December 25-28 Potential Significant Winter Storm (Updated 12/19)

We continue to see the potential for a significant winter storm somewhere in the East US between December 25-28. Let's see what the models are saying today.

I've decided today will be dedicated to the ensembles, because I just don't trust the operational models this far out to handle a storm like this on their own. As we saw with the Dec. 19-22 blizzard, it can take a while before a reliable solution can be developed, so we'll look to the ensembles for the time being. This forecast is for Christmas morning, and we can see our storm system sitting in the Gulf Coast region. The ensembles seem pretty disorganized as far as exact location goes, seeing as there is no defined low pressure area on the image for this body of precipitation.

Moving ahead to the morning of December 27th, we see the ensembles have solidified in a system more confidently, with a low pressure signature now showing up just offshore of New Jersey. We see the rain/snow line hugging the storm as well, a solution that would bring heavy snow to much of the Northeast.

Considering the general neutrality of the North Atlantic Oscillation through this timeframe, with a slight positive PNA, I wouldn't go as far as to completely trust this solution. It will need to be watched, but I'm not totally confident in it.

Moving on to the Canadian GGEM Ensembles, we find our system stationed in the Southwest, producing copious amounts of precipitation on the evening of December 26th. Keep an eye on these timing issues. The GGEM Ensembles appear to be following a track similar to the GFS Ensembles, although they appear to want to pull the storm slightly further north, possibly in response to a weak positive PNA.

The morning of December 28th brings the storm well offshore per the GGEM Ensembles. Some precipitation is reaching the immediate coastal areas as snow, but with the system so far offshore, I would like to disprove that precipitation forecast.

I don't trust the GGEM Ensembles usually due to their base off the GGEM model, but they do seem to agree with the idea of a slight positive PNA having an effect on this system. It will have to be watched.

Morning of December 26th

Morning of December 27th

The ECMWF Ensembles take a totally different track on this system, preferring to have the system start out in southern Tennessee before moving northeast into the Northeast US, not offshore. A sharp cold outbreak then follows the energy. Such a track would likely bring the best snows to the Ohio Valley and central Great Lakes, but because the ECMWF EPS precipitation graphics are so expensive (if the European weather center even produces them), we have no way of knowing.

For those wanting the ECMWF model itself, here's a couple precipitation graphics:

All precipitation above the dark blue line would be snow- and that would be a LOT of snow for the western Great Lakes and Midwest, possibly in excess of 10 inches.

Forecast preference: ECMWF ENS
Confidence: 20%

Andrew

December 19-22 Blizzard NowCast (12/19)

**This post is dedicated to Olivia Engel, age 6, a victim of the Connecticut shootings.**

Surface analysis across the nation shows that our storm system has now ejected from the Plains and is on its way to moving across the Midwest. We already see a wide precipitation shield spread out across the Plains in the presence of this storm system and another disturbance latched onto the north side of this storm system. Blizzard conditions have already been reported in many areas across Kansas and Nebraska in response to this storm's presence in the Plains at this time.

The wide array of winter weather bulletins shows us just how intense this storm system is. We have winter weather advisories in purple, and those stetch from the Front Range in Colorado to central Ohio. Dark blue colors represent Winter Storm Watches, which now only appears valid for the Chicagoland area. Pink implies a Winter Storm Warning, and this is where we will see good accumulations above 6 inches. The reddish shading is the fearsome Blizzard Warning, combining heavy snow and very high winds to make this event a big one, and one worthy of blizzard warnings.

This snowfall forecast fresh off the presses from the 18z GFS shows that accumulations well above 12 inches are likely to fall in southwest Wisconsin and east Illinois as the storm meanders on through. If such a development came true, travel would be paralyzed in cities in this region. Howling winds would not help the snowplow's efforts to keep the roads cleared.

Another, more high resolution graphic from the NAM model shows amounts significantly lower than the GFS is showing. I can't say I have a preference for either based on these single model runs, but if I were to pick one model, I would side with the GFS for its consistency and the definite likelihood of such high snowfall amounts to verify.

Travel will be severely disrupted in many regions across the Midwest, particularly Wisconsin, Iowa, Nebraska and northwest Illinois. Again, very high winds will make travel difficult, and snow days are likely to happen upon these areas, especially Wisconsin and Iowa if the GFS verifies.

More updates as this situation develops.

Andrew

December 25-28 Potential Significant Winter Storm

I have upgraded the title of this potential winter storm from 'Winter Storm' to 'Significant Winter Storm'.

**This post is dedicated to Rachel Davino, age 29, a victim of the Connecticut shootings.**

We'll start with today's 12z GFS model and its forecast for the 500mb height layer. This is the forecast for the evening of December 27th. We see a deep depression in the 500mb height layer in the Midwest, signifying our storm system. It looks to be centered in central Illinois. If this looks familiar to you from the upcoming Dec. 19-22 blizzard forecasts, then you're correct- but the buck stops there. There's one thing that I think will make this system fairly big: The Rex Block.

Theweatherprediction.com

This is an image of a Rex Block from the site of theweatherprediction.com. The Rex Block involves a low pressure system in the Southwest US, and a high pressure system in the Pacific Northwest. In response to this highly meridional flow (north to south wind pattern), storm systems in the presence of a Rex Block are strengthened, as shown in this example of a storm system in a Rex Block-turned-Omega Block during May 2005.

While we're on the topic of this storm system in May, let's see if this storm system has any similarities with the GFS forecast above, and the ECMWF forecast below.

Are you surprised? It's almost like a carbon copy of the May 2005 storm is now in this ECMWF forecast for the morning of December 26th. Putting aside the (eventually crucial) time differences, we see the Rex Block has now transitioned to an Omega Block, as was pictured in the May 2005 picture above.

It should be noted that the ECMWF is showing an Omega Block while the GFS is showing a fair Rex Block, with a system in the Southwest and slight ridge in the Pac. NW. The slight difference in these two, as small as it may seem, is actually quite significant, and something we will need to watch later on.

What does this mean? It means that the models are in rare agreement of high pressure setting up on the West Coast, leading to an Omega Block (possibly Rex Block) over the area that then gives the storm system a much more amplified feel. Naturally, as the Omega block/Rex block produces a much more meridional flow, we see the vorticity centers in the low pressure system enhanced by the meridional flow, and suddenly a much stronger storm is born. Another thing interesting is the level of agreement from the ECMWF and GFS on the placement of the storm. Given, there are some time issues that need to be worked out, but it's reassuring to see this storm now showing up on more than one model.

For those wondering what precipitation forecasts are coming out of the models, here's a few images:

Evening of Dec. 27th precipitation forecast. All precip.
above blue line is snow. Multiply value on legend with 10 to get snow.

Andrew

December 19-22 High-Impact Blizzard Event

**This post is dedicated to Rachel Davino, age 29, a victim of the Connecticut shootings.**

I am holding firm with yesterday's call as far as the track goes, but PLEASE DISREGARD THE SNOWFALL FORECAST. I have a new snowfall forecast below. I still expect the system to move from the Plains through the Midwest, most likely through north Indiana and southern Michigan. These areas will have a tougher time getting any precipitation, as dry slotting issues may occur. As the storm strengthens, it will wrap in air through its southern flank in response to very humid air (the precipitation) wrapping around its northern side. Wherever the low pressure system travels, dry slotting could cut down precipitation of any kind.

I'm still holding with my idea of a rain-to-snow situation in much of north Illinois, southern Michigan, Missouri and Kansas. The main snows for these areas will depend on the strength of the backside snow. Models were earlier indicating a heavy band of backside snow coming through the Midwest, and if such a solution happened, a few inches could fall.

The model consensus has aligned itself overnight with my track, leaving me with the logical option to hold this forecast in place.

Snowfall is expected to be heavy, and, combined with winds, will make travel dangerous (see Impact Scale below). I am anticipating amounts of 1 to 3 inches in the light blue colors. This is the tricky part, because this involves the areas right along the rain-snow line, something that troubles may forecasters and such rain-snow line forecasts are prone to 'busting', or failing. I am confident in a good 3-6 inch accumulation in the darker blue, and a 6-12 inch plus snowfall over southern Wisconsin, much of Iowa, extreme northwest Missouri, extreme southeast Nebraska and northeast Kansas. Amounts will increase as you move northeast through this forecast in response to the storm strengthening. Again, such a strengthening storm and the tricky rain snow line makes forecasts difficult to confirm. Watch for amounts over a foot in Wisconsin.

Green: Low Impact
Yellow: Moderate Impact
Red: Heavy Impact
Pink: Major Impact
Blue: Extreme Impact

We here at The Weather Centre want to put out forecasts that best interest you and your safety. For that reason, earlier this year, we developed our Exclusive Impact Scale, where I can identify areas where travel may be hazardous, among other things, depending on the season. My forecast for this system involves a Low Impact forecast for much of the region that will see some precipitation that is not wintry from this system. Low accumulation spots on the order of 1-3 inches then includes the Moderate Impact spot, where some people could see slippery roads and slightly hazardous travel weather. Heavy Impact status is expected in much of the 3-6 inch accumulation stripe, where there will be fair travel delays. Flights will have issues, and driving will be a 'challenge'. Unnecessary travel is not advised. Finally, I hesitantly put out a pink stripe of Major Impact status for the 6 inch+ areas. The only reason I do this is for the wind- very high winds will make whiteout conditions common in rural areas, and long-lasting travel delays will stem from those areas.

And now, our final graphic is for the kids. This is another tool recently developed here at The Weather Centre. I have made a forecast chance for a snow day based off of my previous snow day experiences and putting myself in the situation of a bus driver, administrator and parent, the big question being "Can kids safely get to school?" Although there are 10 categories, DO NOT use them as a 1/10, or 2/10 chance- that is not the intent.

I marked down generally low probabilities for much of the impacted region simply because of the lack of significant snow. I did put a 'Not Very Good' chance for the 3-6 inch range (you may have to adjust it west a tad, or clip off the southern edge of the yellow zone). The Medium forecast is again based off of blizzard-like conditions (or just plain blizzard conditions). I then went ahead and went up to 'Good' for Wisconsin (you should consider it likely). I did not put likely in there without loss of visuals of the counties.

You can also use a formula I made a year or two ago for snow day potential by clicking here.

Hunker down, folks, this one looks to be a nice introduction to winter. I will have more updates on the Facebook page, which you can find on the right-hand sidebar. If you haven't given it a 'Like', go ahead and do it- you can find my latest thoughts not seen here on the blog.

Andrew

December 26-28 Potential Winter Storm

**This post is dedicated to Daniel Barden, age 7, a victim of the Connecticut shootings.**

The potential for a winter storm in the Plains and general East US is present between the 26th and the 28th, as the ECMWF is now showing such a potential.

The above image is for the morning of December 25th. We see our system as a deep depression in the Plains, in a 'U' shaped depression. Such a deep depression gives me three clues: 

One, this system could be pretty strong when it ejects from the system.

Two, the deep-ness of the depression and neutral tilt tells me it could evolve into a closed low.

Three, a cold air outbreak could follow this system if cold air is available up north.

That ridge present in the eastern US displays a sort of positive PNA feel, with the typical positive PNA ridge on the West Coast also in play. This looks to be a Midwest/Plains storm transitioning to the coast, where the Mid-Atlantic and Northeast may be affected. No guarantees, however.

This is the Hour 240 forecast from the ECMWF, once again dealing with 500mb heights. We see that the storm system has now evolved into a very deep closed low, meaning it has been cut off from the jet stream. Closed lows are notorious for staying in place for days on end, but this does not look like it will happen. We have the system squeezed in between two high pressure systems. One is in New England, prohibiting the storm from going directly northeast, and the other ridge is in the Rockies. Considering the ridge in the Rockies looks much stronger, and the angle of the ridge favors a direct east path, I do not find the idea of the Midwest, Ohio Valley and Northeast getting a winter storm totally off the table. This closed low would indeed bring in a lot of cold air to the Midwest and Great Lakes.

The ECMWF also shows a very nice accumulation of snow in the Midwest from this system on the 26th, shown as the colored values above the dark blue 32 degree temperature line:

And the very high amounts of precipitation in the Mid-Atlantic some time later on Dec. 27 tell a possible severe weather story:

All in all, it's looking pretty interesting. While we are still a while away, we are in a negative PNA pattern favorable for an amplified storm track. If a steep negative NAO evolves, we could have a highly meridional flow more conducive for strong storms.

Andrew