January 28-30 Potentially Historic Southeast Snowstorm

UPDATED POST HERE: http://theweathercentre.blogspot.com/2014/01/january-28-30-potentially-historic_26.html

Things appear to be setting up for a potentially historic snowstorm in the Southeast over the January 28-30 period.

The NAM model is the most aggressive with this storm system, dropping as much as 18.1" inches on the Southeast. As the legend on the right shows, we would be seeing accumulations over 6 inches in far southern Mississippi, and snow amounts over 1 foot slamming southern Alabama, much of Georgia, much of South Carolina, and eastern North Carolina. There is also 12"+ being forecasted in the panhandle of Florida, near Pensacola. Now, the NAM is notorious for over-doing snowfall, and I've got a feeling this situation applies here. If the storm does end up happening, you could probably cut these totals in half or so and end up with what we will see. Regardless, you get the picture: considering any snowfall in the South is a big deal, this much accumulating snow could be not only historic, but disastrous for those not used to snow falling in states like Georgia or Alabama.

The GFS model shows its forecast for all four types of precipitation for this event: ice pellets (top left), freezing rain (top right), rain (bottom left), and snow on the bottom right. The snow panel shows liquid equivalent snow amounts, meaning 0.5" on that bottom right panel would typically equate to 5" of snow. We see the GFS puts down all types of precipitation for this event, with freezing rain and ice pellets targeting southern Louisiana, southern MS/AL, central Georgia and eastern South Carolina. The snowfall hits areas immediately north of those two precipitation types. It looks like the GFS wants to lay down nearly 10" of snow in the Carolinas, though I'd put that at closer to 8 or 9 inches, considering the snow should be more or less heavier than your standard 10:1 ratio snow, where 10 inches of snow equals 1 inch of water. The GFS, while lighter in snow amounts than the NAM, still shows a pretty intense snow event for the South, with all forms of precipitation hitting the southern states.

The final model to look at here is the ECMWF model, which shows a much less significant snow event. We see the heaviest snow hits the coastal regions of South and North Carolina, where 6"+ of snow would be expected. The states of Georgia, Florida, Mississippi, Alabama, Louisiana and even Texas receive only a couple of inches of snow in this scenario. Based on the ECMWF Ensembles, I think the model is a little light on snow amounts across the board, and the more conservative estimates would probably range around the 2-5" range for the states previously mentioned.

This does look like a significant, and possibly historic snowstorm for the Southeast. My thoughts on the system side with the GFS model right now, which shows snow amounts decently as shown below. It would be wise to take off a couple inches here and there, specifically in the hardest hit areas, just to account for likely mixing of precipitation during the event.

Andrew

February 1-5 Potential Winter Storm

There is increasing potential for a winter storm in the February 1-5 period.

Shown above is the 500mb anomaly forecast off the ECMWF model, valid on the morning of January 26th. If you've followed this blog for a while, you know that we can use that area of negative height anomalies over Japan to forecast a storm in the United States. As Joe Renken states, we can extrapolate a storm over East Asia out 6-10 days to arrive at the timeframe for a storm here in the United States. Going that length past January 26th gives us a storm timeframe of February 1-5. That's not the only thing we can take away from this predictor, however. Looping the ECMWF forecast tells me that the storm system will be moving essentially due east as it passes through central Japan. This tells me that we could (keyword here is could) see our February 1-5 storm system also moving on a west-to-east track, not really curving north or dropping south too much. The 12z and 18z GFS models caught on to that idea of a west-to-east storm, but had the storm hitting the US on January 31st to February 1st, which is just outside the timeframe presented here. We'll discuss the models a bit later on in this post.

Adding to the evidence of a possible storm in the early February timeframe is what we saw happen in the Bering Sea on January 18th. The image above shows contoured 500mb height anomalies over the north Pacific on January 18th. We see a storm system moving east from far northeast Russia, which is the westernmost 'X' on the image above. The next day, we saw the storm move nearly due east to the second 'X' in the image, only moving just a bit north in the process. Going back to that link I posted above, there is also something called the Bering Sea Rule, developed by Joe Renken. The Bering Sea Rule states that a storm in the Bering Sea can result in a storm over the United States 17-21 days later. If we extrapolate the January 16th date out using the 17-21 day timeframe, we arrive at a potential storm in the February 2-6 timeframe. This fits well in the February 1-5 timeframe we got from the East Asian correlation, and the nearly west-to-east movement in the Bering Sea tells us the storm in the US may be west-to-east as well, like we saw in the ECMWF forecast.

Now, model guidance is pretty iffy on the timing of this storm, and that is to be expected. The GFS model wants to bring a zonal (west to east) track storm through the US, but on the January 31-February 1 timeframe rather than the early February time periods established by the East Asian correlation and the Bering Sea Rule. The ECMWF model brings a west-to-east track storm system through the US on February 2-3, which fits right into the two time periods we established above. However, the ECMWF is much less robust on snow totals; the GFS allows significant snow in the January 31 forecast above, while the ECMWF only lays down a handful of inches from the Plains to the Great Lakes.

I think we'll end up seeing a compromise, where the GFS slows down the storm to the ECMWF's timeframe of the first days of February, but we see increased precipitation amounts like the 0z ECMWF and 12z + 18z GFS put down, rather than the weak amounts put down by the 12z ECMWF model.

Andrew

Intense Cold Arrives Monday, Tuesday and Wednesday

Arctic air not seen since the early January outbreak is set to arrive on Monday in the Plains before flooding the northern US into Tuesday and Wednesday. Let's break down who will get the cold weather, and when.

WeatherBell

The ECMWF indicates we will see the core of the cold start out in states like Minnesota, North Dakota and Iowa on Monday, as this temperature forecast image for Monday morning shows. Below zero temperatures are expected from central Indiana back northwest, with temperatures below -20 degrees F anticipated in the aforementioned northern Plains states. We are likely to see school closings throughout the upper Plains on Monday as this cold filters in, with widespread cancellations possible.

Tuesday morning is when the Arctic air really flexes its muscles. We see the ECMWF model taking temperatures as low as -30 degrees F to Minnesota and Wisconsin, while giving cities like Indianapolis, Madison and Chicago severe cold that could easily flirt with -20 degrees F. The Northeast will begin to encounter these frigid conditions, as below-zero temperatures are forecasted to hit the entire region, except for areas immediately near the coast. I expect widespread school cancellations across Minnesota, Wisconsin, North Dakota, Illinois, Michigan, Indiana, Ohio, West Virginia, and additional closures throughout nearly each state in the Northeast. The Great Lakes will work to warm up those to the east of the bodies of water, as you can see in western Michigan and western New York.

The cold weather will begin moving out of the Plains on Wednesday, but not before another frigid morning brings well below-zero temperatures to Wisconsin, with other negative temperature values found in Minnesota, Iowa, Michigan, Illinois, Indiana, Ohio, and across New England. School cancellations should be considered a possibility in Wisconsin, Minnesota and western Pennsylvania. Cancellation prospects are a bit more iffy in the other states I mentioned.

A few tips for the upcoming cold blast:

If you absolutely have to go outside, make sure to follow these tips:

-Minimize time spent outdoors. Even if you have heavy winter clothing on, this weather means you must be indoors as much as possible.

-Load up on clothing. Any and all coats, jackets, hats, gloves, mittens... this is extremely dangerous cold weather that is not to be taken lightly. This isn't your average winter cold spell- this is the kind of weather that is more typical of the Arctic tundra.

-Protect your pets. Fur isn't going to suffice this time around- keep your pets indoors at all times. This is just as dangerous to them (if not more dangerous) as it is to you. If you have to take them outside for a walk, put some clothes on them. As silly as it sounds, it might just save their life.

-Stay indoors. As obvious as this tip may seem, some people need that extra reminder that they aren't invincible to all cold weather, even with heavy coats and jackets.

Andrew

Chicago, Detroit, New York City Targeted for Biggest February Snows

It looks like the end of winter will go out like a lion for a good chunk of the north-central and northeast US, with cities like Chicago, Detroit and New York City in the mix for the biggest snow events.

We're going to start with the Lezak Recurring Cycle. The Lezak Recurring Cycle, or LRC, is a tool developed by meteorologist Gary Lezak that, in essence, can enable forecasters to predict the overall weather pattern months in advance. The gist of the LRC involves a cycling weather pattern that develops in October and November of each year; no pattern is the same from year to year. Around mid November, the LRC begins to repeat, meaning we start to see a similar weather pattern in mid November that we saw in early October. This means that the cycling pattern has begun, and it will continue to cycle on a regular (non-changing) 40-60 day interval for the next ~10 months before it dissipates over the following summer.

This season's LRC cycle is on a 57 day cycle length, meaning the weather we saw in December can be expected to "come back" as we enter February. The image above shows the surface condition map on the left and 500mb map on the right from December 14, 2013. If w extrapolate that date out 57 days, we come to the February 7th mark. To make up for natural shifts that occur in the weather pattern, we can reasonably stretch the timeframe out to February 6-8 if we want to. As the 500mb map above shows, we have high pressure over the West Coast, with only slight ridging in the Southeast and the storm system easily working its way across the Plains. Based on the pattern I'm seeing to start February, I'm thinking a storm track similar to the one this system took in mid-December could very well be in the cards, or possibly suppressed a bit to the south (not by much). Regardless of where this storm ends up tracking, we should be watching out for a wintry system to impact the Plains, Midwest and eventually the Northeast in the February 5-10 period (I put February 5-10 instead of 6-8 to cover the timeframe the storm will affect all regions of the US, not just for the image shown here).

The second snow potential to watch would fall around the February 11-15 period, as this December 20th disturbance brought precipitation to the central Great Lakes and northern Ohio Valley, with additional precipitation extended into Kansas and Nebraska. The pattern here had a nearly-closed low in the Southwest, with high pressure in the Southeast. I'm a little cautious of saying that this storm track may repeat exactly as shown- I'm thinking we'll see a slightly different track this time around. If it is to deviate from the December 20th track, I expect it would be more to the south as of right now. Again, we would want to watch February 11-15 for this potential storm.

The ECMWF model then has an intense storm moving north in the far western Bering Sea, and by using the Bering Sea Rule (which states a storm in the Bering Sea results in a storm in the United States 17-21 days later), we can find that this January 27th storm would equate to roughly a February 13-17 period for a storm in the US. By seeing how the storm is in the far western Bering Sea, we can guess that the February 13-17 storm potential would then be based in the Central US. Time will tell, as always, but the evidence is mounting for a storm system in this mid-February timeframe. The storm could also be significant if the projections of a rather strong storm in the western Bering Sea are correct.

Just two days later, on December 22nd, surface and 500mb observation charts show precipitation associated with a disturbance stretching from the Plains to the Northeast. I wouldn't be too surprised to see a similar storm track play out on the expected February 14-16 period based on the LRC's 57 day timeframe, mainly because the pattern I think we'll see in mid-February will continue to resemble the pattern showing up in early February.

The ECMWF model zoomed on the Bering Sea then sees a storm system enter the far western waters of its domain on January 30th, meaning we would then be eyeing the February 16-20 period after extrapolating out 17-21 days using the Bering Sea Rule. Again, since the storm is in the western Bering Sea, we might anticipate a storm in the Central or near-Eastern US in roughly this timeframe.

These LRC and Bering Sea images do seem to favor the Plains, Midwest, Great Lakes and Northeast regions for snow events, especially if the storms are able to transfer offshore (which I believe we may see in February). I'm cautiously optimistic that those in the north-central and northeast may see a full three month winter of not only cold, but snowy weather.

Andrew

Long Range Lookout: Model Confusion Signals Pattern Change Ahead

It's time we start up our weekly Long Range Lookout segment, though this particular post will focus more on the medium range. Nevertheless, this discussion will be as important as the rest of them.

8-10 day 500mb height anomaly forecasts from the ECMWF (left), GFS (middle) and CMC models.

Medium range model guidance above shows rather drastic model differences between each guidance system, with multiple areas of the Northern Hemisphere differing in what exactly is going to happen. We see the ECMWF model on the far left prefers to hold a negative Arctic Oscillation (AO) regime in place, as is shown by stagnant high pressure in the Arctic Circle. In this forecast, the polar vortex appears to be broken into two pieces, with one lobe located in northern Eurasia and the more significant piece of the vortex displaced over Greenland, producing a strong positive North Atlantic Oscillation (NAO). The positive phase of the NAO tends to induce zonal flow over the nation, meaning no huge cold or warm outbreaks anywhere in the nation. Despite this, we still see suppressed high pressure over the West Coast producing a positive Pacific-North American (PNA) index, and that leads directly to suppressed low pressure anomalies over the Central and East US. I'm not buying the positive PNA forecast, and the reason for that will be explained a bit later down in this post. I do agree with the negative AO projection, though I think we may see high pressure near the Gulf of Alaska expanded a bit in the future.

For the GFS forecast in the middle, we see a much stronger negative Arctic Oscillation signature, with high pressure forming a bridge across the North Pole. This splits the vortex fully into two pieces; one part ends up in northeast Asia, while the other part is weakened and stretched out between Greenland and the United States. The GFS also shows a positive NAO forecast, which is why I'm a little hesitant accepting that deep low pressure anomaly in the Eastern US. However, strong high pressure blocking in the upper latitudes over Alaska would support such an idea, though the lack of support from the Pacific North American index gives me the feel the GFS is a bit too ambitious by bringing the vortex down towards the United States. Other than that issue, I believe the remainder of the forecast is rather sound. 

The CMC model on the right has a very similar forecast as the ECMWF model, though in this case we see a closed low offshore Baja California, in what appears to be an attempt at a Rex Block (high pressure directly north of low pressure, see image below). In this particular case, though, adding to the issues I find with the ECMWF forecast, the subtropical jet stream would most likely be strengthened by this closed low. Due to the positive NAO forecast in both the ECMWF and CMC forecasts, and due to how we aren't finding an El Nino in east Pacific waters (as the image below shows), I'm not so sure the pattern is conducive for strengthening of the subtropical jet stream after this northwest flow pattern we are currently in dissipates.

Typical Rex Block pattern. Notice the zonal flow to the east of the blocking pattern.
(TheWeatherPrediction)

Observed SST anomalies over the El Nino-Southern Oscillation (ENSO) monitoring area. The lack of significant warm waters at the bottom of the image tells us there is no El Nino currently in place. (CPC)

Let's take a look over teleconnection forecasts now, and see what they can tell us about the medium range period.

We'll begin with the Arctic Oscillation teleconnection forecast. As I stated above, the Arctic Oscillation has a negative phase, which means stagnant high pressure over the Arctic, and there is also a positive AO phase, which indicates stagnant low pressure over the Arctic. The -AO argues for a colder weather pattern in the United States, while the +AO argues for a warmer weather pattern.
The first thing we notice when looking at the ensemble forecast for the Arctic Oscillation is the wide spread in ensemble members at the medium and end of the forecast period. This tells us that the weather pattern is about to flip after we get out of this northwest flow pattern, and the discrepancies among the medium range models above confirm this idea. So, how can we pull a forecast from here? We can look at the errors previous forecasts made. Looking at the bottom panel of this image, we see the observed Arctic Oscillation value superimposed on the spread of forecast members from previous days, weeks and months of forecasts. Looking at that image, it seems that the observed Arctic Oscillation usually ended up more positive than the forecasts thought it would be. This has been the trend through much of the winter, until recently. Looking at the observed Arctic Oscillation from December 14th onwards, the trend has been for the observed AO to be more negative than forecasts believed it would be. I do not believe the idea that the Arctic Oscillation will be positive in the medium and long range, so it may be best to ride the new trend of lower AO values than forecasted and predict a neutral or negative Arctic Oscillation in the medium and long range time period.

This image shows the Pacific North American index (PNA) forecast over the same timeframe as the AO forecast above. We also see those superimposed panels like we saw in the Arctic Oscillation image. The PNA has two phases: positive, which means high pressure dominates the West Coast and leads to wintry weather over the East US, while a negative PNA depicts low pressures over the West Coast and warmer weather in the Central and East US. The forecast members here appear in sync with the idea of the Pacific North American index trending more negative in the future, and I agree. Some model systems haven't really caught on yet, and are still projecting frigid forecasts for the long range. I anticipate we see a gradual warming of those forecasts, though the Pacific should still be conducive for at least some decently cold weather. If we use that trend idea in the bottom panel here like we did with the Arctic Oscillation image, we don't see a particular trend right now- the PNA has stuck relatively close to its forecasted values in the long range. Thus, I see no reason to back off the trends now. I expect an eventual -PNA to evolve.

Why are models so confused? Well, we can look to the shorter term for that answer. This image shows an ensemble projection from the Climate Prediction Center for 5 days out, unlike the week-plus forecasts we discussed above. On the left is the ensemble projection for one height level (5520m), while the right panel shows the forecast for another height level (5760m). These images are both valid at the same time, they're just forecasting for different parts of the atmosphere, if you will. The pattern is clearly defined in the left panel, with ensemble members agreeing on the northwest flow pattern continuing, but the picture is more muddled with the panel on the right. On the right, we see high model disagreement with the pattern in the waters west of Mexico, with some members wanting to develop a closed low, and other members wanting to retain high pressure over the West US. Still other ensemble members want to keep the pattern more zonal and not contaminated with high or low pressure. So, who do we believe? Well, considering the Pacific-North American (PNA) index looks to still be positive over the next few days, and other ensemble guidance agreeing on intense cold weather staying in the US 5 days away, I would buy more into the high pressure on the West Coast solution than the closed low idea.

Long range ensembles still aren't faring too well.

This four-panel image shows the 500 millibar height anomaly forecast over the Northern Hemisphere from the GFS model (top left), the GFS Ensemble Control (top right), and the GFS Ensemble mean (bottom left). The bottom-right image shows the ensemble spread across the Northern Hemisphere. All four forecasts are more or less similar to the GFS image we discussed at the beginning of this post, but take a look at the ensemble mean. We can observe the continued model confusion with deeper purples emerging over both the Bering Sea region and over Greenland, signaling higher model uncertainty. The model uncertainty in the Bering Sea is likely due to how strong developing high pressure in that area will be, with some forecasts suppressing the ridge of high pressure to the south of the Bering Sea, and other forecasts flooding the body of water with high pressure. Similarly, models don't know how to handle pressure anomalies over Greenland. If there's low pressure present there, as some models indicate, we would see the positive North American Oscillation (NAO), which would support that warmer and zonal weather, while a negative NAO would support a continued northwest flow and cold weather in the East US.

So what do I think will happen in the long range, since models aren't giving too much to work with at face value?

Well, we can probably expect a break from the intense cold we will see to round out February. In its place ought to be a warm Southeast as some ridging takes hold thanks to that negative PNA, but also some cold weather in the Northern US as the negative Arctic Oscillation persists. It is possible the ridge in the Southeast is suppressed if we can get high pressure over Greenland to get a negative North Atlantic Oscillation going, though model guidance everywhere is having trouble with that aspect of the forecast. The West can anticipate a well-deserved break from the persistent high pressure, with stormy weather making a comeback... to some degree. We'll need to see how model uncertainty in the Bering Sea resolves itself before we can be sure the West will get at least a bit of relief.

Andrew