November 27-29 Potential Winter Storm (Updated 11/20)

This is a post regarding the November 27-29 storm potential. Let's not waste any time and get right into it.

This is the 12z GFS model forecast for the early morning hours of November 27th. This chart combines isobars, areas of high and low pressure, the blue rain/snow line, and precipitation all in one. The GFS is forecasting our system to be resting in the Gulf Coast in the early morning of the 27th. Precipitation looks to be ongoing across a wide swath of that area, with Illinois, Indiana and Missouri possibly getting snow. Mississippi, Louisiana and other states in the same area would likely get rain. This low pressure system is forecasted to be as low as 1015 millibars- pretty high, indicating a weak system.

12 hours later, we are in the afternoon of the 27th. At this point, the storm system has moved northeast and strengthened ever so slightly from 1015mb to 1014mb. Not a very significant change. Precipitation is now very much spread out across the East US. There could be snow ongoing in Pennsylvania, Indiana, Ohio and western New York if this verified. The mountain influence would also favor snow in Virginia. Take note of how the blue rain/snow line is not pushing south in the wake of this storm system. If we take a look farther east, we see a bowing formation of observed precipitation, telling me that this is likely a cold front. If it is, why is the cold air not pushing in behind? A very mysterious case.

Now, we have moved forward to the morning of November 28th, when many are just getting up. Our storm system has now jumped ship and is offshore, with a low pressure of 1000mb. A fair drop from earlier, but still a weak system. Precipitation now extends well offshore, but snow is abundant across the Northeast, with New York and Pennsylvania getting the best inland snow. Vermont and New Hampshire also get in on the action. However, as I showed above, the blue rain/snow line is still not filling south behind the system.

I can't say I'm liking the GFS' solution at the moment, because the GFS has not been as good as the ECMWF model, which we will take a look at next.

This is hour 144 of the ECMWF, also known as the morning of November 26th. We are already seeing a large time difference between the ECMWF and GFS, something that tells me these two models are done agreeing and are trying to get their way. The ECMWF has our storm system on the Oklahoma-Kansas border at a minimum pressure of roughly 1001 millibars. The ECMWF has the system further north and stronger than the GFS at this point. Notice the cold air remains to the north. This is not surprising, seeing as the system has just exited the mountains, where it lost strength and thus its ability to pull in air behind it.

Now, we have gone forward to the morning of November 27th, just 24 hours later. The ECMWF is predicting this system to be in Indiana at this point, with a minimum pressure reading of 1006 millibars. This means that the system has actually weakened as it has gone through the Plains. Note the cold air now flowing in behind the system in the Plains. This is what the GFS is not showing, and I believe that it should be. What is also interesting is that this system and another down on the Arklatex borders are sharing isobars, indicating that they may be trying to come together.

So we now have two 'camps' (different solutions by different models are frequently called camps in the weather world, with one camp taking one scenario and another camp taking a different scenario). The ECMWF is in the north camp (takes the storm north), while the GFS is in the south camp (takes the storm south and up the coast).

Sure, models are good, but we can never forget the ensembles.

This is the GFS Ensemble mean for the morning of November 27th. Unfortunately, the blue rain/snow line did not print for this image. The ensembles are showing abundant precipitation through the Ohio Valley and down into the Gulf Coast. A wider coverage of precipitation near the coast tells me that the ensembles may be favoring that area for the system's placement, just like the GFS model itself.

We now fast forward 12 hours to the evening of November 27th, where we find the blue rain/snow line has returned. Precipitation is most abundant from the Ohio Valley into the Mid Atlantic and Southeast. The placement of that abundance tells me that the low pressure system is taking the route that the 12z GFS took. That is a big victory for those in the Northeast wanting snow to fall from this system.

Another 12 hours later, on the morning of the 28th, the low pressure system has indeed moved up the coast, and it looks like heavy precipitation has hit the Northeast (possibly as snow) as the system moved up and out of the region. Because the ensembles were so indecisive today compared to yesterday as far as low pressure placement goes, I'm not prepared to make a comment on these ensembles until I see the ECMWF Ensembles.

North Camp: ECMWF
South Camp: GFS, GFS ENS

The ECMWF ensembles are currently having this storm system in southwest Oklahoma, a bit further south than the ECMWF model itself. The system has a minimum pressure of 1009 millibars, which is about as strong as the GFS model. Keep a very close eye on that ridge in the Southeast. That ridge is what will make or break this storm for one area or another. Right now, the ECMWF Ensembles appear very concentrated on the idea of the ridge being present during this timeframe- a good sign for Midwesterners.

24 hours later, on the morning of November 27th, the ECMWF Ensembles have this system in the Midwest/Ohio Valley with a minimum pressure of 1015 millibars, meaning that it has weakened as it moved from the Rockies to the Midwest. The Southeast ridge has now moved offshore, but its effects are still present so that the storm was moved north. This does make the ECMWF ensembles a member of the North Camp.

Latest Camp Stats:
North Camp: ECMWF, ECMWF ENS
South Camp: GFS, GFS ENS

I took the liberty of looking at a few other models for their take on the situation, and I have sorted them into camps for ease of access. Here is the latest model camp stats.

North Camp: ECMWF, ECMWF ENS, GGEM (CMC), 
South Camp: GFS, GFS ENS, DGEX, 

It would appear that the North Camp has a slight lead. But let's now look at how this could all play out and not just stare at the models.

This is the 500mb height anomaly forecast from the GFS model (the same one we saw above), valid on the morning of November 27th. You might look at it and see lots of colors, but check out the upper right hand corner. See that big red splat to the east of Greenland? Not only is that a negative NAO, it is an east-based negative NAO. What's the difference, you might ask?

Image from NCSU

This image above shows height anomalies of an east-based negative NAO and a west-based negative NAO on the left hand side. The right hand side shows temperature anomalies for their respective bases. When I say 'east-based', it is in relation to Greenland. In this case, an east-based negative NAO would have the ridge of high pressure (the negative NAO) to the east of Greenland. In a similar fashion, a west-based negative NAO allows the ridge to be west of Greenland. Both types of negative NAO are different from each other. In west-based negative NAO's, the Northeast can expect to see the storms the most often. In an east-based negative NAO, the Midwest tends to get the best storms. Do you see why I'm skeptical, with the GFS showing an east-based negative NAO but bringing the storm up the East Coast?

Also, if you look back to the 500mb height anomaly image, you'll see that a positive PNA is trying to form, shown by a ridge in the northeast Pacific. A positive PNA is best for the Midwest for storms, enhancing my suspicion of the 12z GFS.

In conclusion, this system remains quite a ways away. However, I feel that the ECMWF and the ECMWF Ensembles have the best handle on this system at the moment. I pointed out flaws in the GFS, and I will discard its scenario for that reason.

Andrew