December 24-25 Clipper System

**This post is dedicated to Ana Marquez-Greene, age 6, a victim of the Connecticut shootings.**

It is becoming increasingly likely that a small clipper system will deliver precipitation and disturbed weather to the Plains and Midwest during Christmas Eve and the day of Christmas.

Above is the NAM model forecast, valid for the afternoon of Christmas Eve. We see a small, negatively tilted area of vorticity in the upper left hand part of the collage of images above. The system looks to be a clipper, pushing to the southeast from Canada. It's lack of sustained moisture and unimpressive isobar gradient tells me this will not be a winter storm event at all. 6 hour precipitation values in the upper right tell of light snow ongoing in the Upper Midwest in response to this system. High pressure stationed in the Plains will provide a base for cold air in the region.

Two forecasts, both based off of the NAM model but known as parallels (slightly changed from the NAM model itself), show 3 hour snow in north Wisconsin and Minnesota during this system as light, nothing extremely impressive. Based on the low prospect for any big accumulating snows, and the NAM's lack of moisture in the top image collage, this does not look like anything to get excited about. If you're into the traditional snowing on Christmas Morning, those in the West Great Lakes may have a shot at such a prospect as the clipper continues pushing to the southeast.

Andrew

CFS Suggests Cold Opening To January 2013

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

The CFS v2 weeklies from WeatherBell are suggesting that January will open on a very chilly note, with temperature anomalies lower than 7 degrees below normal likely.

The CFS v2 has been showing a cold open to January for quite some time, so it's not surprising to see the forecast still standing today. The ECMWF is projecting widespread warming across North America in the stratosphere in 10 days, enhancing the probability that cold weather will push down to the surface in the next couple weeks, as exhibited by the ECMWF forecast below:

Andrew

December 25-27 Potential Significant Winter Storm (Updated 12/20)

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

We'll start with the GFS model. This forecast is for the morning of December 26th. We can see our storm system in the Southeast, with a pressure at or just below the 1000 millibar mark. There is a strong high pressure system in the Plains keeping the system from wanting to move north and towards the Midwest, so it would seem the only logical solution is to have the system move towards the East Coast.

Moving ahead to the morning of December 27th, we see that the system has bombed out and is moving up the East Coast with a minimum pressure of 988 millibars. The proximity to land suggests much of the Northeast and Mid-Atlantic would be severely affected by this solution, should it happen. We still see our high pressure system in the Plains, keeping the system away from the Central US.

We now move on to the ECMWF model, forecast for the same timeframe of December 26th in the morning. Once again, we see the storm system in the Southeast and a strong high pressure in the Plains in an act known as suppression, where the storm is forced south and thus suppressed from moving north. This is a typical negative NAO pattern, something I will address below.

Moving to the morning of December 27th, the reliable ECMWF model brings the system inland into New York, with a minimum pressure just below 980 millibars. This strength difference does prove key, and will prove key in future forecasts. We still see a high pressure system in the Midwest and Plains, but not as strong as the GFS for this same timeframe.

Here's the issue I see with the forecasts the ECMWF and GFS are putting out. They both suppress the storm to a massive degree and then take it up the East Coast in a strikingly textbook negative NAO pattern. But looking at the teleconnection forecasts for the NAO, we actually see a neutral NAO. Such a neutral NAO would not push the storm up the coast, but rather could push the storm out to sea without going up the coast.

The Pacific North American index (PNA) forecast from the 12z models shows a spread in the forecasts during the time period this forecast is valid for. The GFS/GFS Ensembles take a positive PNA, while the ECMWF goes negative PNA. The ECMWF Ensembles are indifferent to either phase. If such a positive PNA evolves, you are not going to see the storm be suppressed south and then go up the East Coast. A positive PNA induces conditions that prefer a storm track in the Ohio Valley and Midwest, like previous ECMWF runs were showing.

If the PNA were negative like the ECMWF is showing, high pressure would build in over the Southeast and provoke a storm track more tailored to the Plains, like we saw earlier in the month. The ECMWF is not handling its teleconnections correctly. A neutral NAO and negative PNA would not suppress the storm south and bring it up the coast, unless this is an anomaly situation, which it should not be.

My feeling is the models are having troubles. I have found that the timeframe between Hour 120-Hour 204 is the most turbulent time in model forecasts. The system remains out in the Pacific, meaning weather balloon data is unable to sample it, and the model forecasts are thus hindered in their accuracy.

If anything, such a strong storm in the East would try to provoke a ridge in the Rockies, possibly then inciting a slight positive PNA as the GFS is suggesting. I won't believe that big suppression forecast unless I see it.

Andrew

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