December 26-29 Significant Ice Storm and Snowstorm

It appears that a significant snowstorm, as well as a significant ice storm, will unfold in the December 26-29 period.

CMC
VALID: 12/27 at 4 AM CST
All further images with this layout are also from the CMC

The event begins as the storm system exits the Southwest. We will see a swath of moderate to heavy rainfall extending from the Great Lakes into Texas, but the real story will be the wintry precipitation. In this image, the forecasted precipitation type valid for December 27th at 4 AM CST, moderate snow will be falling in northern Texas into the Oklahoma Panhandle, as well as the western portion of Texas into New Mexico. Heavy mixed precipitation will be falling in southwest Oklahoma into northern Texas, draped across the Red River. Freezing rain is forecasted very close to Oklahoma City, while moderate rain falls across the remainder of the state. Mixing will also be a concern in northwest Missouri.

VALID: 12/27 at 5 PM CST

By the evening of December 27th, model guidance foresees snow continuing to fall in northern and western Texas, with continued moderate to heavy mixing in portions of Oklahoma. Oklahoma City now appears at risk for a freezing rain event in this timeframe, with heavy rain located in Arkansas and Missouri. Our storm system is also observed moving across the Southern Plains at this time.

VALID: 12/28 at 1 AM CST

In the early morning hours of December 28th, the system will begin to spread its influence to the north. Moderate snow will be ongoing on the back end of the system in Oklahoma, and now also falling in southern and eastern Kansas into far northwest Missouri. Mixed precipitation will be falling at this time in portions of Oklahoma and Texas, and falling at a heavier rate in northern Missouri into extreme western Illinois. Heavy rain will continue falling in Missouri and Arkansas, setting up a pronounced flooding risk which we will analyze in a follow-up post, which will be published shortly.

VALID: 12/28 at 7 AM CST

As the sun begins to rise, the situation becomes more dicey for those in the Midwest and Great Lakes. Model guidance now foresees moderate snow falling on the back end of the system in central Oklahoma, with a tight gradient from snow to mix to rain in eastern Oklahoma. Moderate snow will be falling in much of Kansas into southeast Nebraska, with heavy snow impacting southern Iowa and northwest Missouri. A large swath of mixed precipitation will be falling from central and northern Missouri into much of northern Illinois and Indiana, including much of metropolitan Chicago, IL. A narrow, but moderate band of freezing rain looks to hit central Illinois into northern Indiana at this time period, setting up a potentially catastrophic rush hour situation for urban areas.

VALID: 12/28 at 2 PM CST

By the early afternoon hours of December 28th, the storm system is now mature, with a defined dry slot shooting into southern Illinois, and a frontal band producing moderate to heavy rain from Alabama and Georgia to Tennessee and Kentucky. Light to moderate snow is projected to be falling from east Kansas into east Nebraska, as well as much of Iowa and northwest Missouri. Heavy snow is outlooked in southern Wisconsin and central Iowa. A heavy band of mixed precipitation is forecasted in central Missouri, eastern Iowa, northern Illinois, extreme southern Wisconsin, southern Michigan, and even portions of Pennsylvania. A narrow but intense band of freezing rain looks to set up immediately south of downtown Chicago, extending east into the heart of Gary, IN, and just making its way into southern Michigan. This would likely set up a treacherous commute home for any workers able to make it in during the morning mess.

Now that we've analyzed the hour-by-hour, let's check out the total accumulations.

Meteocentre - Snow accumulations
Accumulations for this image and the next two accumulation images are in MILLIMETERS.

The Canadian model projects a general swath of 20mm to 40mm of liquid-equivalent snow in the Texas/New Mexico/Oklahoma region. Putting this into a conversion chart tells us this is roughly 0.8" to 1.6" of liquid-equivalent snow. The average snow-to-liquid ratio is 10:1, meaning one inch of liquid would produce 10" of snow in that environment. However, given the lack of a preceding cold air mass, it would not surprise me to see totals lower than that, as the ratio goes down and snow becomes heavier. Thus, using a general ratio of 8:1, we could expect about 6-12" of snow for this region.

A band from northern Missouri to southern Wisconsin shows about the same liquid equivalent values, and again, a lower snow ratio is expected, so 6-12" of snow could be expected if this forecast were to verify as is. These heavier accumulations would just miss downtown Chicago, but would greatly impact Madison, WI and Milwaukee, WI.

Meteocentre - Ice Pellet accumulations

For the projected ice pellet accumulations, a swath of 10-25 millimeters exists in portions of Oklahoma, which is converted to about 0.4" to 1.0" of liquid-equivalent ice pellets. The amounts only increase, with a higher band clocking Chicago and into Detroit. Amounts on the order of 25mm to 40mm of ice pellets, converted to 1.0" to 1.6" of liquid-equivalent ice pellets, would be expected in this band. Additional, albeit lesser amounts are then outlooked for the Northeast.

Meteocentre - Freezing Rain accumulations

Lastly, we examine the projected freezing rain accumulations from this system. The main band appears to be placed immediately south of downtown Chicago, if not slightly inside the main metropolitan area. The swath contains values on the order of 7.5 millimeters to about 15 millimeters, with the highest amounts in Illinois. Using our handy conversions, we can identify that this storm drops 0.3" to 0.6" of freezing rain, a potentially catastrophic scenario with the projected wind fields.

To Summarize:

- A significant snowstorm and ice storm looks to unfold across the country over the December 26-29 period.
- Freezing rain on the order of up to 0.75" could impact portions of Illinois, Indiana, Missouri, and Michigan.
- Snowfall on the order of 6-12" could impact portions of Texas, Oklahoma, Missouri, Iowa, Wisconsin, and Michigan.
- Preparations should begin for those outlooked in snow, mix, ice, and heavy rain areas.

Additional posts on this storm are forthcoming.

Andrew 

March 4-5 Potential Ice Storm

There is an increasing likelihood that accumulating ice will pose a threat to residents in the Southern US.

PSU

High resolution precipitation-type forecasts show rain impacting the states of Mississippi, Georgia, and Alabama into the Carolinas late on March 4th into the 5th, before widespread icing begins to take hold. Areas affected include northern Mississippi, northern Alabama, central Tennessee, eastern Kentucky, and portions of the Mid-Atlantic. Snowfall may then be expected to the north.

WxCaster

Ice accumulations from the GFS model lay down pockets of close to 0.50" in portions of Arkansas and Tennessee, as well as Texas and Louisiana. Model guidance usually doesn't do well with freezing rain forecasts, but amounts over 0.10" are very possible, if not likely, in northern Mississippi into Tennessee.

To summarize:

- An ice storm may impact the South US on March 4th and 5th.
- Accumulations could reach as high as 0.25" of ice in many areas, with isolated higher amounts.

Andrew

February 15-19 Potential Snowstorm and Ice Storm

The February 15 through 19 period is increasing in likelihood that we will see a winter storm, potentially with an ice storm component.

Tropical Tidbits
Click images to enlarge

The image above shows the GFS forecast for precipitation type over the United States for the morning of February 16th. Here, we see precipitation breaking out over the Southern Plains and along the Gulf Coast as a low pressure system advects northeast-ward. In this forecast, we see a snow shield placed from southern Illinois into Missouri, Arkansas, and the Tennessee/Kentucky area, with rain blossoming in Texas, Louisiana, and southern Mississippi. We also see a rather broad swath of sleet, potentially freezing rain in southeast Oklahoma, central Arkansas, and northern Mississippi. This freezing rain/sleet delineation could prove to be a serious issue for those in the South, especially with memories of the ice storm from last year still fresh.

Tropical Tidbits

By the evening of February 16th, the low pressure system responsible for this precipitation is trekking along the Gulf Coast, located along the Mississippi/Alabama border in this graphic. A rain shield encompasses Louisiana, Mississippi, and a good chunk of both Alabama and Georgia. Snow is falling in eastern Tennessee, the western Carolinas, and extreme western Virginia, with a small band of freezing rain / sleet in northern Mississippi and Georgia. It's encouraging to see the freezing rain shield shrink from our last image, but as freezing rain is so tricky to predict, I wouldn't take that part of this forecast verbatim.

Tropical Tidbits

By early morning on February 18th, the storm has transferred offshore and is beginning to strengthen over warm waters along the coast of the East US. Snow continues to fall in much of North and South Carolina, with rain prevailing in southern Georgia and much of Florida. Heavier bands of snow are already impacting coastline locations in the Mid-Atlantic and Northeast, with New Jersey on the northern fringe of this heavy snow shield. From there, the storm continues north and east.

Snow accumulation charts are unreliable in this case, as some methods for snowfall will accidentally count freezing rain and sleet as snow, unrealistically amplifying snow totals. Thus, it would be unwise to show a snow total chart for those in Arkansas, where that unrealistic amplification of totals is likely to occur.

WxCaster

I want to now look at the forecasted freezing rain accumulation chart from the short-range NAM model over the Eastern US. In this chart, we can see where freezing rain is most likely to occur. Again, because freezing rain is so hard to predict in advance, take this with a relative grain of salt. Regardless, let's see who may be affected. The highest freezing rain totals appear in western South Carolina, where accumulations of 0.50" to 0.75" could be found. Significant accumulations of 0.25" to 0.50" extend through the rest of the Carolinas, and isolated spots of similar totals stretch back through northern Mississippi, Georgia and Alabama, all the way to southern Arkansas. While you shouldn't expect to see this chart verify exactly as-is, it gives you a good idea as to who may be affected by freezing rain from this storm.

To summarize:

- A storm system in the Southern US looks to bring wintry precipitation to states such as Arkansas, Mississippi, Alabama, Georgia, Tennessee, Kentucky, and the Carolinas.
- Accumulating snowfall is possible, particularly in Tennessee and Kentucky.
- Accumulating freezing rain is possible, particularly in southern Arkansas, northern Mississippi, Alabama, Georgia, and the Carolinas.

Andrew

December 19-23 Significant Snowstorm & Potential Ice Storm Discussion

This is a new discussion about the impending significant snowstorm and potential ice storm for a wide swath of the US.

The two pieces of energy that are associated with this system are circled in black. We see a main piece of energy located just offshore the Californian coast, with the second piece of energy just scraping the Aleutian Islands.

There are a few things to discuss here.

I took a look back at the December 14th 0z GFS model forecast and compared it to the same image you see above, which is an analysis image of mid-level vorticity values as of 12z today. When I compared the December 14th image, valid for 6 PM CST tonight to today's 12z GFS forecast for 6 PM tonight, I found a couple of interesting tidbits. For one, the energy is a digging south in the Southwest a bit more than what the December 14th forecast had shown. This is good for those wanting a southern trend in the storm, as it helps prevent a more northern solution. Additionally, and this is probably a more important factor, that energy currently skirting the Aleutian Islands is faster in today's 12z GFS than December 14th's forecast. The faster this northern portion of energy goes into the US, the better chances are for this storm being more impressive than recent forecasts have projected it to be.

Secondly, we have reached what I believe is a crucial point in model forecasts. As the storm system offshore California finally drops down into the Southwest, the troubles models have been having with the speed and intensity of said energy dropping into California will be shed. This means that the upcoming 0z model suite should be forming something that may resemble a consensus of this winter storm. Currently, said consensus would bring heavy snow from portions of Kansas, through southeast Iowa and into Michigan. However, the recent southward trend in the models could continue as some have indicated, and if this is true, expect that heavy snow axis to shift south as well.

Here's the most recent ECMWF snowfall map, showing the predicted location of that heavy snow axis. Model guidance in general has been lighter with snowfall amounts, minus the ECMWF. Despite that discrepancy, I do believe that the strength and pure amount of moisture the system is pulling up should see amounts close to the ECMWF verifying. I really can't see locations in the thick of an all-snow event from this system not exceeding 6-8", even though some guidance systems want to place the threshold at 6". I definitely believe we should watch the models for future shifts to the south; I don't really expect a northward shift at this point. Either the snow shifts south, or we see the current idea verify. Not that enthusiastic about a northward shift from here on out.

As far as this ice storm potential, the exact details are pretty tricky to nail down. A primary factor is the track of the storm still relatively unknown, and the location/intensity of any potential freezing rain/sleet will be determined by the storm track. Regardless of who gets hit with any freezing rain, I think it will be a decent icing event, possibly with decent accumulations as well. I would watch out for changing freezing rain forecasts, as the ingredients needed for accumulating freezing rain are rather tricky to pinpoint, especially with a system as dynamic with this one. One thing is clear- there is certainly potential for an accumulating ice event.

I'll probably be ready to issue my next outlook tomorrow, when the model consensus should either have formed or be forming. Until then, here is a summarization of my thoughts.

-A significant winter storm is probable for the United States, with effects including significant snows and a potential icing event.
-Heavy rains are likely in many parts of the Ohio Valley and Midwest.
-Models should begin to get a substantial handle on this system tomorrow, with the first hints of a model consensus appearing tonight.
-A southward tick in the track with upcoming model runs is not out of the question.

Please do not ask for a specific location getting affected by this storm- this is a very fluid situation, and what you see here is about all I'm willing to say with confidence right now.

Andrew

December 19-23 Potential Significant Snowstorm & Ice Storm

I'm feeling pretty confident that there will be a significant winter storm, composed of heavy snow and accumulating ice, between the December 19-23 period.

The following text & images are from yesterday's post on this system. The end of this copied portion will be denoted as (End of December 15th post).

A pretty strong storm system is expected to move across Japan in 48 hours, after dumping multiple storm systems over that area in the past few days. This storm system is looking healthy, as defined by the GFS ensemble mid-level height anomaly forecast above over Japan. As I have mentioned on this blog, there is a 6-10 day correlation between weather that occurs in East Asia, and weather that occurs in the United States. Thus, with this forecast image, we arrive at the December 13th date. Go ahead 6-10 days, and we find the potential storm timeframe of December 19th to 23rd. The reason I believe it may be significant is due to the strength of this system on the image above. Unlike the upcoming weekend storm, which saw only a moderate system in East Asia 6-10 days prior, the current East Asian projection is for a strong storm to hit Japan, and long range model guidance senses that this system will also be a strong one for the US. (End December 11 post).

The teleconnection forecast for the next little while from the Earth System Research Laboratory (ESRL) above shows projections for the Pacific-North American index (PNA), the North Atlantic Oscillation (NAO), the West Pacific Oscillation (WPO), and the East Pacific Oscillation (EPO). For this discussion, we will focus on the PNA and the NAO.

The forecast for the Pacific North American index is negative over the time period for this storm system, even bottoming out to its minimum during when I anticipate this storm system to strike. So what does the atmosphere look like during a negative PNA?

The image above is what is referred to as a negative Pacific-North American index phase (or -PNA for short). The negative PNA results from ridging in the Gulf of Alaska that then produces storminess/troughiness in the Western US. In response to that West US storminess, we then see the development of high pressure in the Southeast. The jet stream in this situation takes storms from the West and runs them north and east, typically delivering snow to the same areas that have been getting snow recently (think Plains, Midwest, Great Lakes regions). An interesting point of interest is how the two arrows on the image above (from NCSU) merge in the Plains. That means that, if the timing is just right, two storm systems could combine (phase) and form a stronger, single storm. History suggests storms that encounter phasing do become anomalously stronger thereafter, and can produce significant wintry precipitation. I'm not implying this will be a phasing situation, just something interesting.

The next item to discuss is the projected positive phase of the North Atlantic Oscillation. During the positive NAO, we see stormy weather over Greenland, which shifts the jet stream so the weather pattern is not conducive to those big high pressure systems you might sometimes see over the Bering Sea or even the Western US. The positive NAO pattern is what is called 'progressive', meaning no atmospheric feature really sticks around in any single area, because the atmosphere wants to keep this moving, so to speak. That said, previous forecasts for this storm had the system going north-northeast from the southern Plains into the Dakotas and Upper Midwest. Because of that progressive pattern, I'm really doubting we see such a spike northward. Will we see northward movement? Yes, because the -PNA will supply that ridging in the Southeast. However, the progressive nature of the positive NAO should limit any NNE or even due-north movement by this storm. I would watch out for the storm to move northeast, or even ENE.

This relative measure of predictability map, from the NCEP, gives us a glimpse at what atmospheric conditions are likely to be unfolding by an assortment of colors, defined in likelihood by the legend at the bottom. In this case, we see that blues have a single or low double-digit percent chance of verifying, while oranges define close to a 65% or better chance of verification. This map is valid on the evening of December 20th, roughly a day or two before this storm is projected to strike. There are three things we have to look at with this graphic.

The first item we'll discuss in this image is the ridging over the southern portion of the Gulf of Alaska. This ridging is encased in solid oranges right over the heart of the big arcing in the waters west of California and the coast, signaling high confidence that this ridge will be present. If that ridge is present, then is essentially guarantees the set-up I described above. Why? Well, that ridge in the waters off the West Coast then creates West US troughing, which provokes high pressure in the Southeast, and suddenly we have a classic negative PNA pattern.

Second, we see a swath of oranges extending from southern Canada into the Rockies. The troughing in the West from that negative PNA pattern tells us that the ensembles are sensing the trough to be our storm system in the December 19-23 timeframe. Like the Gulf of Alaska ridge, the oranges signify over a 60% confidence among the ensemble system with this storm system. Rising confidence among the ensembles for not only the storm itself, but for the NE Pacific ridge tells me that this storm is becoming even more likely.

Third, there is a wide body of oranges in the Southeast US. Going back to that negative PNA pattern, high pressure/ridging in the Southeast is expected, so it really isn't surprising that the ensemble set is going with a >60% chance of that ridge occurring. However, with the aforementioned progressive pattern thanks to the +NAO, it is unlikely that we see this ridge take over the East entirely. (End December 15 post). 

Now, I was running through model solutions and found something that makes me believe this will be a storm for the Midwest and Great Lakes, possibly not as far south as recent solutions have been indicating.

This screenshot shows mid-level vorticity forecasts from the GFS on the left and from the GEM on the right. Both are valid for Sunday, December 22 in the morning. Right off the bat, we notice significant differences among each model. The GFS on the left has closed off our impending storm system in the far Southwest, extending into Mexico, while the GEM has refrained from closing off the low and has instead propelled it eastward into the Plains and Midwest. As a result of the GFS' closed off solution, the storm system is cut off from the jet stream and meanders eastward into Mexico before finally being pushed northward again. It is this closed off low idea that creates the southern storm track (where snow is given to southern IL, IN, OH and into the Northeast), while the GEM's more progressive solution forms the northern track (snow falls across northern IL, IN, MI, OH into southern Ontario).

To be quite honest, I feel that the GEM has the right idea here, at least in not closing it off in the Southwest. In a recorded list of model bias', the NCEP agency notes that medium range models have a tendency to close off low pressure systems too readily. I feel that this is expressed in the GFS system on the left panel above, and in the latest ECMWF forecast. The GEM also closes off the low pressure system in the Southwest, but releases it from that closed position earlier, thus transferring the storm eastward quicker and going with the northern track. The difference between the GFS/ECMWF and the GEM is that the GFS/ECMWF hold the storm in the Southwest for a longer period of time than the GEM, and that leads me to the second model bias. I have mentioned on this blog that there is a bias that indicates models will hold storms in the Southwest for longer than they actually end up there. The GFS and ECMWF are perfect examples of this phenomenon (seen well in the GFS/GEM panel above), whereas the GEM does not keep the energy in the Southwest for such a long period. Based on those two bias', I do feel like the GEM has an accurate handle on this system.

And the interesting part is, it's not alone.

This dual-panel image shows mean sea level pressure (MSLP) values for Hour 168 from the GEM on the left, and the ECMWF Ensemble Prediction System (EPS) mean on the right. Thus, both images are valid for next Sunday morning. Notice the similar tracks in the MSLP values across the GEM and ECMWF EPS. Timing is a bit off, with the ECMWF EPS faster by a good 12 hours with this system moving into the Ohio Valley. However, the main point is evident in that the GEM and ECMWF EPS agree on this faster ejection of the system out of the Southwest. I feel that the GFS and ECMWF operational models are holding the energy in the Southwest for too long due to that bias, and while the GEM is also subjected to the same bias, it would appear that the model is circumventing it and is going with a quicker solution.

Here's the GEM snowfall and freezing rain projections, respectfully (Use Caution):

Measured in millimeters

I do think that one part of the model tendency to hold that energy in the Southwest might be due to the aforementioned negative PNA during this timeframe, but with the PNA gradually rising to positive territory around Christmas time, as well as the previously-discussed model bias', I doubt the storm would be able to stick around in the Southwest for as long as the ECMWF/GFS believe it will.

It's still a little early for any projected outlook due to model confusion, as well as a couple things I want to monitor over coming days to see if a southward shift shown in the GFS/ECMWF may be a valid possibility (though as I said, I'm not sold on it yet).

Andrew

December 8-9 Potential Southeast Snowstorm/Ice Storm

There is a chance that a snow and ice event may evolve across the Southeast over the December 8-9 period.

The GFS model predicts that areas of ice pellets, freezing rain and snow will begin to form across Louisiana, Mississippi, Alabama, Georgia, and North Carolina for the morning hours of December 8th, in conjunction with liquid precipitation falling across the rest of the aforementioned states. It appears that snowfall would be favored the further north you go, meaning North Carolina, far northern Georgia and upper Alabama would be under the gun for some snow.

The real show begins just 12 hours later, in the evening of December 8th, when the GFS model indicates the precipitation switches over to rather heavy frozen precipitation, particularly across the Carolinas and Virginia. This image, provided by Levi Cowan, does not differentiate between different frozen precipitation types like the WeatherBell image did above, so we are only left to fear that this will either be a potent ice storm, or a decent snow event. The whole event itself is still in question, so there is a lot to still be figured out in the next several model runs. As always, updates will be provided when needed here at The Weather Centre.

Andrew