December 10-12 Potentially Significant Winter Storm

 There is increasing potential for accumulating snow across the north-central and Upper Midwest in the coming days, as a strong low-pressure system moves northeast through the Central Plains and Great Lakes. Note that there are two disturbances in this timeframe - one affecting the Midwest, and another affecting the East Coast. This post covers the first region.

Because of relatively large variance between model guidance, I will first examine the GFS model, followed by the ECMWF model. A summary of my own thoughts will conclude.

GFS Model Analysis

Attached above is a look at 3-hour accumulated precipitation (by type) and sea level pressure contours, per weathermodels.com and per the 12z GFS, valid at 12pm Central Time on December 10th. The GFS anticipates a strong surface low (989 millibars) to eject from the Rockies into the Kansas / Nebraska area around this time, tossing out a band of snow well ahead of the low stretching into South Dakota, Minnesota and northern Wisconsin. Snowfall intensifies in SD as the low approaches and allows for the deformation zone to form.

Moving ahead to 12:01 am on December 11th, we find the storm to be a quick-moving one, with the surface low now in the Dubuque, Iowa vicinity and developing into a mature cyclone (evidenced by ongoing rain and thunderstorm activity in the South and a continued band of moderate snows north and northwest of the low). Accumulating snow looks to overspread Minnesota and northern Wisconsin at this point in time.

The primary issue for those hoping for snow appears to be an aggressive warm front associated with the storm system. Indeed, a warm antecedent air mass (a.k.a. relatively warm weather in the Central U.S. in advance of this storm system exiting the Rockies in the first place) will allow the warm front to keep precipitation type as liquid well north and east of the surface low, stretching past Michigan and into south-central Canada!

Taking a look at projected accumulated snowfall for this event (remember, NOT including the East Coast disturbance around this same time) shows a relatively impressive snowstorm for parts of the Upper Midwest and Plains. Assuming a 10-to-1 snow-to-liquid conversion (meaning 10" of snow can be melted down to 1" of water), a swath of South Dakota is at risk for snows exceeding 6", while portions of Minnesota and especially northern Wisconsin could see a run at more than 12" of snow as the disturbance associated with this storm system matures and intensifies. Indeed, taking the GFS verbatim, some locations in northern Wisconsin may have a shot at 18" of snow!

It's also possible these totals are a little under what they may actually turn out to be, as surface temperatures look to be firmly in the mid- to lower-20s during this event, turning that 10-to-1 snow ratio more into a 15-to-1 ratio.

Now that we've gotten a look at what the GFS model wants, let's peek at the ECMWF's interpretation.

ECMWF Model Analysis

We'll be examining the same points in time here as we did in the GFS analysis: for instance, this first graph shows forecasted precipitation type and sea level pressure contours at noon on December 10th. The ECMWF places this surface low somewhat to the south of the GFS, resulting in a southward shift of the moderate snow band. Indeed, rather than South Dakota receiving the moderate snows from this system as in the GFS, we find much of Nebraska (but not Omaha!) under the gun for accumulating snow. The overall precipitation presentation in the eastern Rockies also looks more robust in the ECMWF, implying that the system may put down more snow than what the GFS showed us.

Moving ahead to 12:01am on December 11th, we find the ECMWF placing the surface low pretty close to where the GFS showed it, but the big difference is the marked increase in moisture available immediately north and west of the surface low. While the GFS has snowfall coming to an end not too long after the surface low passes a given line of longitude, the ECMWF allows moderate to heavy snowfall to continue in Nebraska even as the surface low is near Dubuque!

The result is a much snowier storm system:

As depicted above, via WeatherBell, the 12z ECMWF delivers over 12" of snow to the northern tier of Nebraska, even laying down several inches of snow on the back end of the storm system in parts of eastern Nebraska. A sharp gradient near the surface low likely results in a modest icing threat in the Omaha metro area, while continued snow accumulations exceeding 15" track north and east through northwest Iowa, southern Minnesota and northern Wisconsin.

My Thoughts

A look at how each model handles the mid-level pattern gives some insight as to why these two forecasts are markedly different. 

The first thing that is complicating this outlook is the fact that we're dealing with two separate pieces of energy in making this winter storm happen. I've highlighted these two pieces of energy in a 500-millibar relative vorticity image on the right, valid at noon on December 9th. That might all sound a bit complicated, but in a nutshell, the more colors you see, the more likely it is that there is a 'piece' of energy present that can develop into a storm system.

Both models agree that these two pieces of energy will make landfall on the West Coast in the Pacific Northwest and in California, respectively. It's how they mingle and interact when they meet up in the Rockies that is throwing models for a loop. 

The GFS model wants to hold the core of this energy back in the Rockies a little longer than the ECMWF, making the system positively-tilted at the mid-levels (a development that keeps the storm system relatively immature, and making it tougher to get those heavier snow totals). In contrast, the ECMWF allows a core of this energy to track north and east with the surface low and tilt negatively as it does so, amplifying the surface low and resulting in a more enthusiastic snowfall outlook across the Central.

The current strongly-positive state of the Arctic Oscillation (shown above) helps mold the broader weather pattern by making it less likely that we get these big storm systems with big snow totals, and does lead some credibility to the weaker, more progressive GFS model. At the same time, the Pacific-North American index (PNA) is poised to be strongly-negative at this point in time, which helps fire up a ridge of high pressure in the East U.S. and gives any storm systems in the Central a "ramp" in the mid-levels on which these pieces of energy can tilt negatively and mature into powerful storms.

Putting all these pieces together, I am in favor of the GFS' solution right now, but I do think it's pretty realistic we could see a southward adjustment like the ECMWF shows, in which case we would also see a stronger storm system and higher snow totals. The overwhelming negative state of the PNA drives that conclusion, with multiple GFS ensembles joining a majority of ECMWF ensembles in playing that southern shift out and making for a stronger storm.

To Summarize:

• A potentially significant winter storm is expected in the December 10th - 12th timeframe across the Central U.S.
• At this time, South Dakota, northwest Iowa, southern Minnesota and northern Wisconsin may see snowfall totals of 4-12", with those higher amounts much more likely in NW Iowa / S MN and especially northern WI
• Given regional teleconnections, it seems quite plausible that future forecasts shift this snow axis a bit south and increase snowfall totals, as the ECMWF model projects
• It is too early to tell for sure, but there may be a minimal icing threat along the freezing-temperature line. Material ice accretion is not expected from this storm.

Andrew

January 9-11 Potentially Significant Winter Storm

A potentially-significant winter storm appears to be unfolding in the January 9-11 timeframe.

Before we even dive into this storm, it's crucial to acknowledge that there are two model guidance 'camps' for this storm. As a result, there is unusually high uncertainty, so keep that in mind while reading this post.

Tropical Tidbits

The first solution we'll look at is the east-and-weaker idea. This is the latest GFS model's projected snowfall accumulation through Sunday morning. In this forecast, we see a swath of about 4-8" from southwest Missouri into extreme southwest Illinois, with a stripe of 2-4" on a northeast path until about Gary, Indiana. From that point, amounts on the order of 4-8" are outlooked for much of Michigan, particularly from the extreme southwest part of the state into the northeast part of the state.

This solution has been gaining ground in the last few model cycles, especially against the second model solution, which we'll analyze next.

Tropical Tidbits

The other solution being analyzed is a further northwest and stronger storm track. This is the 12z RGEM model, and it shows a solid 4-8" across much of the southern half of Missouri, with a small jackpot zone of 12-16" from extreme southwest Illinois into central Illinois. A zone of about 5-10" is outlooked for the Chicagoland area into the western half of Michigan.

This solution would appease many in the Midwest and Great Lakes regions looking for snowfall, but this particular solution is been losing traction to the weaker/southeast track in the last few model cycles.

Despite high model uncertainty, and the fact that I've seen stranger things happen, I'm expecting that weaker & southeast track to win out.

To summarize:

- A potentially significant winter storm is expected in the January 9-11 timeframe.
- High model uncertainty exists.
- A 2-6" snowfall swath could impact a southwest-to-northeast line from southwest Missouri to northeast Michigan.

Andrew

January 9-12 Potential Winter Storm

There appears to be the risk for a storm system to impact the East US, particularly the Northeast, in the January 9-12 timeframe.

Tropical Tidbits

We'll first go over the forecast of the storm, and then get into it at a more analytical level. This forecast panel shows the 18z GFS forecast of precipitation type and intensity, as well as mean sea level pressure contours and 1000-500mb thickness values for the early morning of January 11th. We see a strong storm system just offshore the Northeast, producing widespread moderate to heavy rain right along the coast from New Jersey on northeastward, with light snow located in inland New York into east Pennsylvania. This solution could give copious amounts of rain and/or snow to the East Coast.

As expected, the internet is biting hard on this storm, given it could be the first real wintry system for the East this winter. Not to burst anyone's bubble, but below explains why I'm not expecting a real snow threat with this storm, and why not to bite on any forecasts yet.

Tropical Tidbits

This graphic shows us the 500-millibar vorticity values, forecasted for 12z (6 AM CST) today, Monday. That red box, all the way out near the Aleutian Islands, encompasses our storm system. That's right, the system that people are looking at is currently hundreds of miles away, closer to Russia than mainland United States. That's the first sign that we shouldn't take any model guidance at face value right now. Heck, the latest 00z GFS run came in and showed the storm pushing well out to sea, not even affecting most of the East US. I just chose to show that 18z GFS graphic so I could show what I'm talking about, and to make a few points, like this one.

Additionally, check out all the pieces of energy out ahead of our main system. If you count closely, just looking at this graphic, I can identify at least three separate storm systems downstream of our system of interest near the Aleutian Islands. Three systems is a lot for model guidance to sort out, and it doesn't help that one of those systems is also well out at sea, not even close to the radiosonde network which would help model accuracy. In sum, models are going to suffer a lot with this storm, both with the distance the storm is from the U.S., and how many systems are downstream of our storm itself.

Tropical Tidbits

This next point is specifically for the snow enthusiasts in the Northeast already getting excited for this storm. This is the 850-millibar temperature forecast for the morning of January 9th. At this point in time, our system is located somewhere in the South Plains, though it is not well-shown on this map. That's alright though, because we're looking at this image for the temperature spread across the Central and East US. All colors green and warmer are temperatures above freezing at this layer of the atmosphere (about 5,000 feet off the ground). That means that most areas east of the Mississippi River, save for Maine, Vermont, and New Hampshire, should be comfortably above freezing in the couple of days, even several hours prior to the storm actually beginning to impact the East (if it does at all).

For a good snow set-up, no matter where you may live, you want an established cold air mass before the storm begins. We saw that guideline in play with our last storm system, which dumped copious amounts of snow in the Plains into Iowa and Wisconsin, even laying down a few inches of sleet near Chicagoland. It was all that sleet and freezing rain that fell because there was no antecedent cold air mass. This go-around, we also will be lacking a pre-established cold air mass, and that really concerns me with snow prospects.

CPC

While I'm not set on the idea of a snowstorm, I am open to the concept of this actually being a storm for the Northeast. The North Atlantic Oscillation (NAO) will be negative when this storm occurs, albeit only slightly negative. This should encourage the storm to curve up the coast, though just how much 'encouragement' there is remains in question. It's completely plausible the storm goes straight out to sea, as the 00z GFS portrayed, and it's also possible it curves up the coast. I'm not confident in it going one way or the other right now, but both solutions are plausible.

To Summarize:

- There is the chance for a wintry storm system in the Northeast between the January 9-12 timeframe.
- Personally, I see a very realistic chance that this storm produces more rain than snow as a result of no antecedent cold air mass.
- Model guidance should not be trusted this far out.
- This storm could plausibly go out to sea rather than up the coast.

Andrew 

December 26-29 Potentially Catastrophic Flooding

A potentially catastrophic flooding situation is unfolding for parts of the South Plains in the next couple of days.

WPC

The Weather Prediction Center has outlooked portions of eastern Oklahoma and extreme northwest Arkansas in a High risk of rainfall exceeding flash flood guidance to the right of a line. In essence, this is the risk of very heavy rainfall in a given area. There is a Moderate risk extending from northern Texas, portions of eastern Oklahoma, northwest Arkansas, central and southern Missouri, and extreme west-central Illinois. Those in the Moderate risk should make preparations for very heavy rainfall, while those in the High risk area should make preparations for potentially-widespread flash flooding.

WPC

The Weather Prediction Center places an 11.87" rainfall maximum over eastern Oklahoma for this storm system over the next 72 hours. Given that Oklahoma went through a record-breaking rainfall event in May of this year, this impending rain event will only add stress to an environment that has already dealt with a very intense rainfall event this year. Amounts on the order of 4-7" are projected for northeast Texas, while central and southern Missouri into northwest Arkansas and portions of east Oklahoma are outlooked for 7-10"+ of rain. Again, this is not something to take lightly. Rainfall on this magnitude, especially in areas that received extreme rainfall several months ago, is expected to cause flash flooding, possibly on a severe scale.

To Summarize:

- Heavy rain is expected to cause flash flooding over the next few days.
- This rain could be as high as 12", with amounts over 6" possible in portions of eastern Oklahoma, southern Missouri, and western Arkansas.
- Preparations should begin now for potentially severe flash flooding, especially in the areas mentioned above.

Andrew 

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 

December 26-29 Significant Winter Storm

There is growing concern that a potentially-historic snowstorm may impact the Southern Plains, with lesser, but still-substantial amounts to the north and east in the December 26-29 timeframe.

Tropical Tidbits

The system of interest is currently in the Gulf of Alaska, pushing towards the Pacific Northwest as of posting time. We see the higher vorticity values in the circled region, indicating the presence of the trough. Looking over the map right now, the pattern downstream (to the east) of this energy is rather zonal (not wavy; flow is west-to-east), which would keep this storm from going too far north. However, the flow immediately upstream (to the west) of this piece of energy is rather meridional (wavy; flow can appear to move in a north-south direction at times), which will likely play a role in the storm track later on. For now though, we can only confirm the presence of our storm system in the Gulf of Alaska.

Tropical Tidbits

The first snowfall map we'll analyze is the 00z GFS model, the most recent as of the time I am writing this post (it's currently 1:11 AM on Thursday, sleep is not my friend right now). The latest GFS shows amounts in the 6-14" range across northern, western and central Texas, with scattered swaths of 14-20", particularly in far western Texas into New Mexico, and into north-central Texas. A narrow, but intense band of amounts exceeding 24" is projected onto the southwest portion of Oklahoma into north-central Texas. Amounts in the range of 12-18" are then outlooked in much of central and northern Oklahoma, before amounts taper off to the 5-10" range from southeast Kansas through Chicago. The energy then appears to transfer east along the East Coast, dropping significant amounts in the Northeast, with New York, Massachusetts, Vermont, and New Hampshire among the states projected to receive up to 18".
This model has been quite variable with snow amounts, but has maintained consistency with the storm actually occurring, and confidence is increasing in the storm occurring in at least some format.

Tropical Tidbits

Next, we'll take a look at the Canadian GEM model forecast for this system. This model paints a different scenario, expressing the variability that still exists with this event. This model drops a general 6-18" swath across northern, western and central Texas, with the highest amounts in the far northern and western regions. Oklahoma also experiences significant snows, with a general 6-12" swath for much of the state, and even a 24" bullseye on the Oklahoma/Texas border just southeast of the Oklahoma Panhandle. Amounts then diminish to a 6-12" range in northern Missouri to northern Illinois and Indiana, southern Michigan, and boosting amounts slightly as the energy hits the Northeast.

While I'm not buying into any particular snowfall solution just yet, I am advocating a particular track. Next, I'll explain why I believe we will see the heavy snow axis fall where it is currently projected, or even further south and east than the above maps show.

Both images from CPC

The first two items to discuss are the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO). The AO measures the strength of the tropospheric polar vortex- a positive AO indicates a stronger than normal polar vortex, while a negative AO indicates a weaker polar vortex. The positive phase of the AO tends to promote a more zonal flow, generally warmer than normal and less stormy. The positive phase of the NAO works similarly, promoting warmer weather in the eastern 2/3rds of the country, and zonal flow. Both of these indices are forecasted to maintain positive strength when this storm comes around, telling me that the storm will be hard-pressed to push northward like earlier model runs had suggested.

CPC

We're also looking at a positive Pacific-North American (PNA) index for when this storm is forecasted to hit. The positive phase of the PNA promotes high pressure in the west, and low pressure in the east. It's typically conducive for storms in the Midwest and Ohio Valley for this reason, and can also lead to these transfer events, which give the Northeast the threat for stormy weather as well. A typical positive PNA set up is shown below, and we are expecting this positive PNA set-up for the storm's occurrence.

NCSU

But the real kicker is a piece of energy that, in a way, is both the least and most important part of this storm.

Tropical Tidbits

The above image shows the 500mb vorticity map, forecasted by the 00z GFS for 84 hours out. We see our storm now well developed and centered in far northern Mexico, beginning to push eastward into the southern Plains. However, the point of discussion here is that area of higher vorticity in the northern Plains and Midwest, and it is this item that tells me we could see the storm track stay to the south.

That band of higher vorticity values is an area of confluence aloft. Confluence is the act of air essentially rushing into a given space- similarly, diffluence is the act of air evacuating a given space. In the atmosphere, confluence aloft can result in high pressure at the surface, which acts to force storm systems away (in this case, further to the south). It is this that completes my belief that the storm will take this more eastern track and give cities like Oklahoma City, perhaps Dallas, Chicago, Detroit, and much of the Northeast a solid snow after Christmas.

To summarize:

- A significant winter storm is forecasted to hit a large portion of the country in the December 26-29 timeframe.
- Snowfall amounts in excess of 12" cannot be ruled out for the Southern Plains, including Texas and Oklahoma.
- Snowfall amounts in excess of 6" cannot be ruled out for portions of the Midwest and Great Lakes, including Chicago and Detroit.
- Snowfall amounts in excess of 12" cannot be ruled out for the Northeast.
- High uncertainty still exists with this system.

Andrew

December 26-29 Potentially Significant Winter Storm

Model guidance is beginning to pick up on the idea of a strong winter storm impacting the Plains.

WxCaster

The above image shows today's 06z GFS snowfall forecast from the time of posting to midnight on December 29th, 2015. We can see the large swath of snowfall extending from northern Texas through Kansas, Nebraska, and into Minnesota. The heaviest-hit region in the deep south appears to be extreme northern Texas, with amounts in the 12" to 24" range, particularly along the border with Oklahoma, as well as in a spot near New Mexico.
Portions of the Oklahoma Panhandle, as well as southwest Kansas and eastern Colorado, may be in line for up to 36" of snow, according to this forecast model. These are very extreme amounts, and although they have been showing up multiple times on previous forecast runs, extreme caution should be used with these maps, especially given the timeframe.
Amounts in excess of 24" are found in spotty areas to the north and east, particularly eastern Nebraska into southeast South Dakota, with lesser (but still significant) amounts in Minnesota.

WxCaster

The above image shows the 12z GFS interpretation of the coming storm, with total snowfall from posting time to 6 AM December 29th shown. Snowfall amounts are still incredibly high in northern Texas, the Oklahoma Panhandle, and much of Kansas into southeast Nebraska, but trail off from there. Amounts in the 12-16" range can still be found in Iowa and Minnesota, but these amounts are substantially lower than those in the 06z forecast. This variability, both in amounts and track (as you'll notice the heavy snow axis has shifted east) expresses the extreme uncertainty still associated with this system.

Tropical Tidbits

Lastly, we'll analyze today's 12z Canadian model, the GEM model. Amounts on the order of 20-24" are found in north and west Texas into the Oklahoma Panhandle, even in south-central Kansas, but overall snowfall totals are drastically less than the two GFS runs we analyzed earlier. This is a red flag that may indicate one model is either over-doing snowfall (GFS) or one model is under-doing snow (GEM). There's also a plausible chance that they're both incorrect- the point is, there's a lot of uncertainty here.

But just where is this system now?

Tropical Tidbits

Our system is currently traversing the Aleutian Islands, and will drop into the west coast of North America to produce a very strong trough, as per latest model guidance. It remains to be seen if this is model error, as medium and long-range guidance has a tendency to over-amplify storm systems, but for now, it's merely a system to monitor.

To Summarize:

- There is the threat of a winter storm in the Plains over the December 26-29 period.
- Model guidance is expressing high snow totals in the South Plains, but high uncertainty indicates that these models should not be taken at face value right now.

Andrew 

November 20-22 Potentially Significant Winter Storm

Model guidance continues to increase snowfall amounts in the winter storm expected to impact Friday through the weekend.

Instant Weather Maps

The latest GFS model has shifted snowfall amounts further to the north since we last analyzed the system yesterday. We now see forecasted totals in the 3-6" range across northern Nebraska into extreme southern South Dakota. Amounts in the 6-10" range are then forecasted in most of Iowa, with lesser amounts in the southern portion of the state. Amounts exceeding 12" are projected to hit southern Wisconsin, including Madison and Milwaukee. Northern Illinois experiences an intense gradient, with the city of Chicago receiving under 3" and areas along the Illinois-Wisconsin border receiving 12".

Instant Weather Maps

The latest NAM model takes a different approach to the storm. It drops 3-10" across southern South Dakota, with the lower end of the gradient in the western portion of the state and heavier amounts in the eastern part of the state. Northeast Nebraska is in line for 6-10" of snow according to this model, with a sharp gradient to the south of that heaviest snow axis. Iowa is projected to be slammed with snow across the board, with heaviest amounts in the western and northern portions of the state. Amounts then drop off as the storm moves into the Midwest. 6-8" amounts are forecasted to hit in a thin line along the Illinois-Wisconsin border, heading right into downtown Chicago, Illinois. Lesser amounts in the 3-6" range are then found in southern and central Michigan.

To summarize:

- A potentially significant winter storm is expected to impact the Plains, Midwest and Great Lakes Friday into Sunday.
- Considerable uncertainty still exists with respect to the track of this system.
- Amounts in excess of 6" are possible, especially in Iowa, southern Wisconsin, and northern Illinois.

Andrew

Thanksgiving Potential Wintry Storm System

There does appear to be at least a chance of a wintry storm system in the Thanksgiving timeframe.

Tropical Tidbits

The GFS model has a messy solution evolving for Thanksgiving Day, as the above image shows. We see two centers of low pressure, one on the Oklahoma/Colorado/Kansas border, and another on the South Dakota/North Dakota/Minnesota border. They are both about the same strength, with the southern one at 1004 millibars and the northern one at 1005 millibars. The graphic here shows widespread (albeit light) snowfall in Montana and North Dakota, with heavier snowfall in eastern North Dakota near the low pressure system.

In the warm sector, we see a line of rain and thunderstorms extending from southern Texas through Oklahoma, Missouri, and into Wisconsin and Minnesota. Going by this image, it's possible we see some stronger storms in the Texas and Oklahoma areas, into Arkansas as the day continues. Heavy rain would still be possible in the Midwest and Great Lakes, making for a soggy Turkey Day for locations such as Chicago, Milwaukee, St. Louis, and Little Rock.

Tropical Tidbits

The GEM model shows a singular storm system placed in southeast Oklahoma in the early morning hours of Friday, immediately after Thanksgiving. This would include heavy rain, and possibly thunderstorms, in Missouri, Arkansas, and along the Gulf Coast. Lighter rain would be anticipated in Iowa and Illinois, while a rain-to-mix solution could evolve for Kansas, Nebraska, and Iowa. A snow-mix situation would unfold across the Upper Midwest and North Plains. It's too early to talk accumulations, especially since there is no model agreement on strength, track, and precipitation.

To summarize:

- Model guidance is indicating the chance for a wintry storm system on Thanksgiving Day.
- Low confidence still exists in this situation.

Andrew

November 20-22 Winter Storm

A winter storm is expected to impact portions of the Plains, Midwest and Great Lakes over the November 20-22 timeframe.

Tropical Tidbits

The GFS model is insistent on a strong, yet compact low pressure system bringing accumulating snowfall to the North US. Using a 10:1 snow-to-liquid ratio, the 12z run of this model drops 4-8" of snow in northeast Nebraska and central Iowa, with 2-5" spread across southern South Dakota, central Nebraska, and the northern and southern sides of Iowa. Amounts in the 6-10" range are then forecasted in northern Illinois, northern Indiana, and southwest Michigan, with lesser amounts into southern Wisconsin, central Illinois, central Indiana, and central Michigan. Some lake effect snow after the storm could enhance totals on the eastern side of Lake Michigan.

Tropical Tidbits

The GEM model has a slightly different scenario. It paints a swath of 5-10" across northeast Nebraska into much of Iowa. Southern Iowa could see amounts near 12" according to this model, before amounts back in the 5-10" range are forecasted in north Illinois, north and central Indiana, and southern Michigan. This model is notorious for over-doing snowfall amounts, and has historically not performed well compared to other model guidance with respect to snowfall amounts, storm strength, and storm track.

It does appear the GFS and GEM models are in a snowier 'camp' together, with both of them laying down a swath of significant snowfall. The NAM model is against this snowier forecast. Since the system responsible for this threat will not be onshore until Thursday evening, and these early-season storms can have low ratio snows, it would be wise to stick with lower-end guidance for the time being. These concerns are also reflected in the Area Forecast Discussion from the NWS office in Chicago:

SOME FACTORS UNDER CONSIDERATION IS
THAT THE MODELS ARE TRENDING TOO HIGH WITH RESPECT TO SNOW TO LIQUID
RATIOS WHILE SFC AND GROUND TEMPERATURES WILL HAVE A BIG IMPACT ON
ACCUMULATION RATES.

The Chicagoland office expects 4 to 5 inches of snow for that area, using 7:1-8:1 to 9:1-12:1 ratios across their forecast office. I do believe this is a reasonable forecast, given the low-ratio nature of the system.

To summarize:

- A winter storm is expected to impact the area over the November 20-22 timeframe.
- Snowfall amounts generally in the 2-6" range can be expected in portions of the Plains, Midwest, and Great Lakes.

Andrew

November 17-19 Potential Winter Storm

There is the potential for a winter storm in the November 17-19 timeframe.

COD

Analysis of absolute vorticity values across the Northern Hemisphere shows a pretty active environment, especially with the energy lifting north from the United States into Canada which brought us the strong low pressure system this past week. The main item we want to look at is the piece of energy circled in red in the northeast Pacific, currently moving into the Gulf of Alaska. It is this energy that will provide the chance for this winter storm next week.

COD

The 42 hour forecast from the GFS shows the energy deepening and digging southward into California by Sunday morning. Ahead of the system ejecting into the Plains, we will see a blast of warm air into the Central US, as the trough digs south and forces warm air to advect north over the prairie.

Instant Weather Maps

The real fun starts on the 17th, when model guidance indicates this storm system will eject east into the Plains in the form of a sub-1000mb low pressure system. Here, we see cold air wrapping into the Rockies, and a deformation zone across Colorado and far western Nebraska. It is here where we should see the highest accumulations, should this storm transpire. It is also worth noting the potential for serious flooding in eastern Texas, the Arkansas/Oklahoma border, Missouri, and parts of Illinois.

Instant Weather Maps

Forecasted accumulations range from 6-12" in the western portion of Nebraska, to as high as 48" along the Nebraska/Wyoming border. Colorado could see amounts in excess of four feet, especially if orographic lifting comes into play. The gradient will be sharp in Nebraska and Colorado, and snow ratios should be rather low, given this is an early-season storm.

To summarize:

- A potential winter storm could drop significant snow in Nebraska and Colorado between November 17-19.
- Serious flooding could arise along the Mississippi River due to heavy rains.

Andrew

March 1-2 Potential Winter Storm

Model guidance has been indicating a winter storm will track across the middle of the country, bringing accumulating snow to those further south of where much of the wintry weather has occurred this winter.

Tropical Tidbits

The image above shows the GFS forecast for precipitation type and intensity, valid for Sunday night. We see light to moderate, possibly even heavy snow falling across the Midwest, Great Lakes, and Ohio Valley. A classic signal of winter finally on its way out, the rain/snow line is displaced pretty far north in comparison to where it has been in recent days and weeks, with rain being a concern in southern Illinois into southern Ohio. A bit of mixed precipitation could impact those in the central Plains.

Instant Weather Maps

Snowfall projections have been waving back and forth with the axis of heaviest accumulations, but the latest GFS pegs a swath of 6-10" accumulations across the central Plains into the Midwest and central Great Lakes, with Kansas, northern Missouri, central Illinois and northern Indiana receiving the most snow.

To summarize:

- Model guidance is anticipating a winter storm to affect the country over the next few days.
- Snowfall on the order of 6-10" may be expected.
- While the track of this storm is still not nailed down yet, the central Plains and Midwest ought to see the most intense snow accumulations.

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