking high pressure from the Bering Sea into the Arctic, something the American ensembles have been hinting at for quite a while now. There is no low pressure in that area, meaning the polar vortex has been exiled from its domain. On a related note, large low pressure in northeast Asia shows where the polar vortex may have been shifted to. The fact that the polar vortex isn't even allowed in its domain means cold air is much more likely to flow south and into lower latitudes, the extent of which is to be determined. We see a solid negative NAO over Greenland, shown by high pressure in that region. Now, I said how I believed the NAO may be more negative than what is currently being forecast, and the reason for this is because of that strong -AO. There is certainly plausible evidence that the negative Arctic Oscillation's high pressure could influence and strengthen the high pressure over Greenland. I will be watching for this in the next week or two to see how it plays out. As far as the negative PNA goes, it would be strengthened by high pressure in the Bering Sea into the Gulf of Alaska. As a result, you can see a small shade of orange in the South. As small as it is being shown to be, I can assure you the high pressure would be much stronger if this forecast comes true.

After reconciling these atmospheric factors and adding in a few more complicated indices, this is my March 2013 forecast by region.

Pacific Northwest: Starts more or less warmer than normal, gradually turns stormier and colder.
Southwest: Continued instances of stormy weather, possibly increasing after mid-month.
South Plains: Turbulent temperatures in first half of March, gradually turning warmer.
Southeast: Chilly start gives way to warm end.
Northeast and Mid-Atlantic: Bouts of cold and chances for snow give way to warmer end to March.
Ohio Valley: Chances for cold and snow lessen as month progresses; turning warmer.
Midwest: Chilly start to month ends up moderating as month goes on. Storm track shifts north.
North Plains: Cold start, stormy end. Wintry region of the month.

Andrew